1<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> 2<html> 3<head> 4<meta http-equiv="Content-Type" content="text/html; charset=UTF-8"> 5<title>Users' Guide</title> 6<link rel="stylesheet" href="../../../doc/src/boostbook.css" type="text/css"> 7<meta name="generator" content="DocBook XSL Stylesheets V1.79.1"> 8<link rel="home" href="../index.html" title="The Boost C++ Libraries BoostBook Documentation Subset"> 9<link rel="up" href="../proto.html" title="Chapter 32. Boost.Proto"> 10<link rel="prev" href="../proto.html" title="Chapter 32. Boost.Proto"> 11<link rel="next" href="reference.html" title="Reference"> 12</head> 13<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"> 14<table cellpadding="2" width="100%"><tr> 15<td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../boost.png"></td> 16<td align="center"><a href="../../../index.html">Home</a></td> 17<td align="center"><a href="../../../libs/libraries.htm">Libraries</a></td> 18<td align="center"><a href="http://www.boost.org/users/people.html">People</a></td> 19<td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td> 20<td align="center"><a href="../../../more/index.htm">More</a></td> 21</tr></table> 22<hr> 23<div class="spirit-nav"> 24<a accesskey="p" href="../proto.html"><img src="../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../proto.html"><img src="../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../index.html"><img src="../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="reference.html"><img src="../../../doc/src/images/next.png" alt="Next"></a> 25</div> 26<div class="section"> 27<div class="titlepage"><div><div><h2 class="title" style="clear: both"> 28<a name="proto.users_guide"></a><a class="link" href="users_guide.html" title="Users' Guide">Users' Guide</a> 29</h2></div></div></div> 30<div class="toc"><dl class="toc"> 31<dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.getting_started">Getting Started</a></span></dt> 32<dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.front_end">Fronts Ends: Defining 33 Terminals and Non-Terminals of Your EDSL</a></span></dt> 34<dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.intermediate_form">Intermediate 35 Form: Understanding and Introspecting Expressions</a></span></dt> 36<dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.back_end">Back Ends: Making Expression 37 Templates Do Useful Work</a></span></dt> 38<dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.examples">Examples</a></span></dt> 39<dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.resources">Background and Resources</a></span></dt> 40<dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.glossary">Glossary</a></span></dt> 41</dl></div> 42<h4> 43<a name="proto.users_guide.h0"></a> 44 <span class="phrase"><a name="proto.users_guide.compilers__compiler_construction_toolkits__and_proto"></a></span><a class="link" href="users_guide.html#proto.users_guide.compilers__compiler_construction_toolkits__and_proto">Compilers, 45 Compiler Construction Toolkits, and Proto</a> 46 </h4> 47<p> 48 Most compilers have front ends and back ends. The front end parses the text 49 of an input program into some intermediate form like an abstract syntax tree, 50 and the back end takes the intermediate form and generates an executable from 51 it. 52 </p> 53<p> 54 A library built with Proto is essentially a compiler for an embedded domain-specific 55 language (EDSL). It also has a front end, an intermediate form, and a back 56 end. The front end is comprised of the symbols (a.k.a., terminals), members, 57 operators and functions that make up the user-visible aspects of the EDSL. 58 The back end is made of evaluation contexts and transforms that give meaning 59 and behavior to the expression templates generated by the front end. In between 60 is the intermediate form: the expression template itself, which is an abstract 61 syntax tree in a very real sense. 62 </p> 63<p> 64 To build a library with Proto, you will first decide what your interface will 65 be; that is, you'll design a programming language for your domain and build 66 the front end with tools provided by Proto. Then you'll design the back end 67 by writing evaluation contexts and/or transforms that accept expression templates 68 and do interesting things with them. 69 </p> 70<p> 71 This users' guide is organized as follows. After a <a class="link" href="users_guide.html#boost_proto.users_guide.getting_started" title="Getting Started">Getting 72 Started guide</a>, we'll cover the tools Proto provides for defining and 73 manipulating the three major parts of a compiler: 74 </p> 75<div class="variablelist"> 76<p class="title"><b></b></p> 77<dl class="variablelist"> 78<dt><span class="term"><a class="link" href="users_guide.html#boost_proto.users_guide.front_end" title="Fronts Ends: Defining Terminals and Non-Terminals of Your EDSL">Front Ends</a></span></dt> 79<dd><p> 80 How to define the aspects of your EDSL with which your users will interact 81 directly. 82 </p></dd> 83<dt><span class="term"><a class="link" href="users_guide.html#boost_proto.users_guide.intermediate_form" title="Intermediate Form: Understanding and Introspecting Expressions">Intermediate 84 Form</a></span></dt> 85<dd><p> 86 What Proto expression templates look like, how to discover their structure 87 and access their constituents. 88 </p></dd> 89<dt><span class="term"><a class="link" href="users_guide.html#boost_proto.users_guide.back_end" title="Back Ends: Making Expression Templates Do Useful Work">Back Ends</a></span></dt> 90<dd><p> 91 How to define evaluation contexts and transforms that make expression 92 templates do interesting things. 93 </p></dd> 94</dl> 95</div> 96<p> 97 After that, you may be interested in seeing some <a class="link" href="users_guide.html#boost_proto.users_guide.examples" title="Examples">Examples</a> 98 to get a better idea of how the pieces all fit together. 99 </p> 100<div class="section"> 101<div class="titlepage"><div><div><h3 class="title"> 102<a name="boost_proto.users_guide.getting_started"></a><a class="link" href="users_guide.html#boost_proto.users_guide.getting_started" title="Getting Started">Getting Started</a> 103</h3></div></div></div> 104<div class="toc"><dl class="toc"> 105<dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.getting_started.installing_proto">Installing 106 Proto</a></span></dt> 107<dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.getting_started.naming">Naming 108 Conventions</a></span></dt> 109<dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.getting_started.hello_world">Hello 110 World</a></span></dt> 111<dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.getting_started.hello_calculator">Hello 112 Calculator</a></span></dt> 113</dl></div> 114<div class="section"> 115<div class="titlepage"><div><div><h4 class="title"> 116<a name="boost_proto.users_guide.getting_started.installing_proto"></a><a class="link" href="users_guide.html#boost_proto.users_guide.getting_started.installing_proto" title="Installing Proto">Installing 117 Proto</a> 118</h4></div></div></div> 119<h6> 120<a name="boost_proto.users_guide.getting_started.installing_proto.h0"></a> 121 <span class="phrase"><a name="boost_proto.users_guide.getting_started.installing_proto.getting_proto"></a></span><a class="link" href="users_guide.html#boost_proto.users_guide.getting_started.installing_proto.getting_proto">Getting 122 Proto</a> 123 </h6> 124<p> 125 You can get Proto by downloading Boost (Proto is in version 1.37 and later), 126 or by accessing Boost's SVN repository on SourceForge.net. Just go to 127 <a href="http://svn.boost.org/trac/boost/wiki/BoostSubversion" target="_top">http://svn.boost.org/trac/boost/wiki/BoostSubversion</a> 128 and follow the instructions there for anonymous SVN access. 129 </p> 130<h6> 131<a name="boost_proto.users_guide.getting_started.installing_proto.h1"></a> 132 <span class="phrase"><a name="boost_proto.users_guide.getting_started.installing_proto.building_with_proto"></a></span><a class="link" href="users_guide.html#boost_proto.users_guide.getting_started.installing_proto.building_with_proto">Building 133 with Proto</a> 134 </h6> 135<p> 136 Proto is a header-only template library, which means you don't need to 137 alter your build scripts or link to any separate lib file to use it. All 138 you need to do is <code class="computeroutput"><span class="preprocessor">#include</span> 139 <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">proto</span><span class="special">/</span><span class="identifier">proto</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span></code>. Or, you might decide to just include 140 the core of Proto (<code class="computeroutput"><span class="preprocessor">#include</span> 141 <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">proto</span><span class="special">/</span><span class="identifier">core</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span></code>) and whichever contexts and transforms 142 you happen to use. 143 </p> 144<h6> 145<a name="boost_proto.users_guide.getting_started.installing_proto.h2"></a> 146 <span class="phrase"><a name="boost_proto.users_guide.getting_started.installing_proto.requirements"></a></span><a class="link" href="users_guide.html#boost_proto.users_guide.getting_started.installing_proto.requirements">Requirements</a> 147 </h6> 148<p> 149 Proto depends on Boost. You must use either Boost version 1.34.1 or higher, 150 or the version in SVN trunk. 151 </p> 152<h6> 153<a name="boost_proto.users_guide.getting_started.installing_proto.h3"></a> 154 <span class="phrase"><a name="boost_proto.users_guide.getting_started.installing_proto.supported_compilers"></a></span><a class="link" href="users_guide.html#boost_proto.users_guide.getting_started.installing_proto.supported_compilers">Supported 155 Compilers</a> 156 </h6> 157<p> 158 Currently, Boost.Proto is known to work on the following compilers: 159 </p> 160<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "> 161<li class="listitem"> 162 Visual C++ 8 and higher 163 </li> 164<li class="listitem"> 165 GNU C++ 3.4 and higher 166 </li> 167<li class="listitem"> 168 Intel on Linux 8.1 and higher 169 </li> 170<li class="listitem"> 171 Intel on Windows 9.1 and higher 172 </li> 173</ul></div> 174<div class="note"><table border="0" summary="Note"> 175<tr> 176<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../doc/src/images/note.png"></td> 177<th align="left">Note</th> 178</tr> 179<tr><td align="left" valign="top"><p> 180 Please send any questions, comments and bug reports to eric <at> 181 boostpro <dot> com. 182 </p></td></tr> 183</table></div> 184</div> 185<div class="section"> 186<div class="titlepage"><div><div><h4 class="title"> 187<a name="boost_proto.users_guide.getting_started.naming"></a><a class="link" href="users_guide.html#boost_proto.users_guide.getting_started.naming" title="Naming Conventions">Naming 188 Conventions</a> 189</h4></div></div></div> 190<p> 191 Proto is a large library and probably quite unlike any library you've used 192 before. Proto uses some consistent naming conventions to make it easier 193 to navigate, and they're described below. 194 </p> 195<h6> 196<a name="boost_proto.users_guide.getting_started.naming.h0"></a> 197 <span class="phrase"><a name="boost_proto.users_guide.getting_started.naming.functions"></a></span><a class="link" href="users_guide.html#boost_proto.users_guide.getting_started.naming.functions">Functions</a> 198 </h6> 199<p> 200 All of Proto's functions are defined in the <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">proto</span></code> 201 namespace. For example, there is a function called <code class="computeroutput"><span class="identifier">value</span><span class="special">()</span></code> defined in <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">proto</span></code> 202 that accepts a terminal expression and returns the terminal's value. 203 </p> 204<h6> 205<a name="boost_proto.users_guide.getting_started.naming.h1"></a> 206 <span class="phrase"><a name="boost_proto.users_guide.getting_started.naming.metafunctions"></a></span><a class="link" href="users_guide.html#boost_proto.users_guide.getting_started.naming.metafunctions">Metafunctions</a> 207 </h6> 208<p> 209 Proto defines <span class="emphasis"><em>metafunctions</em></span> that correspond to each 210 of Proto's free functions. The metafunctions are used to compute the functions' 211 return types. All of Proto's metafunctions live in the <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">result_of</span></code> 212 namespace and have the same name as the functions to which they correspond. 213 For instance, there is a class template <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">value</span><span class="special"><></span></code> that you can use to compute the 214 return type of the <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">value</span><span class="special">()</span></code> function. 215 </p> 216<h6> 217<a name="boost_proto.users_guide.getting_started.naming.h2"></a> 218 <span class="phrase"><a name="boost_proto.users_guide.getting_started.naming.function_objects"></a></span><a class="link" href="users_guide.html#boost_proto.users_guide.getting_started.naming.function_objects">Function 219 Objects</a> 220 </h6> 221<p> 222 Proto defines <span class="emphasis"><em>function object</em></span> equivalents of all of 223 its free functions. (A function object is an instance of a class type that 224 defines an <code class="computeroutput"><span class="keyword">operator</span><span class="special">()</span></code> 225 member function.) All of Proto's function object types are defined in the 226 <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">functional</span></code> namespace and have the same 227 name as their corresponding free functions. For example, <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">functional</span><span class="special">::</span><span class="identifier">value</span></code> is a class that defines a function 228 object that does the same thing as the <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">value</span><span class="special">()</span></code> free function. 229 </p> 230<h6> 231<a name="boost_proto.users_guide.getting_started.naming.h3"></a> 232 <span class="phrase"><a name="boost_proto.users_guide.getting_started.naming.primitive_transforms"></a></span><a class="link" href="users_guide.html#boost_proto.users_guide.getting_started.naming.primitive_transforms">Primitive 233 Transforms</a> 234 </h6> 235<p> 236 Proto also defines <span class="emphasis"><em>primitive transforms</em></span> -- class types 237 that can be used to compose larger transforms for manipulating expression 238 trees. Many of Proto's free functions have corresponding primitive transforms. 239 These live in the <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">proto</span></code> 240 namespace and their names have a leading underscore. For instance, the 241 transform corresponding to the <code class="computeroutput"><span class="identifier">value</span><span class="special">()</span></code> function is called <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_value</span></code>. 242 </p> 243<p> 244 The following table summarizes the discussion above: 245 </p> 246<div class="table"> 247<a name="boost_proto.users_guide.getting_started.naming.t0"></a><p class="title"><b>Table 32.1. Proto Naming Conventions</b></p> 248<div class="table-contents"><table class="table" summary="Proto Naming Conventions"> 249<colgroup> 250<col> 251<col> 252</colgroup> 253<thead><tr> 254<th> 255 <p> 256 Entity 257 </p> 258 </th> 259<th> 260 <p> 261 Example 262 </p> 263 </th> 264</tr></thead> 265<tbody> 266<tr> 267<td> 268 <p> 269 Free Function 270 </p> 271 </td> 272<td> 273 <p> 274 <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">value</span><span class="special">()</span></code> 275 </p> 276 </td> 277</tr> 278<tr> 279<td> 280 <p> 281 Metafunction 282 </p> 283 </td> 284<td> 285 <p> 286 <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">value</span><span class="special"><></span></code> 287 </p> 288 </td> 289</tr> 290<tr> 291<td> 292 <p> 293 Function Object 294 </p> 295 </td> 296<td> 297 <p> 298 <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">functional</span><span class="special">::</span><span class="identifier">value</span></code> 299 </p> 300 </td> 301</tr> 302<tr> 303<td> 304 <p> 305 Transform 306 </p> 307 </td> 308<td> 309 <p> 310 <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_value</span></code> 311 </p> 312 </td> 313</tr> 314</tbody> 315</table></div> 316</div> 317<br class="table-break"> 318</div> 319<div class="section"> 320<div class="titlepage"><div><div><h4 class="title"> 321<a name="boost_proto.users_guide.getting_started.hello_world"></a><a class="link" href="users_guide.html#boost_proto.users_guide.getting_started.hello_world" title="Hello World">Hello 322 World</a> 323</h4></div></div></div> 324<p> 325 Below is a very simple program that uses Proto to build an expression template 326 and then execute it. 327 </p> 328<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">iostream</span><span class="special">></span> 329<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">proto</span><span class="special">/</span><span class="identifier">proto</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> 330<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">typeof</span><span class="special">/</span><span class="identifier">std</span><span class="special">/</span><span class="identifier">ostream</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> 331<span class="keyword">using</span> <span class="keyword">namespace</span> <span class="identifier">boost</span><span class="special">;</span> 332 333<span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special"><</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">ostream</span> <span class="special">&</span> <span class="special">>::</span><span class="identifier">type</span> <span class="identifier">cout_</span> <span class="special">=</span> <span class="special">{</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">};</span> 334 335<span class="keyword">template</span><span class="special"><</span> <span class="keyword">typename</span> <span class="identifier">Expr</span> <span class="special">></span> 336<span class="keyword">void</span> <span class="identifier">evaluate</span><span class="special">(</span> <span class="identifier">Expr</span> <span class="keyword">const</span> <span class="special">&</span> <span class="identifier">expr</span> <span class="special">)</span> 337<span class="special">{</span> 338 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">default_context</span> <span class="identifier">ctx</span><span class="special">;</span> 339 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">eval</span><span class="special">(</span><span class="identifier">expr</span><span class="special">,</span> <span class="identifier">ctx</span><span class="special">);</span> 340<span class="special">}</span> 341 342<span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span> 343<span class="special">{</span> 344 <span class="identifier">evaluate</span><span class="special">(</span> <span class="identifier">cout_</span> <span class="special"><<</span> <span class="string">"hello"</span> <span class="special"><<</span> <span class="char">','</span> <span class="special"><<</span> <span class="string">" world"</span> <span class="special">);</span> 345 <span class="keyword">return</span> <span class="number">0</span><span class="special">;</span> 346<span class="special">}</span> 347</pre> 348<p> 349 This program outputs the following: 350 </p> 351<pre class="programlisting">hello, world 352</pre> 353<p> 354 This program builds an object representing the output operation and passes 355 it to an <code class="computeroutput"><span class="identifier">evaluate</span><span class="special">()</span></code> 356 function, which then executes it. 357 </p> 358<p> 359 The basic idea of expression templates is to overload all the operators 360 so that, rather than evaluating the expression immediately, they build 361 a tree-like representation of the expression so that it can be evaluated 362 later. For each operator in an expression, at least one operand must be 363 Protofied in order for Proto's operator overloads to be found. In the expression 364 ... 365 </p> 366<pre class="programlisting"><span class="identifier">cout_</span> <span class="special"><<</span> <span class="string">"hello"</span> <span class="special"><<</span> <span class="char">','</span> <span class="special"><<</span> <span class="string">" world"</span> 367</pre> 368<p> 369 ... the Protofied sub-expression is <code class="computeroutput"><span class="identifier">cout_</span></code>, 370 which is the Proto-ification of <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span></code>. 371 The presence of <code class="computeroutput"><span class="identifier">cout_</span></code> "infects" 372 the expression, and brings Proto's tree-building operator overloads into 373 consideration. Any literals in the expression are then Protofied by wrapping 374 them in a Proto terminal before they are combined into larger Proto expressions. 375 </p> 376<p> 377 Once Proto's operator overloads have built the expression tree, the expression 378 can be lazily evaluated later by walking the tree. That is what <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">eval</span><span class="special">()</span></code> 379 does. It is a general tree-walking expression evaluator, whose behavior 380 is customizable via a <span class="emphasis"><em>context</em></span> parameter. The use of 381 <code class="computeroutput"><a class="link" href="../boost/proto/context/default_context.html" title="Struct default_context">proto::default_context</a></code> 382 assigns the standard meanings to the operators in the expression. (By using 383 a different context, you could give the operators in your expressions different 384 semantics. By default, Proto makes no assumptions about what operators 385 actually <span class="emphasis"><em>mean</em></span>.) 386 </p> 387<h6> 388<a name="boost_proto.users_guide.getting_started.hello_world.h0"></a> 389 <span class="phrase"><a name="boost_proto.users_guide.getting_started.hello_world.proto_design_philosophy"></a></span><a class="link" href="users_guide.html#boost_proto.users_guide.getting_started.hello_world.proto_design_philosophy">Proto 390 Design Philosophy</a> 391 </h6> 392<p> 393 Before we continue, let's use the above example to illustrate an important 394 design principle of Proto's. The expression template created in the <span class="emphasis"><em>hello 395 world</em></span> example is totally general and abstract. It is not tied 396 in any way to any particular domain or application, nor does it have any 397 particular meaning or behavior on its own, until it is evaluated in a 398 <span class="emphasis"><em>context</em></span>. Expression templates are really just heterogeneous 399 trees, which might mean something in one domain, and something else entirely 400 in a different one. 401 </p> 402<p> 403 As we'll see later, there is a way to create Proto expression trees that 404 are <span class="emphasis"><em>not</em></span> purely abstract, and that have meaning and 405 behaviors independent of any context. There is also a way to control which 406 operators are overloaded for your particular domain. But that is not the 407 default behavior. We'll see later why the default is often a good thing. 408 </p> 409</div> 410<div class="section"> 411<div class="titlepage"><div><div><h4 class="title"> 412<a name="boost_proto.users_guide.getting_started.hello_calculator"></a><a class="link" href="users_guide.html#boost_proto.users_guide.getting_started.hello_calculator" title="Hello Calculator">Hello 413 Calculator</a> 414</h4></div></div></div> 415<p> 416 "Hello, world" is nice, but it doesn't get you very far. Let's 417 use Proto to build a EDSL (embedded domain-specific language) for a lazily-evaluated 418 calculator. We'll see how to define the terminals in your mini-language, 419 how to compose them into larger expressions, and how to define an evaluation 420 context so that your expressions can do useful work. When we're done, we'll 421 have a mini-language that will allow us to declare a lazily-evaluated arithmetic 422 expression, such as <code class="computeroutput"><span class="special">(</span><span class="identifier">_2</span> 423 <span class="special">-</span> <span class="identifier">_1</span><span class="special">)</span> <span class="special">/</span> <span class="identifier">_2</span> 424 <span class="special">*</span> <span class="number">100</span></code>, 425 where <code class="computeroutput"><span class="identifier">_1</span></code> and <code class="computeroutput"><span class="identifier">_2</span></code> are placeholders for values to be 426 passed in when the expression is evaluated. 427 </p> 428<h6> 429<a name="boost_proto.users_guide.getting_started.hello_calculator.h0"></a> 430 <span class="phrase"><a name="boost_proto.users_guide.getting_started.hello_calculator.defining_terminals"></a></span><a class="link" href="users_guide.html#boost_proto.users_guide.getting_started.hello_calculator.defining_terminals">Defining 431 Terminals</a> 432 </h6> 433<p> 434 The first order of business is to define the placeholders <code class="computeroutput"><span class="identifier">_1</span></code> and <code class="computeroutput"><span class="identifier">_2</span></code>. 435 For that, we'll use the <code class="computeroutput"><a class="link" href="../boost/proto/terminal.html" title="Struct template terminal">proto::terminal<></a></code> 436 metafunction. 437 </p> 438<pre class="programlisting"><span class="comment">// Define a placeholder type</span> 439<span class="keyword">template</span><span class="special"><</span><span class="keyword">int</span> <span class="identifier">I</span><span class="special">></span> 440<span class="keyword">struct</span> <span class="identifier">placeholder</span> 441<span class="special">{};</span> 442 443<span class="comment">// Define the Protofied placeholder terminals</span> 444<span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special"><</span><span class="identifier">placeholder</span><span class="special"><</span><span class="number">0</span><span class="special">></span> <span class="special">>::</span><span class="identifier">type</span> <span class="keyword">const</span> <span class="identifier">_1</span> <span class="special">=</span> <span class="special">{{}};</span> 445<span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special"><</span><span class="identifier">placeholder</span><span class="special"><</span><span class="number">1</span><span class="special">></span> <span class="special">>::</span><span class="identifier">type</span> <span class="keyword">const</span> <span class="identifier">_2</span> <span class="special">=</span> <span class="special">{{}};</span> 446</pre> 447<p> 448 The initialization may look a little odd at first, but there is a good 449 reason for doing things this way. The objects <code class="computeroutput"><span class="identifier">_1</span></code> 450 and <code class="computeroutput"><span class="identifier">_2</span></code> above do not require 451 run-time construction -- they are <span class="emphasis"><em>statically initialized</em></span>, 452 which means they are essentially initialized at compile time. See the 453 <a class="link" href="appendices.html#boost_proto.appendices.rationale.static_initialization" title="Static Initialization">Static 454 Initialization</a> section in the <a class="link" href="appendices.html#boost_proto.appendices.rationale" title="Appendix C: Rationale">Rationale</a> 455 appendix for more information. 456 </p> 457<h6> 458<a name="boost_proto.users_guide.getting_started.hello_calculator.h1"></a> 459 <span class="phrase"><a name="boost_proto.users_guide.getting_started.hello_calculator.constructing_expression_trees"></a></span><a class="link" href="users_guide.html#boost_proto.users_guide.getting_started.hello_calculator.constructing_expression_trees">Constructing 460 Expression Trees</a> 461 </h6> 462<p> 463 Now that we have terminals, we can use Proto's operator overloads to combine 464 these terminals into larger expressions. So, for instance, we can immediately 465 say things like: 466 </p> 467<pre class="programlisting"><span class="comment">// This builds an expression template</span> 468<span class="special">(</span><span class="identifier">_2</span> <span class="special">-</span> <span class="identifier">_1</span><span class="special">)</span> <span class="special">/</span> <span class="identifier">_2</span> <span class="special">*</span> <span class="number">100</span><span class="special">;</span> 469</pre> 470<p> 471 This creates an expression tree with a node for each operator. The type 472 of the resulting object is large and complex, but we are not terribly interested 473 in it right now. 474 </p> 475<p> 476 So far, the object is just a tree representing the expression. It has no 477 behavior. In particular, it is not yet a calculator. Below we'll see how 478 to make it a calculator by defining an evaluation context. 479 </p> 480<h6> 481<a name="boost_proto.users_guide.getting_started.hello_calculator.h2"></a> 482 <span class="phrase"><a name="boost_proto.users_guide.getting_started.hello_calculator.evaluating_expression_trees"></a></span><a class="link" href="users_guide.html#boost_proto.users_guide.getting_started.hello_calculator.evaluating_expression_trees">Evaluating 483 Expression Trees</a> 484 </h6> 485<p> 486 No doubt you want your expression templates to actually <span class="emphasis"><em>do</em></span> 487 something. One approach is to define an <span class="emphasis"><em>evaluation context</em></span>. 488 The context is like a function object that associates behaviors with the 489 node types in your expression tree. The following example should make it 490 clear. It is explained below. 491 </p> 492<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">calculator_context</span> 493 <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">callable_context</span><span class="special"><</span> <span class="identifier">calculator_context</span> <span class="keyword">const</span> <span class="special">></span> 494<span class="special">{</span> 495 <span class="comment">// Values to replace the placeholders</span> 496 <span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special"><</span><span class="keyword">double</span><span class="special">></span> <span class="identifier">args</span><span class="special">;</span> 497 498 <span class="comment">// Define the result type of the calculator.</span> 499 <span class="comment">// (This makes the calculator_context "callable".)</span> 500 <span class="keyword">typedef</span> <span class="keyword">double</span> <span class="identifier">result_type</span><span class="special">;</span> 501 502 <span class="comment">// Handle the placeholders:</span> 503 <span class="keyword">template</span><span class="special"><</span><span class="keyword">int</span> <span class="identifier">I</span><span class="special">></span> 504 <span class="keyword">double</span> <span class="keyword">operator</span><span class="special">()(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">,</span> <span class="identifier">placeholder</span><span class="special"><</span><span class="identifier">I</span><span class="special">>)</span> <span class="keyword">const</span> 505 <span class="special">{</span> 506 <span class="keyword">return</span> <span class="keyword">this</span><span class="special">-></span><span class="identifier">args</span><span class="special">[</span><span class="identifier">I</span><span class="special">];</span> 507 <span class="special">}</span> 508<span class="special">};</span> 509</pre> 510<p> 511 In <code class="computeroutput"><span class="identifier">calculator_context</span></code>, 512 we specify how Proto should evaluate the placeholder terminals by defining 513 the appropriate overloads of the function call operator. For any other 514 nodes in the expression tree (e.g., arithmetic operations or non-placeholder 515 terminals), Proto will evaluate the expression in the "default" 516 way. For example, a binary plus node is evaluated by first evaluating the 517 left and right operands and adding the results. Proto's default evaluator 518 uses the <a href="../../../libs/typeof/index.html" target="_top">Boost.Typeof</a> 519 library to compute return types. 520 </p> 521<p> 522 Now that we have an evaluation context for our calculator, we can use it 523 to evaluate our arithmetic expressions, as below: 524 </p> 525<pre class="programlisting"><span class="identifier">calculator_context</span> <span class="identifier">ctx</span><span class="special">;</span> 526<span class="identifier">ctx</span><span class="special">.</span><span class="identifier">args</span><span class="special">.</span><span class="identifier">push_back</span><span class="special">(</span><span class="number">45</span><span class="special">);</span> <span class="comment">// the value of _1 is 45</span> 527<span class="identifier">ctx</span><span class="special">.</span><span class="identifier">args</span><span class="special">.</span><span class="identifier">push_back</span><span class="special">(</span><span class="number">50</span><span class="special">);</span> <span class="comment">// the value of _2 is 50</span> 528 529<span class="comment">// Create an arithmetic expression and immediately evaluate it</span> 530<span class="keyword">double</span> <span class="identifier">d</span> <span class="special">=</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">eval</span><span class="special">(</span> <span class="special">(</span><span class="identifier">_2</span> <span class="special">-</span> <span class="identifier">_1</span><span class="special">)</span> <span class="special">/</span> <span class="identifier">_2</span> <span class="special">*</span> <span class="number">100</span><span class="special">,</span> <span class="identifier">ctx</span> <span class="special">);</span> 531 532<span class="comment">// This prints "10"</span> 533<span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="identifier">d</span> <span class="special"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span> 534</pre> 535<p> 536 Later, we'll see how to define more interesting evaluation contexts and 537 expression transforms that give you total control over how your expressions 538 are evaluated. 539 </p> 540<h6> 541<a name="boost_proto.users_guide.getting_started.hello_calculator.h3"></a> 542 <span class="phrase"><a name="boost_proto.users_guide.getting_started.hello_calculator.customizing_expression_trees"></a></span><a class="link" href="users_guide.html#boost_proto.users_guide.getting_started.hello_calculator.customizing_expression_trees">Customizing 543 Expression Trees</a> 544 </h6> 545<p> 546 Our calculator EDSL is already pretty useful, and for many EDSL scenarios, 547 no more would be needed. But let's keep going. Imagine how much nicer it 548 would be if all calculator expressions overloaded <code class="computeroutput"><span class="keyword">operator</span><span class="special">()</span></code> so that they could be used as function 549 objects. We can do that by creating a calculator <span class="emphasis"><em>domain</em></span> 550 and telling Proto that all expressions in the calculator domain have extra 551 members. Here is how to define a calculator domain: 552 </p> 553<pre class="programlisting"><span class="comment">// Forward-declare an expression wrapper</span> 554<span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">></span> 555<span class="keyword">struct</span> <span class="identifier">calculator</span><span class="special">;</span> 556 557<span class="comment">// Define a calculator domain. Expression within</span> 558<span class="comment">// the calculator domain will be wrapped in the</span> 559<span class="comment">// calculator<> expression wrapper.</span> 560<span class="keyword">struct</span> <span class="identifier">calculator_domain</span> 561 <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">domain</span><span class="special"><</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">generator</span><span class="special"><</span><span class="identifier">calculator</span><span class="special">></span> <span class="special">></span> 562<span class="special">{};</span> 563</pre> 564<p> 565 The <code class="computeroutput"><span class="identifier">calculator</span><span class="special"><></span></code> 566 type will be an expression wrapper. It will behave just like the expression 567 that it wraps, but it will have extra member functions that we will define. 568 The <code class="computeroutput"><span class="identifier">calculator_domain</span></code> is 569 what informs Proto about our wrapper. It is used below in the definition 570 of <code class="computeroutput"><span class="identifier">calculator</span><span class="special"><></span></code>. 571 Read on for a description. 572 </p> 573<pre class="programlisting"><span class="comment">// Define a calculator expression wrapper. It behaves just like</span> 574<span class="comment">// the expression it wraps, but with an extra operator() member</span> 575<span class="comment">// function that evaluates the expression. </span> 576<span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">></span> 577<span class="keyword">struct</span> <span class="identifier">calculator</span> 578 <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">extends</span><span class="special"><</span><span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">calculator</span><span class="special"><</span><span class="identifier">Expr</span><span class="special">>,</span> <span class="identifier">calculator_domain</span><span class="special">></span> 579<span class="special">{</span> 580 <span class="keyword">typedef</span> 581 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">extends</span><span class="special"><</span><span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">calculator</span><span class="special"><</span><span class="identifier">Expr</span><span class="special">>,</span> <span class="identifier">calculator_domain</span><span class="special">></span> 582 <span class="identifier">base_type</span><span class="special">;</span> 583 584 <span class="identifier">calculator</span><span class="special">(</span><span class="identifier">Expr</span> <span class="keyword">const</span> <span class="special">&</span><span class="identifier">expr</span> <span class="special">=</span> <span class="identifier">Expr</span><span class="special">())</span> 585 <span class="special">:</span> <span class="identifier">base_type</span><span class="special">(</span><span class="identifier">expr</span><span class="special">)</span> 586 <span class="special">{}</span> 587 588 <span class="keyword">typedef</span> <span class="keyword">double</span> <span class="identifier">result_type</span><span class="special">;</span> 589 590 <span class="comment">// Overload operator() to invoke proto::eval() with</span> 591 <span class="comment">// our calculator_context.</span> 592 <span class="keyword">double</span> <span class="keyword">operator</span><span class="special">()(</span><span class="keyword">double</span> <span class="identifier">a1</span> <span class="special">=</span> <span class="number">0</span><span class="special">,</span> <span class="keyword">double</span> <span class="identifier">a2</span> <span class="special">=</span> <span class="number">0</span><span class="special">)</span> <span class="keyword">const</span> 593 <span class="special">{</span> 594 <span class="identifier">calculator_context</span> <span class="identifier">ctx</span><span class="special">;</span> 595 <span class="identifier">ctx</span><span class="special">.</span><span class="identifier">args</span><span class="special">.</span><span class="identifier">push_back</span><span class="special">(</span><span class="identifier">a1</span><span class="special">);</span> 596 <span class="identifier">ctx</span><span class="special">.</span><span class="identifier">args</span><span class="special">.</span><span class="identifier">push_back</span><span class="special">(</span><span class="identifier">a2</span><span class="special">);</span> 597 598 <span class="keyword">return</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">eval</span><span class="special">(*</span><span class="keyword">this</span><span class="special">,</span> <span class="identifier">ctx</span><span class="special">);</span> 599 <span class="special">}</span> 600<span class="special">};</span> 601</pre> 602<p> 603 The <code class="computeroutput"><span class="identifier">calculator</span><span class="special"><></span></code> 604 struct is an expression <span class="emphasis"><em>extension</em></span>. It uses <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">extends</span><span class="special"><></span></code> 605 to effectively add additional members to an expression type. When composing 606 larger expressions from smaller ones, Proto notes what domain the smaller 607 expressions are in. The larger expression is in the same domain and is 608 automatically wrapped in the domain's extension wrapper. 609 </p> 610<p> 611 All that remains to be done is to put our placeholders in the calculator 612 domain. We do that by wrapping them in our <code class="computeroutput"><span class="identifier">calculator</span><span class="special"><></span></code> wrapper, as below: 613 </p> 614<pre class="programlisting"><span class="comment">// Define the Protofied placeholder terminals, in the</span> 615<span class="comment">// calculator domain.</span> 616<span class="identifier">calculator</span><span class="special"><</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special"><</span><span class="identifier">placeholder</span><span class="special"><</span><span class="number">0</span><span class="special">></span> <span class="special">>::</span><span class="identifier">type</span><span class="special">></span> <span class="keyword">const</span> <span class="identifier">_1</span><span class="special">;</span> 617<span class="identifier">calculator</span><span class="special"><</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special"><</span><span class="identifier">placeholder</span><span class="special"><</span><span class="number">1</span><span class="special">></span> <span class="special">>::</span><span class="identifier">type</span><span class="special">></span> <span class="keyword">const</span> <span class="identifier">_2</span><span class="special">;</span> 618</pre> 619<p> 620 Any larger expression that contain these placeholders will automatically 621 be wrapped in the <code class="computeroutput"><span class="identifier">calculator</span><span class="special"><></span></code> wrapper and have our <code class="computeroutput"><span class="keyword">operator</span><span class="special">()</span></code> 622 overload. That means we can use them as function objects as follows. 623 </p> 624<pre class="programlisting"><span class="keyword">double</span> <span class="identifier">result</span> <span class="special">=</span> <span class="special">((</span><span class="identifier">_2</span> <span class="special">-</span> <span class="identifier">_1</span><span class="special">)</span> <span class="special">/</span> <span class="identifier">_2</span> <span class="special">*</span> <span class="number">100</span><span class="special">)(</span><span class="number">45.0</span><span class="special">,</span> <span class="number">50.0</span><span class="special">);</span> 625<span class="identifier">assert</span><span class="special">(</span><span class="identifier">result</span> <span class="special">==</span> <span class="special">(</span><span class="number">50.0</span> <span class="special">-</span> <span class="number">45.0</span><span class="special">)</span> <span class="special">/</span> <span class="number">50.0</span> <span class="special">*</span> <span class="number">100</span><span class="special">));</span> 626</pre> 627<p> 628 Since calculator expressions are now valid function objects, we can use 629 them with standard algorithms, as shown below: 630 </p> 631<pre class="programlisting"><span class="keyword">double</span> <span class="identifier">a1</span><span class="special">[</span><span class="number">4</span><span class="special">]</span> <span class="special">=</span> <span class="special">{</span> <span class="number">56</span><span class="special">,</span> <span class="number">84</span><span class="special">,</span> <span class="number">37</span><span class="special">,</span> <span class="number">69</span> <span class="special">};</span> 632<span class="keyword">double</span> <span class="identifier">a2</span><span class="special">[</span><span class="number">4</span><span class="special">]</span> <span class="special">=</span> <span class="special">{</span> <span class="number">65</span><span class="special">,</span> <span class="number">120</span><span class="special">,</span> <span class="number">60</span><span class="special">,</span> <span class="number">70</span> <span class="special">};</span> 633<span class="keyword">double</span> <span class="identifier">a3</span><span class="special">[</span><span class="number">4</span><span class="special">]</span> <span class="special">=</span> <span class="special">{</span> <span class="number">0</span> <span class="special">};</span> 634 635<span class="comment">// Use std::transform() and a calculator expression</span> 636<span class="comment">// to calculate percentages given two input sequences:</span> 637<span class="identifier">std</span><span class="special">::</span><span class="identifier">transform</span><span class="special">(</span><span class="identifier">a1</span><span class="special">,</span> <span class="identifier">a1</span><span class="special">+</span><span class="number">4</span><span class="special">,</span> <span class="identifier">a2</span><span class="special">,</span> <span class="identifier">a3</span><span class="special">,</span> <span class="special">(</span><span class="identifier">_2</span> <span class="special">-</span> <span class="identifier">_1</span><span class="special">)</span> <span class="special">/</span> <span class="identifier">_2</span> <span class="special">*</span> <span class="number">100</span><span class="special">);</span> 638</pre> 639<p> 640 Now, let's use the calculator example to explore some other useful features 641 of Proto. 642 </p> 643<h6> 644<a name="boost_proto.users_guide.getting_started.hello_calculator.h4"></a> 645 <span class="phrase"><a name="boost_proto.users_guide.getting_started.hello_calculator.detecting_invalid_expressions"></a></span><a class="link" href="users_guide.html#boost_proto.users_guide.getting_started.hello_calculator.detecting_invalid_expressions">Detecting 646 Invalid Expressions</a> 647 </h6> 648<p> 649 You may have noticed that you didn't have to define an overloaded <code class="computeroutput"><span class="keyword">operator</span><span class="special">-()</span></code> 650 or <code class="computeroutput"><span class="keyword">operator</span><span class="special">/()</span></code> 651 -- Proto defined them for you. In fact, Proto overloads <span class="emphasis"><em>all</em></span> 652 the operators for you, even though they may not mean anything in your domain-specific 653 language. That means it may be possible to create expressions that are 654 invalid in your domain. You can detect invalid expressions with Proto by 655 defining the <span class="emphasis"><em>grammar</em></span> of your domain-specific language. 656 </p> 657<p> 658 For simplicity, assume that our calculator EDSL should only allow addition, 659 subtraction, multiplication and division. Any expression involving any 660 other operator is invalid. Using Proto, we can state this requirement by 661 defining the grammar of the calculator EDSL. It looks as follows: 662 </p> 663<pre class="programlisting"><span class="comment">// Define the grammar of calculator expressions</span> 664<span class="keyword">struct</span> <span class="identifier">calculator_grammar</span> 665 <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">or_</span><span class="special"><</span> 666 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">plus</span><span class="special"><</span> <span class="identifier">calculator_grammar</span><span class="special">,</span> <span class="identifier">calculator_grammar</span> <span class="special">></span> 667 <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">minus</span><span class="special"><</span> <span class="identifier">calculator_grammar</span><span class="special">,</span> <span class="identifier">calculator_grammar</span> <span class="special">></span> 668 <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">multiplies</span><span class="special"><</span> <span class="identifier">calculator_grammar</span><span class="special">,</span> <span class="identifier">calculator_grammar</span> <span class="special">></span> 669 <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">divides</span><span class="special"><</span> <span class="identifier">calculator_grammar</span><span class="special">,</span> <span class="identifier">calculator_grammar</span> <span class="special">></span> 670 <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special"><</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_</span> <span class="special">></span> 671 <span class="special">></span> 672<span class="special">{};</span> 673</pre> 674<p> 675 You can read the above grammar as follows: an expression tree conforms 676 to the calculator grammar if it is a binary plus, minus, multiplies or 677 divides node, where both child nodes also conform to the calculator grammar; 678 or if it is a terminal. In a Proto grammar, <code class="computeroutput"><a class="link" href="../boost/proto/_.html" title="Struct _">proto::_</a></code> is a wildcard that matches 679 any type, so <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special"><</span> 680 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_</span> <span class="special">></span></code> 681 matches any terminal, whether it is a placeholder or a literal. 682 </p> 683<div class="note"><table border="0" summary="Note"> 684<tr> 685<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../doc/src/images/note.png"></td> 686<th align="left">Note</th> 687</tr> 688<tr><td align="left" valign="top"><p> 689 This grammar is actually a little looser than we would like. Only placeholders 690 and literals that are convertible to doubles are valid terminals. Later 691 on we'll see how to express things like that in Proto grammars. 692 </p></td></tr> 693</table></div> 694<p> 695 Once you have defined the grammar of your EDSL, you can use the <code class="computeroutput"><a class="link" href="../boost/proto/matches.html" title="Struct template matches">proto::matches<></a></code> metafunction to check 696 whether a given expression type conforms to the grammar. For instance, 697 we might add the following to our <code class="computeroutput"><span class="identifier">calculator</span><span class="special">::</span><span class="keyword">operator</span><span class="special">()</span></code> overload: 698 </p> 699<pre class="programlisting"><span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">></span> 700<span class="keyword">struct</span> <span class="identifier">calculator</span> 701 <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">extends</span><span class="special"><</span> <span class="comment">/* ... as before ... */</span> <span class="special">></span> 702<span class="special">{</span> 703 <span class="comment">/* ... */</span> 704 <span class="keyword">double</span> <span class="keyword">operator</span><span class="special">()(</span><span class="keyword">double</span> <span class="identifier">a1</span> <span class="special">=</span> <span class="number">0</span><span class="special">,</span> <span class="keyword">double</span> <span class="identifier">a2</span> <span class="special">=</span> <span class="number">0</span><span class="special">)</span> <span class="keyword">const</span> 705 <span class="special">{</span> 706 <span class="comment">// Check here that the expression we are about to</span> 707 <span class="comment">// evaluate actually conforms to the calculator grammar.</span> 708 <span class="identifier">BOOST_MPL_ASSERT</span><span class="special">((</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">matches</span><span class="special"><</span><span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">calculator_grammar</span><span class="special">>));</span> 709 <span class="comment">/* ... */</span> 710 <span class="special">}</span> 711<span class="special">};</span> 712</pre> 713<p> 714 The addition of the <code class="computeroutput"><span class="identifier">BOOST_MPL_ASSERT</span><span class="special">()</span></code> line enforces at compile time that we 715 only evaluate expressions that conform to the calculator EDSL's grammar. 716 With Proto grammars, <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">matches</span><span class="special"><></span></code> and <code class="computeroutput"><span class="identifier">BOOST_MPL_ASSERT</span><span class="special">()</span></code> it is very easy to give the users of 717 your EDSL short and readable compile-time errors when they accidentally 718 misuse your EDSL. 719 </p> 720<div class="note"><table border="0" summary="Note"> 721<tr> 722<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../doc/src/images/note.png"></td> 723<th align="left">Note</th> 724</tr> 725<tr><td align="left" valign="top"><p> 726 <code class="computeroutput"><span class="identifier">BOOST_MPL_ASSERT</span><span class="special">()</span></code> 727 is part of the Boost Metaprogramming Library. To use it, just <code class="computeroutput"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">mpl</span><span class="special">/</span><span class="identifier">assert</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span></code>. 728 </p></td></tr> 729</table></div> 730<h6> 731<a name="boost_proto.users_guide.getting_started.hello_calculator.h5"></a> 732 <span class="phrase"><a name="boost_proto.users_guide.getting_started.hello_calculator.controlling_operator_overloads"></a></span><a class="link" href="users_guide.html#boost_proto.users_guide.getting_started.hello_calculator.controlling_operator_overloads">Controlling 733 Operator Overloads</a> 734 </h6> 735<p> 736 Grammars and <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">matches</span><span class="special"><></span></code> 737 make it possible to detect when a user has created an invalid expression 738 and issue a compile-time error. But what if you want to prevent users from 739 creating invalid expressions in the first place? By using grammars and 740 domains together, you can disable any of Proto's operator overloads that 741 would create an invalid expression. It is as simple as specifying the EDSL's 742 grammar when you define the domain, as shown below: 743 </p> 744<pre class="programlisting"><span class="comment">// Define a calculator domain. Expression within</span> 745<span class="comment">// the calculator domain will be wrapped in the</span> 746<span class="comment">// calculator<> expression wrapper.</span> 747<span class="comment">// NEW: Any operator overloads that would create an</span> 748<span class="comment">// expression that does not conform to the</span> 749<span class="comment">// calculator grammar is automatically disabled.</span> 750<span class="keyword">struct</span> <span class="identifier">calculator_domain</span> 751 <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">domain</span><span class="special"><</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">generator</span><span class="special"><</span><span class="identifier">calculator</span><span class="special">>,</span> <span class="identifier">calculator_grammar</span> <span class="special">></span> 752<span class="special">{};</span> 753</pre> 754<p> 755 The only thing we changed is we added <code class="computeroutput"><span class="identifier">calculator_grammar</span></code> 756 as the second template parameter to the <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">domain</span><span class="special"><></span></code> template when defining <code class="computeroutput"><span class="identifier">calculator_domain</span></code>. With this simple addition, 757 we disable any of Proto's operator overloads that would create an invalid 758 calculator expression. 759 </p> 760<h6> 761<a name="boost_proto.users_guide.getting_started.hello_calculator.h6"></a> 762 <span class="phrase"><a name="boost_proto.users_guide.getting_started.hello_calculator.____and_much_more"></a></span><a class="link" href="users_guide.html#boost_proto.users_guide.getting_started.hello_calculator.____and_much_more">... 763 And Much More</a> 764 </h6> 765<p> 766 Hopefully, this gives you an idea of what sorts of things Proto can do 767 for you. But this only scratches the surface. The rest of this users' guide 768 will describe all these features and others in more detail. 769 </p> 770<p> 771 Happy metaprogramming! 772 </p> 773</div> 774</div> 775<div class="section"> 776<div class="titlepage"><div><div><h3 class="title"> 777<a name="boost_proto.users_guide.front_end"></a><a class="link" href="users_guide.html#boost_proto.users_guide.front_end" title="Fronts Ends: Defining Terminals and Non-Terminals of Your EDSL">Fronts Ends: Defining 778 Terminals and Non-Terminals of Your EDSL</a> 779</h3></div></div></div> 780<div class="toc"><dl class="toc"> 781<dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.front_end.making_terminals">Making 782 Terminals</a></span></dt> 783<dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.front_end.proto_s_operator_overloads">Proto's 784 Operator Overloads</a></span></dt> 785<dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.front_end.making_lazy_functions">Making 786 Lazy Functions</a></span></dt> 787<dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.front_end.customizing_expressions_in_your_domain">Customizing 788 Expressions in Your Domain</a></span></dt> 789<dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.front_end.define_operators">Adapting 790 Existing Types to Proto</a></span></dt> 791<dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.front_end.code_repetition">Generating 792 Repetitive Code with the Preprocessor</a></span></dt> 793</dl></div> 794<p> 795 Here is the fun part: designing your own mini-programming language. In this 796 section we'll talk about the nuts and bolts of designing an EDSL interface 797 using Proto. We'll cover the definition of terminals and lazy functions that 798 the users of your EDSL will get to program with. We'll also talk about Proto's 799 expression template-building operator overloads, and about ways to add additional 800 members to expressions within your domain. 801 </p> 802<div class="section"> 803<div class="titlepage"><div><div><h4 class="title"> 804<a name="boost_proto.users_guide.front_end.making_terminals"></a><a class="link" href="users_guide.html#boost_proto.users_guide.front_end.making_terminals" title="Making Terminals">Making 805 Terminals</a> 806</h4></div></div></div> 807<p> 808 As we saw with the Calculator example from the Introduction, the simplest 809 way to get an EDSL up and running is simply to define some terminals, as 810 follows. 811 </p> 812<pre class="programlisting"><span class="comment">// Define a literal integer Proto expression.</span> 813<span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special"><</span><span class="keyword">int</span><span class="special">>::</span><span class="identifier">type</span> <span class="identifier">i</span> <span class="special">=</span> <span class="special">{</span><span class="number">0</span><span class="special">};</span> 814 815<span class="comment">// This creates an expression template.</span> 816<span class="identifier">i</span> <span class="special">+</span> <span class="number">1</span><span class="special">;</span> 817</pre> 818<p> 819 With some terminals and Proto's operator overloads, you can immediately 820 start creating expression templates. 821 </p> 822<p> 823 Defining terminals -- with aggregate initialization -- can be a little 824 awkward at times. Proto provides an easier-to-use wrapper for literals 825 that can be used to construct Protofied terminal expressions. It's called 826 <code class="computeroutput"><a class="link" href="../boost/proto/literal.html" title="Struct template literal">proto::literal<></a></code>. 827 </p> 828<pre class="programlisting"><span class="comment">// Define a literal integer Proto expression.</span> 829<span class="identifier">proto</span><span class="special">::</span><span class="identifier">literal</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> <span class="identifier">i</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</span> 830 831<span class="comment">// Proto literals are really just Proto terminal expressions.</span> 832<span class="comment">// For example, this builds a Proto expression template:</span> 833<span class="identifier">i</span> <span class="special">+</span> <span class="number">1</span><span class="special">;</span> 834</pre> 835<p> 836 There is also a <code class="computeroutput"><a class="link" href="../boost/proto/lit.html" title="Function lit">proto::lit()</a></code> function for constructing 837 a <code class="computeroutput"><a class="link" href="../boost/proto/literal.html" title="Struct template literal">proto::literal<></a></code> in-place. The above 838 expression can simply be written as: 839 </p> 840<pre class="programlisting"><span class="comment">// proto::lit(0) creates an integer terminal expression</span> 841<span class="identifier">proto</span><span class="special">::</span><span class="identifier">lit</span><span class="special">(</span><span class="number">0</span><span class="special">)</span> <span class="special">+</span> <span class="number">1</span><span class="special">;</span> 842</pre> 843</div> 844<div class="section"> 845<div class="titlepage"><div><div><h4 class="title"> 846<a name="boost_proto.users_guide.front_end.proto_s_operator_overloads"></a><a class="link" href="users_guide.html#boost_proto.users_guide.front_end.proto_s_operator_overloads" title="Proto's Operator Overloads">Proto's 847 Operator Overloads</a> 848</h4></div></div></div> 849<p> 850 Once we have some Proto terminals, expressions involving those terminals 851 build expression trees for us. Proto defines overloads for each of C++'s 852 overloadable operators in the <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">proto</span></code> 853 namespace. As long as one operand is a Proto expression, the result of 854 the operation is a tree node representing that operation. 855 </p> 856<div class="note"><table border="0" summary="Note"> 857<tr> 858<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../doc/src/images/note.png"></td> 859<th align="left">Note</th> 860</tr> 861<tr><td align="left" valign="top"><p> 862 Proto's operator overloads live in the <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">proto</span></code> 863 namespace and are found via ADL (argument-dependent lookup). That is 864 why expressions must be "tainted" with Proto-ness for Proto 865 to be able to build trees out of expressions. 866 </p></td></tr> 867</table></div> 868<p> 869 As a result of Proto's operator overloads, we can say: 870 </p> 871<pre class="programlisting"><span class="special">-</span><span class="identifier">_1</span><span class="special">;</span> <span class="comment">// OK, build a unary-negate tree node</span> 872<span class="identifier">_1</span> <span class="special">+</span> <span class="number">42</span><span class="special">;</span> <span class="comment">// OK, build a binary-plus tree node</span> 873</pre> 874<p> 875 For the most part, this Just Works and you don't need to think about it, 876 but a few operators are special and it can be helpful to know how Proto 877 handles them. 878 </p> 879<h6> 880<a name="boost_proto.users_guide.front_end.proto_s_operator_overloads.h0"></a> 881 <span class="phrase"><a name="boost_proto.users_guide.front_end.proto_s_operator_overloads.assignment__subscript__and_function_call_operators"></a></span><a class="link" href="users_guide.html#boost_proto.users_guide.front_end.proto_s_operator_overloads.assignment__subscript__and_function_call_operators">Assignment, 882 Subscript, and Function Call Operators</a> 883 </h6> 884<p> 885 Proto also overloads <code class="computeroutput"><span class="keyword">operator</span><span class="special">=</span></code>, <code class="computeroutput"><span class="keyword">operator</span><span class="special">[]</span></code>, and <code class="computeroutput"><span class="keyword">operator</span><span class="special">()</span></code>, but these operators are member functions 886 of the expression template rather than free functions in Proto's namespace. 887 The following are valid Proto expressions: 888 </p> 889<pre class="programlisting"><span class="identifier">_1</span> <span class="special">=</span> <span class="number">5</span><span class="special">;</span> <span class="comment">// OK, builds a binary assign tree node</span> 890<span class="identifier">_1</span><span class="special">[</span><span class="number">6</span><span class="special">];</span> <span class="comment">// OK, builds a binary subscript tree node</span> 891<span class="identifier">_1</span><span class="special">();</span> <span class="comment">// OK, builds a unary function tree node</span> 892<span class="identifier">_1</span><span class="special">(</span><span class="number">7</span><span class="special">);</span> <span class="comment">// OK, builds a binary function tree node</span> 893<span class="identifier">_1</span><span class="special">(</span><span class="number">8</span><span class="special">,</span><span class="number">9</span><span class="special">);</span> <span class="comment">// OK, builds a ternary function tree node</span> 894<span class="comment">// ... etc.</span> 895</pre> 896<p> 897 For the first two lines, assignment and subscript, it should be fairly 898 unsurprising that the resulting expression node should be binary. After 899 all, there are two operands in each expression. It may be surprising at 900 first that what appears to be a function call with no arguments, <code class="computeroutput"><span class="identifier">_1</span><span class="special">()</span></code>, 901 actually creates an expression node with one child. The child is <code class="computeroutput"><span class="identifier">_1</span></code> itself. Likewise, the expression 902 <code class="computeroutput"><span class="identifier">_1</span><span class="special">(</span><span class="number">7</span><span class="special">)</span></code> has two 903 children: <code class="computeroutput"><span class="identifier">_1</span></code> and <code class="computeroutput"><span class="number">7</span></code>. 904 </p> 905<p> 906 Because these operators can only be defined as member functions, the following 907 expressions are invalid: 908 </p> 909<pre class="programlisting"><span class="keyword">int</span> <span class="identifier">i</span><span class="special">;</span> 910<span class="identifier">i</span> <span class="special">=</span> <span class="identifier">_1</span><span class="special">;</span> <span class="comment">// ERROR: cannot assign _1 to an int</span> 911 912<span class="keyword">int</span> <span class="special">*</span><span class="identifier">p</span><span class="special">;</span> 913<span class="identifier">p</span><span class="special">[</span><span class="identifier">_1</span><span class="special">];</span> <span class="comment">// ERROR: cannot use _1 as an index</span> 914 915<span class="identifier">std</span><span class="special">::</span><span class="identifier">sin</span><span class="special">(</span><span class="identifier">_1</span><span class="special">);</span> <span class="comment">// ERROR: cannot call std::sin() with _1</span> 916</pre> 917<p> 918 Also, C++ has special rules for overloads of <code class="computeroutput"><span class="keyword">operator</span><span class="special">-></span></code> that make it useless for building 919 expression templates, so Proto does not overload it. 920 </p> 921<h6> 922<a name="boost_proto.users_guide.front_end.proto_s_operator_overloads.h1"></a> 923 <span class="phrase"><a name="boost_proto.users_guide.front_end.proto_s_operator_overloads.the_address_of_operator"></a></span><a class="link" href="users_guide.html#boost_proto.users_guide.front_end.proto_s_operator_overloads.the_address_of_operator">The 924 Address-Of Operator</a> 925 </h6> 926<p> 927 Proto overloads the address-of operator for expression types, so that the 928 following code creates a new unary address-of tree node: 929 </p> 930<pre class="programlisting"><span class="special">&</span><span class="identifier">_1</span><span class="special">;</span> <span class="comment">// OK, creates a unary address-of tree node</span> 931</pre> 932<p> 933 It does <span class="emphasis"><em>not</em></span> return the address of the <code class="computeroutput"><span class="identifier">_1</span></code> object. However, there is special 934 code in Proto such that a unary address-of node is implicitly convertible 935 to a pointer to its child. In other words, the following code works and 936 does what you might expect, but not in the obvious way: 937 </p> 938<pre class="programlisting"><span class="keyword">typedef</span> 939 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special"><</span> <span class="identifier">placeholder</span><span class="special"><</span><span class="number">0</span><span class="special">></span> <span class="special">>::</span><span class="identifier">type</span> 940<span class="identifier">_1_type</span><span class="special">;</span> 941 942<span class="identifier">_1_type</span> <span class="keyword">const</span> <span class="identifier">_1</span> <span class="special">=</span> <span class="special">{{}};</span> 943<span class="identifier">_1_type</span> <span class="keyword">const</span> <span class="special">*</span> <span class="identifier">p</span> <span class="special">=</span> <span class="special">&</span><span class="identifier">_1</span><span class="special">;</span> <span class="comment">// OK, &_1 implicitly converted</span> 944</pre> 945</div> 946<div class="section"> 947<div class="titlepage"><div><div><h4 class="title"> 948<a name="boost_proto.users_guide.front_end.making_lazy_functions"></a><a class="link" href="users_guide.html#boost_proto.users_guide.front_end.making_lazy_functions" title="Making Lazy Functions">Making 949 Lazy Functions</a> 950</h4></div></div></div> 951<p> 952 If we limited ourselves to nothing but terminals and operator overloads, 953 our embedded domain-specific languages wouldn't be very expressive. Imagine 954 that we wanted to extend our calculator EDSL with a full suite of math 955 functions like <code class="computeroutput"><span class="identifier">sin</span><span class="special">()</span></code> 956 and <code class="computeroutput"><span class="identifier">pow</span><span class="special">()</span></code> 957 that we could invoke lazily as follows. 958 </p> 959<pre class="programlisting"><span class="comment">// A calculator expression that takes one argument</span> 960<span class="comment">// and takes the sine of it.</span> 961<span class="identifier">sin</span><span class="special">(</span><span class="identifier">_1</span><span class="special">);</span> 962</pre> 963<p> 964 We would like the above to create an expression template representing a 965 function invocation. When that expression is evaluated, it should cause 966 the function to be invoked. (At least, that's the meaning of function invocation 967 we'd like the calculator EDSL to have.) You can define <code class="computeroutput"><span class="identifier">sin</span></code> 968 quite simply as follows. 969 </p> 970<pre class="programlisting"><span class="comment">// "sin" is a Proto terminal containing a function pointer</span> 971<span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special"><</span> <span class="keyword">double</span><span class="special">(*)(</span><span class="keyword">double</span><span class="special">)</span> <span class="special">>::</span><span class="identifier">type</span> <span class="keyword">const</span> <span class="identifier">sin</span> <span class="special">=</span> <span class="special">{&</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">sin</span><span class="special">};</span> 972</pre> 973<p> 974 In the above, we define <code class="computeroutput"><span class="identifier">sin</span></code> 975 as a Proto terminal containing a pointer to the <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">sin</span><span class="special">()</span></code> function. Now we can use <code class="computeroutput"><span class="identifier">sin</span></code> as a lazy function. The <code class="computeroutput"><span class="identifier">default_context</span></code> that we saw in the Introduction 976 knows how to evaluate lazy functions. Consider the following: 977 </p> 978<pre class="programlisting"><span class="keyword">double</span> <span class="identifier">pi</span> <span class="special">=</span> <span class="number">3.1415926535</span><span class="special">;</span> 979<span class="identifier">proto</span><span class="special">::</span><span class="identifier">default_context</span> <span class="identifier">ctx</span><span class="special">;</span> 980<span class="comment">// Create a lazy "sin" invocation and immediately evaluate it</span> 981<span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">eval</span><span class="special">(</span> <span class="identifier">sin</span><span class="special">(</span><span class="identifier">pi</span><span class="special">/</span><span class="number">2</span><span class="special">),</span> <span class="identifier">ctx</span> <span class="special">)</span> <span class="special"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span> 982</pre> 983<p> 984 The above code prints out: 985 </p> 986<pre class="programlisting">1</pre> 987<p> 988 I'm no expert at trigonometry, but that looks right to me. 989 </p> 990<p> 991 We can write <code class="computeroutput"><span class="identifier">sin</span><span class="special">(</span><span class="identifier">pi</span><span class="special">/</span><span class="number">2</span><span class="special">)</span></code> because the <code class="computeroutput"><span class="identifier">sin</span></code> 992 object, which is a Proto terminal, has an overloaded <code class="computeroutput"><span class="keyword">operator</span><span class="special">()()</span></code> that builds a node representing a function 993 call invocation. The actual type of <code class="computeroutput"><span class="identifier">sin</span><span class="special">(</span><span class="identifier">pi</span><span class="special">/</span><span class="number">2</span><span class="special">)</span></code> is actually 994 something like this: 995 </p> 996<pre class="programlisting"><span class="comment">// The type of the expression sin(pi/2):</span> 997<span class="identifier">proto</span><span class="special">::</span><span class="identifier">function</span><span class="special"><</span> 998 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special"><</span> <span class="keyword">double</span><span class="special">(*)(</span><span class="keyword">double</span><span class="special">)</span> <span class="special">>::</span><span class="identifier">type</span> <span class="keyword">const</span> <span class="special">&</span> 999 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">as_child</span><span class="special"><</span> <span class="keyword">double</span> <span class="keyword">const</span> <span class="special">>::</span><span class="identifier">type</span> 1000<span class="special">>::</span><span class="identifier">type</span> 1001</pre> 1002<p> 1003 This type further expands to an unsightly node type with a <span class="emphasis"><em>tag</em></span> 1004 type of <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">function</span></code> and two children: the first 1005 representing the function to be invoked, and the second representing the 1006 argument to the function. (Node tag types describe the operation that created 1007 the node. The difference between <code class="computeroutput"><span class="identifier">a</span> 1008 <span class="special">+</span> <span class="identifier">b</span></code> 1009 and <code class="computeroutput"><span class="identifier">a</span> <span class="special">-</span> 1010 <span class="identifier">b</span></code> is that the former has tag 1011 type <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">plus</span></code> and the latter has tag type <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">minus</span></code>. Tag types are pure compile-time 1012 information.) 1013 </p> 1014<div class="note"><table border="0" summary="Note"> 1015<tr> 1016<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../doc/src/images/note.png"></td> 1017<th align="left">Note</th> 1018</tr> 1019<tr><td align="left" valign="top"><p> 1020 In the type computation above, <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">as_child</span><span class="special"><></span></code> is a metafunction that ensures 1021 its argument is a Proto expression type. If it isn't one already, it 1022 becomes a Proto terminal. We'll learn more about this metafunction, along 1023 with <code class="computeroutput"><a class="link" href="../boost/proto/as_child.html" title="Function as_child">proto::as_child()</a></code>, its runtime counterpart, 1024 <a class="link" href="users_guide.html#boost_proto.users_guide.front_end.customizing_expressions_in_your_domain.per_domain_as_child" title="Controlling How Child Expressions Are Captured">later</a>. 1025 For now, you can forget about it. 1026 </p></td></tr> 1027</table></div> 1028<p> 1029 It is important to note that there is nothing special about terminals that 1030 contain function pointers. <span class="emphasis"><em>Any</em></span> Proto expression has 1031 an overloaded function call operator. Consider: 1032 </p> 1033<pre class="programlisting"><span class="comment">// This compiles!</span> 1034<span class="identifier">proto</span><span class="special">::</span><span class="identifier">lit</span><span class="special">(</span><span class="number">1</span><span class="special">)(</span><span class="number">2</span><span class="special">)(</span><span class="number">3</span><span class="special">,</span><span class="number">4</span><span class="special">)(</span><span class="number">5</span><span class="special">,</span><span class="number">6</span><span class="special">,</span><span class="number">7</span><span class="special">,</span><span class="number">8</span><span class="special">);</span> 1035</pre> 1036<p> 1037 That may look strange at first. It creates an integer terminal with <code class="computeroutput"><a class="link" href="../boost/proto/lit.html" title="Function lit">proto::lit()</a></code>, and then invokes it like 1038 a function again and again. What does it mean? Who knows?! You get to decide 1039 when you define an evaluation context or a transform. But more on that 1040 later. 1041 </p> 1042<h6> 1043<a name="boost_proto.users_guide.front_end.making_lazy_functions.h0"></a> 1044 <span class="phrase"><a name="boost_proto.users_guide.front_end.making_lazy_functions.making_lazy_functions__continued"></a></span><a class="link" href="users_guide.html#boost_proto.users_guide.front_end.making_lazy_functions.making_lazy_functions__continued">Making 1045 Lazy Functions, Continued</a> 1046 </h6> 1047<p> 1048 Now, what if we wanted to add a <code class="computeroutput"><span class="identifier">pow</span><span class="special">()</span></code> function to our calculator EDSL that 1049 users could invoke as follows? 1050 </p> 1051<pre class="programlisting"><span class="comment">// A calculator expression that takes one argument</span> 1052<span class="comment">// and raises it to the 2nd power</span> 1053<span class="identifier">pow</span><span class="special"><</span> <span class="number">2</span> <span class="special">>(</span><span class="identifier">_1</span><span class="special">);</span> 1054</pre> 1055<p> 1056 The simple technique described above of making <code class="computeroutput"><span class="identifier">pow</span></code> 1057 a terminal containing a function pointer doesn't work here. If <code class="computeroutput"><span class="identifier">pow</span></code> is an object, then the expression 1058 <code class="computeroutput"><span class="identifier">pow</span><span class="special"><</span> 1059 <span class="number">2</span> <span class="special">>(</span><span class="identifier">_1</span><span class="special">)</span></code> is 1060 not valid C++. (Well, technically it is; it means, <code class="computeroutput"><span class="identifier">pow</span></code> 1061 less than 2, greater than <code class="computeroutput"><span class="special">(</span><span class="identifier">_1</span><span class="special">)</span></code>, 1062 which is nothing at all like what we want.) <code class="computeroutput"><span class="identifier">pow</span></code> 1063 should be a real function template. But it must be an unusual function: 1064 one that returns an expression template. 1065 </p> 1066<p> 1067 With <code class="computeroutput"><span class="identifier">sin</span></code>, we relied on 1068 Proto to provide an overloaded <code class="computeroutput"><span class="keyword">operator</span><span class="special">()()</span></code> to build an expression node with tag 1069 type <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">function</span></code> for us. Now we'll need to do 1070 so ourselves. As before, the node will have two children: the function 1071 to invoke and the function's argument. 1072 </p> 1073<p> 1074 With <code class="computeroutput"><span class="identifier">sin</span></code>, the function 1075 to invoke was a raw function pointer wrapped in a Proto terminal. In the 1076 case of <code class="computeroutput"><span class="identifier">pow</span></code>, we want it 1077 to be a terminal containing TR1-style function object. This will allow 1078 us to parameterize the function on the exponent. Below is the implementation 1079 of a simple TR1-style wrapper for the <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">pow</span></code> 1080 function: 1081 </p> 1082<pre class="programlisting"><span class="comment">// Define a pow_fun function object</span> 1083<span class="keyword">template</span><span class="special"><</span> <span class="keyword">int</span> <span class="identifier">Exp</span> <span class="special">></span> 1084<span class="keyword">struct</span> <span class="identifier">pow_fun</span> 1085<span class="special">{</span> 1086 <span class="keyword">typedef</span> <span class="keyword">double</span> <span class="identifier">result_type</span><span class="special">;</span> 1087 1088 <span class="keyword">double</span> <span class="keyword">operator</span><span class="special">()(</span><span class="keyword">double</span> <span class="identifier">d</span><span class="special">)</span> <span class="keyword">const</span> 1089 <span class="special">{</span> 1090 <span class="keyword">return</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">pow</span><span class="special">(</span><span class="identifier">d</span><span class="special">,</span> <span class="identifier">Exp</span><span class="special">);</span> 1091 <span class="special">}</span> 1092<span class="special">};</span> 1093</pre> 1094<p> 1095 Following the <code class="computeroutput"><span class="identifier">sin</span></code> example, 1096 we want <code class="computeroutput"><span class="identifier">pow</span><span class="special"><</span> 1097 <span class="number">1</span> <span class="special">>(</span> 1098 <span class="identifier">pi</span><span class="special">/</span><span class="number">2</span> <span class="special">)</span></code> to have 1099 a type like this: 1100 </p> 1101<pre class="programlisting"><span class="comment">// The type of the expression pow<1>(pi/2):</span> 1102<span class="identifier">proto</span><span class="special">::</span><span class="identifier">function</span><span class="special"><</span> 1103 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special"><</span> <span class="identifier">pow_fun</span><span class="special"><</span><span class="number">1</span><span class="special">></span> <span class="special">>::</span><span class="identifier">type</span> 1104 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">as_child</span><span class="special"><</span> <span class="keyword">double</span> <span class="keyword">const</span> <span class="special">>::</span><span class="identifier">type</span> 1105<span class="special">>::</span><span class="identifier">type</span> 1106</pre> 1107<p> 1108 We could write a <code class="computeroutput"><span class="identifier">pow</span><span class="special">()</span></code> 1109 function using code like this, but it's verbose and error prone; it's too 1110 easy to introduce subtle bugs by forgetting to call <code class="computeroutput"><a class="link" href="../boost/proto/as_child.html" title="Function as_child">proto::as_child()</a></code> 1111 where necessary, resulting in code that seems to work but sometimes doesn't. 1112 Proto provides a better way to construct expression nodes: <code class="computeroutput"><a class="link" href="../boost/proto/make_expr.html" title="Function make_expr">proto::make_expr()</a></code>. 1113 </p> 1114<h6> 1115<a name="boost_proto.users_guide.front_end.making_lazy_functions.h1"></a> 1116 <span class="phrase"><a name="boost_proto.users_guide.front_end.making_lazy_functions.lazy_functions_made_simple_with__literal_make_expr____literal_"></a></span><a class="link" href="users_guide.html#boost_proto.users_guide.front_end.making_lazy_functions.lazy_functions_made_simple_with__literal_make_expr____literal_">Lazy 1117 Functions Made Simple With <code class="literal">make_expr()</code></a> 1118 </h6> 1119<p> 1120 Proto provides a helper for building expression templates called <code class="computeroutput"><a class="link" href="../boost/proto/make_expr.html" title="Function make_expr">proto::make_expr()</a></code>. We can concisely define 1121 the <code class="computeroutput"><span class="identifier">pow</span><span class="special">()</span></code> 1122 function with it as below. 1123 </p> 1124<pre class="programlisting"><span class="comment">// Define a lazy pow() function for the calculator EDSL.</span> 1125<span class="comment">// Can be used as: pow< 2 >(_1)</span> 1126<span class="keyword">template</span><span class="special"><</span> <span class="keyword">int</span> <span class="identifier">Exp</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Arg</span> <span class="special">></span> 1127<span class="keyword">typename</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">make_expr</span><span class="special"><</span> 1128 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">function</span> <span class="comment">// Tag type</span> 1129 <span class="special">,</span> <span class="identifier">pow_fun</span><span class="special"><</span> <span class="identifier">Exp</span> <span class="special">></span> <span class="comment">// First child (by value)</span> 1130 <span class="special">,</span> <span class="identifier">Arg</span> <span class="keyword">const</span> <span class="special">&</span> <span class="comment">// Second child (by reference)</span> 1131<span class="special">>::</span><span class="identifier">type</span> <span class="keyword">const</span> 1132<span class="identifier">pow</span><span class="special">(</span><span class="identifier">Arg</span> <span class="keyword">const</span> <span class="special">&</span><span class="identifier">arg</span><span class="special">)</span> 1133<span class="special">{</span> 1134 <span class="keyword">return</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">make_expr</span><span class="special"><</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">function</span><span class="special">>(</span> 1135 <span class="identifier">pow_fun</span><span class="special"><</span><span class="identifier">Exp</span><span class="special">>()</span> <span class="comment">// First child (by value)</span> 1136 <span class="special">,</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">ref</span><span class="special">(</span><span class="identifier">arg</span><span class="special">)</span> <span class="comment">// Second child (by reference)</span> 1137 <span class="special">);</span> 1138<span class="special">}</span> 1139</pre> 1140<p> 1141 There are some things to notice about the above code. We use <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">make_expr</span><span class="special"><></span></code> 1142 to calculate the return type. The first template parameter is the tag type 1143 for the expression node we're building -- in this case, <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">function</span></code>. 1144 </p> 1145<p> 1146 Subsequent template parameters to <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">make_expr</span><span class="special"><></span></code> represent child nodes. If a child 1147 type is not already a Proto expression, it is automatically made into a 1148 terminal with <code class="computeroutput"><a class="link" href="../boost/proto/as_child.html" title="Function as_child">proto::as_child()</a></code>. 1149 A type such as <code class="computeroutput"><span class="identifier">pow_fun</span><span class="special"><</span><span class="identifier">Exp</span><span class="special">></span></code> results in terminal that is held by 1150 value, whereas a type like <code class="computeroutput"><span class="identifier">Arg</span> 1151 <span class="keyword">const</span> <span class="special">&</span></code> 1152 (note the reference) indicates that the result should be held by reference. 1153 </p> 1154<p> 1155 In the function body is the runtime invocation of <code class="computeroutput"><a class="link" href="../boost/proto/make_expr.html" title="Function make_expr">proto::make_expr()</a></code>. 1156 It closely mirrors the return type calculation. <code class="computeroutput"><a class="link" href="../boost/proto/make_expr.html" title="Function make_expr">proto::make_expr()</a></code> 1157 requires you to specify the node's tag type as a template parameter. The 1158 arguments to the function become the node's children. When a child should 1159 be stored by value, nothing special needs to be done. When a child should 1160 be stored by reference, you must use the <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">ref</span><span class="special">()</span></code> function to wrap the argument. 1161 </p> 1162<p> 1163 And that's it! <code class="computeroutput"><a class="link" href="../boost/proto/make_expr.html" title="Function make_expr">proto::make_expr()</a></code> 1164 is the lazy person's way to make a lazy funtion. 1165 </p> 1166</div> 1167<div class="section"> 1168<div class="titlepage"><div><div><h4 class="title"> 1169<a name="boost_proto.users_guide.front_end.customizing_expressions_in_your_domain"></a><a class="link" href="users_guide.html#boost_proto.users_guide.front_end.customizing_expressions_in_your_domain" title="Customizing Expressions in Your Domain">Customizing 1170 Expressions in Your Domain</a> 1171</h4></div></div></div> 1172<div class="toc"><dl class="toc"> 1173<dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.front_end.customizing_expressions_in_your_domain.domains">Domains</a></span></dt> 1174<dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.front_end.customizing_expressions_in_your_domain.extends">The 1175 <code class="literal">extends<></code> Expression Wrapper</a></span></dt> 1176<dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.front_end.customizing_expressions_in_your_domain.expression_generators">Expression 1177 Generators</a></span></dt> 1178<dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.front_end.customizing_expressions_in_your_domain.inhibiting_overloads">Controlling 1179 Operator Overloads</a></span></dt> 1180<dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.front_end.customizing_expressions_in_your_domain.per_domain_as_child">Controlling 1181 How Child Expressions Are Captured</a></span></dt> 1182<dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.front_end.customizing_expressions_in_your_domain.subdomains">EDSL 1183 Interoperatability: Sub-Domains</a></span></dt> 1184</dl></div> 1185<p> 1186 In this section, we'll learn all about <span class="emphasis"><em>domains</em></span>. In 1187 particular, we'll learn: 1188 </p> 1189<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "> 1190<li class="listitem"> 1191 How to associate Proto expressions with a domain, 1192 </li> 1193<li class="listitem"> 1194 How to add members to expressions within a domain, 1195 </li> 1196<li class="listitem"> 1197 How to use a <span class="emphasis"><em>generator</em></span> to post-process all new 1198 expressions created in your domain, 1199 </li> 1200<li class="listitem"> 1201 How to control which operators are overloaded in a domain, 1202 </li> 1203<li class="listitem"> 1204 How to specify capturing policies for child expressions and non-Proto 1205 objects, and 1206 </li> 1207<li class="listitem"> 1208 How to make expressions from separate domains interoperate. 1209 </li> 1210</ul></div> 1211<div class="section"> 1212<div class="titlepage"><div><div><h5 class="title"> 1213<a name="boost_proto.users_guide.front_end.customizing_expressions_in_your_domain.domains"></a><a class="link" href="users_guide.html#boost_proto.users_guide.front_end.customizing_expressions_in_your_domain.domains" title="Domains">Domains</a> 1214</h5></div></div></div> 1215<p> 1216 In the <a class="link" href="users_guide.html#boost_proto.users_guide.getting_started.hello_calculator" title="Hello Calculator">Hello 1217 Calculator</a> section, we looked into making calculator expressions 1218 directly usable as lambda expressions in calls to STL algorithms, as 1219 below: 1220 </p> 1221<pre class="programlisting"><span class="keyword">double</span> <span class="identifier">data</span><span class="special">[]</span> <span class="special">=</span> <span class="special">{</span><span class="number">1.</span><span class="special">,</span> <span class="number">2.</span><span class="special">,</span> <span class="number">3.</span><span class="special">,</span> <span class="number">4.</span><span class="special">};</span> 1222 1223<span class="comment">// Use the calculator EDSL to square each element ... HOW?</span> 1224<span class="identifier">std</span><span class="special">::</span><span class="identifier">transform</span><span class="special">(</span> <span class="identifier">data</span><span class="special">,</span> <span class="identifier">data</span> <span class="special">+</span> <span class="number">4</span><span class="special">,</span> <span class="identifier">data</span><span class="special">,</span> <span class="identifier">_1</span> <span class="special">*</span> <span class="identifier">_1</span> <span class="special">);</span> 1225</pre> 1226<p> 1227 The difficulty, if you recall, was that by default Proto expressions 1228 don't have interesting behaviors of their own. They're just trees. In 1229 particular, the expression <code class="computeroutput"><span class="identifier">_1</span> 1230 <span class="special">*</span> <span class="identifier">_1</span></code> 1231 won't have an <code class="computeroutput"><span class="keyword">operator</span><span class="special">()</span></code> 1232 that takes a double and returns a double like <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">transform</span><span class="special">()</span></code> expects -- unless we give it one. To 1233 make this work, we needed to define an expression wrapper type that defined 1234 the <code class="computeroutput"><span class="keyword">operator</span><span class="special">()</span></code> 1235 member function, and we needed to associate the wrapper with the calculator 1236 <span class="emphasis"><em>domain</em></span>. 1237 </p> 1238<p> 1239 In Proto, the term <span class="emphasis"><em>domain</em></span> refers to a type that 1240 associates expressions in that domain to an expression <span class="emphasis"><em>generator</em></span>. 1241 The generator is just a function object that accepts an expression and 1242 does something to it, like wrapping it in an expression wrapper. 1243 </p> 1244<p> 1245 You can also use a domain to associate expressions with a grammar. When 1246 you specify a domain's grammar, Proto ensures that all the expressions 1247 it generates in that domain conform to the domain's grammar. It does 1248 that by disabling any operator overloads that would create invalid expressions. 1249 </p> 1250</div> 1251<div class="section"> 1252<div class="titlepage"><div><div><h5 class="title"> 1253<a name="boost_proto.users_guide.front_end.customizing_expressions_in_your_domain.extends"></a><a class="link" href="users_guide.html#boost_proto.users_guide.front_end.customizing_expressions_in_your_domain.extends" title="The extends<> Expression Wrapper">The 1254 <code class="literal">extends<></code> Expression Wrapper</a> 1255</h5></div></div></div> 1256<p> 1257 The first step to giving your calculator expressions extra behaviors 1258 is to define a calculator domain. All expressions within the calculator 1259 domain will be imbued with calculator-ness, as we'll see. 1260 </p> 1261<pre class="programlisting"><span class="comment">// A type to be used as a domain tag (to be defined below)</span> 1262<span class="keyword">struct</span> <span class="identifier">calculator_domain</span><span class="special">;</span> 1263</pre> 1264<p> 1265 We use this domain type when extending the <code class="computeroutput"><a class="link" href="../boost/proto/expr.html" title="Struct template expr">proto::expr<></a></code> 1266 type, which we do with the <code class="computeroutput"><a class="link" href="../boost/proto/extends.html" title="Struct template extends">proto::extends<></a></code> 1267 class template. Here is our expression wrapper, which imbues an expression 1268 with calculator-ness. It is described below. 1269 </p> 1270<pre class="programlisting"><span class="comment">// The calculator<> expression wrapper makes expressions</span> 1271<span class="comment">// function objects.</span> 1272<span class="keyword">template</span><span class="special"><</span> <span class="keyword">typename</span> <span class="identifier">Expr</span> <span class="special">></span> 1273<span class="keyword">struct</span> <span class="identifier">calculator</span> 1274 <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">extends</span><span class="special"><</span> <span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">calculator</span><span class="special"><</span> <span class="identifier">Expr</span> <span class="special">>,</span> <span class="identifier">calculator_domain</span> <span class="special">></span> 1275<span class="special">{</span> 1276 <span class="keyword">typedef</span> 1277 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">extends</span><span class="special"><</span> <span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">calculator</span><span class="special"><</span> <span class="identifier">Expr</span> <span class="special">>,</span> <span class="identifier">calculator_domain</span> <span class="special">></span> 1278 <span class="identifier">base_type</span><span class="special">;</span> 1279 1280 <span class="identifier">calculator</span><span class="special">(</span> <span class="identifier">Expr</span> <span class="keyword">const</span> <span class="special">&</span><span class="identifier">expr</span> <span class="special">=</span> <span class="identifier">Expr</span><span class="special">()</span> <span class="special">)</span> 1281 <span class="special">:</span> <span class="identifier">base_type</span><span class="special">(</span> <span class="identifier">expr</span> <span class="special">)</span> 1282 <span class="special">{}</span> 1283 1284 <span class="comment">// This is usually needed because by default, the compiler-</span> 1285 <span class="comment">// generated assignment operator hides extends<>::operator=</span> 1286 <span class="identifier">BOOST_PROTO_EXTENDS_USING_ASSIGN</span><span class="special">(</span><span class="identifier">calculator</span><span class="special">)</span> 1287 1288 <span class="keyword">typedef</span> <span class="keyword">double</span> <span class="identifier">result_type</span><span class="special">;</span> 1289 1290 <span class="comment">// Hide base_type::operator() by defining our own which</span> 1291 <span class="comment">// evaluates the calculator expression with a calculator context.</span> 1292 <span class="identifier">result_type</span> <span class="keyword">operator</span><span class="special">()(</span> <span class="keyword">double</span> <span class="identifier">d1</span> <span class="special">=</span> <span class="number">0.0</span><span class="special">,</span> <span class="keyword">double</span> <span class="identifier">d2</span> <span class="special">=</span> <span class="number">0.0</span> <span class="special">)</span> <span class="keyword">const</span> 1293 <span class="special">{</span> 1294 <span class="comment">// As defined in the Hello Calculator section.</span> 1295 <span class="identifier">calculator_context</span> <span class="identifier">ctx</span><span class="special">;</span> 1296 1297 <span class="comment">// ctx.args is a vector<double> that holds the values</span> 1298 <span class="comment">// with which we replace the placeholders (e.g., _1 and _2)</span> 1299 <span class="comment">// in the expression.</span> 1300 <span class="identifier">ctx</span><span class="special">.</span><span class="identifier">args</span><span class="special">.</span><span class="identifier">push_back</span><span class="special">(</span> <span class="identifier">d1</span> <span class="special">);</span> <span class="comment">// _1 gets the value of d1</span> 1301 <span class="identifier">ctx</span><span class="special">.</span><span class="identifier">args</span><span class="special">.</span><span class="identifier">push_back</span><span class="special">(</span> <span class="identifier">d2</span> <span class="special">);</span> <span class="comment">// _2 gets the value of d2</span> 1302 1303 <span class="keyword">return</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">eval</span><span class="special">(*</span><span class="keyword">this</span><span class="special">,</span> <span class="identifier">ctx</span> <span class="special">);</span> <span class="comment">// evaluate the expression</span> 1304 <span class="special">}</span> 1305<span class="special">};</span> 1306</pre> 1307<p> 1308 We want calculator expressions to be function objects, so we have to 1309 define an <code class="computeroutput"><span class="keyword">operator</span><span class="special">()</span></code> 1310 that takes and returns doubles. The <code class="computeroutput"><span class="identifier">calculator</span><span class="special"><></span></code> wrapper above does that with 1311 the help of the <code class="computeroutput"><a class="link" href="../boost/proto/extends.html" title="Struct template extends">proto::extends<></a></code> 1312 template. The first template to <code class="computeroutput"><a class="link" href="../boost/proto/extends.html" title="Struct template extends">proto::extends<></a></code> 1313 parameter is the expression type we are extending. The second is the 1314 type of the wrapped expression. The third parameter is the domain that 1315 this wrapper is associated with. A wrapper type like <code class="computeroutput"><span class="identifier">calculator</span><span class="special"><></span></code> that inherits from <code class="computeroutput"><a class="link" href="../boost/proto/extends.html" title="Struct template extends">proto::extends<></a></code> behaves just like 1316 the expression type it has extended, with any additional behaviors you 1317 choose to give it. 1318 </p> 1319<div class="note"><table border="0" summary="Note"> 1320<tr> 1321<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../doc/src/images/note.png"></td> 1322<th align="left">Note</th> 1323</tr> 1324<tr><td align="left" valign="top"> 1325<p> 1326 <span class="bold"><strong>Why not just inherit from <code class="literal">proto::expr<></code>?</strong></span> 1327 </p> 1328<p> 1329 You might be thinking that this expression extension business is unnecessarily 1330 complicated. After all, isn't this why C++ supports inheritance? Why 1331 can't <code class="literal">calculator<Expr></code> just inherit from 1332 <code class="literal">Expr</code> directly? The reason is because <code class="literal">Expr</code>, 1333 which presumably is an instantiation of <code class="computeroutput"><a class="link" href="../boost/proto/expr.html" title="Struct template expr">proto::expr<></a></code>, 1334 has expression template-building operator overloads that will be incorrect 1335 for derived types. They will store <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code> by reference to <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">expr</span><span class="special"><></span></code>, effectively slicing off any 1336 derived parts. <code class="computeroutput"><a class="link" href="../boost/proto/extends.html" title="Struct template extends">proto::extends<></a></code> 1337 gives your derived types operator overloads that don't slice off your 1338 additional members. 1339 </p> 1340</td></tr> 1341</table></div> 1342<p> 1343 Although not strictly necessary in this case, we bring <code class="computeroutput"><span class="identifier">extends</span><span class="special"><>::</span><span class="keyword">operator</span><span class="special">=</span></code> 1344 into scope with the <code class="computeroutput"><span class="identifier">BOOST_PROTO_EXTENDS_USING_ASSIGN</span><span class="special">()</span></code> macro. This is really only necessary 1345 if you want expressions like <code class="computeroutput"><span class="identifier">_1</span> 1346 <span class="special">=</span> <span class="number">3</span></code> 1347 to create a lazily evaluated assignment. <code class="computeroutput"><a class="link" href="../boost/proto/extends.html" title="Struct template extends">proto::extends<></a></code> 1348 defines the appropriate <code class="computeroutput"><span class="keyword">operator</span><span class="special">=</span></code> for you, but the compiler-generated 1349 <code class="computeroutput"><span class="identifier">calculator</span><span class="special"><>::</span><span class="keyword">operator</span><span class="special">=</span></code> 1350 will hide it unless you make it available with the macro. 1351 </p> 1352<p> 1353 Note that in the implementation of <code class="computeroutput"><span class="identifier">calculator</span><span class="special"><>::</span><span class="keyword">operator</span><span class="special">()</span></code>, we evaluate the expression with the 1354 <code class="computeroutput"><span class="identifier">calculator_context</span></code> we 1355 defined earlier. As we saw before, the context is what gives the operators 1356 their meaning. In the case of the calculator, the context is also what 1357 defines the meaning of the placeholder terminals. 1358 </p> 1359<p> 1360 Now that we have defined the <code class="computeroutput"><span class="identifier">calculator</span><span class="special"><></span></code> expression wrapper, we need to 1361 wrap the placeholders to imbue them with calculator-ness: 1362 </p> 1363<pre class="programlisting"><span class="identifier">calculator</span><span class="special"><</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special"><</span> <span class="identifier">placeholder</span><span class="special"><</span><span class="number">0</span><span class="special">></span> <span class="special">>::</span><span class="identifier">type</span> <span class="special">></span> <span class="keyword">const</span> <span class="identifier">_1</span><span class="special">;</span> 1364<span class="identifier">calculator</span><span class="special"><</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special"><</span> <span class="identifier">placeholder</span><span class="special"><</span><span class="number">1</span><span class="special">></span> <span class="special">>::</span><span class="identifier">type</span> <span class="special">></span> <span class="keyword">const</span> <span class="identifier">_2</span><span class="special">;</span> 1365</pre> 1366<h6> 1367<a name="boost_proto.users_guide.front_end.customizing_expressions_in_your_domain.extends.h0"></a> 1368 <span class="phrase"><a name="boost_proto.users_guide.front_end.customizing_expressions_in_your_domain.extends.retaining_pod_ness_with__literal_boost_proto_extends____literal_"></a></span><a class="link" href="users_guide.html#boost_proto.users_guide.front_end.customizing_expressions_in_your_domain.extends.retaining_pod_ness_with__literal_boost_proto_extends____literal_">Retaining 1369 POD-ness with <code class="literal">BOOST_PROTO_EXTENDS()</code></a> 1370 </h6> 1371<p> 1372 To use <code class="computeroutput"><a class="link" href="../boost/proto/extends.html" title="Struct template extends">proto::extends<></a></code>, your extension type 1373 must derive from <code class="computeroutput"><a class="link" href="../boost/proto/extends.html" title="Struct template extends">proto::extends<></a></code>. 1374 Unfortunately, that means that your extension type is no longer POD and 1375 its instances cannot be <span class="emphasis"><em>statically initialized</em></span>. 1376 (See the <a class="link" href="appendices.html#boost_proto.appendices.rationale.static_initialization" title="Static Initialization">Static 1377 Initialization</a> section in the <a class="link" href="appendices.html#boost_proto.appendices.rationale" title="Appendix C: Rationale">Rationale</a> 1378 appendix for why this matters.) In particular, as defined above, the 1379 global placeholder objects <code class="computeroutput"><span class="identifier">_1</span></code> 1380 and <code class="computeroutput"><span class="identifier">_2</span></code> will need to be 1381 initialized at runtime, which could lead to subtle order of initialization 1382 bugs. 1383 </p> 1384<p> 1385 There is another way to make an expression extension that doesn't sacrifice 1386 POD-ness : the <code class="literal"><code class="computeroutput"><a class="link" href="../BOOST_PROTO_EXTENDS.html" title="Macro BOOST_PROTO_EXTENDS">BOOST_PROTO_EXTENDS</a></code>()</code> 1387 macro. You can use it much like you use <code class="computeroutput"><a class="link" href="../boost/proto/extends.html" title="Struct template extends">proto::extends<></a></code>. 1388 We can use <code class="literal"><code class="computeroutput"><a class="link" href="../BOOST_PROTO_EXTENDS.html" title="Macro BOOST_PROTO_EXTENDS">BOOST_PROTO_EXTENDS</a></code>()</code> 1389 to keep <code class="computeroutput"><span class="identifier">calculator</span><span class="special"><></span></code> 1390 a POD and our placeholders statically initialized. 1391 </p> 1392<pre class="programlisting"><span class="comment">// The calculator<> expression wrapper makes expressions</span> 1393<span class="comment">// function objects.</span> 1394<span class="keyword">template</span><span class="special"><</span> <span class="keyword">typename</span> <span class="identifier">Expr</span> <span class="special">></span> 1395<span class="keyword">struct</span> <span class="identifier">calculator</span> 1396<span class="special">{</span> 1397 <span class="comment">// Use BOOST_PROTO_EXTENDS() instead of proto::extends<> to</span> 1398 <span class="comment">// make this type a Proto expression extension.</span> 1399 <span class="identifier">BOOST_PROTO_EXTENDS</span><span class="special">(</span><span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">calculator</span><span class="special"><</span><span class="identifier">Expr</span><span class="special">>,</span> <span class="identifier">calculator_domain</span><span class="special">)</span> 1400 1401 <span class="keyword">typedef</span> <span class="keyword">double</span> <span class="identifier">result_type</span><span class="special">;</span> 1402 1403 <span class="identifier">result_type</span> <span class="keyword">operator</span><span class="special">()(</span> <span class="keyword">double</span> <span class="identifier">d1</span> <span class="special">=</span> <span class="number">0.0</span><span class="special">,</span> <span class="keyword">double</span> <span class="identifier">d2</span> <span class="special">=</span> <span class="number">0.0</span> <span class="special">)</span> <span class="keyword">const</span> 1404 <span class="special">{</span> 1405 <span class="comment">/* ... as before ... */</span> 1406 <span class="special">}</span> 1407<span class="special">};</span> 1408</pre> 1409<p> 1410 With the new <code class="computeroutput"><span class="identifier">calculator</span><span class="special"><></span></code> type, we can redefine our placeholders 1411 to be statically initialized: 1412 </p> 1413<pre class="programlisting"><span class="identifier">calculator</span><span class="special"><</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special"><</span> <span class="identifier">placeholder</span><span class="special"><</span><span class="number">0</span><span class="special">></span> <span class="special">>::</span><span class="identifier">type</span> <span class="special">></span> <span class="keyword">const</span> <span class="identifier">_1</span> <span class="special">=</span> <span class="special">{{{}}};</span> 1414<span class="identifier">calculator</span><span class="special"><</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special"><</span> <span class="identifier">placeholder</span><span class="special"><</span><span class="number">1</span><span class="special">></span> <span class="special">>::</span><span class="identifier">type</span> <span class="special">></span> <span class="keyword">const</span> <span class="identifier">_2</span> <span class="special">=</span> <span class="special">{{{}}};</span> 1415</pre> 1416<p> 1417 We need to make one additional small change to accommodate the POD-ness 1418 of our expression extension, which we'll describe below in the section 1419 on expression generators. 1420 </p> 1421<p> 1422 What does <code class="literal"><code class="computeroutput"><a class="link" href="../BOOST_PROTO_EXTENDS.html" title="Macro BOOST_PROTO_EXTENDS">BOOST_PROTO_EXTENDS</a></code>()</code> 1423 do? It defines a data member of the expression type being extended; some 1424 nested typedefs that Proto requires; <code class="computeroutput"><span class="keyword">operator</span><span class="special">=</span></code>, <code class="computeroutput"><span class="keyword">operator</span><span class="special">[]</span></code> and <code class="computeroutput"><span class="keyword">operator</span><span class="special">()</span></code> overloads for building expression templates; 1425 and a nested <code class="computeroutput"><span class="identifier">result</span><span class="special"><></span></code> 1426 template for calculating the return type of <code class="computeroutput"><span class="keyword">operator</span><span class="special">()</span></code>. In this case, however, the <code class="computeroutput"><span class="keyword">operator</span><span class="special">()</span></code> 1427 overloads and the <code class="computeroutput"><span class="identifier">result</span><span class="special"><></span></code> template are not needed because 1428 we are defining our own <code class="computeroutput"><span class="keyword">operator</span><span class="special">()</span></code> in the <code class="computeroutput"><span class="identifier">calculator</span><span class="special"><></span></code> type. Proto provides additional 1429 macros for finer control over which member functions are defined. We 1430 could improve our <code class="computeroutput"><span class="identifier">calculator</span><span class="special"><></span></code> type as follows: 1431 </p> 1432<pre class="programlisting"><span class="comment">// The calculator<> expression wrapper makes expressions</span> 1433<span class="comment">// function objects.</span> 1434<span class="keyword">template</span><span class="special"><</span> <span class="keyword">typename</span> <span class="identifier">Expr</span> <span class="special">></span> 1435<span class="keyword">struct</span> <span class="identifier">calculator</span> 1436<span class="special">{</span> 1437 <span class="comment">// Use BOOST_PROTO_BASIC_EXTENDS() instead of proto::extends<> to</span> 1438 <span class="comment">// make this type a Proto expression extension:</span> 1439 <span class="identifier">BOOST_PROTO_BASIC_EXTENDS</span><span class="special">(</span><span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">calculator</span><span class="special"><</span><span class="identifier">Expr</span><span class="special">>,</span> <span class="identifier">calculator_domain</span><span class="special">)</span> 1440 1441 <span class="comment">// Define operator[] to build expression templates:</span> 1442 <span class="identifier">BOOST_PROTO_EXTENDS_SUBSCRIPT</span><span class="special">()</span> 1443 1444 <span class="comment">// Define operator= to build expression templates:</span> 1445 <span class="identifier">BOOST_PROTO_EXTENDS_ASSIGN</span><span class="special">()</span> 1446 1447 <span class="keyword">typedef</span> <span class="keyword">double</span> <span class="identifier">result_type</span><span class="special">;</span> 1448 1449 <span class="identifier">result_type</span> <span class="keyword">operator</span><span class="special">()(</span> <span class="keyword">double</span> <span class="identifier">d1</span> <span class="special">=</span> <span class="number">0.0</span><span class="special">,</span> <span class="keyword">double</span> <span class="identifier">d2</span> <span class="special">=</span> <span class="number">0.0</span> <span class="special">)</span> <span class="keyword">const</span> 1450 <span class="special">{</span> 1451 <span class="comment">/* ... as before ... */</span> 1452 <span class="special">}</span> 1453<span class="special">};</span> 1454</pre> 1455<p> 1456 Notice that we are now using <code class="literal"><code class="computeroutput"><a class="link" href="../BOOST_PROTO_BASIC_EXTENDS.html" title="Macro BOOST_PROTO_BASIC_EXTENDS">BOOST_PROTO_BASIC_EXTENDS</a></code>()</code> 1457 instead of <code class="literal"><code class="computeroutput"><a class="link" href="../BOOST_PROTO_EXTENDS.html" title="Macro BOOST_PROTO_EXTENDS">BOOST_PROTO_EXTENDS</a></code>()</code>. 1458 This just adds the data member and the nested typedefs but not any of 1459 the overloaded operators. Those are added separately with <code class="literal"><code class="computeroutput"><a class="link" href="../BOOST_PROTO_EXTENDS_ASSIGN.html" title="Macro BOOST_PROTO_EXTENDS_ASSIGN">BOOST_PROTO_EXTENDS_ASSIGN</a></code>()</code> 1460 and <code class="literal"><code class="computeroutput"><a class="link" href="../BOOST_PROTO__1_3_33_5_10_7.html" title="Macro BOOST_PROTO_EXTENDS_SUBSCRIPT">BOOST_PROTO_EXTENDS_SUBSCRIPT</a></code>()</code>. 1461 We are leaving out the function call operator and the nested <code class="computeroutput"><span class="identifier">result</span><span class="special"><></span></code> 1462 template that could have been defined with Proto's <code class="literal"><code class="computeroutput"><a class="link" href="../BOOST_PROTO__1_3_33_5_10_6.html" title="Macro BOOST_PROTO_EXTENDS_FUNCTION">BOOST_PROTO_EXTENDS_FUNCTION</a></code>()</code> 1463 macro. 1464 </p> 1465<p> 1466 In summary, here are the macros you can use to define expression extensions, 1467 and a brief description of each. 1468 </p> 1469<div class="table"> 1470<a name="boost_proto.users_guide.front_end.customizing_expressions_in_your_domain.extends.t0"></a><p class="title"><b>Table 32.2. Expression Extension Macros</b></p> 1471<div class="table-contents"><table class="table" summary="Expression Extension Macros"> 1472<colgroup> 1473<col> 1474<col> 1475</colgroup> 1476<thead><tr> 1477<th> 1478 <p> 1479 Macro 1480 </p> 1481 </th> 1482<th> 1483 <p> 1484 Purpose 1485 </p> 1486 </th> 1487</tr></thead> 1488<tbody> 1489<tr> 1490<td> 1491 <p> 1492</p> 1493<pre class="programlisting"><code class="computeroutput"><a class="link" href="../BOOST_PROTO_BASIC_EXTENDS.html" title="Macro BOOST_PROTO_BASIC_EXTENDS">BOOST_PROTO_BASIC_EXTENDS</a></code><span class="special">(</span> 1494 <em class="replaceable"><code>expression</code></em> 1495 <span class="special">,</span> <em class="replaceable"><code>extension</code></em> 1496 <span class="special">,</span> <em class="replaceable"><code>domain</code></em> 1497<span class="special">)</span></pre> 1498<p> 1499 </p> 1500 </td> 1501<td> 1502 <p> 1503 Defines a data member of type <code class="computeroutput"><em class="replaceable"><code>expression</code></em></code> 1504 and some nested typedefs that Proto requires. 1505 </p> 1506 </td> 1507</tr> 1508<tr> 1509<td> 1510 <p> 1511 <code class="literal"><code class="computeroutput"><a class="link" href="../BOOST_PROTO_EXTENDS_ASSIGN.html" title="Macro BOOST_PROTO_EXTENDS_ASSIGN">BOOST_PROTO_EXTENDS_ASSIGN</a></code>()</code> 1512 </p> 1513 </td> 1514<td> 1515 <p> 1516 Defines <code class="computeroutput"><span class="keyword">operator</span><span class="special">=</span></code>. Only valid when preceded 1517 by <code class="literal"><code class="computeroutput"><a class="link" href="../BOOST_PROTO_BASIC_EXTENDS.html" title="Macro BOOST_PROTO_BASIC_EXTENDS">BOOST_PROTO_BASIC_EXTENDS</a></code>()</code>. 1518 </p> 1519 </td> 1520</tr> 1521<tr> 1522<td> 1523 <p> 1524 <code class="literal"><code class="computeroutput"><a class="link" href="../BOOST_PROTO__1_3_33_5_10_7.html" title="Macro BOOST_PROTO_EXTENDS_SUBSCRIPT">BOOST_PROTO_EXTENDS_SUBSCRIPT</a></code>()</code> 1525 </p> 1526 </td> 1527<td> 1528 <p> 1529 Defines <code class="computeroutput"><span class="keyword">operator</span><span class="special">[]</span></code>. Only valid when preceded 1530 by <code class="literal"><code class="computeroutput"><a class="link" href="../BOOST_PROTO_BASIC_EXTENDS.html" title="Macro BOOST_PROTO_BASIC_EXTENDS">BOOST_PROTO_BASIC_EXTENDS</a></code>()</code>. 1531 </p> 1532 </td> 1533</tr> 1534<tr> 1535<td> 1536 <p> 1537 <code class="literal"><code class="computeroutput"><a class="link" href="../BOOST_PROTO__1_3_33_5_10_6.html" title="Macro BOOST_PROTO_EXTENDS_FUNCTION">BOOST_PROTO_EXTENDS_FUNCTION</a></code>()</code> 1538 </p> 1539 </td> 1540<td> 1541 <p> 1542 Defines <code class="computeroutput"><span class="keyword">operator</span><span class="special">()</span></code> and a nested <code class="computeroutput"><span class="identifier">result</span><span class="special"><></span></code> 1543 template for return type calculation. Only valid when preceded 1544 by <code class="literal"><code class="computeroutput"><a class="link" href="../BOOST_PROTO_BASIC_EXTENDS.html" title="Macro BOOST_PROTO_BASIC_EXTENDS">BOOST_PROTO_BASIC_EXTENDS</a></code>()</code>. 1545 </p> 1546 </td> 1547</tr> 1548<tr> 1549<td> 1550 <p> 1551</p> 1552<pre class="programlisting"><code class="computeroutput"><a class="link" href="../BOOST_PROTO_EXTENDS.html" title="Macro BOOST_PROTO_EXTENDS">BOOST_PROTO_EXTENDS</a></code><span class="special">(</span> 1553 <em class="replaceable"><code>expression</code></em> 1554 <span class="special">,</span> <em class="replaceable"><code>extension</code></em> 1555 <span class="special">,</span> <em class="replaceable"><code>domain</code></em> 1556<span class="special">)</span></pre> 1557<p> 1558 </p> 1559 </td> 1560<td> 1561 <p> 1562 Equivalent to: 1563</p> 1564<pre class="programlisting"><code class="computeroutput"><a class="link" href="../BOOST_PROTO_BASIC_EXTENDS.html" title="Macro BOOST_PROTO_BASIC_EXTENDS">BOOST_PROTO_BASIC_EXTENDS</a></code><span class="special">(</span><em class="replaceable"><code>expression</code></em><span class="special">,</span> <em class="replaceable"><code>extension</code></em><span class="special">,</span> <em class="replaceable"><code>domain</code></em><span class="special">)</span> 1565 1566 <code class="literal"><code class="computeroutput"><a class="link" href="../BOOST_PROTO_EXTENDS_ASSIGN.html" title="Macro BOOST_PROTO_EXTENDS_ASSIGN">BOOST_PROTO_EXTENDS_ASSIGN</a></code>()</code> 1567 1568 <code class="literal"><code class="computeroutput"><a class="link" href="../BOOST_PROTO__1_3_33_5_10_7.html" title="Macro BOOST_PROTO_EXTENDS_SUBSCRIPT">BOOST_PROTO_EXTENDS_SUBSCRIPT</a></code>()</code> 1569 1570 <code class="literal"><code class="computeroutput"><a class="link" href="../BOOST_PROTO__1_3_33_5_10_6.html" title="Macro BOOST_PROTO_EXTENDS_FUNCTION">BOOST_PROTO_EXTENDS_FUNCTION</a></code>()</code></pre> 1571<p> 1572 </p> 1573 </td> 1574</tr> 1575</tbody> 1576</table></div> 1577</div> 1578<br class="table-break"><div class="warning"><table border="0" summary="Warning"> 1579<tr> 1580<td rowspan="2" align="center" valign="top" width="25"><img alt="[Warning]" src="../../../doc/src/images/warning.png"></td> 1581<th align="left">Warning</th> 1582</tr> 1583<tr><td align="left" valign="top"> 1584<p> 1585 <span class="bold"><strong>Argument-Dependent Lookup and <code class="literal"><code class="computeroutput"><a class="link" href="../BOOST_PROTO_EXTENDS.html" title="Macro BOOST_PROTO_EXTENDS">BOOST_PROTO_EXTENDS</a></code>()</code></strong></span> 1586 </p> 1587<p> 1588 Proto's operator overloads are defined in the <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">proto</span></code> 1589 namespace and are found by argument-dependent lookup (ADL). This usually 1590 just works because expressions are made up of types that live in the 1591 <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">proto</span></code> namespace. However, sometimes 1592 when you use <code class="literal"><code class="computeroutput"><a class="link" href="../BOOST_PROTO_EXTENDS.html" title="Macro BOOST_PROTO_EXTENDS">BOOST_PROTO_EXTENDS</a></code>()</code> 1593 that is not the case. Consider: 1594 </p> 1595<p> 1596</p> 1597<pre class="programlisting"><span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> 1598<span class="keyword">struct</span> <span class="identifier">my_complex</span> 1599<span class="special">{</span> 1600 <span class="identifier">BOOST_PROTO_EXTENDS</span><span class="special">(</span> 1601 <span class="keyword">typename</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special"><</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">complex</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="special">>::</span><span class="identifier">type</span> 1602 <span class="special">,</span> <span class="identifier">my_complex</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> 1603 <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">default_domain</span> 1604 <span class="special">)</span> 1605<span class="special">};</span> 1606 1607<span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span> 1608<span class="special">{</span> 1609 <span class="identifier">my_complex</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> <span class="identifier">c0</span><span class="special">,</span> <span class="identifier">c1</span><span class="special">;</span> 1610 1611 <span class="identifier">c0</span> <span class="special">+</span> <span class="identifier">c1</span><span class="special">;</span> <span class="comment">// ERROR: operator+ not found</span> 1612<span class="special">}</span> 1613</pre> 1614<p> 1615 </p> 1616<p> 1617 The problem has to do with how argument-dependent lookup works. The 1618 type <code class="computeroutput"><span class="identifier">my_complex</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span></code> 1619 is not associated in any way with the <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">proto</span></code> 1620 namespace, so the operators defined there are not considered. (Had 1621 we inherited from <code class="computeroutput"><a class="link" href="../boost/proto/extends.html" title="Struct template extends">proto::extends<></a></code> 1622 instead of used <code class="literal"><code class="computeroutput"><a class="link" href="../BOOST_PROTO_EXTENDS.html" title="Macro BOOST_PROTO_EXTENDS">BOOST_PROTO_EXTENDS</a></code>()</code>, 1623 we would have avoided the problem because inheriting from a type in 1624 <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">proto</span></code> namespace is enough to get 1625 ADL to kick in.) 1626 </p> 1627<p> 1628 So what can we do? By adding an extra dummy template parameter that 1629 defaults to a type in the <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">proto</span></code> 1630 namespace, we can trick ADL into finding the right operator overloads. 1631 The solution looks like this: 1632 </p> 1633<p> 1634</p> 1635<pre class="programlisting"><span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">,</span> <span class="keyword">class</span> <span class="identifier">Dummy</span> <span class="special">=</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">is_proto_expr</span><span class="special">></span> 1636<span class="keyword">struct</span> <span class="identifier">my_complex</span> 1637<span class="special">{</span> 1638 <span class="identifier">BOOST_PROTO_EXTENDS</span><span class="special">(</span> 1639 <span class="keyword">typename</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special"><</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">complex</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="special">>::</span><span class="identifier">type</span> 1640 <span class="special">,</span> <span class="identifier">my_complex</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> 1641 <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">default_domain</span> 1642 <span class="special">)</span> 1643<span class="special">};</span> 1644 1645<span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span> 1646<span class="special">{</span> 1647 <span class="identifier">my_complex</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> <span class="identifier">c0</span><span class="special">,</span> <span class="identifier">c1</span><span class="special">;</span> 1648 1649 <span class="identifier">c0</span> <span class="special">+</span> <span class="identifier">c1</span><span class="special">;</span> <span class="comment">// OK, operator+ found now!</span> 1650<span class="special">}</span> 1651</pre> 1652<p> 1653 </p> 1654<p> 1655 The type <code class="computeroutput"><a class="link" href="../boost/proto/is_proto_expr.html" title="Struct is_proto_expr">proto::is_proto_expr</a></code> is nothing 1656 but an empty struct, but by making it a template parameter we make 1657 <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">proto</span></code> an associated namespace of 1658 <code class="computeroutput"><span class="identifier">my_complex</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span></code>. 1659 Now ADL can successfully find Proto's operator overloads. 1660 </p> 1661</td></tr> 1662</table></div> 1663</div> 1664<div class="section"> 1665<div class="titlepage"><div><div><h5 class="title"> 1666<a name="boost_proto.users_guide.front_end.customizing_expressions_in_your_domain.expression_generators"></a><a class="link" href="users_guide.html#boost_proto.users_guide.front_end.customizing_expressions_in_your_domain.expression_generators" title="Expression Generators">Expression 1667 Generators</a> 1668</h5></div></div></div> 1669<p> 1670 The last thing that remains to be done is to tell Proto that it needs 1671 to wrap all of our calculator expressions in our <code class="computeroutput"><span class="identifier">calculator</span><span class="special"><></span></code> wrapper. We have already wrapped 1672 the placeholders, but we want <span class="emphasis"><em>all</em></span> expressions that 1673 involve the calculator placeholders to be calculators. We can do that 1674 by specifying an expression generator when we define our <code class="computeroutput"><span class="identifier">calculator_domain</span></code>, as follows: 1675 </p> 1676<pre class="programlisting"><span class="comment">// Define the calculator_domain we forward-declared above.</span> 1677<span class="comment">// Specify that all expression in this domain should be wrapped</span> 1678<span class="comment">// in the calculator<> expression wrapper.</span> 1679<span class="keyword">struct</span> <span class="identifier">calculator_domain</span> 1680 <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">domain</span><span class="special"><</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">generator</span><span class="special"><</span> <span class="identifier">calculator</span> <span class="special">></span> <span class="special">></span> 1681<span class="special">{};</span> 1682</pre> 1683<p> 1684 The first template parameter to <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">domain</span><span class="special"><></span></code> is the generator. "Generator" 1685 is just a fancy name for a function object that accepts an expression 1686 and does something to it. <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">generator</span><span class="special"><></span></code> is a very simple one --- it wraps 1687 an expression in the wrapper you specify. <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">domain</span><span class="special"><></span></code> inherits from its generator parameter, 1688 so all domains are themselves function objects. 1689 </p> 1690<p> 1691 If we used <code class="literal"><code class="computeroutput"><a class="link" href="../BOOST_PROTO_EXTENDS.html" title="Macro BOOST_PROTO_EXTENDS">BOOST_PROTO_EXTENDS</a></code>()</code> 1692 to keep our expression extension type POD, then we need to use <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">pod_generator</span><span class="special"><></span></code> 1693 instead of <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">generator</span><span class="special"><></span></code>, 1694 as follows: 1695 </p> 1696<pre class="programlisting"><span class="comment">// If calculator<> uses BOOST_PROTO_EXTENDS() instead of </span> 1697<span class="comment">// use proto::extends<>, use proto::pod_generator<> instead</span> 1698<span class="comment">// of proto::generator<>.</span> 1699<span class="keyword">struct</span> <span class="identifier">calculator_domain</span> 1700 <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">domain</span><span class="special"><</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">pod_generator</span><span class="special"><</span> <span class="identifier">calculator</span> <span class="special">></span> <span class="special">></span> 1701<span class="special">{};</span> 1702</pre> 1703<p> 1704 After Proto has calculated a new expression type, it checks the domains 1705 of the child expressions. They must match. Assuming they do, Proto creates 1706 the new expression and passes it to <code class="computeroutput"><em class="replaceable"><code>Domain</code></em><span class="special">::</span><span class="keyword">operator</span><span class="special">()</span></code> for any additional processing. If we 1707 don't specify a generator, the new expression gets passed through unchanged. 1708 But since we've specified a generator above, <code class="computeroutput"><span class="identifier">calculator_domain</span><span class="special">::</span><span class="keyword">operator</span><span class="special">()</span></code> returns <code class="computeroutput"><span class="identifier">calculator</span><span class="special"><></span></code> objects. 1709 </p> 1710<p> 1711 Now we can use calculator expressions as function objects to STL algorithms, 1712 as follows: 1713 </p> 1714<pre class="programlisting"><span class="keyword">double</span> <span class="identifier">data</span><span class="special">[]</span> <span class="special">=</span> <span class="special">{</span><span class="number">1.</span><span class="special">,</span> <span class="number">2.</span><span class="special">,</span> <span class="number">3.</span><span class="special">,</span> <span class="number">4.</span><span class="special">};</span> 1715 1716<span class="comment">// Use the calculator EDSL to square each element ... WORKS! :-)</span> 1717<span class="identifier">std</span><span class="special">::</span><span class="identifier">transform</span><span class="special">(</span> <span class="identifier">data</span><span class="special">,</span> <span class="identifier">data</span> <span class="special">+</span> <span class="number">4</span><span class="special">,</span> <span class="identifier">data</span><span class="special">,</span> <span class="identifier">_1</span> <span class="special">*</span> <span class="identifier">_1</span> <span class="special">);</span> 1718</pre> 1719</div> 1720<div class="section"> 1721<div class="titlepage"><div><div><h5 class="title"> 1722<a name="boost_proto.users_guide.front_end.customizing_expressions_in_your_domain.inhibiting_overloads"></a><a class="link" href="users_guide.html#boost_proto.users_guide.front_end.customizing_expressions_in_your_domain.inhibiting_overloads" title="Controlling Operator Overloads">Controlling 1723 Operator Overloads</a> 1724</h5></div></div></div> 1725<p> 1726 By default, Proto defines every possible operator overload for Protofied 1727 expressions. This makes it simple to bang together an EDSL. In some cases, 1728 however, the presence of Proto's promiscuous overloads can lead to confusion 1729 or worse. When that happens, you'll have to disable some of Proto's overloaded 1730 operators. That is done by defining the grammar for your domain and specifying 1731 it as the second parameter of the <code class="computeroutput"><a class="link" href="../boost/proto/domain.html" title="Struct template domain">proto::domain<></a></code> 1732 template. 1733 </p> 1734<p> 1735 In the <a class="link" href="users_guide.html#boost_proto.users_guide.getting_started.hello_calculator" title="Hello Calculator">Hello 1736 Calculator</a> section, we saw an example of a Proto grammar, which 1737 is repeated here: 1738 </p> 1739<pre class="programlisting"><span class="comment">// Define the grammar of calculator expressions</span> 1740<span class="keyword">struct</span> <span class="identifier">calculator_grammar</span> 1741 <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">or_</span><span class="special"><</span> 1742 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">plus</span><span class="special"><</span> <span class="identifier">calculator_grammar</span><span class="special">,</span> <span class="identifier">calculator_grammar</span> <span class="special">></span> 1743 <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">minus</span><span class="special"><</span> <span class="identifier">calculator_grammar</span><span class="special">,</span> <span class="identifier">calculator_grammar</span> <span class="special">></span> 1744 <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">multiplies</span><span class="special"><</span> <span class="identifier">calculator_grammar</span><span class="special">,</span> <span class="identifier">calculator_grammar</span> <span class="special">></span> 1745 <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">divides</span><span class="special"><</span> <span class="identifier">calculator_grammar</span><span class="special">,</span> <span class="identifier">calculator_grammar</span> <span class="special">></span> 1746 <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special"><</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_</span> <span class="special">></span> 1747 <span class="special">></span> 1748<span class="special">{};</span> 1749</pre> 1750<p> 1751 We'll have much more to say about grammars in subsequent sections, but 1752 for now, we'll just say that the <code class="computeroutput"><span class="identifier">calculator_grammar</span></code> 1753 struct describes a subset of all expression types -- the subset that 1754 comprise valid calculator expressions. We would like to prohibit Proto 1755 from creating a calculator expression that does not conform to this grammar. 1756 We do that by changing the definition of the <code class="computeroutput"><span class="identifier">calculator_domain</span></code> 1757 struct. 1758 </p> 1759<pre class="programlisting"><span class="comment">// Define the calculator_domain. Expressions in the calculator</span> 1760<span class="comment">// domain are wrapped in the calculator<> wrapper, and they must</span> 1761<span class="comment">// conform to the calculator_grammar:</span> 1762<span class="keyword">struct</span> <span class="identifier">calculator_domain</span> 1763 <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">domain</span><span class="special"><</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">generator</span><span class="special"><</span> <span class="identifier">calculator</span> <span class="special">>,</span> <span class="bold"><strong>calculator_grammar</strong></span> <span class="special">></span> 1764<span class="special">{};</span> 1765</pre> 1766<p> 1767 The only new addition is <code class="computeroutput"><span class="identifier">calculator_grammar</span></code> 1768 as the second template parameter to the <code class="computeroutput"><a class="link" href="../boost/proto/domain.html" title="Struct template domain">proto::domain<></a></code> 1769 template. That has the effect of disabling any of Proto's operator overloads 1770 that would create an invalid calculator expression. 1771 </p> 1772<p> 1773 Another common use for this feature would be to disable Proto's unary 1774 <code class="computeroutput"><span class="keyword">operator</span><span class="special">&</span></code> 1775 overload. It may be surprising for users of your EDSL that they cannot 1776 take the address of their expressions! You can very easily disable Proto's 1777 unary <code class="computeroutput"><span class="keyword">operator</span><span class="special">&</span></code> 1778 overload for your domain with a very simple grammar, as below: 1779 </p> 1780<pre class="programlisting"><span class="comment">// For expressions in my_domain, disable Proto's</span> 1781<span class="comment">// unary address-of operator.</span> 1782<span class="keyword">struct</span> <span class="identifier">my_domain</span> 1783 <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">domain</span><span class="special"><</span> 1784 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">generator</span><span class="special"><</span> <span class="identifier">my_wrapper</span> <span class="special">></span> 1785 <span class="comment">// A simple grammar that matches any expression that</span> 1786 <span class="comment">// is not a unary address-of expression.</span> 1787 <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">not_</span><span class="special"><</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">address_of</span><span class="special"><</span> <span class="identifier">_</span> <span class="special">></span> <span class="special">></span> 1788 <span class="special">></span> 1789<span class="special">{};</span> 1790</pre> 1791<p> 1792 The type <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">not_</span><span class="special"><</span> 1793 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">address_of</span><span class="special"><</span> 1794 <span class="identifier">_</span> <span class="special">></span> 1795 <span class="special">></span></code> is a very simple grammar 1796 that matches all expressions except unary address-of expressions. In 1797 the section describing Proto's intermediate form, we'll have much more 1798 to say about grammars. 1799 </p> 1800</div> 1801<div class="section"> 1802<div class="titlepage"><div><div><h5 class="title"> 1803<a name="boost_proto.users_guide.front_end.customizing_expressions_in_your_domain.per_domain_as_child"></a><a class="link" href="users_guide.html#boost_proto.users_guide.front_end.customizing_expressions_in_your_domain.per_domain_as_child" title="Controlling How Child Expressions Are Captured">Controlling 1804 How Child Expressions Are Captured</a> 1805</h5></div></div></div> 1806<div class="note"><table border="0" summary="Note"> 1807<tr> 1808<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../doc/src/images/note.png"></td> 1809<th align="left">Note</th> 1810</tr> 1811<tr><td align="left" valign="top"><p> 1812 This is an advanced topic. Feel free to skip this if you're just getting 1813 started with Proto. 1814 </p></td></tr> 1815</table></div> 1816<p> 1817 Proto's operator overloads build expressions from sub-expressions. The 1818 sub-expressions become children of the new expression. By default, the 1819 children are stored in the parent by reference. This section describes 1820 how to change that default. 1821 </p> 1822<h6> 1823<a name="boost_proto.users_guide.front_end.customizing_expressions_in_your_domain.per_domain_as_child.h0"></a> 1824 <span class="phrase"><a name="boost_proto.users_guide.front_end.customizing_expressions_in_your_domain.per_domain_as_child.primer___literal_as_child__literal__vs___literal_as_expr__literal_"></a></span><a class="link" href="users_guide.html#boost_proto.users_guide.front_end.customizing_expressions_in_your_domain.per_domain_as_child.primer___literal_as_child__literal__vs___literal_as_expr__literal_">Primer: 1825 <code class="literal">as_child</code> vs. <code class="literal">as_expr</code></a> 1826 </h6> 1827<p> 1828 Proto lets you independently customize the behavior of <code class="computeroutput"><a class="link" href="../boost/proto/as_child.html" title="Function as_child">proto::as_child()</a></code> and <code class="computeroutput"><a class="link" href="../boost/proto/as_expr.html" title="Function as_expr">proto::as_expr()</a></code>. 1829 Both accept an object <code class="literal">x</code> and return a Proto expression 1830 by turning <code class="literal">x</code> it into a Proto terminal if necessary. 1831 Although similar, the two functions are used in different situations 1832 and have subtly different behavior by default. It's important to understand 1833 the difference so that you know which to customize to achieve the behavior 1834 you want. 1835 </p> 1836<p> 1837 To wit: <code class="computeroutput"><a class="link" href="../boost/proto/as_expr.html" title="Function as_expr">proto::as_expr()</a></code> is typically used by 1838 <span class="emphasis"><em>you</em></span> to turn an object into a Proto expression that 1839 is to be held in a local variable, as so: 1840 </p> 1841<pre class="programlisting"><span class="keyword">auto</span> <span class="identifier">l</span> <span class="special">=</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">as_expr</span><span class="special">(</span><span class="identifier">x</span><span class="special">);</span> <span class="comment">// Turn x into a Proto expression, hold the result in a local</span> 1842</pre> 1843<p> 1844 The above works regardless of whether <code class="computeroutput"><span class="identifier">x</span></code> 1845 is already a Proto expression or not. The object <code class="computeroutput"><span class="identifier">l</span></code> 1846 is guaranteed to be a valid Proto expression. If <code class="computeroutput"><span class="identifier">x</span></code> 1847 is a non-Proto object, it is turned into a terminal expression that holds 1848 <code class="computeroutput"><span class="identifier">x</span></code> <span class="emphasis"><em>by value</em></span>.<a href="#ftn.boost_proto.users_guide.front_end.customizing_expressions_in_your_domain.per_domain_as_child.f0" class="footnote" name="boost_proto.users_guide.front_end.customizing_expressions_in_your_domain.per_domain_as_child.f0"><sup class="footnote">[34]</sup></a> If <code class="computeroutput"><span class="identifier">x</span></code> is a 1849 Proto object already, <code class="computeroutput"><a class="link" href="../boost/proto/as_expr.html" title="Function as_expr">proto::as_expr()</a></code> 1850 returns it <span class="emphasis"><em>by value</em></span> unmodified. 1851 </p> 1852<p> 1853 In contrast, <code class="computeroutput"><a class="link" href="../boost/proto/as_child.html" title="Function as_child">proto::as_child()</a></code> 1854 is used internally by Proto to pre-process objects before making them 1855 children of another expression. Since it's internal to Proto, you don't 1856 see it explicitly, but it's there behind the scenes in expressions like 1857 this: 1858 </p> 1859<pre class="programlisting"><span class="identifier">x</span> <span class="special">+</span> <span class="identifier">y</span><span class="special">;</span> <span class="comment">// Consider that y is a Proto expression, but x may or may not be.</span> 1860</pre> 1861<p> 1862 In this case, Proto builds a plus node from the two children. Both are 1863 pre-processed by passing them to <code class="computeroutput"><a class="link" href="../boost/proto/as_child.html" title="Function as_child">proto::as_child()</a></code> 1864 before making them children of the new node. If <code class="computeroutput"><span class="identifier">x</span></code> 1865 is not a Proto expression, it becomes one by being wrapped in a Proto 1866 terminal that holds it <span class="emphasis"><em>by reference</em></span>. If <code class="computeroutput"><span class="identifier">x</span></code> is already a Proto expression, <code class="computeroutput"><a class="link" href="../boost/proto/as_child.html" title="Function as_child">proto::as_child()</a></code> returns it <span class="emphasis"><em>by 1867 reference</em></span> unmodified. Contrast this with the above description 1868 for <code class="computeroutput"><a class="link" href="../boost/proto/as_expr.html" title="Function as_expr">proto::as_expr()</a></code>. 1869 </p> 1870<p> 1871 The table below summarizes the above description. 1872 </p> 1873<div class="table"> 1874<a name="boost_proto.users_guide.front_end.customizing_expressions_in_your_domain.per_domain_as_child.t0"></a><p class="title"><b>Table 32.3. proto::as_expr() vs. proto::as_child()</b></p> 1875<div class="table-contents"><table class="table" summary="proto::as_expr() vs. proto::as_child()"> 1876<colgroup> 1877<col> 1878<col> 1879<col> 1880</colgroup> 1881<thead><tr> 1882<th> 1883 <p> 1884 <span class="bold"><strong>Function</strong></span> 1885 </p> 1886 </th> 1887<th> 1888 <p> 1889 <span class="bold"><strong>When <code class="literal">t</code> is not a Proto 1890 expr...</strong></span> 1891 </p> 1892 </th> 1893<th> 1894 <p> 1895 <span class="bold"><strong>When <code class="literal">t</code> is a Proto 1896 expr...</strong></span> 1897 </p> 1898 </th> 1899</tr></thead> 1900<tbody> 1901<tr> 1902<td> 1903 <p> 1904 <code class="literal">proto::as_expr(t)</code> 1905 </p> 1906 </td> 1907<td> 1908 <p> 1909 Return (by value) a new Proto terminal holding <code class="literal">t</code> 1910 by value. 1911 </p> 1912 </td> 1913<td> 1914 <p> 1915 Return <code class="literal">t</code> by value unmodified. 1916 </p> 1917 </td> 1918</tr> 1919<tr> 1920<td> 1921 <p> 1922 <code class="literal">proto::as_child(t)</code> 1923 </p> 1924 </td> 1925<td> 1926 <p> 1927 Return (by value) a new Proto terminal holding <code class="literal">t</code> 1928 by reference. 1929 </p> 1930 </td> 1931<td> 1932 <p> 1933 Return <code class="literal">t</code> by reference unmodified. 1934 </p> 1935 </td> 1936</tr> 1937</tbody> 1938</table></div> 1939</div> 1940<br class="table-break"><div class="note"><table border="0" summary="Note"> 1941<tr> 1942<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../doc/src/images/note.png"></td> 1943<th align="left">Note</th> 1944</tr> 1945<tr><td align="left" valign="top"><p> 1946 There is one important place where Proto uses both <code class="computeroutput"><span class="identifier">as_expr</span></code> 1947 <span class="emphasis"><em>and</em></span> <code class="computeroutput"><span class="identifier">as_child</span></code>: 1948 <code class="computeroutput"><a class="link" href="../boost/proto/make_expr.html" title="Function make_expr">proto::make_expr()</a></code>. The <code class="computeroutput"><a class="link" href="../boost/proto/make_expr.html" title="Function make_expr">proto::make_expr()</a></code> function requires 1949 you to specify for each child whether it should be held by value or 1950 by reference. Proto uses <code class="computeroutput"><a class="link" href="../boost/proto/as_expr.html" title="Function as_expr">proto::as_expr()</a></code> 1951 to pre-process the children to be held by value, and <code class="computeroutput"><a class="link" href="../boost/proto/as_child.html" title="Function as_child">proto::as_child()</a></code> for the ones to be 1952 held by reference. 1953 </p></td></tr> 1954</table></div> 1955<p> 1956 Now that you know what <code class="computeroutput"><a class="link" href="../boost/proto/as_child.html" title="Function as_child">proto::as_child()</a></code> 1957 and <code class="computeroutput"><a class="link" href="../boost/proto/as_expr.html" title="Function as_expr">proto::as_expr()</a></code> are, where they are 1958 used, and what they do by default, you may decide that one or both of 1959 these functions should have different behavior for your domain. For instance, 1960 given the above description of <code class="computeroutput"><a class="link" href="../boost/proto/as_child.html" title="Function as_child">proto::as_child()</a></code>, 1961 the following code is always wrong: 1962 </p> 1963<pre class="programlisting"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">literal</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> <span class="identifier">i</span><span class="special">(</span><span class="number">0</span><span class="special">);</span> 1964<span class="keyword">auto</span> <span class="identifier">l</span> <span class="special">=</span> <span class="identifier">i</span> <span class="special">+</span> <span class="number">42</span><span class="special">;</span> <span class="comment">// This is WRONG! Don't do this.</span> 1965</pre> 1966<p> 1967 Why is this wrong? Because <code class="computeroutput"><a class="link" href="../boost/proto/as_child.html" title="Function as_child">proto::as_child()</a></code> 1968 will turn the integer literal 42 into a Proto terminal that holds a reference 1969 to a temporary integer initialized with 42. The lifetime of that temporary 1970 ends at the semicolon, guaranteeing that the local <code class="computeroutput"><span class="identifier">l</span></code> 1971 is left holding a dangling reference to a deceased integer. What to do? 1972 One answer is to use <code class="computeroutput"><a class="link" href="../boost/proto/deep_copy.html" title="Function template deep_copy">proto::deep_copy()</a></code>. 1973 Another is to customize the behavior of <code class="computeroutput"><a class="link" href="../boost/proto/as_child.html" title="Function as_child">proto::as_child()</a></code> 1974 for your domain. Read on for the details. 1975 </p> 1976<h6> 1977<a name="boost_proto.users_guide.front_end.customizing_expressions_in_your_domain.per_domain_as_child.h1"></a> 1978 <span class="phrase"><a name="boost_proto.users_guide.front_end.customizing_expressions_in_your_domain.per_domain_as_child.per_domain__literal_as_child__literal_"></a></span><a class="link" href="users_guide.html#boost_proto.users_guide.front_end.customizing_expressions_in_your_domain.per_domain_as_child.per_domain__literal_as_child__literal_">Per-Domain 1979 <code class="literal">as_child</code></a> 1980 </h6> 1981<p> 1982 To control how Proto builds expressions out of sub-expressions in your 1983 domain, define your domain as usual, and then define a nested <code class="computeroutput"><span class="identifier">as_child</span><span class="special"><></span></code> 1984 class template within it, as follows: 1985 </p> 1986<pre class="programlisting"><span class="keyword">class</span> <span class="identifier">my_domain</span> 1987 <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">domain</span><span class="special"><</span> <span class="identifier">my_generator</span><span class="special">,</span> <span class="identifier">my_grammar</span> <span class="special">></span> 1988<span class="special">{</span> 1989 <span class="comment">// Here is where you define how Proto should handle</span> 1990 <span class="comment">// sub-expressions that are about to be glommed into</span> 1991 <span class="comment">// a larger expression.</span> 1992 <span class="keyword">template</span><span class="special"><</span> <span class="keyword">typename</span> <span class="identifier">T</span> <span class="special">></span> 1993 <span class="keyword">struct</span> <span class="identifier">as_child</span> 1994 <span class="special">{</span> 1995 <span class="keyword">typedef</span> <span class="emphasis"><em><code class="literal">unspecified-Proto-expr-type</code></em></span> <span class="identifier">result_type</span><span class="special">;</span> 1996 1997 <span class="identifier">result_type</span> <span class="keyword">operator</span><span class="special">()(</span> <span class="identifier">T</span> <span class="special">&</span> <span class="identifier">t</span> <span class="special">)</span> <span class="keyword">const</span> 1998 <span class="special">{</span> 1999 <span class="keyword">return</span> <span class="emphasis"><em><code class="literal">unspecified-Proto-expr-object</code></em></span><span class="special">;</span> 2000 <span class="special">}</span> 2001 <span class="special">};</span> 2002<span class="special">};</span> 2003</pre> 2004<p> 2005 There's one important thing to note: in the above code, the template 2006 parameter <code class="literal">T</code> may or may not be a Proto expression type, 2007 but the result <span class="emphasis"><em>must</em></span> be a Proto expression type, 2008 or a reference to one. That means that most user-defined <code class="literal">as_child<></code> 2009 templates will need to check whether <code class="literal">T</code> is an expression 2010 or not (using <code class="computeroutput"><a class="link" href="../boost/proto/is_expr.html" title="Struct template is_expr">proto::is_expr<></a></code>), and then turn non-expressions 2011 into Proto terminals by wrapping them as <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special"><</span> <span class="comment">/* ... */</span> 2012 <span class="special">>::</span><span class="identifier">type</span></code> 2013 or equivalent. 2014 </p> 2015<h6> 2016<a name="boost_proto.users_guide.front_end.customizing_expressions_in_your_domain.per_domain_as_child.h2"></a> 2017 <span class="phrase"><a name="boost_proto.users_guide.front_end.customizing_expressions_in_your_domain.per_domain_as_child.per_domain__literal_as_expr__literal_"></a></span><a class="link" href="users_guide.html#boost_proto.users_guide.front_end.customizing_expressions_in_your_domain.per_domain_as_child.per_domain__literal_as_expr__literal_">Per-Domain 2018 <code class="literal">as_expr</code></a> 2019 </h6> 2020<p> 2021 Although less common, Proto also lets you customize the behavior of 2022 <code class="computeroutput"><a class="link" href="../boost/proto/as_expr.html" title="Function as_expr">proto::as_expr()</a></code> on a per-domain basis. 2023 The technique is identical to that for <code class="literal">as_child</code>. See 2024 below: 2025 </p> 2026<pre class="programlisting"><span class="keyword">class</span> <span class="identifier">my_domain</span> 2027 <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">domain</span><span class="special"><</span> <span class="identifier">my_generator</span><span class="special">,</span> <span class="identifier">my_grammar</span> <span class="special">></span> 2028<span class="special">{</span> 2029 <span class="comment">// Here is where you define how Proto should handle</span> 2030 <span class="comment">// objects that are to be turned into expressions</span> 2031 <span class="comment">// fit for storage in local variables.</span> 2032 <span class="keyword">template</span><span class="special"><</span> <span class="keyword">typename</span> <span class="identifier">T</span> <span class="special">></span> 2033 <span class="keyword">struct</span> <span class="identifier">as_expr</span> 2034 <span class="special">{</span> 2035 <span class="keyword">typedef</span> <span class="emphasis"><em><code class="literal">unspecified-Proto-expr-type</code></em></span> <span class="identifier">result_type</span><span class="special">;</span> 2036 2037 <span class="identifier">result_type</span> <span class="keyword">operator</span><span class="special">()(</span> <span class="identifier">T</span> <span class="special">&</span> <span class="identifier">t</span> <span class="special">)</span> <span class="keyword">const</span> 2038 <span class="special">{</span> 2039 <span class="keyword">return</span> <span class="emphasis"><em><code class="literal">unspecified-Proto-expr-object</code></em></span><span class="special">;</span> 2040 <span class="special">}</span> 2041 <span class="special">};</span> 2042<span class="special">};</span> 2043</pre> 2044<h6> 2045<a name="boost_proto.users_guide.front_end.customizing_expressions_in_your_domain.per_domain_as_child.h3"></a> 2046 <span class="phrase"><a name="boost_proto.users_guide.front_end.customizing_expressions_in_your_domain.per_domain_as_child.making_proto_expressions__literal_auto__literal__safe"></a></span><a class="link" href="users_guide.html#boost_proto.users_guide.front_end.customizing_expressions_in_your_domain.per_domain_as_child.making_proto_expressions__literal_auto__literal__safe">Making 2047 Proto Expressions <code class="literal">auto</code>-safe</a> 2048 </h6> 2049<p> 2050 Let's look again at the problem described above involving the C++11 2051 <code class="computeroutput"><span class="keyword">auto</span></code> keyword and the default 2052 behavior of <code class="computeroutput"><a class="link" href="../boost/proto/as_child.html" title="Function as_child">proto::as_child()</a></code>. 2053 </p> 2054<pre class="programlisting"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">literal</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> <span class="identifier">i</span><span class="special">(</span><span class="number">0</span><span class="special">);</span> 2055<span class="keyword">auto</span> <span class="identifier">l</span> <span class="special">=</span> <span class="identifier">i</span> <span class="special">+</span> <span class="number">42</span><span class="special">;</span> <span class="comment">// This is WRONG! Don't do this.</span> 2056</pre> 2057<p> 2058 Recall that the problem is the lifetime of the temporary integer created 2059 to hold the value 42. The local <code class="computeroutput"><span class="identifier">l</span></code> 2060 will be left holding a dangling reference to it after its lifetime is 2061 over. What if we want Proto to make expressions safe to store this way 2062 in local variables? We can do so very easily by making <code class="computeroutput"><a class="link" href="../boost/proto/as_child.html" title="Function as_child">proto::as_child()</a></code> behave just like <code class="computeroutput"><a class="link" href="../boost/proto/as_expr.html" title="Function as_expr">proto::as_expr()</a></code>. The following code 2063 achieves this: 2064 </p> 2065<pre class="programlisting"><span class="keyword">template</span><span class="special"><</span> <span class="keyword">typename</span> <span class="identifier">E</span> <span class="special">></span> 2066<span class="keyword">struct</span> <span class="identifier">my_expr</span><span class="special">;</span> 2067 2068<span class="keyword">struct</span> <span class="identifier">my_generator</span> 2069 <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">pod_generator</span><span class="special"><</span> <span class="identifier">my_expr</span> <span class="special">></span> 2070<span class="special">{};</span> 2071 2072<span class="keyword">struct</span> <span class="identifier">my_domain</span> 2073 <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">domain</span><span class="special"><</span> <span class="identifier">my_generator</span> <span class="special">></span> 2074<span class="special">{</span> 2075 <span class="comment">// Make as_child() behave like as_expr() in my_domain.</span> 2076 <span class="comment">// (proto_base_domain is a typedef for proto::domain< my_generator ></span> 2077 <span class="comment">// that is defined in proto::domain<>.)</span> 2078 <span class="keyword">template</span><span class="special"><</span> <span class="keyword">typename</span> <span class="identifier">T</span> <span class="special">></span> 2079 <span class="keyword">struct</span> <span class="identifier">as_child</span> 2080 <span class="special">:</span> <span class="identifier">proto_base_domain</span><span class="special">::</span><span class="identifier">as_expr</span><span class="special"><</span> <span class="identifier">T</span> <span class="special">></span> 2081 <span class="special">{};</span> 2082<span class="special">};</span> 2083 2084<span class="keyword">template</span><span class="special"><</span> <span class="keyword">typename</span> <span class="identifier">E</span> <span class="special">></span> 2085<span class="keyword">struct</span> <span class="identifier">my_expr</span> 2086<span class="special">{</span> 2087 <span class="identifier">BOOST_PROTO_EXTENDS</span><span class="special">(</span> <span class="identifier">E</span><span class="special">,</span> <span class="identifier">my_expr</span><span class="special"><</span> <span class="identifier">E</span> <span class="special">>,</span> <span class="identifier">my_domain</span> <span class="special">)</span> 2088<span class="special">};</span> 2089 2090<span class="comment">/* ... */</span> 2091 2092<span class="identifier">proto</span><span class="special">::</span><span class="identifier">literal</span><span class="special"><</span> <span class="keyword">int</span><span class="special">,</span> <span class="identifier">my_domain</span> <span class="special">></span> <span class="identifier">i</span><span class="special">(</span><span class="number">0</span><span class="special">);</span> 2093<span class="keyword">auto</span> <span class="identifier">l</span> <span class="special">=</span> <span class="identifier">i</span> <span class="special">+</span> <span class="number">42</span><span class="special">;</span> <span class="comment">// OK! Everything is stored by value here.</span> 2094</pre> 2095<p> 2096 Notice that <code class="computeroutput"><span class="identifier">my_domain</span><span class="special">::</span><span class="identifier">as_child</span><span class="special"><></span></code> simply defers to the default 2097 implementation of <code class="computeroutput"><span class="identifier">as_expr</span><span class="special"><></span></code> found in <code class="computeroutput"><a class="link" href="../boost/proto/domain.html" title="Struct template domain">proto::domain<></a></code>. 2098 By simply cross-wiring our domain's <code class="computeroutput"><span class="identifier">as_child</span><span class="special"><></span></code> to <code class="computeroutput"><span class="identifier">as_expr</span><span class="special"><></span></code>, we guarantee that all terminals 2099 that can be held by value are, and that all child expressions are also 2100 held by value. This increases copying and may incur a runtime performance 2101 cost, but it eliminates any spector of lifetime management issues. 2102 </p> 2103<p> 2104 For another example, see the definition of <code class="computeroutput"><span class="identifier">lldomain</span></code> 2105 in <code class="literal">libs/proto/example/lambda.hpp</code>. That example is 2106 a complete reimplementation of the Boost Lambda Library (BLL) on top 2107 of Boost.Proto. The function objects the BLL generates are safe to be 2108 stored in local variables. To emulate this with Proto, the <code class="computeroutput"><span class="identifier">lldomain</span></code> cross-wires <code class="computeroutput"><span class="identifier">as_child</span><span class="special"><></span></code> 2109 to <code class="computeroutput"><span class="identifier">as_expr</span><span class="special"><></span></code> 2110 as above, but with one extra twist: objects with array type are also 2111 stored by reference. Check it out. 2112 </p> 2113</div> 2114<div class="section"> 2115<div class="titlepage"><div><div><h5 class="title"> 2116<a name="boost_proto.users_guide.front_end.customizing_expressions_in_your_domain.subdomains"></a><a class="link" href="users_guide.html#boost_proto.users_guide.front_end.customizing_expressions_in_your_domain.subdomains" title="EDSL Interoperatability: Sub-Domains">EDSL 2117 Interoperatability: Sub-Domains</a> 2118</h5></div></div></div> 2119<div class="note"><table border="0" summary="Note"> 2120<tr> 2121<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../doc/src/images/note.png"></td> 2122<th align="left">Note</th> 2123</tr> 2124<tr><td align="left" valign="top"><p> 2125 This is an advanced topic. Feel free to skip this if you're just getting 2126 started with Proto. 2127 </p></td></tr> 2128</table></div> 2129<p> 2130 The ability to <span class="emphasis"><em>compose</em></span> different EDSLs is one of 2131 their most exciting features. Consider how you build a parser using yacc. 2132 You write your grammar rules in yacc's domain-specific language. Then 2133 you embed semantic actions written in C within your grammar. Boost's 2134 Spirit parser generator gives you the same ability. You write grammar 2135 rules using Spirit.Qi and embed semantic actions using the Phoenix library. 2136 Phoenix and Spirit are both Proto-based domain-specific languages with 2137 their own distinct syntax and semantics. But you can freely embed Phoenix 2138 expressions within Spirit expressions. This section describes Proto's 2139 <span class="emphasis"><em>sub-domain</em></span> feature that lets you define families 2140 of interoperable domains. 2141 </p> 2142<h6> 2143<a name="boost_proto.users_guide.front_end.customizing_expressions_in_your_domain.subdomains.h0"></a> 2144 <span class="phrase"><a name="boost_proto.users_guide.front_end.customizing_expressions_in_your_domain.subdomains.dueling_domains"></a></span><a class="link" href="users_guide.html#boost_proto.users_guide.front_end.customizing_expressions_in_your_domain.subdomains.dueling_domains">Dueling 2145 Domains</a> 2146 </h6> 2147<p> 2148 When you try to create an expression from two sub-expressions in different 2149 domains, what is the domain of the resulting expression? This is the 2150 fundamental problem that is addressed by sub-domains. Consider the following 2151 code: 2152 </p> 2153<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">proto</span><span class="special">/</span><span class="identifier">proto</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> 2154<span class="keyword">namespace</span> <span class="identifier">proto</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">proto</span><span class="special">;</span> 2155 2156<span class="comment">// Forward-declare two expression wrappers</span> 2157<span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">E</span><span class="special">></span> <span class="keyword">struct</span> <span class="identifier">spirit_expr</span><span class="special">;</span> 2158<span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">E</span><span class="special">></span> <span class="keyword">struct</span> <span class="identifier">phoenix_expr</span><span class="special">;</span> 2159 2160<span class="comment">// Define two domains</span> 2161<span class="keyword">struct</span> <span class="identifier">spirit_domain</span> <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">domain</span><span class="special"><</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">generator</span><span class="special"><</span><span class="identifier">spirit_expr</span><span class="special">></span> <span class="special">></span> <span class="special">{};</span> 2162<span class="keyword">struct</span> <span class="identifier">phoenix_domain</span> <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">domain</span><span class="special"><</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">generator</span><span class="special"><</span><span class="identifier">phoenix_expr</span><span class="special">></span> <span class="special">></span> <span class="special">{};</span> 2163 2164<span class="comment">// Implement the two expression wrappers</span> 2165<span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">E</span><span class="special">></span> 2166<span class="keyword">struct</span> <span class="identifier">spirit_expr</span> 2167 <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">extends</span><span class="special"><</span><span class="identifier">E</span><span class="special">,</span> <span class="identifier">spirit_expr</span><span class="special"><</span><span class="identifier">E</span><span class="special">>,</span> <span class="identifier">spirit_domain</span><span class="special">></span> 2168<span class="special">{</span> 2169 <span class="identifier">spirit_expr</span><span class="special">(</span><span class="identifier">E</span> <span class="keyword">const</span> <span class="special">&</span><span class="identifier">e</span> <span class="special">=</span> <span class="identifier">E</span><span class="special">())</span> <span class="special">:</span> <span class="identifier">spirit_expr</span><span class="special">::</span><span class="identifier">proto_extends</span><span class="special">(</span><span class="identifier">e</span><span class="special">)</span> <span class="special">{}</span> 2170<span class="special">};</span> 2171 2172<span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">E</span><span class="special">></span> 2173<span class="keyword">struct</span> <span class="identifier">phoenix_expr</span> 2174 <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">extends</span><span class="special"><</span><span class="identifier">E</span><span class="special">,</span> <span class="identifier">phoenix_expr</span><span class="special"><</span><span class="identifier">E</span><span class="special">>,</span> <span class="identifier">phoenix_domain</span><span class="special">></span> 2175<span class="special">{</span> 2176 <span class="identifier">phoenix_expr</span><span class="special">(</span><span class="identifier">E</span> <span class="keyword">const</span> <span class="special">&</span><span class="identifier">e</span> <span class="special">=</span> <span class="identifier">E</span><span class="special">())</span> <span class="special">:</span> <span class="identifier">phoenix_expr</span><span class="special">::</span><span class="identifier">proto_extends</span><span class="special">(</span><span class="identifier">e</span><span class="special">)</span> <span class="special">{}</span> 2177<span class="special">};</span> 2178 2179<span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span> 2180<span class="special">{</span> 2181 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">literal</span><span class="special"><</span><span class="keyword">int</span><span class="special">,</span> <span class="identifier">spirit_domain</span><span class="special">></span> <span class="identifier">sp</span><span class="special">(</span><span class="number">0</span><span class="special">);</span> 2182 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">literal</span><span class="special"><</span><span class="keyword">int</span><span class="special">,</span> <span class="identifier">phoenix_domain</span><span class="special">></span> <span class="identifier">phx</span><span class="special">(</span><span class="number">0</span><span class="special">);</span> 2183 2184 <span class="comment">// Whoops! What does it mean to add two expressions in different domains?</span> 2185 <span class="identifier">sp</span> <span class="special">+</span> <span class="identifier">phx</span><span class="special">;</span> <span class="comment">// ERROR</span> 2186<span class="special">}</span> 2187</pre> 2188<p> 2189 Above, we define two domains called <code class="computeroutput"><span class="identifier">spirit_domain</span></code> 2190 and <code class="computeroutput"><span class="identifier">phoenix_domain</span></code> and 2191 declare two int literals in each. Then we try to compose them into a 2192 larger expression using Proto's binary plus operator, and it fails. Proto 2193 can't figure out whether the resulting expression should be in the Spirit 2194 domain or the Phoenix domain, and thus whether it should be an instance 2195 of <code class="computeroutput"><span class="identifier">spirit_expr</span><span class="special"><></span></code> 2196 or <code class="computeroutput"><span class="identifier">phoenix_expr</span><span class="special"><></span></code>. 2197 We have to tell Proto how to resolve the conflict. We can do that by 2198 declaring that Phoenix is a sub-domain of Spirit as in the following 2199 definition of <code class="computeroutput"><span class="identifier">phoenix_domain</span></code>: 2200 </p> 2201<pre class="programlisting"><span class="comment">// Declare that phoenix_domain is a sub-domain of spirit_domain</span> 2202<span class="keyword">struct</span> <span class="identifier">phoenix_domain</span> 2203 <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">domain</span><span class="special"><</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">generator</span><span class="special"><</span><span class="identifier">phoenix_expr</span><span class="special">>,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_</span><span class="special">,</span> <span class="bold"><strong>spirit_domain</strong></span><span class="special">></span> 2204<span class="special">{};</span> 2205</pre> 2206<p> 2207 The third template parameter to <code class="computeroutput"><a class="link" href="../boost/proto/domain.html" title="Struct template domain">proto::domain<></a></code> 2208 is the super-domain. By defining <code class="computeroutput"><span class="identifier">phoenix_domain</span></code> 2209 as above, we are saying that Phoenix expressions can be combined with 2210 Spirit expressions, and that when that happens, the resulting expression 2211 should be a Spirit expression. 2212 </p> 2213<div class="note"><table border="0" summary="Note"> 2214<tr> 2215<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../doc/src/images/note.png"></td> 2216<th align="left">Note</th> 2217</tr> 2218<tr><td align="left" valign="top"><p> 2219 If you are wondering what the purpose of <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_</span></code> 2220 is in the definition of <code class="computeroutput"><span class="identifier">phoenix_domain</span></code> 2221 above, recall that the second template parameter to <code class="computeroutput"><a class="link" href="../boost/proto/domain.html" title="Struct template domain">proto::domain<></a></code> 2222 is the domain's grammar. <span class="quote">“<span class="quote"><code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_</span></code></span>”</span> 2223 is the default and signifies that the domain places no restrictions 2224 on the expressions that are valid within it. 2225 </p></td></tr> 2226</table></div> 2227<h6> 2228<a name="boost_proto.users_guide.front_end.customizing_expressions_in_your_domain.subdomains.h1"></a> 2229 <span class="phrase"><a name="boost_proto.users_guide.front_end.customizing_expressions_in_your_domain.subdomains.domain_resolution"></a></span><a class="link" href="users_guide.html#boost_proto.users_guide.front_end.customizing_expressions_in_your_domain.subdomains.domain_resolution">Domain 2230 Resolution</a> 2231 </h6> 2232<p> 2233 When there are multiple domains in play within a given expression, Proto 2234 uses some rules to figure out which domain "wins". The rules 2235 are loosely modeled on the rules for C++ inheritance. <code class="computeroutput"><span class="identifier">Phoenix_domain</span></code> 2236 is a sub-domain of <code class="computeroutput"><span class="identifier">spirit_domain</span></code>. 2237 You can liken that to a derived/base relationship that gives Phoenix 2238 expressions a kind of implicit conversion to Spirit expressions. And 2239 since Phoenix expressions can be "converted" to Spirit expressions, 2240 they can be freely combined with Spirit expressions and the result is 2241 a Spirit expression. 2242 </p> 2243<div class="note"><table border="0" summary="Note"> 2244<tr> 2245<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../doc/src/images/note.png"></td> 2246<th align="left">Note</th> 2247</tr> 2248<tr><td align="left" valign="top"><p> 2249 Super- and sub-domains are not actually implemented using inheritance. 2250 This is only a helpful mental model. 2251 </p></td></tr> 2252</table></div> 2253<p> 2254 The analogy with inheritance holds even in the case of three domains 2255 when two are sub-domains of the third. Imagine another domain called 2256 <code class="computeroutput"><span class="identifier">foobar_domain</span></code> that was 2257 also a sub-domain of <code class="computeroutput"><span class="identifier">spirit_domain</span></code>. 2258 Expressions in the <code class="computeroutput"><span class="identifier">foobar_domain</span></code> 2259 could be combined with expressions in the <code class="computeroutput"><span class="identifier">phoenix_domain</span></code> 2260 and the resulting expression would be in the <code class="computeroutput"><span class="identifier">spirit_domain</span></code>. 2261 That's because expressions in the two sub-domains both have "conversions" 2262 to the super-domain, so the operation is allowed and the super-domain 2263 wins. 2264 </p> 2265<h6> 2266<a name="boost_proto.users_guide.front_end.customizing_expressions_in_your_domain.subdomains.h2"></a> 2267 <span class="phrase"><a name="boost_proto.users_guide.front_end.customizing_expressions_in_your_domain.subdomains.the_default_domain"></a></span><a class="link" href="users_guide.html#boost_proto.users_guide.front_end.customizing_expressions_in_your_domain.subdomains.the_default_domain">The 2268 Default Domain</a> 2269 </h6> 2270<p> 2271 When you don't assign a Proto expression to a particular domain, Proto 2272 considers it a member of the so-called default domain, <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">default_domain</span></code>. Even non-Proto objects 2273 are treated as terminals in the default domain. Consider: 2274 </p> 2275<pre class="programlisting"><span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span> 2276<span class="special">{</span> 2277 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">literal</span><span class="special"><</span><span class="keyword">int</span><span class="special">,</span> <span class="identifier">spirit_domain</span><span class="special">></span> <span class="identifier">sp</span><span class="special">(</span><span class="number">0</span><span class="special">);</span> 2278 2279 <span class="comment">// Add 1 to a spirit expression. Result is a spirit expression.</span> 2280 <span class="identifier">sp</span> <span class="special">+</span> <span class="number">1</span><span class="special">;</span> 2281<span class="special">}</span> 2282</pre> 2283<p> 2284 Expressions in the default domain (or non-expressions like <code class="literal">1</code>) 2285 have a kind of implicit conversion to expressions every other domain 2286 type. What's more, you can define your domain to be a sub-domain of the 2287 default domain. In so doing, you give expressions in your domain conversions 2288 to expressions in every other domain. This is like a <span class="quote">“<span class="quote">free love</span>”</span> 2289 domain, because it will freely mix with all other domains. 2290 </p> 2291<p> 2292 Let's think again about the Phoenix EDSL. Since it provides generally 2293 useful lambda functionality, it's reasonable to assume that lots of other 2294 EDSLs besides Spirit might want the ability to embed Phoenix expressions. 2295 In other words, <code class="computeroutput"><span class="identifier">phoenix_domain</span></code> 2296 should be a sub-domain of <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">default_domain</span></code>, 2297 not <code class="computeroutput"><span class="identifier">spirit_domain</span></code>: 2298 </p> 2299<pre class="programlisting"><span class="comment">// Declare that phoenix_domain is a sub-domain of proto::default_domain</span> 2300<span class="keyword">struct</span> <span class="identifier">phoenix_domain</span> 2301 <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">domain</span><span class="special"><</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">generator</span><span class="special"><</span><span class="identifier">phoenix_expr</span><span class="special">>,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_</span><span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">default_domain</span><span class="special">></span> 2302<span class="special">{};</span> 2303</pre> 2304<p> 2305 That's much better. Phoenix expressions can now be put anywhere. 2306 </p> 2307<h6> 2308<a name="boost_proto.users_guide.front_end.customizing_expressions_in_your_domain.subdomains.h3"></a> 2309 <span class="phrase"><a name="boost_proto.users_guide.front_end.customizing_expressions_in_your_domain.subdomains.sub_domain_summary"></a></span><a class="link" href="users_guide.html#boost_proto.users_guide.front_end.customizing_expressions_in_your_domain.subdomains.sub_domain_summary">Sub-Domain 2310 Summary</a> 2311 </h6> 2312<p> 2313 Use Proto sub-domains to make it possible to mix expressions from multiple 2314 domains. And when you want expressions in your domain to freely combine 2315 with <span class="emphasis"><em>all</em></span> expressions, make it a sub-domain of <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">default_domain</span></code>. 2316 </p> 2317</div> 2318</div> 2319<div class="section"> 2320<div class="titlepage"><div><div><h4 class="title"> 2321<a name="boost_proto.users_guide.front_end.define_operators"></a><a class="link" href="users_guide.html#boost_proto.users_guide.front_end.define_operators" title="Adapting Existing Types to Proto">Adapting 2322 Existing Types to Proto</a> 2323</h4></div></div></div> 2324<p> 2325 The preceding discussions of defining Proto front ends have all made a 2326 big assumption: that you have the luxury of defining everything from scratch. 2327 What happens if you have existing types, say a matrix type and a vector 2328 type, that you would like to treat as if they were Proto terminals? Proto 2329 usually trades only in its own expression types, but with <code class="literal"><code class="computeroutput"><a class="link" href="../BOOST_PROTO__1_3_33_5_33_3.html" title="Macro BOOST_PROTO_DEFINE_OPERATORS">BOOST_PROTO_DEFINE_OPERATORS</a></code>()</code>, 2330 it can accomodate your custom terminal types, too. 2331 </p> 2332<p> 2333 Let's say, for instance, that you have the following types and that you 2334 can't modify then to make them <span class="quote">“<span class="quote">native</span>”</span> Proto terminal types. 2335 </p> 2336<pre class="programlisting"><span class="keyword">namespace</span> <span class="identifier">math</span> 2337<span class="special">{</span> 2338 <span class="comment">// A matrix type ...</span> 2339 <span class="keyword">struct</span> <span class="identifier">matrix</span> <span class="special">{</span> <span class="comment">/*...*/</span> <span class="special">};</span> 2340 2341 <span class="comment">// A vector type ...</span> 2342 <span class="keyword">struct</span> <span class="identifier">vector</span> <span class="special">{</span> <span class="comment">/*...*/</span> <span class="special">};</span> 2343<span class="special">}</span> 2344</pre> 2345<p> 2346 You can non-intrusively make objects of these types Proto terminals by 2347 defining the proper operator overloads using <code class="literal"><code class="computeroutput"><a class="link" href="../BOOST_PROTO__1_3_33_5_33_3.html" title="Macro BOOST_PROTO_DEFINE_OPERATORS">BOOST_PROTO_DEFINE_OPERATORS</a></code>()</code>. 2348 The basic procedure is as follows: 2349 </p> 2350<div class="orderedlist"><ol class="orderedlist" type="1"> 2351<li class="listitem"> 2352 Define a trait that returns true for your types and false for all others. 2353 </li> 2354<li class="listitem"> 2355 Reopen the namespace of your types and use <code class="literal"><code class="computeroutput"><a class="link" href="../BOOST_PROTO__1_3_33_5_33_3.html" title="Macro BOOST_PROTO_DEFINE_OPERATORS">BOOST_PROTO_DEFINE_OPERATORS</a></code>()</code> 2356 to define a set of operator overloads, passing the name of the trait 2357 as the first macro parameter, and the name of a Proto domain (e.g., 2358 <code class="computeroutput"><a class="link" href="../boost/proto/default_domain.html" title="Struct default_domain">proto::default_domain</a></code>) 2359 as the second. 2360 </li> 2361</ol></div> 2362<p> 2363 The following code demonstrates how it works. 2364 </p> 2365<pre class="programlisting"><span class="keyword">namespace</span> <span class="identifier">math</span> 2366<span class="special">{</span> 2367 <span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">></span> 2368 <span class="keyword">struct</span> <span class="identifier">is_terminal</span> 2369 <span class="special">:</span> <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">false_</span> 2370 <span class="special">{};</span> 2371 2372 <span class="comment">// OK, "matrix" is a custom terminal type</span> 2373 <span class="keyword">template</span><span class="special"><></span> 2374 <span class="keyword">struct</span> <span class="identifier">is_terminal</span><span class="special"><</span><span class="identifier">matrix</span><span class="special">></span> 2375 <span class="special">:</span> <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">true_</span> 2376 <span class="special">{};</span> 2377 2378 <span class="comment">// OK, "vector" is a custom terminal type</span> 2379 <span class="keyword">template</span><span class="special"><></span> 2380 <span class="keyword">struct</span> <span class="identifier">is_terminal</span><span class="special"><</span><span class="identifier">vector</span><span class="special">></span> 2381 <span class="special">:</span> <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">true_</span> 2382 <span class="special">{};</span> 2383 2384 <span class="comment">// Define all the operator overloads to construct Proto</span> 2385 <span class="comment">// expression templates, treating "matrix" and "vector"</span> 2386 <span class="comment">// objects as if they were Proto terminals.</span> 2387 <span class="identifier">BOOST_PROTO_DEFINE_OPERATORS</span><span class="special">(</span><span class="identifier">is_terminal</span><span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">default_domain</span><span class="special">)</span> 2388<span class="special">}</span> 2389</pre> 2390<p> 2391 The invocation of the <code class="literal"><code class="computeroutput"><a class="link" href="../BOOST_PROTO__1_3_33_5_33_3.html" title="Macro BOOST_PROTO_DEFINE_OPERATORS">BOOST_PROTO_DEFINE_OPERATORS</a></code>()</code> 2392 macro defines a complete set of operator overloads that treat <code class="computeroutput"><span class="identifier">matrix</span></code> and <code class="computeroutput"><span class="identifier">vector</span></code> 2393 objects as if they were Proto terminals. And since the operators are defined 2394 in the same namespace as the <code class="computeroutput"><span class="identifier">matrix</span></code> 2395 and <code class="computeroutput"><span class="identifier">vector</span></code> types, the operators 2396 will be found by argument-dependent lookup. With the code above, we can 2397 now construct expression templates with matrices and vectors, as shown 2398 below. 2399 </p> 2400<pre class="programlisting"><span class="identifier">math</span><span class="special">::</span><span class="identifier">matrix</span> <span class="identifier">m1</span><span class="special">;</span> 2401<span class="identifier">math</span><span class="special">::</span><span class="identifier">vector</span> <span class="identifier">v1</span><span class="special">;</span> 2402<span class="identifier">proto</span><span class="special">::</span><span class="identifier">literal</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> <span class="identifier">i</span><span class="special">(</span><span class="number">0</span><span class="special">);</span> 2403 2404<span class="identifier">m1</span> <span class="special">*</span> <span class="number">1</span><span class="special">;</span> <span class="comment">// custom terminal and literals are OK</span> 2405<span class="identifier">m1</span> <span class="special">*</span> <span class="identifier">i</span><span class="special">;</span> <span class="comment">// custom terminal and Proto expressions are OK</span> 2406<span class="identifier">m1</span> <span class="special">*</span> <span class="identifier">v1</span><span class="special">;</span> <span class="comment">// two custom terminals are OK, too.</span> 2407</pre> 2408</div> 2409<div class="section"> 2410<div class="titlepage"><div><div><h4 class="title"> 2411<a name="boost_proto.users_guide.front_end.code_repetition"></a><a class="link" href="users_guide.html#boost_proto.users_guide.front_end.code_repetition" title="Generating Repetitive Code with the Preprocessor">Generating 2412 Repetitive Code with the Preprocessor</a> 2413</h4></div></div></div> 2414<p> 2415 Sometimes as an EDSL designer, to make the lives of your users easy, you 2416 have to make your own life hard. Giving your users natural and flexible 2417 syntax often involves writing large numbers of repetitive function overloads. 2418 It can be enough to give you repetitive stress injury! Before you hurt 2419 yourself, check out the macros Proto provides for automating many repetitive 2420 code-generation chores. 2421 </p> 2422<p> 2423 Imagine that we are writing a lambda EDSL, and we would like to enable 2424 syntax for constructing temporary objects of any type using the following 2425 syntax: 2426 </p> 2427<pre class="programlisting"><span class="comment">// A lambda expression that takes two arguments and</span> 2428<span class="comment">// uses them to construct a temporary std::complex<></span> 2429<span class="identifier">construct</span><span class="special"><</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">complex</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> <span class="special">>(</span> <span class="identifier">_1</span><span class="special">,</span> <span class="identifier">_2</span> <span class="special">)</span> 2430</pre> 2431<p> 2432 For the sake of the discussion, imagine that we already have a function 2433 object template <code class="computeroutput"><span class="identifier">construct_impl</span><span class="special"><></span></code> that accepts arguments and constructs 2434 new objects from them. We would want the above lambda expression to be 2435 equivalent to the following: 2436 </p> 2437<pre class="programlisting"><span class="comment">// The above lambda expression should be roughly equivalent</span> 2438<span class="comment">// to the following:</span> 2439<span class="identifier">proto</span><span class="special">::</span><span class="identifier">make_expr</span><span class="special"><</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">function</span><span class="special">>(</span> 2440 <span class="identifier">construct_impl</span><span class="special"><</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">complex</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> <span class="special">>()</span> <span class="comment">// The function to invoke lazily</span> 2441 <span class="special">,</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">ref</span><span class="special">(</span><span class="identifier">_1</span><span class="special">)</span> <span class="comment">// The first argument to the function</span> 2442 <span class="special">,</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">ref</span><span class="special">(</span><span class="identifier">_2</span><span class="special">)</span> <span class="comment">// The second argument to the function</span> 2443<span class="special">);</span> 2444</pre> 2445<p> 2446 We can define our <code class="computeroutput"><span class="identifier">construct</span><span class="special">()</span></code> function template as follows: 2447 </p> 2448<pre class="programlisting"><span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">A0</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">A1</span><span class="special">></span> 2449<span class="keyword">typename</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">make_expr</span><span class="special"><</span> 2450 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">function</span> 2451 <span class="special">,</span> <span class="identifier">construct_impl</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> 2452 <span class="special">,</span> <span class="identifier">A0</span> <span class="keyword">const</span> <span class="special">&</span> 2453 <span class="special">,</span> <span class="identifier">A1</span> <span class="keyword">const</span> <span class="special">&</span> 2454<span class="special">>::</span><span class="identifier">type</span> <span class="keyword">const</span> 2455<span class="identifier">construct</span><span class="special">(</span><span class="identifier">A0</span> <span class="keyword">const</span> <span class="special">&</span><span class="identifier">a0</span><span class="special">,</span> <span class="identifier">A1</span> <span class="keyword">const</span> <span class="special">&</span><span class="identifier">a1</span><span class="special">)</span> 2456<span class="special">{</span> 2457 <span class="keyword">return</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">make_expr</span><span class="special"><</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">function</span><span class="special">>(</span> 2458 <span class="identifier">construct_impl</span><span class="special"><</span><span class="identifier">T</span><span class="special">>()</span> 2459 <span class="special">,</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">ref</span><span class="special">(</span><span class="identifier">a0</span><span class="special">)</span> 2460 <span class="special">,</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">ref</span><span class="special">(</span><span class="identifier">a1</span><span class="special">)</span> 2461 <span class="special">);</span> 2462<span class="special">}</span> 2463</pre> 2464<p> 2465 This works for two arguments, but we would like it to work for any number 2466 of arguments, up to ( <code class="literal"><code class="computeroutput"><a class="link" href="../BOOST_PROTO_MAX_ARITY.html" title="Macro BOOST_PROTO_MAX_ARITY">BOOST_PROTO_MAX_ARITY</a></code></code> 2467 - 1). (Why "- 1"? Because one child is taken up by the <code class="computeroutput"><span class="identifier">construct_impl</span><span class="special"><</span><span class="identifier">T</span><span class="special">>()</span></code> 2468 terminal leaving room for only ( <code class="literal"><code class="computeroutput"><a class="link" href="../BOOST_PROTO_MAX_ARITY.html" title="Macro BOOST_PROTO_MAX_ARITY">BOOST_PROTO_MAX_ARITY</a></code></code> 2469 - 1) other children.) 2470 </p> 2471<p> 2472 For cases like this, Proto provides the <code class="literal"><code class="computeroutput"><a class="link" href="../BOOST_PROTO_REPEAT.html" title="Macro BOOST_PROTO_REPEAT">BOOST_PROTO_REPEAT</a></code>()</code> 2473 and <code class="literal"><code class="computeroutput"><a class="link" href="../BOOST_PROTO_REPEAT_FROM_TO.html" title="Macro BOOST_PROTO_REPEAT_FROM_TO">BOOST_PROTO_REPEAT_FROM_TO</a></code>()</code> 2474 macros. To use it, we turn the function definition above into a macro as 2475 follows: 2476 </p> 2477<pre class="programlisting"><span class="preprocessor">#define</span> <span class="identifier">M0</span><span class="special">(</span><span class="identifier">N</span><span class="special">,</span> <span class="identifier">typename_A</span><span class="special">,</span> <span class="identifier">A_const_ref</span><span class="special">,</span> <span class="identifier">A_const_ref_a</span><span class="special">,</span> <span class="identifier">ref_a</span><span class="special">)</span> <span class="special">\</span> 2478<span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">,</span> <span class="identifier">typename_A</span><span class="special">(</span><span class="identifier">N</span><span class="special">)></span> <span class="special">\</span> 2479<span class="keyword">typename</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">make_expr</span><span class="special"><</span> <span class="special">\</span> 2480 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">function</span> <span class="special">\</span> 2481 <span class="special">,</span> <span class="identifier">construct_impl</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="special">\</span> 2482 <span class="special">,</span> <span class="identifier">A_const_ref</span><span class="special">(</span><span class="identifier">N</span><span class="special">)</span> <span class="special">\</span> 2483<span class="special">>::</span><span class="identifier">type</span> <span class="keyword">const</span> <span class="special">\</span> 2484<span class="identifier">construct</span><span class="special">(</span><span class="identifier">A_const_ref_a</span><span class="special">(</span><span class="identifier">N</span><span class="special">))</span> <span class="special">\</span> 2485<span class="special">{</span> <span class="special">\</span> 2486 <span class="keyword">return</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">make_expr</span><span class="special"><</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">function</span><span class="special">>(</span> <span class="special">\</span> 2487 <span class="identifier">construct_impl</span><span class="special"><</span><span class="identifier">T</span><span class="special">>()</span> <span class="special">\</span> 2488 <span class="special">,</span> <span class="identifier">ref_a</span><span class="special">(</span><span class="identifier">N</span><span class="special">)</span> <span class="special">\</span> 2489 <span class="special">);</span> <span class="special">\</span> 2490<span class="special">}</span> 2491</pre> 2492<p> 2493 Notice that we turned the function into a macro that takes 5 arguments. 2494 The first is the current iteration number. The rest are the names of other 2495 macros that generate different sequences. For instance, Proto passes as 2496 the second parameter the name of a macro that will expand to <code class="computeroutput"><span class="keyword">typename</span> <span class="identifier">A0</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">A1</span><span class="special">,</span> <span class="special">...</span></code>. 2497 </p> 2498<p> 2499 Now that we have turned our function into a macro, we can pass the macro 2500 to <code class="literal"><code class="computeroutput"><a class="link" href="../BOOST_PROTO_REPEAT_FROM_TO.html" title="Macro BOOST_PROTO_REPEAT_FROM_TO">BOOST_PROTO_REPEAT_FROM_TO</a></code>()</code>. 2501 Proto will invoke it iteratively, generating all the function overloads 2502 for us. 2503 </p> 2504<pre class="programlisting"><span class="comment">// Generate overloads of construct() that accept from</span> 2505<span class="comment">// 1 to BOOST_PROTO_MAX_ARITY-1 arguments:</span> 2506<span class="identifier">BOOST_PROTO_REPEAT_FROM_TO</span><span class="special">(</span><span class="number">1</span><span class="special">,</span> <span class="identifier">BOOST_PROTO_MAX_ARITY</span><span class="special">,</span> <span class="identifier">M0</span><span class="special">)</span> 2507<span class="preprocessor">#undef</span> <span class="identifier">M0</span> 2508</pre> 2509<h6> 2510<a name="boost_proto.users_guide.front_end.code_repetition.h0"></a> 2511 <span class="phrase"><a name="boost_proto.users_guide.front_end.code_repetition.non_default_sequences"></a></span><a class="link" href="users_guide.html#boost_proto.users_guide.front_end.code_repetition.non_default_sequences">Non-Default 2512 Sequences</a> 2513 </h6> 2514<p> 2515 As mentioned above, Proto passes as the last 4 arguments to your macro 2516 the names of other macros that generate various sequences. The macros 2517 <code class="literal"><code class="computeroutput"><a class="link" href="../BOOST_PROTO_REPEAT.html" title="Macro BOOST_PROTO_REPEAT">BOOST_PROTO_REPEAT</a></code>()</code> 2518 and <code class="literal"><code class="computeroutput"><a class="link" href="../BOOST_PROTO_REPEAT_FROM_TO.html" title="Macro BOOST_PROTO_REPEAT_FROM_TO">BOOST_PROTO_REPEAT_FROM_TO</a></code>()</code> 2519 select defaults for these parameters. If the defaults do not meet your 2520 needs, you can use <code class="literal"><code class="computeroutput"><a class="link" href="../BOOST_PROTO_REPEAT_EX.html" title="Macro BOOST_PROTO_REPEAT_EX">BOOST_PROTO_REPEAT_EX</a></code>()</code> 2521 and <code class="literal"><code class="computeroutput"><a class="link" href="../BOOST_PROTO__1_3_33_5_37_5.html" title="Macro BOOST_PROTO_REPEAT_FROM_TO_EX">BOOST_PROTO_REPEAT_FROM_TO_EX</a></code>()</code> 2522 and pass different macros that generate different sequences. Proto defines 2523 a number of such macros for use as parameters to <code class="literal"><code class="computeroutput"><a class="link" href="../BOOST_PROTO_REPEAT_EX.html" title="Macro BOOST_PROTO_REPEAT_EX">BOOST_PROTO_REPEAT_EX</a></code>()</code> 2524 and <code class="literal"><code class="computeroutput"><a class="link" href="../BOOST_PROTO__1_3_33_5_37_5.html" title="Macro BOOST_PROTO_REPEAT_FROM_TO_EX">BOOST_PROTO_REPEAT_FROM_TO_EX</a></code>()</code>. 2525 Check the reference section for <code class="computeroutput"><a class="link" href="reference.html#header.boost.proto.repeat_hpp" title="Header <boost/proto/repeat.hpp>">boost/proto/repeat.hpp</a></code> 2526 for all the details. 2527 </p> 2528<p> 2529 Also, check out <code class="literal"><code class="computeroutput"><a class="link" href="../BOOST_PROTO_LOCAL_ITERATE.html" title="Macro BOOST_PROTO_LOCAL_ITERATE">BOOST_PROTO_LOCAL_ITERATE</a></code>()</code>. 2530 It works similarly to <code class="literal"><code class="computeroutput"><a class="link" href="../BOOST_PROTO_REPEAT.html" title="Macro BOOST_PROTO_REPEAT">BOOST_PROTO_REPEAT</a></code>()</code> 2531 and friends, but it can be easier to use when you want to change one macro 2532 argument and accept defaults for the others. 2533 </p> 2534</div> 2535</div> 2536<div class="section"> 2537<div class="titlepage"><div><div><h3 class="title"> 2538<a name="boost_proto.users_guide.intermediate_form"></a><a class="link" href="users_guide.html#boost_proto.users_guide.intermediate_form" title="Intermediate Form: Understanding and Introspecting Expressions">Intermediate 2539 Form: Understanding and Introspecting Expressions</a> 2540</h3></div></div></div> 2541<div class="toc"><dl class="toc"> 2542<dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.intermediate_form.left_right_child">Accessing 2543 Parts of an Expression</a></span></dt> 2544<dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.intermediate_form.deep_copying_expressions">Deep-copying 2545 Expressions</a></span></dt> 2546<dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.intermediate_form.debugging_expressions">Debugging 2547 Expressions</a></span></dt> 2548<dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.intermediate_form.tags_and_metafunctions">Operator 2549 Tags and Metafunctions</a></span></dt> 2550<dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.intermediate_form.expressions_as_fusion_sequences">Expressions 2551 as Fusion Sequences</a></span></dt> 2552<dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.intermediate_form.expression_introspection">Expression 2553 Introspection: Defining a Grammar</a></span></dt> 2554</dl></div> 2555<p> 2556 By now, you know a bit about how to build a front-end for your EDSL "compiler" 2557 -- you can define terminals and functions that generate expression templates. 2558 But we haven't said anything about the expression templates themselves. What 2559 do they look like? What can you do with them? In this section we'll see. 2560 </p> 2561<h5> 2562<a name="boost_proto.users_guide.intermediate_form.h0"></a> 2563 <span class="phrase"><a name="boost_proto.users_guide.intermediate_form.the__literal_expr_lt__gt___literal__type"></a></span><a class="link" href="users_guide.html#boost_proto.users_guide.intermediate_form.the__literal_expr_lt__gt___literal__type">The 2564 <code class="literal">expr<></code> Type</a> 2565 </h5> 2566<p> 2567 All Proto expressions are an instantiation of a template called <code class="computeroutput"><a class="link" href="../boost/proto/expr.html" title="Struct template expr">proto::expr<></a></code> (or a wrapper around 2568 such an instantiation). When we define a terminal as below, we are really 2569 initializing an instance of the <code class="computeroutput"><a class="link" href="../boost/proto/expr.html" title="Struct template expr">proto::expr<></a></code> 2570 template. 2571 </p> 2572<pre class="programlisting"><span class="comment">// Define a placeholder type</span> 2573<span class="keyword">template</span><span class="special"><</span><span class="keyword">int</span> <span class="identifier">I</span><span class="special">></span> 2574<span class="keyword">struct</span> <span class="identifier">placeholder</span> 2575<span class="special">{};</span> 2576 2577<span class="comment">// Define the Protofied placeholder terminal</span> 2578<span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special"><</span> <span class="identifier">placeholder</span><span class="special"><</span><span class="number">0</span><span class="special">></span> <span class="special">>::</span><span class="identifier">type</span> <span class="keyword">const</span> <span class="identifier">_1</span> <span class="special">=</span> <span class="special">{{}};</span> 2579</pre> 2580<p> 2581 The actual type of <code class="computeroutput"><span class="identifier">_1</span></code> looks 2582 like this: 2583 </p> 2584<pre class="programlisting"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">expr</span><span class="special"><</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">term</span><span class="special"><</span> <span class="identifier">placeholder</span><span class="special"><</span><span class="number">0</span><span class="special">></span> <span class="special">>,</span> <span class="number">0</span> <span class="special">></span> 2585</pre> 2586<p> 2587 The <code class="computeroutput"><a class="link" href="../boost/proto/expr.html" title="Struct template expr">proto::expr<></a></code> template is the most 2588 important type in Proto. Although you will rarely need to deal with it directly, 2589 it's always there behind the scenes holding your expression trees together. 2590 In fact, <code class="computeroutput"><a class="link" href="../boost/proto/expr.html" title="Struct template expr">proto::expr<></a></code> <span class="emphasis"><em>is</em></span> 2591 the expression tree -- branches, leaves and all. 2592 </p> 2593<p> 2594 The <code class="computeroutput"><a class="link" href="../boost/proto/expr.html" title="Struct template expr">proto::expr<></a></code> template makes up the 2595 nodes in expression trees. The first template parameter is the node type; 2596 in this case, <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">terminal</span></code>. 2597 That means that <code class="computeroutput"><span class="identifier">_1</span></code> is a leaf-node 2598 in the expression tree. The second template parameter is a list of child 2599 types, or in the case of terminals, the terminal's value type. Terminals 2600 will always have only one type in the type list. The last parameter is the 2601 arity of the expression. Terminals have arity 0, unary expressions have arity 2602 1, etc. 2603 </p> 2604<p> 2605 The <code class="computeroutput"><a class="link" href="../boost/proto/expr.html" title="Struct template expr">proto::expr<></a></code> struct is defined as 2606 follows: 2607 </p> 2608<pre class="programlisting"><span class="keyword">template</span><span class="special"><</span> <span class="keyword">typename</span> <span class="identifier">Tag</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Args</span><span class="special">,</span> <span class="keyword">long</span> <span class="identifier">Arity</span> <span class="special">=</span> <span class="identifier">Args</span><span class="special">::</span><span class="identifier">arity</span> <span class="special">></span> 2609<span class="keyword">struct</span> <span class="identifier">expr</span><span class="special">;</span> 2610 2611<span class="keyword">template</span><span class="special"><</span> <span class="keyword">typename</span> <span class="identifier">Tag</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Args</span> <span class="special">></span> 2612<span class="keyword">struct</span> <span class="identifier">expr</span><span class="special"><</span> <span class="identifier">Tag</span><span class="special">,</span> <span class="identifier">Args</span><span class="special">,</span> <span class="number">1</span> <span class="special">></span> 2613<span class="special">{</span> 2614 <span class="keyword">typedef</span> <span class="keyword">typename</span> <span class="identifier">Args</span><span class="special">::</span><span class="identifier">child0</span> <span class="identifier">proto_child0</span><span class="special">;</span> 2615 <span class="identifier">proto_child0</span> <span class="identifier">child0</span><span class="special">;</span> 2616 <span class="comment">// ...</span> 2617<span class="special">};</span> 2618</pre> 2619<p> 2620 The <code class="computeroutput"><a class="link" href="../boost/proto/expr.html" title="Struct template expr">proto::expr<></a></code> struct does not define 2621 a constructor, or anything else that would prevent static initialization. 2622 All <code class="computeroutput"><a class="link" href="../boost/proto/expr.html" title="Struct template expr">proto::expr<></a></code> objects are initialized 2623 using <span class="emphasis"><em>aggregate initialization</em></span>, with curly braces. In 2624 our example, <code class="computeroutput"><span class="identifier">_1</span></code> is initialized 2625 with the initializer <code class="computeroutput"><span class="special">{{}}</span></code>. The 2626 outer braces are the initializer for the <code class="computeroutput"><a class="link" href="../boost/proto/expr.html" title="Struct template expr">proto::expr<></a></code> 2627 struct, and the inner braces are for the member <code class="computeroutput"><span class="identifier">_1</span><span class="special">.</span><span class="identifier">child0</span></code> 2628 which is of type <code class="computeroutput"><span class="identifier">placeholder</span><span class="special"><</span><span class="number">0</span><span class="special">></span></code>. 2629 Note that we use braces to initialize <code class="computeroutput"><span class="identifier">_1</span><span class="special">.</span><span class="identifier">child0</span></code> 2630 because <code class="computeroutput"><span class="identifier">placeholder</span><span class="special"><</span><span class="number">0</span><span class="special">></span></code> is also 2631 an aggregate. 2632 </p> 2633<h5> 2634<a name="boost_proto.users_guide.intermediate_form.h1"></a> 2635 <span class="phrase"><a name="boost_proto.users_guide.intermediate_form.building_expression_trees"></a></span><a class="link" href="users_guide.html#boost_proto.users_guide.intermediate_form.building_expression_trees">Building 2636 Expression Trees</a> 2637 </h5> 2638<p> 2639 The <code class="computeroutput"><span class="identifier">_1</span></code> node is an instantiation 2640 of <code class="computeroutput"><a class="link" href="../boost/proto/expr.html" title="Struct template expr">proto::expr<></a></code>, and expressions containing 2641 <code class="computeroutput"><span class="identifier">_1</span></code> are also instantiations 2642 of <code class="computeroutput"><a class="link" href="../boost/proto/expr.html" title="Struct template expr">proto::expr<></a></code>. To use Proto effectively, 2643 you won't have to bother yourself with the actual types that Proto generates. 2644 These are details, but you're likely to encounter these types in compiler 2645 error messages, so it's helpful to be familiar with them. The types look 2646 like this: 2647 </p> 2648<pre class="programlisting"><span class="comment">// The type of the expression -_1</span> 2649<span class="keyword">typedef</span> 2650 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">expr</span><span class="special"><</span> 2651 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">negate</span> 2652 <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">list1</span><span class="special"><</span> 2653 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">expr</span><span class="special"><</span> 2654 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">terminal</span> 2655 <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">term</span><span class="special"><</span> <span class="identifier">placeholder</span><span class="special"><</span><span class="number">0</span><span class="special">></span> <span class="special">></span> 2656 <span class="special">,</span> <span class="number">0</span> 2657 <span class="special">></span> <span class="keyword">const</span> <span class="special">&</span> 2658 <span class="special">></span> 2659 <span class="special">,</span> <span class="number">1</span> 2660 <span class="special">></span> 2661<span class="identifier">negate_placeholder_type</span><span class="special">;</span> 2662 2663<span class="identifier">negate_placeholder_type</span> <span class="identifier">x</span> <span class="special">=</span> <span class="special">-</span><span class="identifier">_1</span><span class="special">;</span> 2664 2665<span class="comment">// The type of the expression _1 + 42</span> 2666<span class="keyword">typedef</span> 2667 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">expr</span><span class="special"><</span> 2668 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">plus</span> 2669 <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">list2</span><span class="special"><</span> 2670 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">expr</span><span class="special"><</span> 2671 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">terminal</span> 2672 <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">term</span><span class="special"><</span> <span class="identifier">placeholder</span><span class="special"><</span><span class="number">0</span><span class="special">></span> <span class="special">></span> 2673 <span class="special">,</span> <span class="number">0</span> 2674 <span class="special">></span> <span class="keyword">const</span> <span class="special">&</span> 2675 <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">expr</span><span class="special"><</span> 2676 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">terminal</span> 2677 <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">term</span><span class="special"><</span> <span class="keyword">int</span> <span class="keyword">const</span> <span class="special">&</span> <span class="special">></span> 2678 <span class="special">,</span> <span class="number">0</span> 2679 <span class="special">></span> 2680 <span class="special">></span> 2681 <span class="special">,</span> <span class="number">2</span> 2682 <span class="special">></span> 2683<span class="identifier">placeholder_plus_int_type</span><span class="special">;</span> 2684 2685<span class="identifier">placeholder_plus_int_type</span> <span class="identifier">y</span> <span class="special">=</span> <span class="identifier">_1</span> <span class="special">+</span> <span class="number">42</span><span class="special">;</span> 2686</pre> 2687<p> 2688 There are a few things to note about these types: 2689 </p> 2690<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "> 2691<li class="listitem"> 2692 Terminals have arity zero, unary expressions have arity one and binary 2693 expressions have arity two. 2694 </li> 2695<li class="listitem"> 2696 When one Proto expression is made a child node of another Proto expression, 2697 it is held by reference, <span class="emphasis"><em>even if it is a temporary object</em></span>. 2698 This last point becomes important later. 2699 </li> 2700<li class="listitem"> 2701 Non-Proto expressions, such as the integer literal, are turned into Proto 2702 expressions by wrapping them in new <code class="computeroutput"><span class="identifier">expr</span><span class="special"><></span></code> terminal objects. These new wrappers 2703 are not themselves held by reference, but the object wrapped <span class="emphasis"><em>is</em></span>. 2704 Notice that the type of the Protofied <code class="computeroutput"><span class="number">42</span></code> 2705 literal is <code class="computeroutput"><span class="keyword">int</span> <span class="keyword">const</span> 2706 <span class="special">&</span></code> -- held by reference. 2707 </li> 2708</ul></div> 2709<p> 2710 The types make it clear: everything in a Proto expression tree is held by 2711 reference. That means that building an expression tree is exceptionally cheap. 2712 It involves no copying at all. 2713 </p> 2714<div class="note"><table border="0" summary="Note"> 2715<tr> 2716<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../doc/src/images/note.png"></td> 2717<th align="left">Note</th> 2718</tr> 2719<tr><td align="left" valign="top"><p> 2720 An astute reader will notice that the object <code class="computeroutput"><span class="identifier">y</span></code> 2721 defined above will be left holding a dangling reference to a temporary 2722 int. In the sorts of high-performance applications Proto addresses, it 2723 is typical to build and evaluate an expression tree before any temporary 2724 objects go out of scope, so this dangling reference situation often doesn't 2725 arise, but it is certainly something to be aware of. Proto provides utilities 2726 for deep-copying expression trees so they can be passed around as value 2727 types without concern for dangling references. 2728 </p></td></tr> 2729</table></div> 2730<div class="section"> 2731<div class="titlepage"><div><div><h4 class="title"> 2732<a name="boost_proto.users_guide.intermediate_form.left_right_child"></a><a class="link" href="users_guide.html#boost_proto.users_guide.intermediate_form.left_right_child" title="Accessing Parts of an Expression">Accessing 2733 Parts of an Expression</a> 2734</h4></div></div></div> 2735<p> 2736 After assembling an expression into a tree, you'll naturally want to be 2737 able to do the reverse, and access a node's children. You may even want 2738 to be able to iterate over the children with algorithms from the Boost.Fusion 2739 library. This section shows how. 2740 </p> 2741<h6> 2742<a name="boost_proto.users_guide.intermediate_form.left_right_child.h0"></a> 2743 <span class="phrase"><a name="boost_proto.users_guide.intermediate_form.left_right_child.getting_expression_tags_and_arities"></a></span><a class="link" href="users_guide.html#boost_proto.users_guide.intermediate_form.left_right_child.getting_expression_tags_and_arities">Getting 2744 Expression Tags and Arities</a> 2745 </h6> 2746<p> 2747 Every node in an expression tree has both a <span class="emphasis"><em>tag</em></span> type 2748 that describes the node, and an <span class="emphasis"><em>arity</em></span> corresponding 2749 to the number of child nodes it has. You can use the <code class="computeroutput"><a class="link" href="../boost/proto/tag_of.html" title="Struct template tag_of">proto::tag_of<></a></code> 2750 and <code class="computeroutput"><a class="link" href="../boost/proto/arity_of.html" title="Struct template arity_of">proto::arity_of<></a></code> metafunctions to fetch 2751 them. Consider the following: 2752 </p> 2753<pre class="programlisting"><span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">></span> 2754<span class="keyword">void</span> <span class="identifier">check_plus_node</span><span class="special">(</span><span class="identifier">Expr</span> <span class="keyword">const</span> <span class="special">&)</span> 2755<span class="special">{</span> 2756 <span class="comment">// Assert that the tag type is proto::tag::plus</span> 2757 <span class="identifier">BOOST_STATIC_ASSERT</span><span class="special">((</span> 2758 <span class="identifier">boost</span><span class="special">::</span><span class="identifier">is_same</span><span class="special"><</span> 2759 <span class="keyword">typename</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag_of</span><span class="special"><</span><span class="identifier">Expr</span><span class="special">>::</span><span class="identifier">type</span> 2760 <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">plus</span> 2761 <span class="special">>::</span><span class="identifier">value</span> 2762 <span class="special">));</span> 2763 2764 <span class="comment">// Assert that the arity is 2</span> 2765 <span class="identifier">BOOST_STATIC_ASSERT</span><span class="special">(</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">arity_of</span><span class="special"><</span><span class="identifier">Expr</span><span class="special">>::</span><span class="identifier">value</span> <span class="special">==</span> <span class="number">2</span> <span class="special">);</span> 2766<span class="special">}</span> 2767 2768<span class="comment">// Create a binary plus node and use check_plus_node()</span> 2769<span class="comment">// to verify its tag type and arity:</span> 2770<span class="identifier">check_plus_node</span><span class="special">(</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">lit</span><span class="special">(</span><span class="number">1</span><span class="special">)</span> <span class="special">+</span> <span class="number">2</span> <span class="special">);</span> 2771</pre> 2772<p> 2773 For a given type <code class="computeroutput"><span class="identifier">Expr</span></code>, 2774 you could access the tag and arity directly as <code class="computeroutput"><span class="identifier">Expr</span><span class="special">::</span><span class="identifier">proto_tag</span></code> 2775 and <code class="computeroutput"><span class="identifier">Expr</span><span class="special">::</span><span class="identifier">proto_arity</span></code>, where <code class="computeroutput"><span class="identifier">Expr</span><span class="special">::</span><span class="identifier">proto_arity</span></code> 2776 is an MPL Integral Constant. 2777 </p> 2778<h6> 2779<a name="boost_proto.users_guide.intermediate_form.left_right_child.h1"></a> 2780 <span class="phrase"><a name="boost_proto.users_guide.intermediate_form.left_right_child.getting_terminal_values"></a></span><a class="link" href="users_guide.html#boost_proto.users_guide.intermediate_form.left_right_child.getting_terminal_values">Getting 2781 Terminal Values</a> 2782 </h6> 2783<p> 2784 There is no simpler expression than a terminal, and no more basic operation 2785 than extracting its value. As we've already seen, that is what <code class="computeroutput"><a class="link" href="../boost/proto/value.html" title="Function value">proto::value()</a></code> is for. 2786 </p> 2787<pre class="programlisting"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special"><</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">ostream</span> <span class="special">&</span> <span class="special">>::</span><span class="identifier">type</span> <span class="identifier">cout_</span> <span class="special">=</span> <span class="special">{</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span><span class="special">};</span> 2788 2789<span class="comment">// Get the value of the cout_ terminal:</span> 2790<span class="identifier">std</span><span class="special">::</span><span class="identifier">ostream</span> <span class="special">&</span> <span class="identifier">sout</span> <span class="special">=</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">value</span><span class="special">(</span> <span class="identifier">cout_</span> <span class="special">);</span> 2791 2792<span class="comment">// Assert that we got back what we put in:</span> 2793<span class="identifier">assert</span><span class="special">(</span> <span class="special">&</span><span class="identifier">sout</span> <span class="special">==</span> <span class="special">&</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">);</span> 2794</pre> 2795<p> 2796 To compute the return type of the <code class="computeroutput"><a class="link" href="../boost/proto/value.html" title="Function value">proto::value()</a></code> 2797 function, you can use <code class="computeroutput"><a class="link" href="../boost/proto/result_of/value.html" title="Struct template value">proto::result_of::value<></a></code>. 2798 When the parameter to <code class="computeroutput"><a class="link" href="../boost/proto/result_of/value.html" title="Struct template value">proto::result_of::value<></a></code> 2799 is a non-reference type, the result type of the metafunction is the type 2800 of the value as suitable for storage by value; that is, top-level reference 2801 and qualifiers are stripped from it. But when instantiated with a reference 2802 type, the result type has a reference <span class="emphasis"><em>added</em></span> to it, 2803 yielding a type suitable for storage by reference. If you want to know 2804 the actual type of the terminal's value including whether it is stored 2805 by value or reference, you can use <code class="computeroutput"><span class="identifier">fusion</span><span class="special">::</span><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">value_at</span><span class="special"><</span><span class="identifier">Expr</span><span class="special">,</span> <span class="number">0</span><span class="special">>::</span><span class="identifier">type</span></code>. 2806 </p> 2807<p> 2808 The following table summarizes the above paragraph. 2809 </p> 2810<div class="table"> 2811<a name="boost_proto.users_guide.intermediate_form.left_right_child.t0"></a><p class="title"><b>Table 32.4. Accessing Value Types</b></p> 2812<div class="table-contents"><table class="table" summary="Accessing Value Types"> 2813<colgroup> 2814<col> 2815<col> 2816<col> 2817</colgroup> 2818<thead><tr> 2819<th> 2820 <p> 2821 Metafunction Invocation 2822 </p> 2823 </th> 2824<th> 2825 <p> 2826 When the Value Type Is ... 2827 </p> 2828 </th> 2829<th> 2830 <p> 2831 The Result Is ... 2832 </p> 2833 </th> 2834</tr></thead> 2835<tbody> 2836<tr> 2837<td> 2838 <p> 2839 <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">value</span><span class="special"><</span><span class="identifier">Expr</span><span class="special">>::</span><span class="identifier">type</span></code> 2840 </p> 2841 </td> 2842<td> 2843 <p> 2844 <code class="computeroutput"><span class="identifier">T</span></code> 2845 </p> 2846 </td> 2847<td> 2848 <p> 2849</p> 2850<pre class="programlisting"><span class="keyword">typename</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">remove_const</span><span class="special"><</span> 2851 <span class="keyword">typename</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">remove_reference</span><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="identifier">type</span> 2852<span class="special">>::</span><span class="identifier">type</span> <a href="#ftn.boost_proto.users_guide.intermediate_form.left_right_child.f0" class="footnote" name="boost_proto.users_guide.intermediate_form.left_right_child.f0"><sup class="footnote">[a]</sup></a></pre> 2853<p> 2854 </p> 2855 </td> 2856</tr> 2857<tr> 2858<td> 2859 <p> 2860 <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">value</span><span class="special"><</span><span class="identifier">Expr</span> <span class="special">&>::</span><span class="identifier">type</span></code> 2861 </p> 2862 </td> 2863<td> 2864 <p> 2865 <code class="computeroutput"><span class="identifier">T</span></code> 2866 </p> 2867 </td> 2868<td> 2869 <p> 2870</p> 2871<pre class="programlisting"><span class="keyword">typename</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">add_reference</span><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="identifier">type</span></pre> 2872<p> 2873 </p> 2874 </td> 2875</tr> 2876<tr> 2877<td> 2878 <p> 2879 <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">value</span><span class="special"><</span><span class="identifier">Expr</span> <span class="keyword">const</span> 2880 <span class="special">&>::</span><span class="identifier">type</span></code> 2881 </p> 2882 </td> 2883<td> 2884 <p> 2885 <code class="computeroutput"><span class="identifier">T</span></code> 2886 </p> 2887 </td> 2888<td> 2889 <p> 2890</p> 2891<pre class="programlisting"><span class="keyword">typename</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">add_reference</span><span class="special"><</span> 2892 <span class="keyword">typename</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">add_const</span><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="identifier">type</span> 2893<span class="special">>::</span><span class="identifier">type</span></pre> 2894<p> 2895 </p> 2896 </td> 2897</tr> 2898<tr> 2899<td> 2900 <p> 2901 <code class="computeroutput"><span class="identifier">fusion</span><span class="special">::</span><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">value_at</span><span class="special"><</span><span class="identifier">Expr</span><span class="special">,</span> 2902 <span class="number">0</span><span class="special">>::</span><span class="identifier">type</span></code> 2903 </p> 2904 </td> 2905<td> 2906 <p> 2907 <code class="computeroutput"><span class="identifier">T</span></code> 2908 </p> 2909 </td> 2910<td> 2911 <p> 2912 <code class="computeroutput"><span class="identifier">T</span></code> 2913 </p> 2914 </td> 2915</tr> 2916</tbody> 2917<tbody class="footnotes"><tr><td colspan="3"><div id="ftn.boost_proto.users_guide.intermediate_form.left_right_child.f0" class="footnote"><p><a href="#boost_proto.users_guide.intermediate_form.left_right_child.f0" class="para"><sup class="para">[a] </sup></a>If <code class="computeroutput"><span class="identifier">T</span></code> is a reference-to-function type, then the result type is simply <code class="computeroutput"><span class="identifier">T</span></code>.</p></div></td></tr></tbody> 2918</table></div> 2919</div> 2920<br class="table-break"><h6> 2921<a name="boost_proto.users_guide.intermediate_form.left_right_child.h2"></a> 2922 <span class="phrase"><a name="boost_proto.users_guide.intermediate_form.left_right_child.getting_child_expressions"></a></span><a class="link" href="users_guide.html#boost_proto.users_guide.intermediate_form.left_right_child.getting_child_expressions">Getting 2923 Child Expressions</a> 2924 </h6> 2925<p> 2926 Each non-terminal node in an expression tree corresponds to an operator 2927 in an expression, and the children correspond to the operands, or arguments 2928 of the operator. To access them, you can use the <code class="computeroutput"><a class="link" href="../boost/proto/child_c.html" title="Function child_c">proto::child_c()</a></code> 2929 function template, as demonstrated below: 2930 </p> 2931<pre class="programlisting"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special"><</span><span class="keyword">int</span><span class="special">>::</span><span class="identifier">type</span> <span class="identifier">i</span> <span class="special">=</span> <span class="special">{</span><span class="number">42</span><span class="special">};</span> 2932 2933<span class="comment">// Get the 0-th operand of an addition operation:</span> 2934<span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special"><</span><span class="keyword">int</span><span class="special">>::</span><span class="identifier">type</span> <span class="special">&</span><span class="identifier">ri</span> <span class="special">=</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">child_c</span><span class="special"><</span><span class="number">0</span><span class="special">>(</span> <span class="identifier">i</span> <span class="special">+</span> <span class="number">2</span> <span class="special">);</span> 2935 2936<span class="comment">// Assert that we got back what we put in:</span> 2937<span class="identifier">assert</span><span class="special">(</span> <span class="special">&</span><span class="identifier">i</span> <span class="special">==</span> <span class="special">&</span><span class="identifier">ri</span> <span class="special">);</span> 2938</pre> 2939<p> 2940 You can use the <code class="computeroutput"><a class="link" href="../boost/proto/result_of/child_c.html" title="Struct template child_c">proto::result_of::child_c<></a></code> 2941 metafunction to get the type of the Nth child of an expression node. Usually 2942 you don't care to know whether a child is stored by value or by reference, 2943 so when you ask for the type of the Nth child of an expression <code class="computeroutput"><span class="identifier">Expr</span></code> (where <code class="computeroutput"><span class="identifier">Expr</span></code> 2944 is not a reference type), you get the child's type after references and 2945 cv-qualifiers have been stripped from it. 2946 </p> 2947<pre class="programlisting"><span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">></span> 2948<span class="keyword">void</span> <span class="identifier">test_result_of_child_c</span><span class="special">(</span><span class="identifier">Expr</span> <span class="keyword">const</span> <span class="special">&</span><span class="identifier">expr</span><span class="special">)</span> 2949<span class="special">{</span> 2950 <span class="keyword">typedef</span> <span class="keyword">typename</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">child_c</span><span class="special"><</span><span class="identifier">Expr</span><span class="special">,</span> <span class="number">0</span><span class="special">>::</span><span class="identifier">type</span> <span class="identifier">type</span><span class="special">;</span> 2951 2952 <span class="comment">// Since Expr is not a reference type,</span> 2953 <span class="comment">// result_of::child_c<Expr, 0>::type is a</span> 2954 <span class="comment">// non-cv qualified, non-reference type:</span> 2955 <span class="identifier">BOOST_MPL_ASSERT</span><span class="special">((</span> 2956 <span class="identifier">boost</span><span class="special">::</span><span class="identifier">is_same</span><span class="special"><</span> <span class="identifier">type</span><span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special"><</span><span class="keyword">int</span><span class="special">>::</span><span class="identifier">type</span> <span class="special">></span> 2957 <span class="special">));</span> 2958<span class="special">}</span> 2959 2960<span class="comment">// ...</span> 2961<span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special"><</span><span class="keyword">int</span><span class="special">>::</span><span class="identifier">type</span> <span class="identifier">i</span> <span class="special">=</span> <span class="special">{</span><span class="number">42</span><span class="special">};</span> 2962<span class="identifier">test_result_of_child_c</span><span class="special">(</span> <span class="identifier">i</span> <span class="special">+</span> <span class="number">2</span> <span class="special">);</span> 2963</pre> 2964<p> 2965 However, if you ask for the type of the Nth child of <code class="computeroutput"><span class="identifier">Expr</span> 2966 <span class="special">&</span></code> or <code class="computeroutput"><span class="identifier">Expr</span> 2967 <span class="keyword">const</span> <span class="special">&</span></code> 2968 (note the reference), the result type will be a reference, regardless of 2969 whether the child is actually stored by reference or not. If you need to 2970 know exactly how the child is stored in the node, whether by reference 2971 or by value, you can use <code class="computeroutput"><span class="identifier">fusion</span><span class="special">::</span><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">value_at</span><span class="special"><</span><span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">N</span><span class="special">>::</span><span class="identifier">type</span></code>. The following table summarizes 2972 the behavior of the <code class="computeroutput"><a class="link" href="../boost/proto/result_of/child_c.html" title="Struct template child_c">proto::result_of::child_c<></a></code> 2973 metafunction. 2974 </p> 2975<div class="table"> 2976<a name="boost_proto.users_guide.intermediate_form.left_right_child.t1"></a><p class="title"><b>Table 32.5. Accessing Child Types</b></p> 2977<div class="table-contents"><table class="table" summary="Accessing Child Types"> 2978<colgroup> 2979<col> 2980<col> 2981<col> 2982</colgroup> 2983<thead><tr> 2984<th> 2985 <p> 2986 Metafunction Invocation 2987 </p> 2988 </th> 2989<th> 2990 <p> 2991 When the Child Is ... 2992 </p> 2993 </th> 2994<th> 2995 <p> 2996 The Result Is ... 2997 </p> 2998 </th> 2999</tr></thead> 3000<tbody> 3001<tr> 3002<td> 3003 <p> 3004 <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">child_c</span><span class="special"><</span><span class="identifier">Expr</span><span class="special">,</span> 3005 <span class="identifier">N</span><span class="special">>::</span><span class="identifier">type</span></code> 3006 </p> 3007 </td> 3008<td> 3009 <p> 3010 <code class="computeroutput"><span class="identifier">T</span></code> 3011 </p> 3012 </td> 3013<td> 3014 <p> 3015</p> 3016<pre class="programlisting"><span class="keyword">typename</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">remove_const</span><span class="special"><</span> 3017 <span class="keyword">typename</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">remove_reference</span><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="identifier">type</span> 3018<span class="special">>::</span><span class="identifier">type</span></pre> 3019<p> 3020 </p> 3021 </td> 3022</tr> 3023<tr> 3024<td> 3025 <p> 3026 <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">child_c</span><span class="special"><</span><span class="identifier">Expr</span> <span class="special">&,</span> 3027 <span class="identifier">N</span><span class="special">>::</span><span class="identifier">type</span></code> 3028 </p> 3029 </td> 3030<td> 3031 <p> 3032 <code class="computeroutput"><span class="identifier">T</span></code> 3033 </p> 3034 </td> 3035<td> 3036 <p> 3037</p> 3038<pre class="programlisting"><span class="keyword">typename</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">add_reference</span><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="identifier">type</span></pre> 3039<p> 3040 </p> 3041 </td> 3042</tr> 3043<tr> 3044<td> 3045 <p> 3046 <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">child_c</span><span class="special"><</span><span class="identifier">Expr</span> <span class="keyword">const</span> 3047 <span class="special">&,</span> <span class="identifier">N</span><span class="special">>::</span><span class="identifier">type</span></code> 3048 </p> 3049 </td> 3050<td> 3051 <p> 3052 <code class="computeroutput"><span class="identifier">T</span></code> 3053 </p> 3054 </td> 3055<td> 3056 <p> 3057</p> 3058<pre class="programlisting"><span class="keyword">typename</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">add_reference</span><span class="special"><</span> 3059 <span class="keyword">typename</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">add_const</span><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="identifier">type</span> 3060<span class="special">>::</span><span class="identifier">type</span></pre> 3061<p> 3062 </p> 3063 </td> 3064</tr> 3065<tr> 3066<td> 3067 <p> 3068 <code class="computeroutput"><span class="identifier">fusion</span><span class="special">::</span><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">value_at</span><span class="special"><</span><span class="identifier">Expr</span><span class="special">,</span> 3069 <span class="identifier">N</span><span class="special">>::</span><span class="identifier">type</span></code> 3070 </p> 3071 </td> 3072<td> 3073 <p> 3074 <code class="computeroutput"><span class="identifier">T</span></code> 3075 </p> 3076 </td> 3077<td> 3078 <p> 3079 <code class="computeroutput"><span class="identifier">T</span></code> 3080 </p> 3081 </td> 3082</tr> 3083</tbody> 3084</table></div> 3085</div> 3086<br class="table-break"><h6> 3087<a name="boost_proto.users_guide.intermediate_form.left_right_child.h3"></a> 3088 <span class="phrase"><a name="boost_proto.users_guide.intermediate_form.left_right_child.common_shortcuts"></a></span><a class="link" href="users_guide.html#boost_proto.users_guide.intermediate_form.left_right_child.common_shortcuts">Common 3089 Shortcuts</a> 3090 </h6> 3091<p> 3092 Most operators in C++ are unary or binary, so accessing the only operand, 3093 or the left and right operands, are very common operations. For this reason, 3094 Proto provides the <code class="computeroutput"><a class="link" href="../boost/proto/child.html" title="Function child">proto::child()</a></code>, 3095 <code class="computeroutput"><a class="link" href="../boost/proto/left.html" title="Function left">proto::left()</a></code>, and <code class="computeroutput"><a class="link" href="../boost/proto/right.html" title="Function right">proto::right()</a></code> 3096 functions. <code class="computeroutput"><a class="link" href="../boost/proto/child.html" title="Function child">proto::child()</a></code> and <code class="computeroutput"><a class="link" href="../boost/proto/left.html" title="Function left">proto::left()</a></code> 3097 are synonymous with <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">child_c</span><span class="special"><</span><span class="number">0</span><span class="special">>()</span></code>, 3098 and <code class="computeroutput"><a class="link" href="../boost/proto/right.html" title="Function right">proto::right()</a></code> is synonymous with <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">child_c</span><span class="special"><</span><span class="number">1</span><span class="special">>()</span></code>. 3099 </p> 3100<p> 3101 There are also <code class="computeroutput"><a class="link" href="../boost/proto/result_of/child.html" title="Struct template child">proto::result_of::child<></a></code>, 3102 <code class="computeroutput"><a class="link" href="../boost/proto/result_of/left.html" title="Struct template left">proto::result_of::left<></a></code>, and <code class="computeroutput"><a class="link" href="../boost/proto/result_of/right.html" title="Struct template right">proto::result_of::right<></a></code> 3103 metafunctions that merely forward to their <code class="computeroutput"><a class="link" href="../boost/proto/result_of/child_c.html" title="Struct template child_c">proto::result_of::child_c<></a></code> 3104 counterparts. 3105 </p> 3106</div> 3107<div class="section"> 3108<div class="titlepage"><div><div><h4 class="title"> 3109<a name="boost_proto.users_guide.intermediate_form.deep_copying_expressions"></a><a class="link" href="users_guide.html#boost_proto.users_guide.intermediate_form.deep_copying_expressions" title="Deep-copying Expressions">Deep-copying 3110 Expressions</a> 3111</h4></div></div></div> 3112<p> 3113 When you build an expression template with Proto, all the intermediate 3114 child nodes are held <span class="emphasis"><em>by reference</em></span>. The avoids needless 3115 copies, which is crucial if you want your EDSL to perform well at runtime. 3116 Naturally, there is a danger if the temporary objects go out of scope before 3117 you try to evaluate your expression template. This is especially a problem 3118 in C++0x with the new <code class="computeroutput"><span class="keyword">decltype</span></code> 3119 and <code class="computeroutput"><span class="keyword">auto</span></code> keywords. Consider: 3120 </p> 3121<pre class="programlisting"><span class="comment">// OOPS: "ex" is left holding dangling references</span> 3122<span class="keyword">auto</span> <span class="identifier">ex</span> <span class="special">=</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">lit</span><span class="special">(</span><span class="number">1</span><span class="special">)</span> <span class="special">+</span> <span class="number">2</span><span class="special">;</span> 3123</pre> 3124<p> 3125 The problem can happen in today's C++ also if you use <code class="computeroutput"><span class="identifier">BOOST_TYPEOF</span><span class="special">()</span></code> or <code class="computeroutput"><span class="identifier">BOOST_AUTO</span><span class="special">()</span></code>, or if you try to pass an expression 3126 template outside the scope of its constituents. 3127 </p> 3128<p> 3129 In these cases, you want to deep-copy your expression template so that 3130 all intermediate nodes and the terminals are held <span class="emphasis"><em>by value</em></span>. 3131 That way, you can safely assign the expression template to a local variable 3132 or return it from a function without worrying about dangling references. 3133 You can do this with <code class="computeroutput"><a class="link" href="../boost/proto/deep_copy.html" title="Function template deep_copy">proto::deep_copy()</a></code> 3134 as fo llows: 3135 </p> 3136<pre class="programlisting"><span class="comment">// OK, "ex" has no dangling references</span> 3137<span class="keyword">auto</span> <span class="identifier">ex</span> <span class="special">=</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">deep_copy</span><span class="special">(</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">lit</span><span class="special">(</span><span class="number">1</span><span class="special">)</span> <span class="special">+</span> <span class="number">2</span> <span class="special">);</span> 3138</pre> 3139<p> 3140 If you are using <a href="../../../libs/typeof/index.html" target="_top">Boost.Typeof</a>, 3141 it would look like this: 3142 </p> 3143<pre class="programlisting"><span class="comment">// OK, use BOOST_AUTO() and proto::deep_copy() to</span> 3144<span class="comment">// store an expression template in a local variable </span> 3145<span class="identifier">BOOST_AUTO</span><span class="special">(</span> <span class="identifier">ex</span><span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">deep_copy</span><span class="special">(</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">lit</span><span class="special">(</span><span class="number">1</span><span class="special">)</span> <span class="special">+</span> <span class="number">2</span> <span class="special">)</span> <span class="special">);</span> 3146</pre> 3147<p> 3148 For the above code to work, you must include the <code class="computeroutput"><a class="link" href="reference.html#header.boost.proto.proto_typeof_hpp" title="Header <boost/proto/proto_typeof.hpp>">boost/proto/proto_typeof.hpp</a></code> 3149 header, which also defines the <code class="literal"><code class="computeroutput"><a class="link" href="../BOOST_PROTO_AUTO.html" title="Macro BOOST_PROTO_AUTO">BOOST_PROTO_AUTO</a></code>()</code> 3150 macro which automatically deep-copies its argument. With <code class="literal"><code class="computeroutput"><a class="link" href="../BOOST_PROTO_AUTO.html" title="Macro BOOST_PROTO_AUTO">BOOST_PROTO_AUTO</a></code>()</code>, the above 3151 code can be writen as: 3152 </p> 3153<pre class="programlisting"><span class="comment">// OK, BOOST_PROTO_AUTO() automatically deep-copies</span> 3154<span class="comment">// its argument: </span> 3155<span class="identifier">BOOST_PROTO_AUTO</span><span class="special">(</span> <span class="identifier">ex</span><span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">lit</span><span class="special">(</span><span class="number">1</span><span class="special">)</span> <span class="special">+</span> <span class="number">2</span> <span class="special">);</span> 3156</pre> 3157<p> 3158 When deep-copying an expression tree, all intermediate nodes and all terminals 3159 are stored by value. The only exception is terminals that are function 3160 references, which are left alone. 3161 </p> 3162<div class="note"><table border="0" summary="Note"> 3163<tr> 3164<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../doc/src/images/note.png"></td> 3165<th align="left">Note</th> 3166</tr> 3167<tr><td align="left" valign="top"><p> 3168 <code class="computeroutput"><a class="link" href="../boost/proto/deep_copy.html" title="Function template deep_copy">proto::deep_copy()</a></code> makes no exception for 3169 arrays, which it stores by value. That can potentially cause a large 3170 amount of data to be copied. 3171 </p></td></tr> 3172</table></div> 3173</div> 3174<div class="section"> 3175<div class="titlepage"><div><div><h4 class="title"> 3176<a name="boost_proto.users_guide.intermediate_form.debugging_expressions"></a><a class="link" href="users_guide.html#boost_proto.users_guide.intermediate_form.debugging_expressions" title="Debugging Expressions">Debugging 3177 Expressions</a> 3178</h4></div></div></div> 3179<p> 3180 Proto provides a utility for pretty-printing expression trees that comes 3181 in very handy when you're trying to debug your EDSL. It's called <code class="computeroutput"><a class="link" href="../boost/proto/display_expr.html" title="Function display_expr">proto::display_expr()</a></code>, and you pass it the expression 3182 to print and optionally, an <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">ostream</span></code> 3183 to which to send the output. Consider: 3184 </p> 3185<pre class="programlisting"><span class="comment">// Use display_expr() to pretty-print an expression tree</span> 3186<span class="identifier">proto</span><span class="special">::</span><span class="identifier">display_expr</span><span class="special">(</span> 3187 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">lit</span><span class="special">(</span><span class="string">"hello"</span><span class="special">)</span> <span class="special">+</span> <span class="number">42</span> 3188<span class="special">);</span> 3189</pre> 3190<p> 3191 The above code writes this to <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span></code>: 3192 </p> 3193<pre class="programlisting">plus( 3194 terminal(hello) 3195 , terminal(42) 3196)</pre> 3197<p> 3198 In order to call <code class="computeroutput"><a class="link" href="../boost/proto/display_expr.html" title="Function display_expr">proto::display_expr()</a></code>, 3199 all the terminals in the expression must be Streamable (that is, they can 3200 be written to a <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">ostream</span></code>). In addition, the tag types 3201 must all be Streamable as well. Here is an example that includes a custom 3202 terminal type and a custom tag: 3203 </p> 3204<pre class="programlisting"><span class="comment">// A custom tag type that is Streamable</span> 3205<span class="keyword">struct</span> <span class="identifier">MyTag</span> 3206<span class="special">{</span> 3207 <span class="keyword">friend</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">ostream</span> <span class="special">&</span><span class="keyword">operator</span><span class="special"><<(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">ostream</span> <span class="special">&</span><span class="identifier">s</span><span class="special">,</span> <span class="identifier">MyTag</span><span class="special">)</span> 3208 <span class="special">{</span> 3209 <span class="keyword">return</span> <span class="identifier">s</span> <span class="special"><<</span> <span class="string">"MyTag"</span><span class="special">;</span> 3210 <span class="special">}</span> 3211<span class="special">};</span> 3212 3213<span class="comment">// Some other Streamable type</span> 3214<span class="keyword">struct</span> <span class="identifier">MyTerminal</span> 3215<span class="special">{</span> 3216 <span class="keyword">friend</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">ostream</span> <span class="special">&</span><span class="keyword">operator</span><span class="special"><<(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">ostream</span> <span class="special">&</span><span class="identifier">s</span><span class="special">,</span> <span class="identifier">MyTerminal</span><span class="special">)</span> 3217 <span class="special">{</span> 3218 <span class="keyword">return</span> <span class="identifier">s</span> <span class="special"><<</span> <span class="string">"MyTerminal"</span><span class="special">;</span> 3219 <span class="special">}</span> 3220<span class="special">};</span> 3221 3222<span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span> 3223<span class="special">{</span> 3224 <span class="comment">// Display an expression tree that contains a custom</span> 3225 <span class="comment">// tag and a user-defined type in a terminal</span> 3226 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">display_expr</span><span class="special">(</span> 3227 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">make_expr</span><span class="special"><</span><span class="identifier">MyTag</span><span class="special">>(</span><span class="identifier">MyTerminal</span><span class="special">())</span> <span class="special">+</span> <span class="number">42</span> 3228 <span class="special">);</span> 3229<span class="special">}</span> 3230</pre> 3231<p> 3232 The above code prints the following: 3233 </p> 3234<pre class="programlisting">plus( 3235 MyTag( 3236 terminal(MyTerminal) 3237 ) 3238 , terminal(42) 3239)</pre> 3240</div> 3241<div class="section"> 3242<div class="titlepage"><div><div><h4 class="title"> 3243<a name="boost_proto.users_guide.intermediate_form.tags_and_metafunctions"></a><a class="link" href="users_guide.html#boost_proto.users_guide.intermediate_form.tags_and_metafunctions" title="Operator Tags and Metafunctions">Operator 3244 Tags and Metafunctions</a> 3245</h4></div></div></div> 3246<p> 3247 The following table lists the overloadable C++ operators, the Proto tag 3248 types for each, and the name of the metafunctions for generating the corresponding 3249 Proto expression types. And as we'll see later, the metafunctions are also 3250 usable as grammars for matching such nodes, as well as pass-through transforms. 3251 </p> 3252<div class="table"> 3253<a name="boost_proto.users_guide.intermediate_form.tags_and_metafunctions.t0"></a><p class="title"><b>Table 32.6. Operators, Tags and Metafunctions</b></p> 3254<div class="table-contents"><table class="table" summary="Operators, Tags and Metafunctions"> 3255<colgroup> 3256<col> 3257<col> 3258<col> 3259</colgroup> 3260<thead><tr> 3261<th> 3262 <p> 3263 Operator 3264 </p> 3265 </th> 3266<th> 3267 <p> 3268 Proto Tag 3269 </p> 3270 </th> 3271<th> 3272 <p> 3273 Proto Metafunction 3274 </p> 3275 </th> 3276</tr></thead> 3277<tbody> 3278<tr> 3279<td> 3280 <p> 3281 unary <code class="computeroutput"><span class="special">+</span></code> 3282 </p> 3283 </td> 3284<td> 3285 <p> 3286 <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">unary_plus</span></code> 3287 </p> 3288 </td> 3289<td> 3290 <p> 3291 <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">unary_plus</span><span class="special"><></span></code> 3292 </p> 3293 </td> 3294</tr> 3295<tr> 3296<td> 3297 <p> 3298 unary <code class="computeroutput"><span class="special">-</span></code> 3299 </p> 3300 </td> 3301<td> 3302 <p> 3303 <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">negate</span></code> 3304 </p> 3305 </td> 3306<td> 3307 <p> 3308 <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">negate</span><span class="special"><></span></code> 3309 </p> 3310 </td> 3311</tr> 3312<tr> 3313<td> 3314 <p> 3315 unary <code class="computeroutput"><span class="special">*</span></code> 3316 </p> 3317 </td> 3318<td> 3319 <p> 3320 <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">dereference</span></code> 3321 </p> 3322 </td> 3323<td> 3324 <p> 3325 <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">dereference</span><span class="special"><></span></code> 3326 </p> 3327 </td> 3328</tr> 3329<tr> 3330<td> 3331 <p> 3332 unary <code class="computeroutput"><span class="special">~</span></code> 3333 </p> 3334 </td> 3335<td> 3336 <p> 3337 <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">complement</span></code> 3338 </p> 3339 </td> 3340<td> 3341 <p> 3342 <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">complement</span><span class="special"><></span></code> 3343 </p> 3344 </td> 3345</tr> 3346<tr> 3347<td> 3348 <p> 3349 unary <code class="computeroutput"><span class="special">&</span></code> 3350 </p> 3351 </td> 3352<td> 3353 <p> 3354 <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">address_of</span></code> 3355 </p> 3356 </td> 3357<td> 3358 <p> 3359 <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">address_of</span><span class="special"><></span></code> 3360 </p> 3361 </td> 3362</tr> 3363<tr> 3364<td> 3365 <p> 3366 unary <code class="computeroutput"><span class="special">!</span></code> 3367 </p> 3368 </td> 3369<td> 3370 <p> 3371 <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">logical_not</span></code> 3372 </p> 3373 </td> 3374<td> 3375 <p> 3376 <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">logical_not</span><span class="special"><></span></code> 3377 </p> 3378 </td> 3379</tr> 3380<tr> 3381<td> 3382 <p> 3383 unary prefix <code class="computeroutput"><span class="special">++</span></code> 3384 </p> 3385 </td> 3386<td> 3387 <p> 3388 <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">pre_inc</span></code> 3389 </p> 3390 </td> 3391<td> 3392 <p> 3393 <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">pre_inc</span><span class="special"><></span></code> 3394 </p> 3395 </td> 3396</tr> 3397<tr> 3398<td> 3399 <p> 3400 unary prefix <code class="computeroutput"><span class="special">--</span></code> 3401 </p> 3402 </td> 3403<td> 3404 <p> 3405 <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">pre_dec</span></code> 3406 </p> 3407 </td> 3408<td> 3409 <p> 3410 <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">pre_dec</span><span class="special"><></span></code> 3411 </p> 3412 </td> 3413</tr> 3414<tr> 3415<td> 3416 <p> 3417 unary postfix <code class="computeroutput"><span class="special">++</span></code> 3418 </p> 3419 </td> 3420<td> 3421 <p> 3422 <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">post_inc</span></code> 3423 </p> 3424 </td> 3425<td> 3426 <p> 3427 <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">post_inc</span><span class="special"><></span></code> 3428 </p> 3429 </td> 3430</tr> 3431<tr> 3432<td> 3433 <p> 3434 unary postfix <code class="computeroutput"><span class="special">--</span></code> 3435 </p> 3436 </td> 3437<td> 3438 <p> 3439 <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">post_dec</span></code> 3440 </p> 3441 </td> 3442<td> 3443 <p> 3444 <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">post_dec</span><span class="special"><></span></code> 3445 </p> 3446 </td> 3447</tr> 3448<tr> 3449<td> 3450 <p> 3451 binary <code class="computeroutput"><span class="special"><<</span></code> 3452 </p> 3453 </td> 3454<td> 3455 <p> 3456 <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">shift_left</span></code> 3457 </p> 3458 </td> 3459<td> 3460 <p> 3461 <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">shift_left</span><span class="special"><></span></code> 3462 </p> 3463 </td> 3464</tr> 3465<tr> 3466<td> 3467 <p> 3468 binary <code class="computeroutput"><span class="special">>></span></code> 3469 </p> 3470 </td> 3471<td> 3472 <p> 3473 <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">shift_right</span></code> 3474 </p> 3475 </td> 3476<td> 3477 <p> 3478 <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">shift_right</span><span class="special"><></span></code> 3479 </p> 3480 </td> 3481</tr> 3482<tr> 3483<td> 3484 <p> 3485 binary <code class="computeroutput"><span class="special">*</span></code> 3486 </p> 3487 </td> 3488<td> 3489 <p> 3490 <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">multiplies</span></code> 3491 </p> 3492 </td> 3493<td> 3494 <p> 3495 <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">multiplies</span><span class="special"><></span></code> 3496 </p> 3497 </td> 3498</tr> 3499<tr> 3500<td> 3501 <p> 3502 binary <code class="computeroutput"><span class="special">/</span></code> 3503 </p> 3504 </td> 3505<td> 3506 <p> 3507 <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">divides</span></code> 3508 </p> 3509 </td> 3510<td> 3511 <p> 3512 <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">divides</span><span class="special"><></span></code> 3513 </p> 3514 </td> 3515</tr> 3516<tr> 3517<td> 3518 <p> 3519 binary <code class="computeroutput"><span class="special">%</span></code> 3520 </p> 3521 </td> 3522<td> 3523 <p> 3524 <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">modulus</span></code> 3525 </p> 3526 </td> 3527<td> 3528 <p> 3529 <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">modulus</span><span class="special"><></span></code> 3530 </p> 3531 </td> 3532</tr> 3533<tr> 3534<td> 3535 <p> 3536 binary <code class="computeroutput"><span class="special">+</span></code> 3537 </p> 3538 </td> 3539<td> 3540 <p> 3541 <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">plus</span></code> 3542 </p> 3543 </td> 3544<td> 3545 <p> 3546 <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">plus</span><span class="special"><></span></code> 3547 </p> 3548 </td> 3549</tr> 3550<tr> 3551<td> 3552 <p> 3553 binary <code class="computeroutput"><span class="special">-</span></code> 3554 </p> 3555 </td> 3556<td> 3557 <p> 3558 <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">minus</span></code> 3559 </p> 3560 </td> 3561<td> 3562 <p> 3563 <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">minus</span><span class="special"><></span></code> 3564 </p> 3565 </td> 3566</tr> 3567<tr> 3568<td> 3569 <p> 3570 binary <code class="computeroutput"><span class="special"><</span></code> 3571 </p> 3572 </td> 3573<td> 3574 <p> 3575 <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">less</span></code> 3576 </p> 3577 </td> 3578<td> 3579 <p> 3580 <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">less</span><span class="special"><></span></code> 3581 </p> 3582 </td> 3583</tr> 3584<tr> 3585<td> 3586 <p> 3587 binary <code class="computeroutput"><span class="special">></span></code> 3588 </p> 3589 </td> 3590<td> 3591 <p> 3592 <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">greater</span></code> 3593 </p> 3594 </td> 3595<td> 3596 <p> 3597 <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">greater</span><span class="special"><></span></code> 3598 </p> 3599 </td> 3600</tr> 3601<tr> 3602<td> 3603 <p> 3604 binary <code class="computeroutput"><span class="special"><=</span></code> 3605 </p> 3606 </td> 3607<td> 3608 <p> 3609 <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">less_equal</span></code> 3610 </p> 3611 </td> 3612<td> 3613 <p> 3614 <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">less_equal</span><span class="special"><></span></code> 3615 </p> 3616 </td> 3617</tr> 3618<tr> 3619<td> 3620 <p> 3621 binary <code class="computeroutput"><span class="special">>=</span></code> 3622 </p> 3623 </td> 3624<td> 3625 <p> 3626 <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">greater_equal</span></code> 3627 </p> 3628 </td> 3629<td> 3630 <p> 3631 <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">greater_equal</span><span class="special"><></span></code> 3632 </p> 3633 </td> 3634</tr> 3635<tr> 3636<td> 3637 <p> 3638 binary <code class="computeroutput"><span class="special">==</span></code> 3639 </p> 3640 </td> 3641<td> 3642 <p> 3643 <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">equal_to</span></code> 3644 </p> 3645 </td> 3646<td> 3647 <p> 3648 <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">equal_to</span><span class="special"><></span></code> 3649 </p> 3650 </td> 3651</tr> 3652<tr> 3653<td> 3654 <p> 3655 binary <code class="computeroutput"><span class="special">!=</span></code> 3656 </p> 3657 </td> 3658<td> 3659 <p> 3660 <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">not_equal_to</span></code> 3661 </p> 3662 </td> 3663<td> 3664 <p> 3665 <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">not_equal_to</span><span class="special"><></span></code> 3666 </p> 3667 </td> 3668</tr> 3669<tr> 3670<td> 3671 <p> 3672 binary <code class="computeroutput"><span class="special">||</span></code> 3673 </p> 3674 </td> 3675<td> 3676 <p> 3677 <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">logical_or</span></code> 3678 </p> 3679 </td> 3680<td> 3681 <p> 3682 <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">logical_or</span><span class="special"><></span></code> 3683 </p> 3684 </td> 3685</tr> 3686<tr> 3687<td> 3688 <p> 3689 binary <code class="computeroutput"><span class="special">&&</span></code> 3690 </p> 3691 </td> 3692<td> 3693 <p> 3694 <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">logical_and</span></code> 3695 </p> 3696 </td> 3697<td> 3698 <p> 3699 <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">logical_and</span><span class="special"><></span></code> 3700 </p> 3701 </td> 3702</tr> 3703<tr> 3704<td> 3705 <p> 3706 binary <code class="computeroutput"><span class="special">&</span></code> 3707 </p> 3708 </td> 3709<td> 3710 <p> 3711 <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">bitwise_and</span></code> 3712 </p> 3713 </td> 3714<td> 3715 <p> 3716 <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">bitwise_and</span><span class="special"><></span></code> 3717 </p> 3718 </td> 3719</tr> 3720<tr> 3721<td> 3722 <p> 3723 binary <code class="computeroutput"><span class="special">|</span></code> 3724 </p> 3725 </td> 3726<td> 3727 <p> 3728 <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">bitwise_or</span></code> 3729 </p> 3730 </td> 3731<td> 3732 <p> 3733 <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">bitwise_or</span><span class="special"><></span></code> 3734 </p> 3735 </td> 3736</tr> 3737<tr> 3738<td> 3739 <p> 3740 binary <code class="computeroutput"><span class="special">^</span></code> 3741 </p> 3742 </td> 3743<td> 3744 <p> 3745 <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">bitwise_xor</span></code> 3746 </p> 3747 </td> 3748<td> 3749 <p> 3750 <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">bitwise_xor</span><span class="special"><></span></code> 3751 </p> 3752 </td> 3753</tr> 3754<tr> 3755<td> 3756 <p> 3757 binary <code class="computeroutput"><span class="special">,</span></code> 3758 </p> 3759 </td> 3760<td> 3761 <p> 3762 <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">comma</span></code> 3763 </p> 3764 </td> 3765<td> 3766 <p> 3767 <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">comma</span><span class="special"><></span></code> 3768 </p> 3769 </td> 3770</tr> 3771<tr> 3772<td> 3773 <p> 3774 binary <code class="computeroutput"><span class="special">->*</span></code> 3775 </p> 3776 </td> 3777<td> 3778 <p> 3779 <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">mem_ptr</span></code> 3780 </p> 3781 </td> 3782<td> 3783 <p> 3784 <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">mem_ptr</span><span class="special"><></span></code> 3785 </p> 3786 </td> 3787</tr> 3788<tr> 3789<td> 3790 <p> 3791 binary <code class="computeroutput"><span class="special">=</span></code> 3792 </p> 3793 </td> 3794<td> 3795 <p> 3796 <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">assign</span></code> 3797 </p> 3798 </td> 3799<td> 3800 <p> 3801 <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">assign</span><span class="special"><></span></code> 3802 </p> 3803 </td> 3804</tr> 3805<tr> 3806<td> 3807 <p> 3808 binary <code class="computeroutput"><span class="special"><<=</span></code> 3809 </p> 3810 </td> 3811<td> 3812 <p> 3813 <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">shift_left_assign</span></code> 3814 </p> 3815 </td> 3816<td> 3817 <p> 3818 <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">shift_left_assign</span><span class="special"><></span></code> 3819 </p> 3820 </td> 3821</tr> 3822<tr> 3823<td> 3824 <p> 3825 binary <code class="computeroutput"><span class="special">>>=</span></code> 3826 </p> 3827 </td> 3828<td> 3829 <p> 3830 <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">shift_right_assign</span></code> 3831 </p> 3832 </td> 3833<td> 3834 <p> 3835 <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">shift_right_assign</span><span class="special"><></span></code> 3836 </p> 3837 </td> 3838</tr> 3839<tr> 3840<td> 3841 <p> 3842 binary <code class="computeroutput"><span class="special">*=</span></code> 3843 </p> 3844 </td> 3845<td> 3846 <p> 3847 <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">multiplies_assign</span></code> 3848 </p> 3849 </td> 3850<td> 3851 <p> 3852 <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">multiplies_assign</span><span class="special"><></span></code> 3853 </p> 3854 </td> 3855</tr> 3856<tr> 3857<td> 3858 <p> 3859 binary <code class="computeroutput"><span class="special">/=</span></code> 3860 </p> 3861 </td> 3862<td> 3863 <p> 3864 <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">divides_assign</span></code> 3865 </p> 3866 </td> 3867<td> 3868 <p> 3869 <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">divides_assign</span><span class="special"><></span></code> 3870 </p> 3871 </td> 3872</tr> 3873<tr> 3874<td> 3875 <p> 3876 binary <code class="computeroutput"><span class="special">%=</span></code> 3877 </p> 3878 </td> 3879<td> 3880 <p> 3881 <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">modulus_assign</span></code> 3882 </p> 3883 </td> 3884<td> 3885 <p> 3886 <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">modulus_assign</span><span class="special"><></span></code> 3887 </p> 3888 </td> 3889</tr> 3890<tr> 3891<td> 3892 <p> 3893 binary <code class="computeroutput"><span class="special">+=</span></code> 3894 </p> 3895 </td> 3896<td> 3897 <p> 3898 <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">plus_assign</span></code> 3899 </p> 3900 </td> 3901<td> 3902 <p> 3903 <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">plus_assign</span><span class="special"><></span></code> 3904 </p> 3905 </td> 3906</tr> 3907<tr> 3908<td> 3909 <p> 3910 binary <code class="computeroutput"><span class="special">-=</span></code> 3911 </p> 3912 </td> 3913<td> 3914 <p> 3915 <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">minus_assign</span></code> 3916 </p> 3917 </td> 3918<td> 3919 <p> 3920 <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">minus_assign</span><span class="special"><></span></code> 3921 </p> 3922 </td> 3923</tr> 3924<tr> 3925<td> 3926 <p> 3927 binary <code class="computeroutput"><span class="special">&=</span></code> 3928 </p> 3929 </td> 3930<td> 3931 <p> 3932 <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">bitwise_and_assign</span></code> 3933 </p> 3934 </td> 3935<td> 3936 <p> 3937 <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">bitwise_and_assign</span><span class="special"><></span></code> 3938 </p> 3939 </td> 3940</tr> 3941<tr> 3942<td> 3943 <p> 3944 binary <code class="computeroutput"><span class="special">|=</span></code> 3945 </p> 3946 </td> 3947<td> 3948 <p> 3949 <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">bitwise_or_assign</span></code> 3950 </p> 3951 </td> 3952<td> 3953 <p> 3954 <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">bitwise_or_assign</span><span class="special"><></span></code> 3955 </p> 3956 </td> 3957</tr> 3958<tr> 3959<td> 3960 <p> 3961 binary <code class="computeroutput"><span class="special">^=</span></code> 3962 </p> 3963 </td> 3964<td> 3965 <p> 3966 <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">bitwise_xor_assign</span></code> 3967 </p> 3968 </td> 3969<td> 3970 <p> 3971 <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">bitwise_xor_assign</span><span class="special"><></span></code> 3972 </p> 3973 </td> 3974</tr> 3975<tr> 3976<td> 3977 <p> 3978 binary subscript 3979 </p> 3980 </td> 3981<td> 3982 <p> 3983 <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">subscript</span></code> 3984 </p> 3985 </td> 3986<td> 3987 <p> 3988 <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">subscript</span><span class="special"><></span></code> 3989 </p> 3990 </td> 3991</tr> 3992<tr> 3993<td> 3994 <p> 3995 ternary <code class="computeroutput"><span class="special">?:</span></code> 3996 </p> 3997 </td> 3998<td> 3999 <p> 4000 <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">if_else_</span></code> 4001 </p> 4002 </td> 4003<td> 4004 <p> 4005 <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">if_else_</span><span class="special"><></span></code> 4006 </p> 4007 </td> 4008</tr> 4009<tr> 4010<td> 4011 <p> 4012 n-ary function call 4013 </p> 4014 </td> 4015<td> 4016 <p> 4017 <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">function</span></code> 4018 </p> 4019 </td> 4020<td> 4021 <p> 4022 <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">function</span><span class="special"><></span></code> 4023 </p> 4024 </td> 4025</tr> 4026</tbody> 4027</table></div> 4028</div> 4029<br class="table-break"> 4030</div> 4031<div class="section"> 4032<div class="titlepage"><div><div><h4 class="title"> 4033<a name="boost_proto.users_guide.intermediate_form.expressions_as_fusion_sequences"></a><a class="link" href="users_guide.html#boost_proto.users_guide.intermediate_form.expressions_as_fusion_sequences" title="Expressions as Fusion Sequences">Expressions 4034 as Fusion Sequences</a> 4035</h4></div></div></div> 4036<p> 4037 Boost.Fusion is a library of iterators, algorithms, containers and adaptors 4038 for manipulating heterogeneous sequences. In essence, a Proto expression 4039 is just a heterogeneous sequence of its child expressions, and so Proto 4040 expressions are valid Fusion random-access sequences. That means you can 4041 apply Fusion algorithms to them, transform them, apply Fusion filters and 4042 views to them, and access their elements using <code class="computeroutput"><span class="identifier">fusion</span><span class="special">::</span><span class="identifier">at</span><span class="special">()</span></code>. The things Fusion can do to heterogeneous 4043 sequences are beyond the scope of this users' guide, but below is a simple 4044 example. It takes a lazy function invocation like <code class="computeroutput"><span class="identifier">fun</span><span class="special">(</span><span class="number">1</span><span class="special">,</span><span class="number">2</span><span class="special">,</span><span class="number">3</span><span class="special">,</span><span class="number">4</span><span class="special">)</span></code> 4045 and uses Fusion to print the function arguments in order. 4046 </p> 4047<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">display</span> 4048<span class="special">{</span> 4049 <span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">></span> 4050 <span class="keyword">void</span> <span class="keyword">operator</span><span class="special">()(</span><span class="identifier">T</span> <span class="keyword">const</span> <span class="special">&</span><span class="identifier">t</span><span class="special">)</span> <span class="keyword">const</span> 4051 <span class="special">{</span> 4052 <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="identifier">t</span> <span class="special"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span> 4053 <span class="special">}</span> 4054<span class="special">};</span> 4055 4056<span class="keyword">struct</span> <span class="identifier">fun_t</span> <span class="special">{};</span> 4057<span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special"><</span><span class="identifier">fun_t</span><span class="special">>::</span><span class="identifier">type</span> <span class="keyword">const</span> <span class="identifier">fun</span> <span class="special">=</span> <span class="special">{{}};</span> 4058 4059<span class="comment">// ...</span> 4060<span class="identifier">fusion</span><span class="special">::</span><span class="identifier">for_each</span><span class="special">(</span> 4061 <span class="identifier">fusion</span><span class="special">::</span><span class="identifier">transform</span><span class="special">(</span> 4062 <span class="comment">// pop_front() removes the "fun" child</span> 4063 <span class="identifier">fusion</span><span class="special">::</span><span class="identifier">pop_front</span><span class="special">(</span><span class="identifier">fun</span><span class="special">(</span><span class="number">1</span><span class="special">,</span><span class="number">2</span><span class="special">,</span><span class="number">3</span><span class="special">,</span><span class="number">4</span><span class="special">))</span> 4064 <span class="comment">// Extract the ints from the terminal nodes</span> 4065 <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">functional</span><span class="special">::</span><span class="identifier">value</span><span class="special">()</span> 4066 <span class="special">)</span> 4067 <span class="special">,</span> <span class="identifier">display</span><span class="special">()</span> 4068<span class="special">);</span> 4069</pre> 4070<p> 4071 Recall from the Introduction that types in the <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">functional</span></code> 4072 namespace define function objects that correspond to Proto's free functions. 4073 So <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">functional</span><span class="special">::</span><span class="identifier">value</span><span class="special">()</span></code> 4074 creates a function object that is equivalent to the <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">value</span><span class="special">()</span></code> function. The above invocation of <code class="computeroutput"><span class="identifier">fusion</span><span class="special">::</span><span class="identifier">for_each</span><span class="special">()</span></code> 4075 displays the following: 4076 </p> 4077<pre class="programlisting">1 40782 40793 40804 4081</pre> 4082<p> 4083 Terminals are also valid Fusion sequences. They contain exactly one element: 4084 their value. 4085 </p> 4086<h6> 4087<a name="boost_proto.users_guide.intermediate_form.expressions_as_fusion_sequences.h0"></a> 4088 <span class="phrase"><a name="boost_proto.users_guide.intermediate_form.expressions_as_fusion_sequences.flattening_proto_expression_tress"></a></span><a class="link" href="users_guide.html#boost_proto.users_guide.intermediate_form.expressions_as_fusion_sequences.flattening_proto_expression_tress">Flattening 4089 Proto Expression Tress</a> 4090 </h6> 4091<p> 4092 Imagine a slight variation of the above example where, instead of iterating 4093 over the arguments of a lazy function invocation, we would like to iterate 4094 over the terminals in an addition expression: 4095 </p> 4096<pre class="programlisting"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special"><</span><span class="keyword">int</span><span class="special">>::</span><span class="identifier">type</span> <span class="keyword">const</span> <span class="identifier">_1</span> <span class="special">=</span> <span class="special">{</span><span class="number">1</span><span class="special">};</span> 4097 4098<span class="comment">// ERROR: this doesn't work! Why?</span> 4099<span class="identifier">fusion</span><span class="special">::</span><span class="identifier">for_each</span><span class="special">(</span> 4100 <span class="identifier">fusion</span><span class="special">::</span><span class="identifier">transform</span><span class="special">(</span> 4101 <span class="identifier">_1</span> <span class="special">+</span> <span class="number">2</span> <span class="special">+</span> <span class="number">3</span> <span class="special">+</span> <span class="number">4</span> 4102 <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">functional</span><span class="special">::</span><span class="identifier">value</span><span class="special">()</span> 4103 <span class="special">)</span> 4104 <span class="special">,</span> <span class="identifier">display</span><span class="special">()</span> 4105<span class="special">);</span> 4106</pre> 4107<p> 4108 The reason this doesn't work is because the expression <code class="computeroutput"><span class="identifier">_1</span> 4109 <span class="special">+</span> <span class="number">2</span> <span class="special">+</span> <span class="number">3</span> <span class="special">+</span> 4110 <span class="number">4</span></code> does not describe a flat sequence 4111 of terminals --- it describes a binary tree. We can treat it as a flat 4112 sequence of terminals, however, using Proto's <code class="computeroutput"><a class="link" href="../boost/proto/flatten.html" title="Function flatten">proto::flatten()</a></code> 4113 function. <code class="computeroutput"><a class="link" href="../boost/proto/flatten.html" title="Function flatten">proto::flatten()</a></code> returns a view which makes 4114 a tree appear as a flat Fusion sequence. If the top-most node has a tag 4115 type <code class="computeroutput"><span class="identifier">T</span></code>, then the elements 4116 of the flattened sequence are the child nodes that do <span class="emphasis"><em>not</em></span> 4117 have tag type <code class="computeroutput"><span class="identifier">T</span></code>. This process 4118 is evaluated recursively. So the above can correctly be written as: 4119 </p> 4120<pre class="programlisting"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special"><</span><span class="keyword">int</span><span class="special">>::</span><span class="identifier">type</span> <span class="keyword">const</span> <span class="identifier">_1</span> <span class="special">=</span> <span class="special">{</span><span class="number">1</span><span class="special">};</span> 4121 4122<span class="comment">// OK, iterate over a flattened view</span> 4123<span class="identifier">fusion</span><span class="special">::</span><span class="identifier">for_each</span><span class="special">(</span> 4124 <span class="identifier">fusion</span><span class="special">::</span><span class="identifier">transform</span><span class="special">(</span> 4125 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">flatten</span><span class="special">(</span><span class="identifier">_1</span> <span class="special">+</span> <span class="number">2</span> <span class="special">+</span> <span class="number">3</span> <span class="special">+</span> <span class="number">4</span><span class="special">)</span> 4126 <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">functional</span><span class="special">::</span><span class="identifier">value</span><span class="special">()</span> 4127 <span class="special">)</span> 4128 <span class="special">,</span> <span class="identifier">display</span><span class="special">()</span> 4129<span class="special">);</span> 4130</pre> 4131<p> 4132 The above invocation of <code class="computeroutput"><span class="identifier">fusion</span><span class="special">::</span><span class="identifier">for_each</span><span class="special">()</span></code> displays the following: 4133 </p> 4134<pre class="programlisting">1 41352 41363 41374 4138</pre> 4139</div> 4140<div class="section"> 4141<div class="titlepage"><div><div><h4 class="title"> 4142<a name="boost_proto.users_guide.intermediate_form.expression_introspection"></a><a class="link" href="users_guide.html#boost_proto.users_guide.intermediate_form.expression_introspection" title="Expression Introspection: Defining a Grammar">Expression 4143 Introspection: Defining a Grammar</a> 4144</h4></div></div></div> 4145<div class="toc"><dl class="toc"> 4146<dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.intermediate_form.expression_introspection.patterns">Finding 4147 Patterns in Expressions</a></span></dt> 4148<dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.intermediate_form.expression_introspection.fuzzy_and_exact_matches_of_terminals">Fuzzy 4149 and Exact Matches of Terminals</a></span></dt> 4150<dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.intermediate_form.expression_introspection.if_and_not"><code class="literal">if_<></code>, 4151 <code class="literal">and_<></code>, and <code class="literal">not_<></code></a></span></dt> 4152<dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.intermediate_form.expression_introspection.switch">Improving 4153 Compile Times With <code class="literal">switch_<></code></a></span></dt> 4154<dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.intermediate_form.expression_introspection.matching_vararg_expressions">Matching 4155 Vararg Expressions</a></span></dt> 4156<dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.intermediate_form.expression_introspection.defining_edsl_grammars">Defining 4157 EDSL Grammars</a></span></dt> 4158</dl></div> 4159<p> 4160 Expression trees can have a very rich and complicated structure. Often, 4161 you need to know some things about an expression's structure before you 4162 can process it. This section describes the tools Proto provides for peering 4163 inside an expression tree and discovering its structure. And as you'll 4164 see in later sections, all the really interesting things you can do with 4165 Proto begin right here. 4166 </p> 4167<div class="section"> 4168<div class="titlepage"><div><div><h5 class="title"> 4169<a name="boost_proto.users_guide.intermediate_form.expression_introspection.patterns"></a><a class="link" href="users_guide.html#boost_proto.users_guide.intermediate_form.expression_introspection.patterns" title="Finding Patterns in Expressions">Finding 4170 Patterns in Expressions</a> 4171</h5></div></div></div> 4172<p> 4173 Imagine your EDSL is a miniature I/O facility, with iostream operations 4174 that execute lazily. You might want expressions representing input operations 4175 to be processed by one function, and output operations to be processed 4176 by a different function. How would you do that? 4177 </p> 4178<p> 4179 The answer is to write patterns (a.k.a, <span class="emphasis"><em>grammars</em></span>) 4180 that match the structure of input and output expressions. Proto provides 4181 utilities for defining the grammars, and the <code class="computeroutput"><a class="link" href="../boost/proto/matches.html" title="Struct template matches">proto::matches<></a></code> 4182 template for checking whether a given expression type matches the grammar. 4183 </p> 4184<p> 4185 First, let's define some terminals we can use in our lazy I/O expressions: 4186 </p> 4187<pre class="programlisting"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special"><</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">istream</span> <span class="special">&</span> <span class="special">>::</span><span class="identifier">type</span> <span class="identifier">cin_</span> <span class="special">=</span> <span class="special">{</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">cin</span> <span class="special">};</span> 4188<span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special"><</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">ostream</span> <span class="special">&</span> <span class="special">>::</span><span class="identifier">type</span> <span class="identifier">cout_</span> <span class="special">=</span> <span class="special">{</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">};</span> 4189</pre> 4190<p> 4191 Now, we can use <code class="computeroutput"><span class="identifier">cout_</span></code> 4192 instead of <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span></code>, and get I/O expression trees 4193 that we can execute later. To define grammars that match input and output 4194 expressions of the form <code class="computeroutput"><span class="identifier">cin_</span> 4195 <span class="special">>></span> <span class="identifier">i</span></code> 4196 and <code class="computeroutput"><span class="identifier">cout_</span> <span class="special"><<</span> 4197 <span class="number">1</span></code> we do this: 4198 </p> 4199<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">Input</span> 4200 <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">shift_right</span><span class="special"><</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special"><</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">istream</span> <span class="special">&</span> <span class="special">>,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_</span> <span class="special">></span> 4201<span class="special">{};</span> 4202 4203<span class="keyword">struct</span> <span class="identifier">Output</span> 4204 <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">shift_left</span><span class="special"><</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special"><</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">ostream</span> <span class="special">&</span> <span class="special">>,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_</span> <span class="special">></span> 4205<span class="special">{};</span> 4206</pre> 4207<p> 4208 We've seen the template <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special"><></span></code> before, but here we're using 4209 it without accessing the nested <code class="computeroutput"><span class="special">::</span><span class="identifier">type</span></code>. When used like this, it is a 4210 very simple grammar, as are <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">shift_right</span><span class="special"><></span></code> and <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">shift_left</span><span class="special"><></span></code>. The newcomer here is <code class="computeroutput"><span class="identifier">_</span></code> in the <code class="computeroutput"><span class="identifier">proto</span></code> 4211 namespace. It is a wildcard that matches anything. The <code class="computeroutput"><span class="identifier">Input</span></code> struct is a grammar that matches 4212 any right-shift expression that has a <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">istream</span></code> 4213 terminal as its left operand. 4214 </p> 4215<p> 4216 We can use these grammars together with the <code class="computeroutput"><a class="link" href="../boost/proto/matches.html" title="Struct template matches">proto::matches<></a></code> 4217 template to query at compile time whether a given I/O expression type 4218 is an input or output operation. Consider the following: 4219 </p> 4220<pre class="programlisting"><span class="keyword">template</span><span class="special"><</span> <span class="keyword">typename</span> <span class="identifier">Expr</span> <span class="special">></span> 4221<span class="keyword">void</span> <span class="identifier">input_output</span><span class="special">(</span> <span class="identifier">Expr</span> <span class="keyword">const</span> <span class="special">&</span> <span class="identifier">expr</span> <span class="special">)</span> 4222<span class="special">{</span> 4223 <span class="keyword">if</span><span class="special">(</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">matches</span><span class="special"><</span> <span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">Input</span> <span class="special">>::</span><span class="identifier">value</span> <span class="special">)</span> 4224 <span class="special">{</span> 4225 <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="string">"Input!\n"</span><span class="special">;</span> 4226 <span class="special">}</span> 4227 4228 <span class="keyword">if</span><span class="special">(</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">matches</span><span class="special"><</span> <span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">Output</span> <span class="special">>::</span><span class="identifier">value</span> <span class="special">)</span> 4229 <span class="special">{</span> 4230 <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="string">"Output!\n"</span><span class="special">;</span> 4231 <span class="special">}</span> 4232<span class="special">}</span> 4233 4234<span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span> 4235<span class="special">{</span> 4236 <span class="keyword">int</span> <span class="identifier">i</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</span> 4237 <span class="identifier">input_output</span><span class="special">(</span> <span class="identifier">cout_</span> <span class="special"><<</span> <span class="number">1</span> <span class="special">);</span> 4238 <span class="identifier">input_output</span><span class="special">(</span> <span class="identifier">cin_</span> <span class="special">>></span> <span class="identifier">i</span> <span class="special">);</span> 4239 4240 <span class="keyword">return</span> <span class="number">0</span><span class="special">;</span> 4241<span class="special">}</span> 4242</pre> 4243<p> 4244 This program prints the following: 4245 </p> 4246<pre class="programlisting">Output! 4247Input! 4248</pre> 4249<p> 4250 If we wanted to break the <code class="computeroutput"><span class="identifier">input_output</span><span class="special">()</span></code> function into two functions, one that 4251 handles input expressions and one for output expressions, we can use 4252 <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">enable_if</span><span class="special"><></span></code>, 4253 as follows: 4254 </p> 4255<pre class="programlisting"><span class="keyword">template</span><span class="special"><</span> <span class="keyword">typename</span> <span class="identifier">Expr</span> <span class="special">></span> 4256<span class="keyword">typename</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">enable_if</span><span class="special"><</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">matches</span><span class="special"><</span> <span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">Input</span> <span class="special">></span> <span class="special">>::</span><span class="identifier">type</span> 4257<span class="identifier">input_output</span><span class="special">(</span> <span class="identifier">Expr</span> <span class="keyword">const</span> <span class="special">&</span> <span class="identifier">expr</span> <span class="special">)</span> 4258<span class="special">{</span> 4259 <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="string">"Input!\n"</span><span class="special">;</span> 4260<span class="special">}</span> 4261 4262<span class="keyword">template</span><span class="special"><</span> <span class="keyword">typename</span> <span class="identifier">Expr</span> <span class="special">></span> 4263<span class="keyword">typename</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">enable_if</span><span class="special"><</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">matches</span><span class="special"><</span> <span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">Output</span> <span class="special">></span> <span class="special">>::</span><span class="identifier">type</span> 4264<span class="identifier">input_output</span><span class="special">(</span> <span class="identifier">Expr</span> <span class="keyword">const</span> <span class="special">&</span> <span class="identifier">expr</span> <span class="special">)</span> 4265<span class="special">{</span> 4266 <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="string">"Output!\n"</span><span class="special">;</span> 4267<span class="special">}</span> 4268</pre> 4269<p> 4270 This works as the previous version did. However, the following does not 4271 compile at all: 4272 </p> 4273<pre class="programlisting"><span class="identifier">input_output</span><span class="special">(</span> <span class="identifier">cout_</span> <span class="special"><<</span> <span class="number">1</span> <span class="special"><<</span> <span class="number">2</span> <span class="special">);</span> <span class="comment">// oops!</span> 4274</pre> 4275<p> 4276 What's wrong? The problem is that this expression does not match our 4277 grammar. The expression groups as if it were written like <code class="computeroutput"><span class="special">(</span><span class="identifier">cout_</span> <span class="special"><<</span> <span class="number">1</span><span class="special">)</span> <span class="special"><<</span> <span class="number">2</span></code>. It will not match the <code class="computeroutput"><span class="identifier">Output</span></code> grammar, which expects the left 4278 operand to be a terminal, not another left-shift operation. We need to 4279 fix the grammar. 4280 </p> 4281<p> 4282 We notice that in order to verify an expression as input or output, we'll 4283 need to recurse down to the bottom-left-most leaf and check that it is 4284 a <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">istream</span></code> or <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">ostream</span></code>. 4285 When we get to the terminal, we must stop recursing. We can express this 4286 in our grammar using <code class="computeroutput"><a class="link" href="../boost/proto/or_.html" title="Struct template or_">proto::or_<></a></code>. 4287 Here are the correct <code class="computeroutput"><span class="identifier">Input</span></code> 4288 and <code class="computeroutput"><span class="identifier">Output</span></code> grammars: 4289 </p> 4290<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">Input</span> 4291 <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">or_</span><span class="special"><</span> 4292 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">shift_right</span><span class="special"><</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special"><</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">istream</span> <span class="special">&</span> <span class="special">>,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_</span> <span class="special">></span> 4293 <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">shift_right</span><span class="special"><</span> <span class="identifier">Input</span><span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_</span> <span class="special">></span> 4294 <span class="special">></span> 4295<span class="special">{};</span> 4296 4297<span class="keyword">struct</span> <span class="identifier">Output</span> 4298 <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">or_</span><span class="special"><</span> 4299 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">shift_left</span><span class="special"><</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special"><</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">ostream</span> <span class="special">&</span> <span class="special">>,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_</span> <span class="special">></span> 4300 <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">shift_left</span><span class="special"><</span> <span class="identifier">Output</span><span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_</span> <span class="special">></span> 4301 <span class="special">></span> 4302<span class="special">{};</span> 4303</pre> 4304<p> 4305 This may look a little odd at first. We seem to be defining the <code class="computeroutput"><span class="identifier">Input</span></code> and <code class="computeroutput"><span class="identifier">Output</span></code> 4306 types in terms of themselves. This is perfectly OK, actually. At the 4307 point in the grammar that the <code class="computeroutput"><span class="identifier">Input</span></code> 4308 and <code class="computeroutput"><span class="identifier">Output</span></code> types are 4309 being used, they are <span class="emphasis"><em>incomplete</em></span>, but by the time 4310 we actually evaluate the grammar with <code class="computeroutput"><a class="link" href="../boost/proto/matches.html" title="Struct template matches">proto::matches<></a></code>, 4311 the types will be complete. These are recursive grammars, and rightly 4312 so because they must match a recursive data structure! 4313 </p> 4314<p> 4315 Matching an expression such as <code class="computeroutput"><span class="identifier">cout_</span> 4316 <span class="special"><<</span> <span class="number">1</span> 4317 <span class="special"><<</span> <span class="number">2</span></code> 4318 against the <code class="computeroutput"><span class="identifier">Output</span></code> grammar 4319 procedes as follows: 4320 </p> 4321<div class="orderedlist"><ol class="orderedlist" type="1"> 4322<li class="listitem"> 4323 The first alternate of the <code class="computeroutput"><a class="link" href="../boost/proto/or_.html" title="Struct template or_">proto::or_<></a></code> 4324 is tried first. It will fail, because the expression <code class="computeroutput"><span class="identifier">cout_</span> <span class="special"><<</span> 4325 <span class="number">1</span> <span class="special"><<</span> 4326 <span class="number">2</span></code> does not match the grammar 4327 <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">shift_left</span><span class="special"><</span> 4328 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special"><</span> 4329 <span class="identifier">std</span><span class="special">::</span><span class="identifier">ostream</span> <span class="special">&</span> 4330 <span class="special">>,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_</span> <span class="special">></span></code>. 4331 </li> 4332<li class="listitem"> 4333 Then the second alternate is tried next. We match the expression 4334 against <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">shift_left</span><span class="special"><</span> 4335 <span class="identifier">Output</span><span class="special">,</span> 4336 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_</span> <span class="special">></span></code>. 4337 The expression is a left-shift, so we next try to match the operands. 4338 </li> 4339<li class="listitem"> 4340 The right operand <code class="computeroutput"><span class="number">2</span></code> matches 4341 <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_</span></code> trivially. 4342 </li> 4343<li class="listitem"> 4344 To see if the left operand <code class="computeroutput"><span class="identifier">cout_</span> 4345 <span class="special"><<</span> <span class="number">1</span></code> 4346 matches <code class="computeroutput"><span class="identifier">Output</span></code>, we 4347 must recursively evaluate the <code class="computeroutput"><span class="identifier">Output</span></code> 4348 grammar. This time we succeed, because <code class="computeroutput"><span class="identifier">cout_</span> 4349 <span class="special"><<</span> <span class="number">1</span></code> 4350 will match the first alternate of the <code class="computeroutput"><a class="link" href="../boost/proto/or_.html" title="Struct template or_">proto::or_<></a></code>. 4351 </li> 4352</ol></div> 4353<p> 4354 We're done -- the grammar matches successfully. 4355 </p> 4356</div> 4357<div class="section"> 4358<div class="titlepage"><div><div><h5 class="title"> 4359<a name="boost_proto.users_guide.intermediate_form.expression_introspection.fuzzy_and_exact_matches_of_terminals"></a><a class="link" href="users_guide.html#boost_proto.users_guide.intermediate_form.expression_introspection.fuzzy_and_exact_matches_of_terminals" title="Fuzzy and Exact Matches of Terminals">Fuzzy 4360 and Exact Matches of Terminals</a> 4361</h5></div></div></div> 4362<p> 4363 The terminals in an expression tree could be const or non-const references, 4364 or they might not be references at all. When writing grammars, you usually 4365 don't have to worry about it because <code class="computeroutput"><a class="link" href="../boost/proto/matches.html" title="Struct template matches">proto::matches<></a></code> 4366 gives you a little wiggle room when matching terminals. A grammar such 4367 as <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span></code> 4368 will match a terminal of type <code class="computeroutput"><span class="keyword">int</span></code>, 4369 <code class="computeroutput"><span class="keyword">int</span> <span class="special">&</span></code>, 4370 or <code class="computeroutput"><span class="keyword">int</span> <span class="keyword">const</span> 4371 <span class="special">&</span></code>. 4372 </p> 4373<p> 4374 You can explicitly specify that you want to match a reference type. If 4375 you do, the type must match exactly. For instance, a grammar such as 4376 <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special"><</span><span class="keyword">int</span> <span class="special">&></span></code> 4377 will only match an <code class="computeroutput"><span class="keyword">int</span> <span class="special">&</span></code>. It will not match an <code class="computeroutput"><span class="keyword">int</span></code> or an <code class="computeroutput"><span class="keyword">int</span> 4378 <span class="keyword">const</span> <span class="special">&</span></code>. 4379 </p> 4380<p> 4381 The table below shows how Proto matches terminals. The simple rule is: 4382 if you want to match only reference types, you must specify the reference 4383 in your grammar. Otherwise, leave it off and Proto will ignore const 4384 and references. 4385 </p> 4386<div class="table"> 4387<a name="boost_proto.users_guide.intermediate_form.expression_introspection.fuzzy_and_exact_matches_of_terminals.t0"></a><p class="title"><b>Table 32.7. proto::matches<> and Reference / CV-Qualification of Terminals</b></p> 4388<div class="table-contents"><table class="table" summary="proto::matches<> and Reference / CV-Qualification of Terminals"> 4389<colgroup> 4390<col> 4391<col> 4392<col> 4393</colgroup> 4394<thead><tr> 4395<th> 4396 <p> 4397 Terminal 4398 </p> 4399 </th> 4400<th> 4401 <p> 4402 Grammar 4403 </p> 4404 </th> 4405<th> 4406 <p> 4407 Matches? 4408 </p> 4409 </th> 4410</tr></thead> 4411<tbody> 4412<tr> 4413<td> 4414 <p> 4415 T 4416 </p> 4417 </td> 4418<td> 4419 <p> 4420 T 4421 </p> 4422 </td> 4423<td> 4424 <p> 4425 yes 4426 </p> 4427 </td> 4428</tr> 4429<tr> 4430<td> 4431 <p> 4432 T & 4433 </p> 4434 </td> 4435<td> 4436 <p> 4437 T 4438 </p> 4439 </td> 4440<td> 4441 <p> 4442 yes 4443 </p> 4444 </td> 4445</tr> 4446<tr> 4447<td> 4448 <p> 4449 T const & 4450 </p> 4451 </td> 4452<td> 4453 <p> 4454 T 4455 </p> 4456 </td> 4457<td> 4458 <p> 4459 yes 4460 </p> 4461 </td> 4462</tr> 4463<tr> 4464<td> 4465 <p> 4466 T 4467 </p> 4468 </td> 4469<td> 4470 <p> 4471 T & 4472 </p> 4473 </td> 4474<td> 4475 <p> 4476 no 4477 </p> 4478 </td> 4479</tr> 4480<tr> 4481<td> 4482 <p> 4483 T & 4484 </p> 4485 </td> 4486<td> 4487 <p> 4488 T & 4489 </p> 4490 </td> 4491<td> 4492 <p> 4493 yes 4494 </p> 4495 </td> 4496</tr> 4497<tr> 4498<td> 4499 <p> 4500 T const & 4501 </p> 4502 </td> 4503<td> 4504 <p> 4505 T & 4506 </p> 4507 </td> 4508<td> 4509 <p> 4510 no 4511 </p> 4512 </td> 4513</tr> 4514<tr> 4515<td> 4516 <p> 4517 T 4518 </p> 4519 </td> 4520<td> 4521 <p> 4522 T const & 4523 </p> 4524 </td> 4525<td> 4526 <p> 4527 no 4528 </p> 4529 </td> 4530</tr> 4531<tr> 4532<td> 4533 <p> 4534 T & 4535 </p> 4536 </td> 4537<td> 4538 <p> 4539 T const & 4540 </p> 4541 </td> 4542<td> 4543 <p> 4544 no 4545 </p> 4546 </td> 4547</tr> 4548<tr> 4549<td> 4550 <p> 4551 T const & 4552 </p> 4553 </td> 4554<td> 4555 <p> 4556 T const & 4557 </p> 4558 </td> 4559<td> 4560 <p> 4561 yes 4562 </p> 4563 </td> 4564</tr> 4565</tbody> 4566</table></div> 4567</div> 4568<br class="table-break"><p> 4569 This begs the question: What if you want to match an <code class="computeroutput"><span class="keyword">int</span></code>, 4570 but not an <code class="computeroutput"><span class="keyword">int</span> <span class="special">&</span></code> 4571 or an <code class="computeroutput"><span class="keyword">int</span> <span class="keyword">const</span> 4572 <span class="special">&</span></code>? For forcing exact matches, 4573 Proto provides the <code class="computeroutput"><a class="link" href="../boost/proto/exact.html" title="Struct template exact">proto::exact<></a></code> 4574 template. For instance, <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special"><</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">exact</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> <span class="special">></span></code> 4575 would only match an <code class="computeroutput"><span class="keyword">int</span></code> 4576 held by value. 4577 </p> 4578<p> 4579 Proto gives you extra wiggle room when matching array types. Array types 4580 match themselves or the pointer types they decay to. This is especially 4581 useful with character arrays. The type returned by <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">as_expr</span><span class="special">(</span><span class="string">"hello"</span><span class="special">)</span></code> is <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special"><</span><span class="keyword">char</span> <span class="keyword">const</span><span class="special">[</span><span class="number">6</span><span class="special">]>::</span><span class="identifier">type</span></code>. That's a terminal containing 4582 a 6-element character array. Naturally, you can match this terminal with 4583 the grammar <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special"><</span><span class="keyword">char</span> <span class="keyword">const</span><span class="special">[</span><span class="number">6</span><span class="special">]></span></code>, 4584 but the grammar <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special"><</span><span class="keyword">char</span> <span class="keyword">const</span> <span class="special">*></span></code> 4585 will match it as well, as the following code fragment illustrates. 4586 </p> 4587<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">CharString</span> 4588 <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special"><</span> <span class="keyword">char</span> <span class="keyword">const</span> <span class="special">*</span> <span class="special">></span> 4589<span class="special">{};</span> 4590 4591<span class="keyword">typedef</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special"><</span> <span class="keyword">char</span> <span class="keyword">const</span><span class="special">[</span><span class="number">6</span><span class="special">]</span> <span class="special">>::</span><span class="identifier">type</span> <span class="identifier">char_array</span><span class="special">;</span> 4592 4593<span class="identifier">BOOST_MPL_ASSERT</span><span class="special">((</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">matches</span><span class="special"><</span> <span class="identifier">char_array</span><span class="special">,</span> <span class="identifier">CharString</span> <span class="special">></span> <span class="special">));</span> 4594</pre> 4595<p> 4596 What if we only wanted <code class="computeroutput"><span class="identifier">CharString</span></code> 4597 to match terminals of exactly the type <code class="computeroutput"><span class="keyword">char</span> 4598 <span class="keyword">const</span> <span class="special">*</span></code>? 4599 You can use <code class="computeroutput"><a class="link" href="../boost/proto/exact.html" title="Struct template exact">proto::exact<></a></code> here to turn off 4600 the fuzzy matching of terminals, as follows: 4601 </p> 4602<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">CharString</span> 4603 <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special"><</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">exact</span><span class="special"><</span> <span class="keyword">char</span> <span class="keyword">const</span> <span class="special">*</span> <span class="special">></span> <span class="special">></span> 4604<span class="special">{};</span> 4605 4606<span class="keyword">typedef</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special"><</span><span class="keyword">char</span> <span class="keyword">const</span><span class="special">[</span><span class="number">6</span><span class="special">]>::</span><span class="identifier">type</span> <span class="identifier">char_array</span><span class="special">;</span> 4607<span class="keyword">typedef</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special"><</span><span class="keyword">char</span> <span class="keyword">const</span> <span class="special">*>::</span><span class="identifier">type</span> <span class="identifier">char_string</span><span class="special">;</span> 4608 4609<span class="identifier">BOOST_MPL_ASSERT</span><span class="special">((</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">matches</span><span class="special"><</span> <span class="identifier">char_string</span><span class="special">,</span> <span class="identifier">CharString</span> <span class="special">></span> <span class="special">));</span> 4610<span class="identifier">BOOST_MPL_ASSERT_NOT</span><span class="special">((</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">matches</span><span class="special"><</span> <span class="identifier">char_array</span><span class="special">,</span> <span class="identifier">CharString</span> <span class="special">></span> <span class="special">));</span> 4611</pre> 4612<p> 4613 Now, <code class="computeroutput"><span class="identifier">CharString</span></code> does 4614 not match array types, only character string pointers. 4615 </p> 4616<p> 4617 The inverse problem is a little trickier: what if you wanted to match 4618 all character arrays, but not character pointers? As mentioned above, 4619 the expression <code class="computeroutput"><span class="identifier">as_expr</span><span class="special">(</span><span class="string">"hello"</span><span class="special">)</span></code> has the type <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special"><</span> <span class="keyword">char</span> <span class="keyword">const</span><span class="special">[</span> <span class="number">6</span> <span class="special">]</span> <span class="special">>::</span><span class="identifier">type</span></code>. If you wanted to match character 4620 arrays of arbitrary size, you could use <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">N</span></code>, 4621 which is an array-size wildcard. The following grammar would match any 4622 string literal: <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special"><</span> <span class="keyword">char</span> <span class="keyword">const</span><span class="special">[</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">N</span> <span class="special">]</span> <span class="special">></span></code>. 4623 </p> 4624<p> 4625 Sometimes you need even more wiggle room when matching terminals. For 4626 example, maybe you're building a calculator EDSL and you want to allow 4627 any terminals that are convertible to <code class="computeroutput"><span class="keyword">double</span></code>. 4628 For that, Proto provides the <code class="computeroutput"><a class="link" href="../boost/proto/convertible_to.html" title="Struct template convertible_to">proto::convertible_to<></a></code> 4629 template. You can use it as: <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special"><</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">convertible_to</span><span class="special"><</span> <span class="keyword">double</span> 4630 <span class="special">></span> <span class="special">></span></code>. 4631 </p> 4632<p> 4633 There is one more way you can perform a fuzzy match on terminals. Consider 4634 the problem of trying to match a <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">complex</span><span class="special"><></span></code> terminal. You can easily match 4635 a <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">complex</span><span class="special"><</span><span class="keyword">float</span><span class="special">></span></code> 4636 or a <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">complex</span><span class="special"><</span><span class="keyword">double</span><span class="special">></span></code>, 4637 but how would you match any instantiation of <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">complex</span><span class="special"><></span></code>? You can use <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_</span></code> 4638 here to solve this problem. Here is the grammar to match any <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">complex</span><span class="special"><></span></code> 4639 instantiation: 4640 </p> 4641<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">StdComplex</span> 4642 <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special"><</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">complex</span><span class="special"><</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_</span> <span class="special">></span> <span class="special">></span> 4643<span class="special">{};</span> 4644</pre> 4645<p> 4646 When given a grammar like this, Proto will deconstruct the grammar and 4647 the terminal it is being matched against and see if it can match all 4648 the constituents. 4649 </p> 4650</div> 4651<div class="section"> 4652<div class="titlepage"><div><div><h5 class="title"> 4653<a name="boost_proto.users_guide.intermediate_form.expression_introspection.if_and_not"></a><a class="link" href="users_guide.html#boost_proto.users_guide.intermediate_form.expression_introspection.if_and_not" title="if_<>, and_<>, and not_<>"><code class="literal">if_<></code>, 4654 <code class="literal">and_<></code>, and <code class="literal">not_<></code></a> 4655</h5></div></div></div> 4656<p> 4657 We've already seen how to use expression generators like <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special"><></span></code> 4658 and <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">shift_right</span><span class="special"><></span></code> 4659 as grammars. We've also seen <code class="computeroutput"><a class="link" href="../boost/proto/or_.html" title="Struct template or_">proto::or_<></a></code>, 4660 which we can use to express a set of alternate grammars. There are a 4661 few others of interest; in particular, <code class="computeroutput"><a class="link" href="../boost/proto/if_.html" title="Struct template if_">proto::if_<></a></code>, 4662 <code class="computeroutput"><a class="link" href="../boost/proto/and_.html" title="Struct template and_">proto::and_<></a></code> and <code class="computeroutput"><a class="link" href="../boost/proto/not_.html" title="Struct template not_">proto::not_<></a></code>. 4663 </p> 4664<p> 4665 The <code class="computeroutput"><a class="link" href="../boost/proto/not_.html" title="Struct template not_">proto::not_<></a></code> template is the simplest. 4666 It takes a grammar as a template parameter and logically negates it; 4667 <code class="computeroutput"><span class="identifier">not_</span><span class="special"><</span><span class="identifier">Grammar</span><span class="special">></span></code> 4668 will match any expression that <code class="computeroutput"><span class="identifier">Grammar</span></code> 4669 does <span class="emphasis"><em>not</em></span> match. 4670 </p> 4671<p> 4672 The <code class="computeroutput"><a class="link" href="../boost/proto/if_.html" title="Struct template if_">proto::if_<></a></code> template is used 4673 together with a Proto transform that is evaluated against expression 4674 types to find matches. (Proto transforms will be described later.) 4675 </p> 4676<p> 4677 The <code class="computeroutput"><a class="link" href="../boost/proto/and_.html" title="Struct template and_">proto::and_<></a></code> template is like 4678 <code class="computeroutput"><a class="link" href="../boost/proto/or_.html" title="Struct template or_">proto::or_<></a></code>, except that each 4679 argument of the <code class="computeroutput"><a class="link" href="../boost/proto/and_.html" title="Struct template and_">proto::and_<></a></code> must match in order 4680 for the <code class="computeroutput"><a class="link" href="../boost/proto/and_.html" title="Struct template and_">proto::and_<></a></code> to match. As an example, 4681 consider the definition of <code class="computeroutput"><span class="identifier">CharString</span></code> 4682 above that uses <code class="computeroutput"><a class="link" href="../boost/proto/exact.html" title="Struct template exact">proto::exact<></a></code>. It could have been 4683 written without <code class="computeroutput"><a class="link" href="../boost/proto/exact.html" title="Struct template exact">proto::exact<></a></code> as follows: 4684 </p> 4685<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">CharString</span> 4686 <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">and_</span><span class="special"><</span> 4687 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special"><</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_</span> <span class="special">></span> 4688 <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">if_</span><span class="special"><</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">is_same</span><span class="special"><</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_value</span><span class="special">,</span> <span class="keyword">char</span> <span class="keyword">const</span> <span class="special">*</span> <span class="special">>()</span> <span class="special">></span> 4689 <span class="special">></span> 4690<span class="special">{};</span> 4691</pre> 4692<p> 4693 This says that a <code class="computeroutput"><span class="identifier">CharString</span></code> 4694 must be a terminal, <span class="emphasis"><em>and</em></span> its value type must be the 4695 same as <code class="computeroutput"><span class="keyword">char</span> <span class="keyword">const</span> 4696 <span class="special">*</span></code>. Notice the template argument 4697 of <code class="computeroutput"><a class="link" href="../boost/proto/if_.html" title="Struct template if_">proto::if_<></a></code>: <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">is_same</span><span class="special"><</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_value</span><span class="special">,</span> <span class="keyword">char</span> <span class="keyword">const</span> <span class="special">*</span> <span class="special">>()</span></code>. This is Proto transform that compares 4698 the value type of a terminal to <code class="computeroutput"><span class="keyword">char</span> 4699 <span class="keyword">const</span> <span class="special">*</span></code>. 4700 </p> 4701<p> 4702 The <code class="computeroutput"><a class="link" href="../boost/proto/if_.html" title="Struct template if_">proto::if_<></a></code> template has a couple 4703 of variants. In addition to <code class="computeroutput"><span class="identifier">if_</span><span class="special"><</span><span class="identifier">Condition</span><span class="special">></span></code> you can also say <code class="computeroutput"><span class="identifier">if_</span><span class="special"><</span><span class="identifier">Condition</span><span class="special">,</span> <span class="identifier">ThenGrammar</span><span class="special">></span></code> and <code class="computeroutput"><span class="identifier">if_</span><span class="special"><</span><span class="identifier">Condition</span><span class="special">,</span> <span class="identifier">ThenGrammar</span><span class="special">,</span> <span class="identifier">ElseGrammar</span><span class="special">></span></code>. These let you select one sub-grammar 4704 or another based on the <code class="computeroutput"><span class="identifier">Condition</span></code>. 4705 </p> 4706</div> 4707<div class="section"> 4708<div class="titlepage"><div><div><h5 class="title"> 4709<a name="boost_proto.users_guide.intermediate_form.expression_introspection.switch"></a><a class="link" href="users_guide.html#boost_proto.users_guide.intermediate_form.expression_introspection.switch" title="Improving Compile Times With switch_<>">Improving 4710 Compile Times With <code class="literal">switch_<></code></a> 4711</h5></div></div></div> 4712<p> 4713 When your Proto grammar gets large, you'll start to run into some scalability 4714 problems with <code class="computeroutput"><a class="link" href="../boost/proto/or_.html" title="Struct template or_">proto::or_<></a></code>, the construct you 4715 use to specify alternate sub-grammars. First, due to limitations in C++, 4716 <code class="computeroutput"><a class="link" href="../boost/proto/or_.html" title="Struct template or_">proto::or_<></a></code> can only accept up 4717 to a certain number of sub-grammars, controlled by the <code class="computeroutput"><span class="identifier">BOOST_PROTO_MAX_LOGICAL_ARITY</span></code> macro. 4718 This macro defaults to eight, and you can set it higher, but doing so 4719 will aggravate another scalability problem: long compile times. With 4720 <code class="computeroutput"><a class="link" href="../boost/proto/or_.html" title="Struct template or_">proto::or_<></a></code>, alternate sub-grammars 4721 are tried in order -- like a series of cascading <code class="computeroutput"><span class="keyword">if</span></code>'s 4722 -- leading to lots of unnecessary template instantiations. What you would 4723 prefer instead is something like <code class="computeroutput"><span class="keyword">switch</span></code> 4724 that avoids the expense of cascading <code class="computeroutput"><span class="keyword">if</span></code>'s. 4725 That's the purpose of <code class="computeroutput"><a class="link" href="../boost/proto/switch_.html" title="Struct template switch_">proto::switch_<></a></code>; 4726 although less convenient than <code class="computeroutput"><a class="link" href="../boost/proto/or_.html" title="Struct template or_">proto::or_<></a></code>, 4727 it improves compile times for larger grammars and does not have an arbitrary 4728 fixed limit on the number of sub-grammars. 4729 </p> 4730<p> 4731 Let's illustrate how to use <code class="computeroutput"><a class="link" href="../boost/proto/switch_.html" title="Struct template switch_">proto::switch_<></a></code> 4732 by first writing a big grammar with <code class="computeroutput"><a class="link" href="../boost/proto/or_.html" title="Struct template or_">proto::or_<></a></code> 4733 and then translating it to an equivalent grammar using <code class="computeroutput"><a class="link" href="../boost/proto/switch_.html" title="Struct template switch_">proto::switch_<></a></code>: 4734 </p> 4735<pre class="programlisting"><span class="comment">// Here is a big, inefficient grammar</span> 4736<span class="keyword">struct</span> <span class="identifier">ABigGrammar</span> 4737 <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">or_</span><span class="special"><</span> 4738 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> 4739 <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special"><</span><span class="keyword">double</span><span class="special">></span> 4740 <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">unary_plus</span><span class="special"><</span><span class="identifier">ABigGrammar</span><span class="special">></span> 4741 <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">negate</span><span class="special"><</span><span class="identifier">ABigGrammar</span><span class="special">></span> 4742 <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">complement</span><span class="special"><</span><span class="identifier">ABigGrammar</span><span class="special">></span> 4743 <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">plus</span><span class="special"><</span><span class="identifier">ABigGrammar</span><span class="special">,</span> <span class="identifier">ABigGrammar</span><span class="special">></span> 4744 <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">minus</span><span class="special"><</span><span class="identifier">ABigGrammar</span><span class="special">,</span> <span class="identifier">ABigGrammar</span><span class="special">></span> 4745 <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">or_</span><span class="special"><</span> 4746 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">multiplies</span><span class="special"><</span><span class="identifier">ABigGrammar</span><span class="special">,</span> <span class="identifier">ABigGrammar</span><span class="special">></span> 4747 <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">divides</span><span class="special"><</span><span class="identifier">ABigGrammar</span><span class="special">,</span> <span class="identifier">ABigGrammar</span><span class="special">></span> 4748 <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">modulus</span><span class="special"><</span><span class="identifier">ABigGrammar</span><span class="special">,</span> <span class="identifier">ABigGrammar</span><span class="special">></span> 4749 <span class="special">></span> 4750 <span class="special">></span> 4751<span class="special">{};</span> 4752</pre> 4753<p> 4754 The above might be the grammar to a more elaborate calculator EDSL. Notice 4755 that since there are more than eight sub-grammars, we had to chain the 4756 sub-grammars with a nested <code class="computeroutput"><a class="link" href="../boost/proto/or_.html" title="Struct template or_">proto::or_<></a></code> 4757 -- not very nice. 4758 </p> 4759<p> 4760 The idea behind <code class="computeroutput"><a class="link" href="../boost/proto/switch_.html" title="Struct template switch_">proto::switch_<></a></code> 4761 is to dispatch based on an expression's tag type to a sub-grammar that 4762 handles expressions of that type. To use <code class="computeroutput"><a class="link" href="../boost/proto/switch_.html" title="Struct template switch_">proto::switch_<></a></code>, 4763 you define a struct with a nested <code class="computeroutput"><span class="identifier">case_</span><span class="special"><></span></code> template, specialized on tag 4764 types. The above grammar can be expressed using <code class="computeroutput"><a class="link" href="../boost/proto/switch_.html" title="Struct template switch_">proto::switch_<></a></code> 4765 as follows. It is described below. 4766 </p> 4767<pre class="programlisting"><span class="comment">// Redefine ABigGrammar more efficiently using proto::switch_<></span> 4768<span class="keyword">struct</span> <span class="identifier">ABigGrammar</span><span class="special">;</span> 4769 4770<span class="keyword">struct</span> <span class="identifier">ABigGrammarCases</span> 4771<span class="special">{</span> 4772 <span class="comment">// The primary template matches nothing:</span> 4773 <span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">Tag</span><span class="special">></span> 4774 <span class="keyword">struct</span> <span class="identifier">case_</span> 4775 <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">not_</span><span class="special"><</span><span class="identifier">_</span><span class="special">></span> 4776 <span class="special">{};</span> 4777<span class="special">};</span> 4778 4779<span class="comment">// Terminal expressions are handled here</span> 4780<span class="keyword">template</span><span class="special"><></span> 4781<span class="keyword">struct</span> <span class="identifier">ABigGrammarCases</span><span class="special">::</span><span class="identifier">case_</span><span class="special"><</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">></span> 4782 <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">or_</span><span class="special"><</span> 4783 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> 4784 <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special"><</span><span class="keyword">double</span><span class="special">></span> 4785 <span class="special">></span> 4786<span class="special">{};</span> 4787 4788<span class="comment">// Non-terminals are handled similarly</span> 4789<span class="keyword">template</span><span class="special"><></span> 4790<span class="keyword">struct</span> <span class="identifier">ABigGrammarCases</span><span class="special">::</span><span class="identifier">case_</span><span class="special"><</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">unary_plus</span><span class="special">></span> 4791 <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">unary_plus</span><span class="special"><</span><span class="identifier">ABigGrammar</span><span class="special">></span> 4792<span class="special">{};</span> 4793 4794<span class="keyword">template</span><span class="special"><></span> 4795<span class="keyword">struct</span> <span class="identifier">ABigGrammarCases</span><span class="special">::</span><span class="identifier">case_</span><span class="special"><</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">negate</span><span class="special">></span> 4796 <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">negate</span><span class="special"><</span><span class="identifier">ABigGrammar</span><span class="special">></span> 4797<span class="special">{};</span> 4798 4799<span class="keyword">template</span><span class="special"><></span> 4800<span class="keyword">struct</span> <span class="identifier">ABigGrammarCases</span><span class="special">::</span><span class="identifier">case_</span><span class="special"><</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">complement</span><span class="special">></span> 4801 <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">complement</span><span class="special"><</span><span class="identifier">ABigGrammar</span><span class="special">></span> 4802<span class="special">{};</span> 4803 4804<span class="keyword">template</span><span class="special"><></span> 4805<span class="keyword">struct</span> <span class="identifier">ABigGrammarCases</span><span class="special">::</span><span class="identifier">case_</span><span class="special"><</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">plus</span><span class="special">></span> 4806 <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">plus</span><span class="special"><</span><span class="identifier">ABigGrammar</span><span class="special">,</span> <span class="identifier">ABigGrammar</span><span class="special">></span> 4807<span class="special">{};</span> 4808 4809<span class="keyword">template</span><span class="special"><></span> 4810<span class="keyword">struct</span> <span class="identifier">ABigGrammarCases</span><span class="special">::</span><span class="identifier">case_</span><span class="special"><</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">minus</span><span class="special">></span> 4811 <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">minus</span><span class="special"><</span><span class="identifier">ABigGrammar</span><span class="special">,</span> <span class="identifier">ABigGrammar</span><span class="special">></span> 4812<span class="special">{};</span> 4813 4814<span class="keyword">template</span><span class="special"><></span> 4815<span class="keyword">struct</span> <span class="identifier">ABigGrammarCases</span><span class="special">::</span><span class="identifier">case_</span><span class="special"><</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">multiplies</span><span class="special">></span> 4816 <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">multiplies</span><span class="special"><</span><span class="identifier">ABigGrammar</span><span class="special">,</span> <span class="identifier">ABigGrammar</span><span class="special">></span> 4817<span class="special">{};</span> 4818 4819<span class="keyword">template</span><span class="special"><></span> 4820<span class="keyword">struct</span> <span class="identifier">ABigGrammarCases</span><span class="special">::</span><span class="identifier">case_</span><span class="special"><</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">divides</span><span class="special">></span> 4821 <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">divides</span><span class="special"><</span><span class="identifier">ABigGrammar</span><span class="special">,</span> <span class="identifier">ABigGrammar</span><span class="special">></span> 4822<span class="special">{};</span> 4823 4824<span class="keyword">template</span><span class="special"><></span> 4825<span class="keyword">struct</span> <span class="identifier">ABigGrammarCases</span><span class="special">::</span><span class="identifier">case_</span><span class="special"><</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">modulus</span><span class="special">></span> 4826 <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">modulus</span><span class="special"><</span><span class="identifier">ABigGrammar</span><span class="special">,</span> <span class="identifier">ABigGrammar</span><span class="special">></span> 4827<span class="special">{};</span> 4828 4829<span class="comment">// Define ABigGrammar in terms of ABigGrammarCases</span> 4830<span class="comment">// using proto::switch_<></span> 4831<span class="keyword">struct</span> <span class="identifier">ABigGrammar</span> 4832 <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">switch_</span><span class="special"><</span><span class="identifier">ABigGrammarCases</span><span class="special">></span> 4833<span class="special">{};</span> 4834</pre> 4835<p> 4836 Matching an expression type <code class="computeroutput"><span class="identifier">E</span></code> 4837 against <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">switch_</span><span class="special"><</span><span class="identifier">C</span><span class="special">></span></code> 4838 is equivalent to matching it against <code class="computeroutput"><span class="identifier">C</span><span class="special">::</span><span class="identifier">case_</span><span class="special"><</span><span class="identifier">E</span><span class="special">::</span><span class="identifier">proto_tag</span><span class="special">></span></code>. By dispatching on the expression's 4839 tag type, we can jump to the sub-grammar that handles expressions of 4840 that type, skipping over all the other sub-grammars that couldn't possibly 4841 match. If there is no specialization of <code class="computeroutput"><span class="identifier">case_</span><span class="special"><></span></code> for a particular tag type, we 4842 select the primary template. In this case, the primary template inherits 4843 from <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">not_</span><span class="special"><</span><span class="identifier">_</span><span class="special">></span></code> 4844 which matches no expressions. 4845 </p> 4846<p> 4847 Notice the specialization that handles terminals: 4848 </p> 4849<pre class="programlisting"><span class="comment">// Terminal expressions are handled here</span> 4850<span class="keyword">template</span><span class="special"><></span> 4851<span class="keyword">struct</span> <span class="identifier">ABigGrammarCases</span><span class="special">::</span><span class="identifier">case_</span><span class="special"><</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">></span> 4852 <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">or_</span><span class="special"><</span> 4853 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> 4854 <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special"><</span><span class="keyword">double</span><span class="special">></span> 4855 <span class="special">></span> 4856<span class="special">{};</span> 4857</pre> 4858<p> 4859 The <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">terminal</span></code> type by itself isn't enough 4860 to select an appropriate sub-grammar, so we use <code class="computeroutput"><a class="link" href="../boost/proto/or_.html" title="Struct template or_">proto::or_<></a></code> 4861 to list the alternate sub-grammars that match terminals. 4862 </p> 4863<div class="note"><table border="0" summary="Note"> 4864<tr> 4865<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../doc/src/images/note.png"></td> 4866<th align="left">Note</th> 4867</tr> 4868<tr><td align="left" valign="top"> 4869<p> 4870 You might be tempted to define your <code class="computeroutput"><span class="identifier">case_</span><span class="special"><></span></code> specializations <span class="emphasis"><em>in 4871 situ</em></span> as follows: 4872 </p> 4873<p> 4874</p> 4875<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">ABigGrammarCases</span> 4876<span class="special">{</span> 4877 <span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">Tag</span><span class="special">></span> 4878 <span class="keyword">struct</span> <span class="identifier">case_</span> <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">not_</span><span class="special"><</span><span class="identifier">_</span><span class="special">></span> <span class="special">{};</span> 4879 4880 <span class="comment">// ERROR: not legal C++</span> 4881 <span class="keyword">template</span><span class="special"><></span> 4882 <span class="keyword">struct</span> <span class="identifier">case_</span><span class="special"><</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">></span> 4883 <span class="comment">/* ... */</span> 4884<span class="special">};</span> 4885</pre> 4886<p> 4887 </p> 4888<p> 4889 Unfortunately, for arcane reasons, it is not legal to define an explicit 4890 nested specialization <span class="emphasis"><em>in situ</em></span> like this. It is, 4891 however, perfectly legal to define <span class="emphasis"><em>partial</em></span> specializations 4892 <span class="emphasis"><em>in situ</em></span>, so you can add a extra dummy template 4893 parameter that has a default, as follows: 4894 </p> 4895<p> 4896</p> 4897<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">ABigGrammarCases</span> 4898<span class="special">{</span> 4899 <span class="comment">// Note extra "Dummy" template parameter here:</span> 4900 <span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">Tag</span><span class="special">,</span> <span class="keyword">int</span> <span class="identifier">Dummy</span> <span class="special">=</span> <span class="number">0</span><span class="special">></span> 4901 <span class="keyword">struct</span> <span class="identifier">case_</span> <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">not_</span><span class="special"><</span><span class="identifier">_</span><span class="special">></span> <span class="special">{};</span> 4902 4903 <span class="comment">// OK: "Dummy" makes this a partial specialization</span> 4904 <span class="comment">// instead of an explicit specialization.</span> 4905 <span class="keyword">template</span><span class="special"><</span><span class="keyword">int</span> <span class="identifier">Dummy</span><span class="special">></span> 4906 <span class="keyword">struct</span> <span class="identifier">case_</span><span class="special"><</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">,</span> <span class="identifier">Dummy</span><span class="special">></span> 4907 <span class="comment">/* ... */</span> 4908<span class="special">};</span> 4909</pre> 4910<p> 4911 </p> 4912<p> 4913 You might find this cleaner than defining explicit <code class="computeroutput"><span class="identifier">case_</span><span class="special"><></span></code> specializations outside of 4914 their enclosing struct. 4915 </p> 4916</td></tr> 4917</table></div> 4918</div> 4919<div class="section"> 4920<div class="titlepage"><div><div><h5 class="title"> 4921<a name="boost_proto.users_guide.intermediate_form.expression_introspection.matching_vararg_expressions"></a><a class="link" href="users_guide.html#boost_proto.users_guide.intermediate_form.expression_introspection.matching_vararg_expressions" title="Matching Vararg Expressions">Matching 4922 Vararg Expressions</a> 4923</h5></div></div></div> 4924<p> 4925 Not all of C++'s overloadable operators are unary or binary. There is 4926 the oddball <code class="computeroutput"><span class="keyword">operator</span><span class="special">()</span></code> 4927 -- the function call operator -- which can have any number of arguments. 4928 Likewise, with Proto you may define your own "operators" that 4929 could also take more that two arguments. As a result, there may be nodes 4930 in your Proto expression tree that have an arbitrary number of children 4931 (up to <code class="literal"><code class="computeroutput"><a class="link" href="../BOOST_PROTO_MAX_ARITY.html" title="Macro BOOST_PROTO_MAX_ARITY">BOOST_PROTO_MAX_ARITY</a></code></code>, 4932 which is configurable). How do you write a grammar to match such a node? 4933 </p> 4934<p> 4935 For such cases, Proto provides the <code class="computeroutput"><a class="link" href="../boost/proto/vararg.html" title="Struct template vararg">proto::vararg<></a></code> 4936 class template. Its template argument is a grammar, and the <code class="computeroutput"><a class="link" href="../boost/proto/vararg.html" title="Struct template vararg">proto::vararg<></a></code> will match the grammar 4937 zero or more times. Consider a Proto lazy function called <code class="computeroutput"><span class="identifier">fun</span><span class="special">()</span></code> 4938 that can take zero or more characters as arguments, as follows: 4939 </p> 4940<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">fun_tag</span> <span class="special">{};</span> 4941<span class="keyword">struct</span> <span class="identifier">FunTag</span> <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special"><</span> <span class="identifier">fun_tag</span> <span class="special">></span> <span class="special">{};</span> 4942<span class="identifier">FunTag</span><span class="special">::</span><span class="identifier">type</span> <span class="keyword">const</span> <span class="identifier">fun</span> <span class="special">=</span> <span class="special">{{}};</span> 4943 4944<span class="comment">// example usage:</span> 4945<span class="identifier">fun</span><span class="special">();</span> 4946<span class="identifier">fun</span><span class="special">(</span><span class="char">'a'</span><span class="special">);</span> 4947<span class="identifier">fun</span><span class="special">(</span><span class="char">'a'</span><span class="special">,</span> <span class="char">'b'</span><span class="special">);</span> 4948<span class="special">...</span> 4949</pre> 4950<p> 4951 Below is the grammar that matches all the allowable invocations of <code class="computeroutput"><span class="identifier">fun</span><span class="special">()</span></code>: 4952 </p> 4953<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">FunCall</span> 4954 <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">function</span><span class="special"><</span> <span class="identifier">FunTag</span><span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">vararg</span><span class="special"><</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special"><</span> <span class="keyword">char</span> <span class="special">></span> <span class="special">></span> <span class="special">></span> 4955<span class="special">{};</span> 4956</pre> 4957<p> 4958 The <code class="computeroutput"><span class="identifier">FunCall</span></code> grammar uses 4959 <code class="computeroutput"><a class="link" href="../boost/proto/vararg.html" title="Struct template vararg">proto::vararg<></a></code> to match zero or 4960 more character literals as arguments of the <code class="computeroutput"><span class="identifier">fun</span><span class="special">()</span></code> function. 4961 </p> 4962<p> 4963 As another example, can you guess what the following grammar matches? 4964 </p> 4965<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">Foo</span> 4966 <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">or_</span><span class="special"><</span> 4967 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special"><</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_</span> <span class="special">></span> 4968 <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">nary_expr</span><span class="special"><</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_</span><span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">vararg</span><span class="special"><</span> <span class="identifier">Foo</span> <span class="special">></span> <span class="special">></span> 4969 <span class="special">></span> 4970<span class="special">{};</span> 4971</pre> 4972<p> 4973 Here's a hint: the first template parameter to <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">nary_expr</span><span class="special"><></span></code> represents the node type, and 4974 any additional template parameters represent child nodes. The answer 4975 is that this is a degenerate grammar that matches every possible expression 4976 tree, from root to leaves. 4977 </p> 4978</div> 4979<div class="section"> 4980<div class="titlepage"><div><div><h5 class="title"> 4981<a name="boost_proto.users_guide.intermediate_form.expression_introspection.defining_edsl_grammars"></a><a class="link" href="users_guide.html#boost_proto.users_guide.intermediate_form.expression_introspection.defining_edsl_grammars" title="Defining EDSL Grammars">Defining 4982 EDSL Grammars</a> 4983</h5></div></div></div> 4984<p> 4985 In this section we'll see how to use Proto to define a grammar for your 4986 EDSL and use it to validate expression templates, giving short, readable 4987 compile-time errors for invalid expressions. 4988 </p> 4989<div class="tip"><table border="0" summary="Tip"> 4990<tr> 4991<td rowspan="2" align="center" valign="top" width="25"><img alt="[Tip]" src="../../../doc/src/images/tip.png"></td> 4992<th align="left">Tip</th> 4993</tr> 4994<tr><td align="left" valign="top"> 4995<p> 4996 You might think that this is a backwards way of doing things. <span class="quote">“<span class="quote">If 4997 Proto let me select which operators to overload, my users wouldn't 4998 be able to create invalid expressions in the first place, and I wouldn't 4999 need a grammar at all!</span>”</span> That may be true, but there are reasons 5000 for preferring to do things this way. 5001 </p> 5002<p> 5003 First, it lets you develop your EDSL rapidly -- all the operators are 5004 there for you already! -- and worry about invalid syntax later. 5005 </p> 5006<p> 5007 Second, it might be the case that some operators are only allowed in 5008 certain contexts within your EDSL. This is easy to express with a grammar, 5009 and hard to do with straight operator overloading. 5010 </p> 5011<p> 5012 Third, using an EDSL grammar to flag invalid expressions can often 5013 yield better errors than manually selecting the overloaded operators. 5014 </p> 5015<p> 5016 Fourth, the grammar can be used for more than just validation. You 5017 can use your grammar to define <span class="emphasis"><em>tree transformations</em></span> 5018 that convert expression templates into other more useful objects. 5019 </p> 5020<p> 5021 If none of the above convinces you, you actually <span class="emphasis"><em>can</em></span> 5022 use Proto to control which operators are overloaded within your domain. 5023 And to do it, you need to define a grammar! 5024 </p> 5025</td></tr> 5026</table></div> 5027<p> 5028 In a previous section, we used Proto to define an EDSL for a lazily evaluated 5029 calculator that allowed any combination of placeholders, floating-point 5030 literals, addition, subtraction, multiplication, division and grouping. 5031 If we were to write the grammar for this EDSL in <a href="http://en.wikipedia.org/wiki/Extended_Backus_Naur_Form" target="_top">EBNF</a>, 5032 it might look like this: 5033 </p> 5034<pre class="programlisting">group ::= '(' expression ')' 5035factor ::= double | '_1' | '_2' | group 5036term ::= factor (('*' factor) | ('/' factor))* 5037expression ::= term (('+' term) | ('-' term))* 5038</pre> 5039<p> 5040 This captures the syntax, associativity and precedence rules of a calculator. 5041 Writing the grammar for our calculator EDSL using Proto is <span class="emphasis"><em>even 5042 simpler</em></span>. Since we are using C++ as the host language, we are 5043 bound to the associativity and precedence rules for the C++ operators. 5044 Our grammar can assume them. Also, in C++ grouping is already handled 5045 for us with the use of parenthesis, so we don't have to code that into 5046 our grammar. 5047 </p> 5048<p> 5049 Let's begin our grammar for forward-declaring it: 5050 </p> 5051<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">CalculatorGrammar</span><span class="special">;</span> 5052</pre> 5053<p> 5054 It's an incomplete type at this point, but we'll still be able to use 5055 it to define the rules of our grammar. Let's define grammar rules for 5056 the terminals: 5057 </p> 5058<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">Double</span> 5059 <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special"><</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">convertible_to</span><span class="special"><</span> <span class="keyword">double</span> <span class="special">></span> <span class="special">></span> 5060<span class="special">{};</span> 5061 5062<span class="keyword">struct</span> <span class="identifier">Placeholder1</span> 5063 <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special"><</span> <span class="identifier">placeholder</span><span class="special"><</span><span class="number">0</span><span class="special">></span> <span class="special">></span> 5064<span class="special">{};</span> 5065 5066<span class="keyword">struct</span> <span class="identifier">Placeholder2</span> 5067 <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special"><</span> <span class="identifier">placeholder</span><span class="special"><</span><span class="number">1</span><span class="special">></span> <span class="special">></span> 5068<span class="special">{};</span> 5069 5070<span class="keyword">struct</span> <span class="identifier">Terminal</span> 5071 <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">or_</span><span class="special"><</span> <span class="identifier">Double</span><span class="special">,</span> <span class="identifier">Placeholder1</span><span class="special">,</span> <span class="identifier">Placeholder2</span> <span class="special">></span> 5072<span class="special">{};</span> 5073</pre> 5074<p> 5075 Now let's define the rules for addition, subtraction, multiplication 5076 and division. Here, we can ignore issues of associativity and precedence 5077 -- the C++ compiler will enforce that for us. We only must enforce that 5078 the arguments to the operators must themselves conform to the <code class="computeroutput"><span class="identifier">CalculatorGrammar</span></code> that we forward-declared 5079 above. 5080 </p> 5081<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">Plus</span> 5082 <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">plus</span><span class="special"><</span> <span class="identifier">CalculatorGrammar</span><span class="special">,</span> <span class="identifier">CalculatorGrammar</span> <span class="special">></span> 5083<span class="special">{};</span> 5084 5085<span class="keyword">struct</span> <span class="identifier">Minus</span> 5086 <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">minus</span><span class="special"><</span> <span class="identifier">CalculatorGrammar</span><span class="special">,</span> <span class="identifier">CalculatorGrammar</span> <span class="special">></span> 5087<span class="special">{};</span> 5088 5089<span class="keyword">struct</span> <span class="identifier">Multiplies</span> 5090 <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">multiplies</span><span class="special"><</span> <span class="identifier">CalculatorGrammar</span><span class="special">,</span> <span class="identifier">CalculatorGrammar</span> <span class="special">></span> 5091<span class="special">{};</span> 5092 5093<span class="keyword">struct</span> <span class="identifier">Divides</span> 5094 <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">divides</span><span class="special"><</span> <span class="identifier">CalculatorGrammar</span><span class="special">,</span> <span class="identifier">CalculatorGrammar</span> <span class="special">></span> 5095<span class="special">{};</span> 5096</pre> 5097<p> 5098 Now that we've defined all the parts of the grammar, we can define <code class="computeroutput"><span class="identifier">CalculatorGrammar</span></code>: 5099 </p> 5100<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">CalculatorGrammar</span> 5101 <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">or_</span><span class="special"><</span> 5102 <span class="identifier">Terminal</span> 5103 <span class="special">,</span> <span class="identifier">Plus</span> 5104 <span class="special">,</span> <span class="identifier">Minus</span> 5105 <span class="special">,</span> <span class="identifier">Multiplies</span> 5106 <span class="special">,</span> <span class="identifier">Divides</span> 5107 <span class="special">></span> 5108<span class="special">{};</span> 5109</pre> 5110<p> 5111 That's it! Now we can use <code class="computeroutput"><span class="identifier">CalculatorGrammar</span></code> 5112 to enforce that an expression template conforms to our grammar. We can 5113 use <code class="computeroutput"><a class="link" href="../boost/proto/matches.html" title="Struct template matches">proto::matches<></a></code> and <code class="computeroutput"><span class="identifier">BOOST_MPL_ASSERT</span><span class="special">()</span></code> 5114 to issue readable compile-time errors for invalid expressions, as below: 5115 </p> 5116<pre class="programlisting"><span class="keyword">template</span><span class="special"><</span> <span class="keyword">typename</span> <span class="identifier">Expr</span> <span class="special">></span> 5117<span class="keyword">void</span> <span class="identifier">evaluate</span><span class="special">(</span> <span class="identifier">Expr</span> <span class="keyword">const</span> <span class="special">&</span> <span class="identifier">expr</span> <span class="special">)</span> 5118<span class="special">{</span> 5119 <span class="identifier">BOOST_MPL_ASSERT</span><span class="special">((</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">matches</span><span class="special"><</span> <span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">CalculatorGrammar</span> <span class="special">></span> <span class="special">));</span> 5120 <span class="comment">// ...</span> 5121<span class="special">}</span> 5122</pre> 5123</div> 5124</div> 5125</div> 5126<div class="section"> 5127<div class="titlepage"><div><div><h3 class="title"> 5128<a name="boost_proto.users_guide.back_end"></a><a class="link" href="users_guide.html#boost_proto.users_guide.back_end" title="Back Ends: Making Expression Templates Do Useful Work">Back Ends: Making Expression 5129 Templates Do Useful Work</a> 5130</h3></div></div></div> 5131<div class="toc"><dl class="toc"> 5132<dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.back_end.expression_evaluation">Expression 5133 Evaluation: Imparting Behaviors with a Context</a></span></dt> 5134<dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.back_end.expression_transformation">Expression 5135 Transformation: Semantic Actions</a></span></dt> 5136</dl></div> 5137<p> 5138 Now that you've written the front end for your EDSL compiler, and you've 5139 learned a bit about the intermediate form it produces, it's time to think 5140 about what to <span class="emphasis"><em>do</em></span> with the intermediate form. This is 5141 where you put your domain-specific algorithms and optimizations. Proto gives 5142 you two ways to evaluate and manipulate expression templates: contexts and 5143 transforms. 5144 </p> 5145<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "> 5146<li class="listitem"> 5147 A <span class="emphasis"><em>context</em></span> is like a function object that you pass 5148 along with an expression to the <code class="computeroutput"><a class="link" href="../boost/proto/eval.html" title="Function eval">proto::eval()</a></code> 5149 function. It associates behaviors with node types. <code class="computeroutput"><a class="link" href="../boost/proto/eval.html" title="Function eval">proto::eval()</a></code> 5150 walks the expression and invokes your context at each node. 5151 </li> 5152<li class="listitem"> 5153 A <span class="emphasis"><em>transform</em></span> is a way to associate behaviors, not 5154 with node types in an expression, but with rules in a Proto grammar. 5155 In this way, they are like semantic actions in other compiler-construction 5156 toolkits. 5157 </li> 5158</ul></div> 5159<p> 5160 Two ways to evaluate expressions! How to choose? Since contexts are largely 5161 procedural, they are a bit simpler to understand and debug so they are a 5162 good place to start. But although transforms are more advanced, they are 5163 also more powerful; since they are associated with rules in your grammar, 5164 you can select the proper transform based on the entire <span class="emphasis"><em>structure</em></span> 5165 of a sub-expression rather than simply on the type of its top-most node. 5166 </p> 5167<p> 5168 Also, transforms have a concise and declarative syntax that can be confusing 5169 at first, but highly expressive and fungible once you become accustomed to 5170 it. And -- this is admittedly very subjective -- the author finds programming 5171 with Proto transforms to be an inordinate amount of <span class="emphasis"><em>fun!</em></span> 5172 Your mileage may vary. 5173 </p> 5174<div class="section"> 5175<div class="titlepage"><div><div><h4 class="title"> 5176<a name="boost_proto.users_guide.back_end.expression_evaluation"></a><a class="link" href="users_guide.html#boost_proto.users_guide.back_end.expression_evaluation" title="Expression Evaluation: Imparting Behaviors with a Context">Expression 5177 Evaluation: Imparting Behaviors with a Context</a> 5178</h4></div></div></div> 5179<div class="toc"><dl class="toc"> 5180<dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.back_end.expression_evaluation.proto_eval">Evaluating 5181 an Expression with <code class="literal">proto::eval()</code></a></span></dt> 5182<dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.back_end.expression_evaluation.contexts">Defining 5183 an Evaluation Context</a></span></dt> 5184<dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.back_end.expression_evaluation.canned_contexts">Proto's 5185 Built-In Contexts</a></span></dt> 5186</dl></div> 5187<p> 5188 Once you have constructed a Proto expression tree, either by using Proto's 5189 operator overloads or with <code class="computeroutput"><a class="link" href="../boost/proto/make_expr.html" title="Function make_expr">proto::make_expr()</a></code> 5190 and friends, you probably want to actually <span class="emphasis"><em>do</em></span> something 5191 with it. The simplest option is to use <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">eval</span><span class="special">()</span></code>, a generic expression evaluator. To use 5192 <code class="computeroutput"><a class="link" href="../boost/proto/eval.html" title="Function eval">proto::eval()</a></code>, you'll need to define 5193 a <span class="emphasis"><em>context</em></span> that tells <code class="computeroutput"><a class="link" href="../boost/proto/eval.html" title="Function eval">proto::eval()</a></code> 5194 how each node should be evaluated. This section goes through the nuts and 5195 bolts of using <code class="computeroutput"><a class="link" href="../boost/proto/eval.html" title="Function eval">proto::eval()</a></code>, defining evaluation contexts, 5196 and using the contexts that Proto provides. 5197 </p> 5198<div class="note"><table border="0" summary="Note"> 5199<tr> 5200<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../doc/src/images/note.png"></td> 5201<th align="left">Note</th> 5202</tr> 5203<tr><td align="left" valign="top"><p> 5204 <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">eval</span><span class="special">()</span></code> 5205 is a less powerful but easier-to-use evaluation technique than Proto 5206 transforms, which are covered later. Although very powerful, transforms 5207 have a steep learning curve and can be more difficult to debug. <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">eval</span><span class="special">()</span></code> 5208 is a rather weak tree traversal algorithm. Dan Marsden has been working 5209 on a more general and powerful tree traversal library. When it is ready, 5210 I anticipate that it will eliminate the need for <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">eval</span><span class="special">()</span></code>. 5211 </p></td></tr> 5212</table></div> 5213<div class="section"> 5214<div class="titlepage"><div><div><h5 class="title"> 5215<a name="boost_proto.users_guide.back_end.expression_evaluation.proto_eval"></a><a class="link" href="users_guide.html#boost_proto.users_guide.back_end.expression_evaluation.proto_eval" title="Evaluating an Expression with proto::eval()">Evaluating 5216 an Expression with <code class="literal">proto::eval()</code></a> 5217</h5></div></div></div> 5218<div class="blockquote"><blockquote class="blockquote"><p> 5219 <span class="bold"><strong>Synopsis:</strong></span> 5220 </p></blockquote></div> 5221<pre class="programlisting"><span class="keyword">namespace</span> <span class="identifier">proto</span> 5222<span class="special">{</span> 5223 <span class="keyword">namespace</span> <span class="identifier">result_of</span> 5224 <span class="special">{</span> 5225 <span class="comment">// A metafunction for calculating the return</span> 5226 <span class="comment">// type of proto::eval() given certain Expr</span> 5227 <span class="comment">// and Context types.</span> 5228 <span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Context</span><span class="special">></span> 5229 <span class="keyword">struct</span> <span class="identifier">eval</span> 5230 <span class="special">{</span> 5231 <span class="keyword">typedef</span> 5232 <span class="keyword">typename</span> <span class="identifier">Context</span><span class="special">::</span><span class="keyword">template</span> <span class="identifier">eval</span><span class="special"><</span><span class="identifier">Expr</span><span class="special">>::</span><span class="identifier">result_type</span> 5233 <span class="identifier">type</span><span class="special">;</span> 5234 <span class="special">};</span> 5235 <span class="special">}</span> 5236 5237 <span class="keyword">namespace</span> <span class="identifier">functional</span> 5238 <span class="special">{</span> 5239 <span class="comment">// A callable function object type for evaluating</span> 5240 <span class="comment">// a Proto expression with a certain context.</span> 5241 <span class="keyword">struct</span> <span class="identifier">eval</span> <span class="special">:</span> <span class="identifier">callable</span> 5242 <span class="special">{</span> 5243 <span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">Sig</span><span class="special">></span> 5244 <span class="keyword">struct</span> <span class="identifier">result</span><span class="special">;</span> 5245 5246 <span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Context</span><span class="special">></span> 5247 <span class="keyword">typename</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">eval</span><span class="special"><</span><span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">Context</span><span class="special">>::</span><span class="identifier">type</span> 5248 <span class="keyword">operator</span> <span class="special">()(</span><span class="identifier">Expr</span> <span class="special">&</span><span class="identifier">expr</span><span class="special">,</span> <span class="identifier">Context</span> <span class="special">&</span><span class="identifier">context</span><span class="special">)</span> <span class="keyword">const</span><span class="special">;</span> 5249 5250 <span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Context</span><span class="special">></span> 5251 <span class="keyword">typename</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">eval</span><span class="special"><</span><span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">Context</span><span class="special">>::</span><span class="identifier">type</span> 5252 <span class="keyword">operator</span> <span class="special">()(</span><span class="identifier">Expr</span> <span class="special">&</span><span class="identifier">expr</span><span class="special">,</span> <span class="identifier">Context</span> <span class="keyword">const</span> <span class="special">&</span><span class="identifier">context</span><span class="special">)</span> <span class="keyword">const</span><span class="special">;</span> 5253 <span class="special">};</span> 5254 <span class="special">}</span> 5255 5256 <span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Context</span><span class="special">></span> 5257 <span class="keyword">typename</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">eval</span><span class="special"><</span><span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">Context</span><span class="special">>::</span><span class="identifier">type</span> 5258 <span class="identifier">eval</span><span class="special">(</span><span class="identifier">Expr</span> <span class="special">&</span><span class="identifier">expr</span><span class="special">,</span> <span class="identifier">Context</span> <span class="special">&</span><span class="identifier">context</span><span class="special">);</span> 5259 5260 <span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Context</span><span class="special">></span> 5261 <span class="keyword">typename</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">eval</span><span class="special"><</span><span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">Context</span><span class="special">>::</span><span class="identifier">type</span> 5262 <span class="identifier">eval</span><span class="special">(</span><span class="identifier">Expr</span> <span class="special">&</span><span class="identifier">expr</span><span class="special">,</span> <span class="identifier">Context</span> <span class="keyword">const</span> <span class="special">&</span><span class="identifier">context</span><span class="special">);</span> 5263<span class="special">}</span> 5264</pre> 5265<p> 5266 Given an expression and an evaluation context, using <code class="computeroutput"><a class="link" href="../boost/proto/eval.html" title="Function eval">proto::eval()</a></code> 5267 is quite simple. Simply pass the expression and the context to <code class="computeroutput"><a class="link" href="../boost/proto/eval.html" title="Function eval">proto::eval()</a></code> and it does the rest 5268 and returns the result. You can use the <code class="computeroutput"><span class="identifier">eval</span><span class="special"><></span></code> metafunction in the <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">result_of</span></code> namespace to compute the 5269 return type of <code class="computeroutput"><a class="link" href="../boost/proto/eval.html" title="Function eval">proto::eval()</a></code>. The following demonstrates 5270 a use of <code class="computeroutput"><a class="link" href="../boost/proto/eval.html" title="Function eval">proto::eval()</a></code>: 5271 </p> 5272<pre class="programlisting"><span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">></span> 5273<span class="keyword">typename</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">eval</span><span class="special"><</span><span class="identifier">Expr</span> <span class="keyword">const</span><span class="special">,</span> <span class="identifier">MyContext</span><span class="special">>::</span><span class="identifier">type</span> 5274<span class="identifier">MyEvaluate</span><span class="special">(</span><span class="identifier">Expr</span> <span class="keyword">const</span> <span class="special">&</span><span class="identifier">expr</span><span class="special">)</span> 5275<span class="special">{</span> 5276 <span class="comment">// Some user-defined context type</span> 5277 <span class="identifier">MyContext</span> <span class="identifier">ctx</span><span class="special">;</span> 5278 5279 <span class="comment">// Evaluate an expression with the context</span> 5280 <span class="keyword">return</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">eval</span><span class="special">(</span><span class="identifier">expr</span><span class="special">,</span> <span class="identifier">ctx</span><span class="special">);</span> 5281<span class="special">}</span> 5282</pre> 5283<p> 5284 What <code class="computeroutput"><a class="link" href="../boost/proto/eval.html" title="Function eval">proto::eval()</a></code> does is also very simple. 5285 It defers most of the work to the context itself. Here essentially is 5286 the implementation of <code class="computeroutput"><a class="link" href="../boost/proto/eval.html" title="Function eval">proto::eval()</a></code>: 5287 </p> 5288<pre class="programlisting"><span class="comment">// eval() dispatches to a nested "eval<>" function</span> 5289<span class="comment">// object within the Context:</span> 5290<span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Context</span><span class="special">></span> 5291<span class="keyword">typename</span> <span class="identifier">Context</span><span class="special">::</span><span class="keyword">template</span> <span class="identifier">eval</span><span class="special"><</span><span class="identifier">Expr</span><span class="special">>::</span><span class="identifier">result_type</span> 5292<span class="identifier">eval</span><span class="special">(</span><span class="identifier">Expr</span> <span class="special">&</span><span class="identifier">expr</span><span class="special">,</span> <span class="identifier">Context</span> <span class="special">&</span><span class="identifier">ctx</span><span class="special">)</span> 5293<span class="special">{</span> 5294 <span class="keyword">typename</span> <span class="identifier">Context</span><span class="special">::</span><span class="keyword">template</span> <span class="identifier">eval</span><span class="special"><</span><span class="identifier">Expr</span><span class="special">></span> <span class="identifier">eval_fun</span><span class="special">;</span> 5295 <span class="keyword">return</span> <span class="identifier">eval_fun</span><span class="special">(</span><span class="identifier">expr</span><span class="special">,</span> <span class="identifier">ctx</span><span class="special">);</span> 5296<span class="special">}</span> 5297</pre> 5298<p> 5299 Really, <code class="computeroutput"><a class="link" href="../boost/proto/eval.html" title="Function eval">proto::eval()</a></code> is nothing more than 5300 a thin wrapper that dispatches to the appropriate handler within the 5301 context class. In the next section, we'll see how to implement a context 5302 class from scratch. 5303 </p> 5304</div> 5305<div class="section"> 5306<div class="titlepage"><div><div><h5 class="title"> 5307<a name="boost_proto.users_guide.back_end.expression_evaluation.contexts"></a><a class="link" href="users_guide.html#boost_proto.users_guide.back_end.expression_evaluation.contexts" title="Defining an Evaluation Context">Defining 5308 an Evaluation Context</a> 5309</h5></div></div></div> 5310<p> 5311 As we saw in the previous section, there is really not much to the <code class="computeroutput"><a class="link" href="../boost/proto/eval.html" title="Function eval">proto::eval()</a></code> function. Rather, all 5312 the interesting expression evaluation goes on within a context class. 5313 This section shows how to implement one from scratch. 5314 </p> 5315<p> 5316 All context classes have roughly the following form: 5317 </p> 5318<pre class="programlisting"><span class="comment">// A prototypical user-defined context.</span> 5319<span class="keyword">struct</span> <span class="identifier">MyContext</span> 5320<span class="special">{</span> 5321 <span class="comment">// A nested eval<> class template</span> 5322 <span class="keyword">template</span><span class="special"><</span> 5323 <span class="keyword">typename</span> <span class="identifier">Expr</span> 5324 <span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Tag</span> <span class="special">=</span> <span class="keyword">typename</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag_of</span><span class="special"><</span><span class="identifier">Expr</span><span class="special">>::</span><span class="identifier">type</span> 5325 <span class="special">></span> 5326 <span class="keyword">struct</span> <span class="identifier">eval</span><span class="special">;</span> 5327 5328 <span class="comment">// Handle terminal nodes here...</span> 5329 <span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">></span> 5330 <span class="keyword">struct</span> <span class="identifier">eval</span><span class="special"><</span><span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">></span> 5331 <span class="special">{</span> 5332 <span class="comment">// Must have a nested result_type typedef.</span> 5333 <span class="keyword">typedef</span> <span class="special">...</span> <span class="identifier">result_type</span><span class="special">;</span> 5334 5335 <span class="comment">// Must have a function call operator that takes</span> 5336 <span class="comment">// an expression and the context.</span> 5337 <span class="identifier">result_type</span> <span class="keyword">operator</span><span class="special">()(</span><span class="identifier">Expr</span> <span class="special">&</span><span class="identifier">expr</span><span class="special">,</span> <span class="identifier">MyContext</span> <span class="special">&</span><span class="identifier">ctx</span><span class="special">)</span> <span class="keyword">const</span> 5338 <span class="special">{</span> 5339 <span class="keyword">return</span> <span class="special">...;</span> 5340 <span class="special">}</span> 5341 <span class="special">};</span> 5342 5343 <span class="comment">// ... other specializations of struct eval<> ...</span> 5344<span class="special">};</span> 5345</pre> 5346<p> 5347 Context classes are nothing more than a collection of specializations 5348 of a nested <code class="computeroutput"><span class="identifier">eval</span><span class="special"><></span></code> 5349 class template. Each specialization handles a different expression type. 5350 </p> 5351<p> 5352 In the <a class="link" href="users_guide.html#boost_proto.users_guide.getting_started.hello_calculator" title="Hello Calculator">Hello 5353 Calculator</a> section, we saw an example of a user-defined context 5354 class for evaluating calculator expressions. That context class was implemented 5355 with the help of Proto's <code class="computeroutput"><a class="link" href="../boost/proto/context/callable_context.html" title="Struct template callable_context">proto::callable_context<></a></code>. 5356 If we were to implement it from scratch, it would look something like 5357 this: 5358 </p> 5359<pre class="programlisting"><span class="comment">// The calculator_context from the "Hello Calculator" section,</span> 5360<span class="comment">// implemented from scratch.</span> 5361<span class="keyword">struct</span> <span class="identifier">calculator_context</span> 5362<span class="special">{</span> 5363 <span class="comment">// The values with which we'll replace the placeholders</span> 5364 <span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special"><</span><span class="keyword">double</span><span class="special">></span> <span class="identifier">args</span><span class="special">;</span> 5365 5366 <span class="keyword">template</span><span class="special"><</span> 5367 <span class="keyword">typename</span> <span class="identifier">Expr</span> 5368 <span class="comment">// defaulted template parameters, so we can</span> 5369 <span class="comment">// specialize on the expressions that need</span> 5370 <span class="comment">// special handling.</span> 5371 <span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Tag</span> <span class="special">=</span> <span class="keyword">typename</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag_of</span><span class="special"><</span><span class="identifier">Expr</span><span class="special">>::</span><span class="identifier">type</span> 5372 <span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Arg0</span> <span class="special">=</span> <span class="keyword">typename</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">child_c</span><span class="special"><</span><span class="identifier">Expr</span><span class="special">,</span> <span class="number">0</span><span class="special">>::</span><span class="identifier">type</span> 5373 <span class="special">></span> 5374 <span class="keyword">struct</span> <span class="identifier">eval</span><span class="special">;</span> 5375 5376 <span class="comment">// Handle placeholder terminals here...</span> 5377 <span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">,</span> <span class="keyword">int</span> <span class="identifier">I</span><span class="special">></span> 5378 <span class="keyword">struct</span> <span class="identifier">eval</span><span class="special"><</span><span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">,</span> <span class="identifier">placeholder</span><span class="special"><</span><span class="identifier">I</span><span class="special">></span> <span class="special">></span> 5379 <span class="special">{</span> 5380 <span class="keyword">typedef</span> <span class="keyword">double</span> <span class="identifier">result_type</span><span class="special">;</span> 5381 5382 <span class="identifier">result_type</span> <span class="keyword">operator</span><span class="special">()(</span><span class="identifier">Expr</span> <span class="special">&,</span> <span class="identifier">MyContext</span> <span class="special">&</span><span class="identifier">ctx</span><span class="special">)</span> <span class="keyword">const</span> 5383 <span class="special">{</span> 5384 <span class="keyword">return</span> <span class="identifier">ctx</span><span class="special">.</span><span class="identifier">args</span><span class="special">[</span><span class="identifier">I</span><span class="special">];</span> 5385 <span class="special">}</span> 5386 <span class="special">};</span> 5387 5388 <span class="comment">// Handle other terminals here...</span> 5389 <span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Arg0</span><span class="special">></span> 5390 <span class="keyword">struct</span> <span class="identifier">eval</span><span class="special"><</span><span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">,</span> <span class="identifier">Arg0</span><span class="special">></span> 5391 <span class="special">{</span> 5392 <span class="keyword">typedef</span> <span class="keyword">double</span> <span class="identifier">result_type</span><span class="special">;</span> 5393 5394 <span class="identifier">result_type</span> <span class="keyword">operator</span><span class="special">()(</span><span class="identifier">Expr</span> <span class="special">&</span><span class="identifier">expr</span><span class="special">,</span> <span class="identifier">MyContext</span> <span class="special">&)</span> <span class="keyword">const</span> 5395 <span class="special">{</span> 5396 <span class="keyword">return</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">child</span><span class="special">(</span><span class="identifier">expr</span><span class="special">);</span> 5397 <span class="special">}</span> 5398 <span class="special">};</span> 5399 5400 <span class="comment">// Handle addition here...</span> 5401 <span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Arg0</span><span class="special">></span> 5402 <span class="keyword">struct</span> <span class="identifier">eval</span><span class="special"><</span><span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">plus</span><span class="special">,</span> <span class="identifier">Arg0</span><span class="special">></span> 5403 <span class="special">{</span> 5404 <span class="keyword">typedef</span> <span class="keyword">double</span> <span class="identifier">result_type</span><span class="special">;</span> 5405 5406 <span class="identifier">result_type</span> <span class="keyword">operator</span><span class="special">()(</span><span class="identifier">Expr</span> <span class="special">&</span><span class="identifier">expr</span><span class="special">,</span> <span class="identifier">MyContext</span> <span class="special">&</span><span class="identifier">ctx</span><span class="special">)</span> <span class="keyword">const</span> 5407 <span class="special">{</span> 5408 <span class="keyword">return</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">eval</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">left</span><span class="special">(</span><span class="identifier">expr</span><span class="special">),</span> <span class="identifier">ctx</span><span class="special">)</span> 5409 <span class="special">+</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">eval</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">right</span><span class="special">(</span><span class="identifier">expr</span><span class="special">),</span> <span class="identifier">ctx</span><span class="special">);</span> 5410 <span class="special">}</span> 5411 <span class="special">};</span> 5412 5413 <span class="comment">// ... other eval<> specializations for other node types ...</span> 5414<span class="special">};</span> 5415</pre> 5416<p> 5417 Now we can use <code class="computeroutput"><a class="link" href="../boost/proto/eval.html" title="Function eval">proto::eval()</a></code> with the context class 5418 above to evaluate calculator expressions as follows: 5419 </p> 5420<pre class="programlisting"><span class="comment">// Evaluate an expression with a calculator_context</span> 5421<span class="identifier">calculator_context</span> <span class="identifier">ctx</span><span class="special">;</span> 5422<span class="identifier">ctx</span><span class="special">.</span><span class="identifier">args</span><span class="special">.</span><span class="identifier">push_back</span><span class="special">(</span><span class="number">5</span><span class="special">);</span> 5423<span class="identifier">ctx</span><span class="special">.</span><span class="identifier">args</span><span class="special">.</span><span class="identifier">push_back</span><span class="special">(</span><span class="number">6</span><span class="special">);</span> 5424<span class="keyword">double</span> <span class="identifier">d</span> <span class="special">=</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">eval</span><span class="special">(</span><span class="identifier">_1</span> <span class="special">+</span> <span class="identifier">_2</span><span class="special">,</span> <span class="identifier">ctx</span><span class="special">);</span> 5425<span class="identifier">assert</span><span class="special">(</span><span class="number">11</span> <span class="special">==</span> <span class="identifier">d</span><span class="special">);</span> 5426</pre> 5427<p> 5428 Defining a context from scratch this way is tedious and verbose, but 5429 it gives you complete control over how the expression is evaluated. The 5430 context class in the <a class="link" href="users_guide.html#boost_proto.users_guide.getting_started.hello_calculator" title="Hello Calculator">Hello 5431 Calculator</a> example was much simpler. In the next section we'll 5432 see the helper class Proto provides to ease the job of implementing context 5433 classes. 5434 </p> 5435</div> 5436<div class="section"> 5437<div class="titlepage"><div><div><h5 class="title"> 5438<a name="boost_proto.users_guide.back_end.expression_evaluation.canned_contexts"></a><a class="link" href="users_guide.html#boost_proto.users_guide.back_end.expression_evaluation.canned_contexts" title="Proto's Built-In Contexts">Proto's 5439 Built-In Contexts</a> 5440</h5></div></div></div> 5441<p> 5442 Proto provides some ready-made context classes that you can use as-is, 5443 or that you can use to help while implementing your own contexts. They 5444 are: 5445 </p> 5446<div class="variablelist"> 5447<p class="title"><b></b></p> 5448<dl class="variablelist"> 5449<dt><span class="term"><a class="link" href="users_guide.html#boost_proto.users_guide.back_end.expression_evaluation.canned_contexts.default_context" title="default_context"><code class="literal">default_context</code></a></span></dt> 5450<dd><p> 5451 An evaluation context that assigns the usual C++ meanings to all 5452 the operators. For example, addition nodes are handled by evaluating 5453 the left and right children and then adding the results. The <code class="computeroutput"><a class="link" href="../boost/proto/context/default_context.html" title="Struct default_context">proto::default_context</a></code> 5454 uses Boost.Typeof to deduce the types of the expressions it evaluates. 5455 </p></dd> 5456<dt><span class="term"><a class="link" href="users_guide.html#boost_proto.users_guide.back_end.expression_evaluation.canned_contexts.null_context" title="null_context"><code class="literal">null_context</code></a></span></dt> 5457<dd><p> 5458 A simple context that recursively evaluates children but does not 5459 combine the results in any way and returns void. 5460 </p></dd> 5461<dt><span class="term"><a class="link" href="users_guide.html#boost_proto.users_guide.back_end.expression_evaluation.canned_contexts.callable_context" title="callable_context<>"><code class="literal">callable_context<></code></a></span></dt> 5462<dd><p> 5463 A helper that simplifies the job of writing context classes. Rather 5464 than writing template specializations, with <code class="computeroutput"><a class="link" href="../boost/proto/context/callable_context.html" title="Struct template callable_context">proto::callable_context<></a></code> 5465 you write a function object with an overloaded function call operator. 5466 Any expressions not handled by an overload are automatically dispatched 5467 to a default evaluation context that you can specify. 5468 </p></dd> 5469</dl> 5470</div> 5471<div class="section"> 5472<div class="titlepage"><div><div><h6 class="title"> 5473<a name="boost_proto.users_guide.back_end.expression_evaluation.canned_contexts.default_context"></a><a class="link" href="users_guide.html#boost_proto.users_guide.back_end.expression_evaluation.canned_contexts.default_context" title="default_context"><code class="literal">default_context</code></a> 5474</h6></div></div></div> 5475<p> 5476 The <code class="computeroutput"><a class="link" href="../boost/proto/context/default_context.html" title="Struct default_context">proto::default_context</a></code> is an 5477 evaluation context that assigns the usual C++ meanings to all the operators. 5478 For example, addition nodes are handled by evaluating the left and 5479 right children and then adding the results. The <code class="computeroutput"><a class="link" href="../boost/proto/context/default_context.html" title="Struct default_context">proto::default_context</a></code> uses 5480 Boost.Typeof to deduce the types of the expressions it evaluates. 5481 </p> 5482<p> 5483 For example, consider the following "Hello World" example: 5484 </p> 5485<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">iostream</span><span class="special">></span> 5486<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">proto</span><span class="special">/</span><span class="identifier">proto</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> 5487<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">proto</span><span class="special">/</span><span class="identifier">context</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> 5488<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">typeof</span><span class="special">/</span><span class="identifier">std</span><span class="special">/</span><span class="identifier">ostream</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> 5489<span class="keyword">using</span> <span class="keyword">namespace</span> <span class="identifier">boost</span><span class="special">;</span> 5490 5491<span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special"><</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">ostream</span> <span class="special">&</span> <span class="special">>::</span><span class="identifier">type</span> <span class="identifier">cout_</span> <span class="special">=</span> <span class="special">{</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">};</span> 5492 5493<span class="keyword">template</span><span class="special"><</span> <span class="keyword">typename</span> <span class="identifier">Expr</span> <span class="special">></span> 5494<span class="keyword">void</span> <span class="identifier">evaluate</span><span class="special">(</span> <span class="identifier">Expr</span> <span class="keyword">const</span> <span class="special">&</span> <span class="identifier">expr</span> <span class="special">)</span> 5495<span class="special">{</span> 5496 <span class="comment">// Evaluate the expression with default_context,</span> 5497 <span class="comment">// to give the operators their C++ meanings:</span> 5498 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">default_context</span> <span class="identifier">ctx</span><span class="special">;</span> 5499 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">eval</span><span class="special">(</span><span class="identifier">expr</span><span class="special">,</span> <span class="identifier">ctx</span><span class="special">);</span> 5500<span class="special">}</span> 5501 5502<span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span> 5503<span class="special">{</span> 5504 <span class="identifier">evaluate</span><span class="special">(</span> <span class="identifier">cout_</span> <span class="special"><<</span> <span class="string">"hello"</span> <span class="special"><<</span> <span class="char">','</span> <span class="special"><<</span> <span class="string">" world"</span> <span class="special">);</span> 5505 <span class="keyword">return</span> <span class="number">0</span><span class="special">;</span> 5506<span class="special">}</span> 5507</pre> 5508<p> 5509 This program outputs the following: 5510 </p> 5511<pre class="programlisting">hello, world 5512</pre> 5513<p> 5514 <code class="computeroutput"><a class="link" href="../boost/proto/context/default_context.html" title="Struct default_context">proto::default_context</a></code> is trivially 5515 defined in terms of a <code class="computeroutput"><span class="identifier">default_eval</span><span class="special"><></span></code> template, as follows: 5516 </p> 5517<pre class="programlisting"><span class="comment">// Definition of default_context</span> 5518<span class="keyword">struct</span> <span class="identifier">default_context</span> 5519<span class="special">{</span> 5520 <span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">></span> 5521 <span class="keyword">struct</span> <span class="identifier">eval</span> 5522 <span class="special">:</span> <span class="identifier">default_eval</span><span class="special"><</span> 5523 <span class="identifier">Expr</span> 5524 <span class="special">,</span> <span class="identifier">default_context</span> <span class="keyword">const</span> 5525 <span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">tag_of</span><span class="special"><</span><span class="identifier">Expr</span><span class="special">>::</span><span class="identifier">type</span> 5526 <span class="special">></span> 5527 <span class="special">{};</span> 5528<span class="special">};</span> 5529</pre> 5530<p> 5531 There are a bunch of <code class="computeroutput"><span class="identifier">default_eval</span><span class="special"><></span></code> specializations, each of which 5532 handles a different C++ operator. Here, for instance, is the specialization 5533 for binary addition: 5534 </p> 5535<pre class="programlisting"><span class="comment">// A default expression evaluator for binary addition</span> 5536<span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Context</span><span class="special">></span> 5537<span class="keyword">struct</span> <span class="identifier">default_eval</span><span class="special"><</span><span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">Context</span><span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">plus</span><span class="special">></span> 5538<span class="special">{</span> 5539<span class="keyword">private</span><span class="special">:</span> 5540 <span class="keyword">static</span> <span class="identifier">Expr</span> <span class="special">&</span> <span class="identifier">s_expr</span><span class="special">;</span> 5541 <span class="keyword">static</span> <span class="identifier">Context</span> <span class="special">&</span> <span class="identifier">s_ctx</span><span class="special">;</span> 5542 5543<span class="keyword">public</span><span class="special">:</span> 5544 <span class="keyword">typedef</span> 5545 <span class="keyword">decltype</span><span class="special">(</span> 5546 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">eval</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">child_c</span><span class="special"><</span><span class="number">0</span><span class="special">>(</span><span class="identifier">s_expr</span><span class="special">),</span> <span class="identifier">s_ctx</span><span class="special">)</span> 5547 <span class="special">+</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">eval</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">child_c</span><span class="special"><</span><span class="number">1</span><span class="special">>(</span><span class="identifier">s_expr</span><span class="special">),</span> <span class="identifier">s_ctx</span><span class="special">)</span> 5548 <span class="special">)</span> 5549 <span class="identifier">result_type</span><span class="special">;</span> 5550 5551 <span class="identifier">result_type</span> <span class="keyword">operator</span> <span class="special">()(</span><span class="identifier">Expr</span> <span class="special">&</span><span class="identifier">expr</span><span class="special">,</span> <span class="identifier">Context</span> <span class="special">&</span><span class="identifier">ctx</span><span class="special">)</span> <span class="keyword">const</span> 5552 <span class="special">{</span> 5553 <span class="keyword">return</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">eval</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">child_c</span><span class="special"><</span><span class="number">0</span><span class="special">>(</span><span class="identifier">expr</span><span class="special">),</span> <span class="identifier">ctx</span><span class="special">)</span> 5554 <span class="special">+</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">eval</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">child_c</span><span class="special"><</span><span class="number">1</span><span class="special">>(</span><span class="identifier">expr</span><span class="special">),</span> <span class="identifier">ctx</span><span class="special">);</span> 5555 <span class="special">}</span> 5556<span class="special">};</span> 5557</pre> 5558<p> 5559 The above code uses <code class="computeroutput"><span class="keyword">decltype</span></code> 5560 to calculate the return type of the function call operator. <code class="computeroutput"><span class="keyword">decltype</span></code> is a new keyword in the next 5561 version of C++ that gets the type of any expression. Most compilers 5562 do not yet support <code class="computeroutput"><span class="keyword">decltype</span></code> 5563 directly, so <code class="computeroutput"><span class="identifier">default_eval</span><span class="special"><></span></code> uses the Boost.Typeof library 5564 to emulate it. On some compilers, that may mean that <code class="computeroutput"><span class="identifier">default_context</span></code> either doesn't work 5565 or that it requires you to register your types with the Boost.Typeof 5566 library. Check the documentation for Boost.Typeof to see. 5567 </p> 5568</div> 5569<div class="section"> 5570<div class="titlepage"><div><div><h6 class="title"> 5571<a name="boost_proto.users_guide.back_end.expression_evaluation.canned_contexts.null_context"></a><a class="link" href="users_guide.html#boost_proto.users_guide.back_end.expression_evaluation.canned_contexts.null_context" title="null_context"><code class="literal">null_context</code></a> 5572</h6></div></div></div> 5573<p> 5574 The <code class="computeroutput"><a class="link" href="../boost/proto/context/null_context.html" title="Struct null_context">proto::null_context<></a></code> 5575 is a simple context that recursively evaluates children but does not 5576 combine the results in any way and returns void. It is useful in conjunction 5577 with <code class="computeroutput"><span class="identifier">callable_context</span><span class="special"><></span></code>, or when defining your own 5578 contexts which mutate an expression tree in-place rather than accumulate 5579 a result, as we'll see below. 5580 </p> 5581<p> 5582 <code class="computeroutput"><a class="link" href="../boost/proto/context/null_context.html" title="Struct null_context">proto::null_context<></a></code> 5583 is trivially implemented in terms of <code class="computeroutput"><span class="identifier">null_eval</span><span class="special"><></span></code> as follows: 5584 </p> 5585<pre class="programlisting"><span class="comment">// Definition of null_context</span> 5586<span class="keyword">struct</span> <span class="identifier">null_context</span> 5587<span class="special">{</span> 5588 <span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">></span> 5589 <span class="keyword">struct</span> <span class="identifier">eval</span> 5590 <span class="special">:</span> <span class="identifier">null_eval</span><span class="special"><</span><span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">null_context</span> <span class="keyword">const</span><span class="special">,</span> <span class="identifier">Expr</span><span class="special">::</span><span class="identifier">proto_arity</span><span class="special">::</span><span class="identifier">value</span><span class="special">></span> 5591 <span class="special">{};</span> 5592<span class="special">};</span> 5593</pre> 5594<p> 5595 And <code class="computeroutput"><span class="identifier">null_eval</span><span class="special"><></span></code> 5596 is also trivially implemented. Here, for instance is a binary <code class="computeroutput"><span class="identifier">null_eval</span><span class="special"><></span></code>: 5597 </p> 5598<pre class="programlisting"><span class="comment">// Binary null_eval<></span> 5599<span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Context</span><span class="special">></span> 5600<span class="keyword">struct</span> <span class="identifier">null_eval</span><span class="special"><</span><span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">Context</span><span class="special">,</span> <span class="number">2</span><span class="special">></span> 5601<span class="special">{</span> 5602 <span class="keyword">typedef</span> <span class="keyword">void</span> <span class="identifier">result_type</span><span class="special">;</span> 5603 5604 <span class="keyword">void</span> <span class="keyword">operator</span><span class="special">()(</span><span class="identifier">Expr</span> <span class="special">&</span><span class="identifier">expr</span><span class="special">,</span> <span class="identifier">Context</span> <span class="special">&</span><span class="identifier">ctx</span><span class="special">)</span> <span class="keyword">const</span> 5605 <span class="special">{</span> 5606 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">eval</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">child_c</span><span class="special"><</span><span class="number">0</span><span class="special">>(</span><span class="identifier">expr</span><span class="special">),</span> <span class="identifier">ctx</span><span class="special">);</span> 5607 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">eval</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">child_c</span><span class="special"><</span><span class="number">1</span><span class="special">>(</span><span class="identifier">expr</span><span class="special">),</span> <span class="identifier">ctx</span><span class="special">);</span> 5608 <span class="special">}</span> 5609<span class="special">};</span> 5610</pre> 5611<p> 5612 When would such classes be useful? Imagine you have an expression tree 5613 with integer terminals, and you would like to increment each integer 5614 in-place. You might define an evaluation context as follows: 5615 </p> 5616<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">increment_ints</span> 5617<span class="special">{</span> 5618 <span class="comment">// By default, just evaluate all children by delegating</span> 5619 <span class="comment">// to the null_eval<></span> 5620 <span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Arg</span> <span class="special">=</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">child</span><span class="special"><</span><span class="identifier">Expr</span><span class="special">>::</span><span class="identifier">type</span><span class="special">></span> 5621 <span class="keyword">struct</span> <span class="identifier">eval</span> 5622 <span class="special">:</span> <span class="identifier">null_eval</span><span class="special"><</span><span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">increment_ints</span> <span class="keyword">const</span><span class="special">></span> 5623 <span class="special">{};</span> 5624 5625 <span class="comment">// Increment integer terminals</span> 5626 <span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">></span> 5627 <span class="keyword">struct</span> <span class="identifier">eval</span><span class="special"><</span><span class="identifier">Expr</span><span class="special">,</span> <span class="keyword">int</span><span class="special">></span> 5628 <span class="special">{</span> 5629 <span class="keyword">typedef</span> <span class="keyword">void</span> <span class="identifier">result_type</span><span class="special">;</span> 5630 5631 <span class="keyword">void</span> <span class="keyword">operator</span><span class="special">()(</span><span class="identifier">Expr</span> <span class="special">&</span><span class="identifier">expr</span><span class="special">,</span> <span class="identifier">increment_ints</span> <span class="keyword">const</span> <span class="special">&)</span> <span class="keyword">const</span> 5632 <span class="special">{</span> 5633 <span class="special">++</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">child</span><span class="special">(</span><span class="identifier">expr</span><span class="special">);</span> 5634 <span class="special">}</span> 5635 <span class="special">};</span> 5636<span class="special">};</span> 5637</pre> 5638<p> 5639 In the next section on <code class="computeroutput"><a class="link" href="../boost/proto/context/callable_context.html" title="Struct template callable_context">proto::callable_context<></a></code>, 5640 we'll see an even simpler way to achieve the same thing. 5641 </p> 5642</div> 5643<div class="section"> 5644<div class="titlepage"><div><div><h6 class="title"> 5645<a name="boost_proto.users_guide.back_end.expression_evaluation.canned_contexts.callable_context"></a><a class="link" href="users_guide.html#boost_proto.users_guide.back_end.expression_evaluation.canned_contexts.callable_context" title="callable_context<>"><code class="literal">callable_context<></code></a> 5646</h6></div></div></div> 5647<p> 5648 The <code class="computeroutput"><a class="link" href="../boost/proto/context/callable_context.html" title="Struct template callable_context">proto::callable_context<></a></code> 5649 is a helper that simplifies the job of writing context classes. Rather 5650 than writing template specializations, with <code class="computeroutput"><a class="link" href="../boost/proto/context/callable_context.html" title="Struct template callable_context">proto::callable_context<></a></code> 5651 you write a function object with an overloaded function call operator. 5652 Any expressions not handled by an overload are automatically dispatched 5653 to a default evaluation context that you can specify. 5654 </p> 5655<p> 5656 Rather than an evaluation context in its own right, <code class="computeroutput"><a class="link" href="../boost/proto/context/callable_context.html" title="Struct template callable_context">proto::callable_context<></a></code> 5657 is more properly thought of as a context adaptor. To use it, you must 5658 define your own context that inherits from <code class="computeroutput"><a class="link" href="../boost/proto/context/callable_context.html" title="Struct template callable_context">proto::callable_context<></a></code>. 5659 </p> 5660<p> 5661 In the <a class="link" href="users_guide.html#boost_proto.users_guide.back_end.expression_evaluation.canned_contexts.null_context" title="null_context"><code class="literal">null_context</code></a> 5662 section, we saw how to implement an evaluation context that increments 5663 all the integers within an expression tree. Here is how to do the same 5664 thing with the <code class="computeroutput"><a class="link" href="../boost/proto/context/callable_context.html" title="Struct template callable_context">proto::callable_context<></a></code>: 5665 </p> 5666<pre class="programlisting"><span class="comment">// An evaluation context that increments all</span> 5667<span class="comment">// integer terminals in-place.</span> 5668<span class="keyword">struct</span> <span class="identifier">increment_ints</span> 5669 <span class="special">:</span> <span class="identifier">callable_context</span><span class="special"><</span> 5670 <span class="identifier">increment_ints</span> <span class="keyword">const</span> <span class="comment">// derived context</span> 5671 <span class="special">,</span> <span class="identifier">null_context</span> <span class="keyword">const</span> <span class="comment">// fall-back context</span> 5672 <span class="special">></span> 5673<span class="special">{</span> 5674 <span class="keyword">typedef</span> <span class="keyword">void</span> <span class="identifier">result_type</span><span class="special">;</span> 5675 5676 <span class="comment">// Handle int terminals here:</span> 5677 <span class="keyword">void</span> <span class="keyword">operator</span><span class="special">()(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">,</span> <span class="keyword">int</span> <span class="special">&</span><span class="identifier">i</span><span class="special">)</span> <span class="keyword">const</span> 5678 <span class="special">{</span> 5679 <span class="special">++</span><span class="identifier">i</span><span class="special">;</span> 5680 <span class="special">}</span> 5681<span class="special">};</span> 5682</pre> 5683<p> 5684 With such a context, we can do the following: 5685 </p> 5686<pre class="programlisting"><span class="identifier">literal</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> <span class="identifier">i</span> <span class="special">=</span> <span class="number">0</span><span class="special">,</span> <span class="identifier">j</span> <span class="special">=</span> <span class="number">10</span><span class="special">;</span> 5687<span class="identifier">proto</span><span class="special">::</span><span class="identifier">eval</span><span class="special">(</span> <span class="identifier">i</span> <span class="special">-</span> <span class="identifier">j</span> <span class="special">*</span> <span class="number">3.14</span><span class="special">,</span> <span class="identifier">increment_ints</span><span class="special">()</span> <span class="special">);</span> 5688 5689<span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="string">"i = "</span> <span class="special"><<</span> <span class="identifier">i</span><span class="special">.</span><span class="identifier">get</span><span class="special">()</span> <span class="special"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span> 5690<span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="string">"j = "</span> <span class="special"><<</span> <span class="identifier">j</span><span class="special">.</span><span class="identifier">get</span><span class="special">()</span> <span class="special"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span> 5691</pre> 5692<p> 5693 This program outputs the following, which shows that the integers 5694 <code class="computeroutput"><span class="identifier">i</span></code> and <code class="computeroutput"><span class="identifier">j</span></code> have been incremented by <code class="computeroutput"><span class="number">1</span></code>: 5695 </p> 5696<pre class="programlisting">i = 1 5697j = 11 5698</pre> 5699<p> 5700 In the <code class="computeroutput"><span class="identifier">increment_ints</span></code> 5701 context, we didn't have to define any nested <code class="computeroutput"><span class="identifier">eval</span><span class="special"><></span></code> templates. That's because 5702 <code class="computeroutput"><a class="link" href="../boost/proto/context/callable_context.html" title="Struct template callable_context">proto::callable_context<></a></code> 5703 implements them for us. <code class="computeroutput"><a class="link" href="../boost/proto/context/callable_context.html" title="Struct template callable_context">proto::callable_context<></a></code> 5704 takes two template parameters: the derived context and a fall-back 5705 context. For each node in the expression tree being evaluated, <code class="computeroutput"><a class="link" href="../boost/proto/context/callable_context.html" title="Struct template callable_context">proto::callable_context<></a></code> checks to see if 5706 there is an overloaded <code class="computeroutput"><span class="keyword">operator</span><span class="special">()</span></code> in the derived context that accepts 5707 it. Given some expression <code class="computeroutput"><span class="identifier">expr</span></code> 5708 of type <code class="computeroutput"><span class="identifier">Expr</span></code>, and a 5709 context <code class="computeroutput"><span class="identifier">ctx</span></code>, it attempts 5710 to call: 5711 </p> 5712<pre class="programlisting"><span class="identifier">ctx</span><span class="special">(</span> 5713 <span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">::</span><span class="identifier">proto_tag</span><span class="special">()</span> 5714 <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">child_c</span><span class="special"><</span><span class="number">0</span><span class="special">>(</span><span class="identifier">expr</span><span class="special">)</span> 5715 <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">child_c</span><span class="special"><</span><span class="number">1</span><span class="special">>(</span><span class="identifier">expr</span><span class="special">)</span> 5716 <span class="special">...</span> 5717<span class="special">);</span> 5718</pre> 5719<p> 5720 Using function overloading and metaprogramming tricks, <code class="computeroutput"><a class="link" href="../boost/proto/context/callable_context.html" title="Struct template callable_context">proto::callable_context<></a></code> 5721 can detect at compile-time whether such a function exists or not. If 5722 so, that function is called. If not, the current expression is passed 5723 to the fall-back evaluation context to be processed. 5724 </p> 5725<p> 5726 We saw another example of the <code class="computeroutput"><a class="link" href="../boost/proto/context/callable_context.html" title="Struct template callable_context">proto::callable_context<></a></code> 5727 when we looked at the simple calculator expression evaluator. There, 5728 we wanted to customize the evaluation of placeholder terminals, and 5729 delegate the handling of all other nodes to the <code class="computeroutput"><a class="link" href="../boost/proto/context/default_context.html" title="Struct default_context">proto::default_context</a></code>. We did 5730 that as follows: 5731 </p> 5732<pre class="programlisting"><span class="comment">// An evaluation context for calculator expressions that</span> 5733<span class="comment">// explicitly handles placeholder terminals, but defers the</span> 5734<span class="comment">// processing of all other nodes to the default_context.</span> 5735<span class="keyword">struct</span> <span class="identifier">calculator_context</span> 5736 <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">callable_context</span><span class="special"><</span> <span class="identifier">calculator_context</span> <span class="keyword">const</span> <span class="special">></span> 5737<span class="special">{</span> 5738 <span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special"><</span><span class="keyword">double</span><span class="special">></span> <span class="identifier">args</span><span class="special">;</span> 5739 5740 <span class="comment">// Define the result type of the calculator.</span> 5741 <span class="keyword">typedef</span> <span class="keyword">double</span> <span class="identifier">result_type</span><span class="special">;</span> 5742 5743 <span class="comment">// Handle the placeholders:</span> 5744 <span class="keyword">template</span><span class="special"><</span><span class="keyword">int</span> <span class="identifier">I</span><span class="special">></span> 5745 <span class="keyword">double</span> <span class="keyword">operator</span><span class="special">()(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">,</span> <span class="identifier">placeholder</span><span class="special"><</span><span class="identifier">I</span><span class="special">>)</span> <span class="keyword">const</span> 5746 <span class="special">{</span> 5747 <span class="keyword">return</span> <span class="keyword">this</span><span class="special">-></span><span class="identifier">args</span><span class="special">[</span><span class="identifier">I</span><span class="special">];</span> 5748 <span class="special">}</span> 5749<span class="special">};</span> 5750</pre> 5751<p> 5752 In this case, we didn't specify a fall-back context. In that case, 5753 <code class="computeroutput"><a class="link" href="../boost/proto/context/callable_context.html" title="Struct template callable_context">proto::callable_context<></a></code> 5754 uses the <code class="computeroutput"><a class="link" href="../boost/proto/context/default_context.html" title="Struct default_context">proto::default_context</a></code>. With 5755 the above <code class="computeroutput"><span class="identifier">calculator_context</span></code> 5756 and a couple of appropriately defined placeholder terminals, we can 5757 evaluate calculator expressions, as demonstrated below: 5758 </p> 5759<pre class="programlisting"><span class="keyword">template</span><span class="special"><</span><span class="keyword">int</span> <span class="identifier">I</span><span class="special">></span> 5760<span class="keyword">struct</span> <span class="identifier">placeholder</span> 5761<span class="special">{};</span> 5762 5763<span class="identifier">terminal</span><span class="special"><</span><span class="identifier">placeholder</span><span class="special"><</span><span class="number">0</span><span class="special">></span> <span class="special">>::</span><span class="identifier">type</span> <span class="keyword">const</span> <span class="identifier">_1</span> <span class="special">=</span> <span class="special">{{}};</span> 5764<span class="identifier">terminal</span><span class="special"><</span><span class="identifier">placeholder</span><span class="special"><</span><span class="number">1</span><span class="special">></span> <span class="special">>::</span><span class="identifier">type</span> <span class="keyword">const</span> <span class="identifier">_2</span> <span class="special">=</span> <span class="special">{{}};</span> 5765<span class="comment">// ...</span> 5766 5767<span class="identifier">calculator_context</span> <span class="identifier">ctx</span><span class="special">;</span> 5768<span class="identifier">ctx</span><span class="special">.</span><span class="identifier">args</span><span class="special">.</span><span class="identifier">push_back</span><span class="special">(</span><span class="number">4</span><span class="special">);</span> 5769<span class="identifier">ctx</span><span class="special">.</span><span class="identifier">args</span><span class="special">.</span><span class="identifier">push_back</span><span class="special">(</span><span class="number">5</span><span class="special">);</span> 5770 5771<span class="keyword">double</span> <span class="identifier">j</span> <span class="special">=</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">eval</span><span class="special">(</span> <span class="special">(</span><span class="identifier">_2</span> <span class="special">-</span> <span class="identifier">_1</span><span class="special">)</span> <span class="special">/</span> <span class="identifier">_2</span> <span class="special">*</span> <span class="number">100</span><span class="special">,</span> <span class="identifier">ctx</span> <span class="special">);</span> 5772<span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="string">"j = "</span> <span class="special"><<</span> <span class="identifier">j</span> <span class="special"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span> 5773</pre> 5774<p> 5775 The above code displays the following: 5776 </p> 5777<pre class="programlisting">j = 20 5778</pre> 5779</div> 5780</div> 5781</div> 5782<div class="section"> 5783<div class="titlepage"><div><div><h4 class="title"> 5784<a name="boost_proto.users_guide.back_end.expression_transformation"></a><a class="link" href="users_guide.html#boost_proto.users_guide.back_end.expression_transformation" title="Expression Transformation: Semantic Actions">Expression 5785 Transformation: Semantic Actions</a> 5786</h4></div></div></div> 5787<div class="toc"><dl class="toc"> 5788<dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.back_end.expression_transformation.__activating__your_grammars"><span class="quote">“<span class="quote">Activating</span>”</span> 5789 Your Grammars</a></span></dt> 5790<dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.back_end.expression_transformation.handling_alternation_and_recursion">Handling 5791 Alternation and Recursion</a></span></dt> 5792<dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.back_end.expression_transformation.callable_transforms">Callable 5793 Transforms</a></span></dt> 5794<dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.back_end.expression_transformation.object_transforms">Object 5795 Transforms</a></span></dt> 5796<dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.back_end.expression_transformation.example__calculator_arity">Example: 5797 Calculator Arity</a></span></dt> 5798<dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.back_end.expression_transformation.state">Transforms 5799 With State Accumulation</a></span></dt> 5800<dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.back_end.expression_transformation.data">Passing 5801 Auxiliary Data to Transforms</a></span></dt> 5802<dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.back_end.expression_transformation.implicit_params">Implicit 5803 Parameters to Primitive Transforms</a></span></dt> 5804<dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.back_end.expression_transformation.unpacking_expressions">Unpacking 5805 Expressions</a></span></dt> 5806<dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.back_end.expression_transformation.external_transforms">Separating 5807 Grammars And Transforms</a></span></dt> 5808<dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.back_end.expression_transformation.canned_transforms">Proto's 5809 Built-In Transforms</a></span></dt> 5810<dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.back_end.expression_transformation.primitives">Building 5811 Custom Primitive Transforms</a></span></dt> 5812<dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.back_end.expression_transformation.is_callable">Making 5813 Your Transform Callable</a></span></dt> 5814</dl></div> 5815<p> 5816 If you have ever built a parser with the help of a tool like Antlr, yacc 5817 or Boost.Spirit, you might be familiar with <span class="emphasis"><em>semantic actions</em></span>. 5818 In addition to allowing you to define the grammar of the language recognized 5819 by the parser, these tools let you embed code within your grammar that 5820 executes when parts of the grammar participate in a parse. Proto has the 5821 equivalent of semantic actions. They are called <span class="emphasis"><em>transforms</em></span>. 5822 This section describes how to embed transforms within your Proto grammars, 5823 turning your grammars into function objects that can manipulate or evaluate 5824 expressions in powerful ways. 5825 </p> 5826<p> 5827 Proto transforms are an advanced topic. We'll take it slow, using examples 5828 to illustrate the key concepts, starting simple. 5829 </p> 5830<div class="section"> 5831<div class="titlepage"><div><div><h5 class="title"> 5832<a name="boost_proto.users_guide.back_end.expression_transformation.__activating__your_grammars"></a><a class="link" href="users_guide.html#boost_proto.users_guide.back_end.expression_transformation.__activating__your_grammars" title="“Activating” Your Grammars"><span class="quote">“<span class="quote">Activating</span>”</span> 5833 Your Grammars</a> 5834</h5></div></div></div> 5835<p> 5836 The Proto grammars we've seen so far are static. You can check at compile-time 5837 to see if an expression type matches a grammar, but that's it. Things 5838 get more interesting when you give them runtime behaviors. A grammar 5839 with embedded transforms is more than just a static grammar. It is a 5840 function object that accepts expressions that match the grammar and does 5841 <span class="emphasis"><em>something</em></span> with them. 5842 </p> 5843<p> 5844 Below is a very simple grammar. It matches terminal expressions. 5845 </p> 5846<pre class="programlisting"><span class="comment">// A simple Proto grammar that matches all terminals</span> 5847<span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special"><</span> <span class="identifier">_</span> <span class="special">></span> 5848</pre> 5849<p> 5850 Here is the same grammar with a transform that extracts the value from 5851 the terminal: 5852 </p> 5853<pre class="programlisting"><span class="comment">// A simple Proto grammar that matches all terminals</span> 5854<span class="comment">// *and* a function object that extracts the value from</span> 5855<span class="comment">// the terminal</span> 5856<span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special"><</span> 5857 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special"><</span> <span class="identifier">_</span> <span class="special">></span> 5858 <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_value</span> <span class="comment">// <-- Look, a transform!</span> 5859<span class="special">></span> 5860</pre> 5861<p> 5862 You can read this as follows: when you match a terminal expression, extract 5863 the value. The type <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_value</span></code> 5864 is a so-called transform. Later we'll see what makes it a transform, 5865 but for now just think of it as a kind of function object. Note the use 5866 of <code class="computeroutput"><a class="link" href="../boost/proto/when.html" title="Struct template when">proto::when<></a></code>: the first template 5867 parameter is the grammar to match and the second is the transform to 5868 execute. The result is both a grammar that matches terminal expressions 5869 and a function object that accepts terminal expressions and extracts 5870 their values. 5871 </p> 5872<p> 5873 As with ordinary grammars, we can define an empty struct that inherits 5874 from a grammar+transform to give us an easy way to refer back to the 5875 thing we're defining, as follows: 5876 </p> 5877<pre class="programlisting"><span class="comment">// A grammar and a function object, as before</span> 5878<span class="keyword">struct</span> <span class="identifier">Value</span> 5879 <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special"><</span> 5880 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special"><</span> <span class="identifier">_</span> <span class="special">></span> 5881 <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_value</span> 5882 <span class="special">></span> 5883<span class="special">{};</span> 5884 5885<span class="comment">// "Value" is a grammar that matches terminal expressions</span> 5886<span class="identifier">BOOST_MPL_ASSERT</span><span class="special">((</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">matches</span><span class="special"><</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special"><</span><span class="keyword">int</span><span class="special">>::</span><span class="identifier">type</span><span class="special">,</span> <span class="identifier">Value</span> <span class="special">></span> <span class="special">));</span> 5887 5888<span class="comment">// "Value" also defines a function object that accepts terminals</span> 5889<span class="comment">// and extracts their value.</span> 5890<span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special"><</span><span class="keyword">int</span><span class="special">>::</span><span class="identifier">type</span> <span class="identifier">answer</span> <span class="special">=</span> <span class="special">{</span><span class="number">42</span><span class="special">};</span> 5891<span class="identifier">Value</span> <span class="identifier">get_value</span><span class="special">;</span> 5892<span class="keyword">int</span> <span class="identifier">i</span> <span class="special">=</span> <span class="identifier">get_value</span><span class="special">(</span> <span class="identifier">answer</span> <span class="special">);</span> 5893</pre> 5894<p> 5895 As already mentioned, <code class="computeroutput"><span class="identifier">Value</span></code> 5896 is a grammar that matches terminal expressions and a function object 5897 that operates on terminal expressions. It would be an error to pass a 5898 non-terminal expression to the <code class="computeroutput"><span class="identifier">Value</span></code> 5899 function object. This is a general property of grammars with transforms; 5900 when using them as function objects, expressions passed to them must 5901 match the grammar. 5902 </p> 5903<p> 5904 Proto grammars are valid TR1-style function objects. That means you can 5905 use <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">result_of</span><span class="special"><></span></code> 5906 to ask a grammar what its return type will be, given a particular expression 5907 type. For instance, we can access the <code class="computeroutput"><span class="identifier">Value</span></code> 5908 grammar's return type as follows: 5909 </p> 5910<pre class="programlisting"><span class="comment">// We can use boost::result_of<> to get the return type</span> 5911<span class="comment">// of a Proto grammar.</span> 5912<span class="keyword">typedef</span> 5913 <span class="keyword">typename</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">result_of</span><span class="special"><</span><span class="identifier">Value</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special"><</span><span class="keyword">int</span><span class="special">>::</span><span class="identifier">type</span><span class="special">)>::</span><span class="identifier">type</span> 5914<span class="identifier">result_type</span><span class="special">;</span> 5915 5916<span class="comment">// Check that we got the type we expected</span> 5917<span class="identifier">BOOST_MPL_ASSERT</span><span class="special">((</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">is_same</span><span class="special"><</span><span class="identifier">result_type</span><span class="special">,</span> <span class="keyword">int</span><span class="special">></span> <span class="special">));</span> 5918</pre> 5919<div class="note"><table border="0" summary="Note"> 5920<tr> 5921<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../doc/src/images/note.png"></td> 5922<th align="left">Note</th> 5923</tr> 5924<tr><td align="left" valign="top"><p> 5925 A grammar with embedded transforms is both a grammar and a function 5926 object. Calling these things "grammars with transforms" would 5927 get tedious. We could call them something like "active grammars", 5928 but as we'll see <span class="emphasis"><em>every</em></span> grammar that you can define 5929 with Proto is "active"; that is, every grammar has some behavior 5930 when used as a function object. So we'll continue calling these things 5931 plain "grammars". The term "transform" is reserved 5932 for the thing that is used as the second parameter to the <code class="computeroutput"><a class="link" href="../boost/proto/when.html" title="Struct template when">proto::when<></a></code> template. 5933 </p></td></tr> 5934</table></div> 5935</div> 5936<div class="section"> 5937<div class="titlepage"><div><div><h5 class="title"> 5938<a name="boost_proto.users_guide.back_end.expression_transformation.handling_alternation_and_recursion"></a><a class="link" href="users_guide.html#boost_proto.users_guide.back_end.expression_transformation.handling_alternation_and_recursion" title="Handling Alternation and Recursion">Handling 5939 Alternation and Recursion</a> 5940</h5></div></div></div> 5941<p> 5942 Most grammars are a little more complicated than the one in the preceding 5943 section. For the sake of illustration, let's define a rather nonsensical 5944 grammar that matches any expression and recurses to the leftmost terminal 5945 and returns its value. It will demonstrate how two key concepts of Proto 5946 grammars -- alternation and recursion -- interact with transforms. The 5947 grammar is described below. 5948 </p> 5949<pre class="programlisting"><span class="comment">// A grammar that matches any expression, and a function object</span> 5950<span class="comment">// that returns the value of the leftmost terminal.</span> 5951<span class="keyword">struct</span> <span class="identifier">LeftmostLeaf</span> 5952 <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">or_</span><span class="special"><</span> 5953 <span class="comment">// If the expression is a terminal, return its value</span> 5954 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special"><</span> 5955 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special"><</span> <span class="identifier">_</span> <span class="special">></span> 5956 <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_value</span> 5957 <span class="special">></span> 5958 <span class="comment">// Otherwise, it is a non-terminal. Return the result</span> 5959 <span class="comment">// of invoking LeftmostLeaf on the 0th (leftmost) child.</span> 5960 <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special"><</span> 5961 <span class="identifier">_</span> 5962 <span class="special">,</span> <span class="identifier">LeftmostLeaf</span><span class="special">(</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_child0</span> <span class="special">)</span> 5963 <span class="special">></span> 5964 <span class="special">></span> 5965<span class="special">{};</span> 5966 5967<span class="comment">// A Proto terminal wrapping std::cout</span> 5968<span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special"><</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">ostream</span> <span class="special">&</span> <span class="special">>::</span><span class="identifier">type</span> <span class="identifier">cout_</span> <span class="special">=</span> <span class="special">{</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">};</span> 5969 5970<span class="comment">// Create an expression and use LeftmostLeaf to extract the</span> 5971<span class="comment">// value of the leftmost terminal, which will be std::cout.</span> 5972<span class="identifier">std</span><span class="special">::</span><span class="identifier">ostream</span> <span class="special">&</span> <span class="identifier">sout</span> <span class="special">=</span> <span class="identifier">LeftmostLeaf</span><span class="special">()(</span> <span class="identifier">cout_</span> <span class="special"><<</span> <span class="string">"the answer: "</span> <span class="special"><<</span> <span class="number">42</span> <span class="special"><<</span> <span class="char">'\n'</span> <span class="special">);</span> 5973</pre> 5974<p> 5975 We've seen <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">or_</span><span class="special"><></span></code> 5976 before. Here it is serving two roles. First, it is a grammar that matches 5977 any of its alternate sub-grammars; in this case, either a terminal or 5978 a non-terminal. Second, it is also a function object that accepts an 5979 expression, finds the alternate sub-grammar that matches the expression, 5980 and applies its transform. And since <code class="computeroutput"><span class="identifier">LeftmostLeaf</span></code> 5981 inherits from <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">or_</span><span class="special"><></span></code>, 5982 <code class="computeroutput"><span class="identifier">LeftmostLeaf</span></code> is also 5983 both a grammar and a function object. 5984 </p> 5985<div class="note"><table border="0" summary="Note"> 5986<tr> 5987<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../doc/src/images/note.png"></td> 5988<th align="left">Note</th> 5989</tr> 5990<tr><td align="left" valign="top"><p> 5991 The second alternate uses <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_</span></code> 5992 as its grammar. Recall that <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_</span></code> 5993 is the wildcard grammar that matches any expression. Since alternates 5994 in <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">or_</span><span class="special"><></span></code> 5995 are tried in order, and since the first alternate handles all terminals, 5996 the second alternate handles all (and only) non-terminals. Often enough, 5997 <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special"><</span> 5998 <span class="identifier">_</span><span class="special">,</span> 5999 <em class="replaceable"><code>some-transform</code></em> <span class="special">></span></code> 6000 is the last alternate in a grammar, so for improved readability, you 6001 could use the equivalent <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">otherwise</span><span class="special"><</span> <em class="replaceable"><code>some-transform</code></em> 6002 <span class="special">></span></code>. 6003 </p></td></tr> 6004</table></div> 6005<p> 6006 The next section describes this grammar further. 6007 </p> 6008</div> 6009<div class="section"> 6010<div class="titlepage"><div><div><h5 class="title"> 6011<a name="boost_proto.users_guide.back_end.expression_transformation.callable_transforms"></a><a class="link" href="users_guide.html#boost_proto.users_guide.back_end.expression_transformation.callable_transforms" title="Callable Transforms">Callable 6012 Transforms</a> 6013</h5></div></div></div> 6014<p> 6015 In the grammar defined in the preceding section, the transform associated 6016 with non-terminals is a little strange-looking: 6017 </p> 6018<pre class="programlisting"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special"><</span> 6019 <span class="identifier">_</span> 6020 <span class="special">,</span> <span class="bold"><strong>LeftmostLeaf( proto::_child0 )</strong></span> <span class="comment">// <-- a "callable" transform</span> 6021<span class="special">></span> 6022</pre> 6023<p> 6024 It has the effect of accepting non-terminal expressions, taking the 0th 6025 (leftmost) child and recursively invoking the <code class="computeroutput"><span class="identifier">LeftmostLeaf</span></code> 6026 function on it. But <code class="computeroutput"><span class="identifier">LeftmostLeaf</span><span class="special">(</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_child0</span> 6027 <span class="special">)</span></code> is actually a <span class="emphasis"><em>function 6028 type</em></span>. Literally, it is the type of a function that accepts 6029 an object of type <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_child0</span></code> 6030 and returns an object of type <code class="computeroutput"><span class="identifier">LeftmostLeaf</span></code>. 6031 So how do we make sense of this transform? Clearly, there is no function 6032 that actually has this signature, nor would such a function be useful. 6033 The key is in understanding how <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special"><></span></code> <span class="emphasis"><em>interprets</em></span> 6034 its second template parameter. 6035 </p> 6036<p> 6037 When the second template parameter to <code class="computeroutput"><a class="link" href="../boost/proto/when.html" title="Struct template when">proto::when<></a></code> 6038 is a function type, <code class="computeroutput"><a class="link" href="../boost/proto/when.html" title="Struct template when">proto::when<></a></code> 6039 interprets the function type as a transform. In this case, <code class="computeroutput"><span class="identifier">LeftmostLeaf</span></code> is treated as the type 6040 of a function object to invoke, and <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_child0</span></code> 6041 is treated as a transform. First, <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_child0</span></code> 6042 is applied to the current expression (the non-terminal that matched this 6043 alternate sub-grammar), and the result (the 0th child) is passed as an 6044 argument to <code class="computeroutput"><span class="identifier">LeftmostLeaf</span></code>. 6045 </p> 6046<div class="note"><table border="0" summary="Note"> 6047<tr> 6048<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../doc/src/images/note.png"></td> 6049<th align="left">Note</th> 6050</tr> 6051<tr><td align="left" valign="top"> 6052<p> 6053 <span class="bold"><strong>Transforms are a Domain-Specific Language</strong></span> 6054 </p> 6055<p> 6056 <code class="computeroutput"><span class="identifier">LeftmostLeaf</span><span class="special">(</span> 6057 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_child0</span> <span class="special">)</span></code> 6058 <span class="emphasis"><em>looks</em></span> like an invocation of the <code class="computeroutput"><span class="identifier">LeftmostLeaf</span></code> function object, but 6059 it's not, but then it actually is! Why this confusing subterfuge? Function 6060 types give us a natural and concise syntax for composing more complicated 6061 transforms from simpler ones. The fact that the syntax is suggestive 6062 of a function invocation is on purpose. It is an embedded domain-specific 6063 language for defining expression transformations. If the subterfuge 6064 worked, it may have fooled you into thinking the transform is doing 6065 exactly what it actually does! And that's the point. 6066 </p> 6067</td></tr> 6068</table></div> 6069<p> 6070 The type <code class="computeroutput"><span class="identifier">LeftmostLeaf</span><span class="special">(</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_child0</span> 6071 <span class="special">)</span></code> is an example of a <span class="emphasis"><em>callable 6072 transform</em></span>. It is a function type that represents a function 6073 object to call and its arguments. The types <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_child0</span></code> 6074 and <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_value</span></code> are <span class="emphasis"><em>primitive transforms</em></span>. 6075 They are plain structs, not unlike function objects, from which callable 6076 transforms can be composed. There is one other type of transform, <span class="emphasis"><em>object 6077 transforms</em></span>, that we'll encounter next. 6078 </p> 6079</div> 6080<div class="section"> 6081<div class="titlepage"><div><div><h5 class="title"> 6082<a name="boost_proto.users_guide.back_end.expression_transformation.object_transforms"></a><a class="link" href="users_guide.html#boost_proto.users_guide.back_end.expression_transformation.object_transforms" title="Object Transforms">Object 6083 Transforms</a> 6084</h5></div></div></div> 6085<p> 6086 The very first transform we looked at simply extracted the value of terminals. 6087 Let's do the same thing, but this time we'll promote all ints to longs 6088 first. (Please forgive the contrived-ness of the examples so far; they 6089 get more interesting later.) Here's the grammar: 6090 </p> 6091<pre class="programlisting"><span class="comment">// A simple Proto grammar that matches all terminals,</span> 6092<span class="comment">// and a function object that extracts the value from</span> 6093<span class="comment">// the terminal, promoting ints to longs:</span> 6094<span class="keyword">struct</span> <span class="identifier">ValueWithPomote</span> 6095 <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">or_</span><span class="special"><</span> 6096 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special"><</span> 6097 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special"><</span> <span class="keyword">int</span> <span class="special">></span> 6098 <span class="special">,</span> <span class="keyword">long</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_value</span><span class="special">)</span> <span class="comment">// <-- an "object" transform</span> 6099 <span class="special">></span> 6100 <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special"><</span> 6101 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special"><</span> <span class="identifier">_</span> <span class="special">></span> 6102 <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_value</span> 6103 <span class="special">></span> 6104 <span class="special">></span> 6105<span class="special">{};</span> 6106</pre> 6107<p> 6108 You can read the above grammar as follows: when you match an int terminal, 6109 extract the value from the terminal and use it to initialize a long; 6110 otherwise, when you match another kind of terminal, just extract the 6111 value. The type <code class="computeroutput"><span class="keyword">long</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_value</span><span class="special">)</span></code> 6112 is a so-called <span class="emphasis"><em>object</em></span> transform. It looks like the 6113 creation of a temporary long, but it's really a function type. Just as 6114 a callable transform is a function type that represents a function to 6115 call and its arguments, an object transforms is a function type that 6116 represents an object to construct and the arguments to its constructor. 6117 </p> 6118<div class="note"><table border="0" summary="Note"> 6119<tr> 6120<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../doc/src/images/note.png"></td> 6121<th align="left">Note</th> 6122</tr> 6123<tr><td align="left" valign="top"> 6124<p> 6125 <span class="bold"><strong>Object Transforms vs. Callable Transforms</strong></span> 6126 </p> 6127<p> 6128 When using function types as Proto transforms, they can either represent 6129 an object to construct or a function to call. It is similar to "normal" 6130 C++ where the syntax <code class="computeroutput"><span class="identifier">foo</span><span class="special">(</span><span class="string">"arg"</span><span class="special">)</span></code> can either be interpreted as an object 6131 to construct or a function to call, depending on whether <code class="computeroutput"><span class="identifier">foo</span></code> is a type or a function. But 6132 consider two of the transforms we've seen so far: 6133 </p> 6134<p> 6135</p> 6136<pre class="programlisting"><span class="identifier">LeftmostLeaf</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_child0</span><span class="special">)</span> <span class="comment">// <-- a callable transform</span> 6137<span class="keyword">long</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_value</span><span class="special">)</span> <span class="comment">// <-- an object transform</span> 6138</pre> 6139<p> 6140 </p> 6141<p> 6142 Proto can't know in general which is which, so it uses a trait, <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">is_callable</span><span class="special"><></span></code>, 6143 to differentiate. <code class="computeroutput"><span class="identifier">is_callable</span><span class="special"><</span> <span class="keyword">long</span> 6144 <span class="special">>::</span><span class="identifier">value</span></code> 6145 is false so <code class="computeroutput"><span class="keyword">long</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_value</span><span class="special">)</span></code> 6146 is an object to construct, but <code class="computeroutput"><span class="identifier">is_callable</span><span class="special"><</span> <span class="identifier">LeftmostLeaf</span> 6147 <span class="special">>::</span><span class="identifier">value</span></code> 6148 is true so <code class="computeroutput"><span class="identifier">LeftmostLeaf</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_child0</span><span class="special">)</span></code> is a function to call. Later on, we'll 6149 see how Proto recognizes a type as "callable". 6150 </p> 6151</td></tr> 6152</table></div> 6153</div> 6154<div class="section"> 6155<div class="titlepage"><div><div><h5 class="title"> 6156<a name="boost_proto.users_guide.back_end.expression_transformation.example__calculator_arity"></a><a class="link" href="users_guide.html#boost_proto.users_guide.back_end.expression_transformation.example__calculator_arity" title="Example: Calculator Arity">Example: 6157 Calculator Arity</a> 6158</h5></div></div></div> 6159<p> 6160 Now that we have the basics of Proto transforms down, let's consider 6161 a slightly more realistic example. We can use transforms to improve the 6162 type-safety of the <a class="link" href="users_guide.html#boost_proto.users_guide.getting_started.hello_calculator" title="Hello Calculator">calculator 6163 EDSL</a>. If you recall, it lets you write infix arithmetic expressions 6164 involving argument placeholders like <code class="computeroutput"><span class="identifier">_1</span></code> 6165 and <code class="computeroutput"><span class="identifier">_2</span></code> and pass them 6166 to STL algorithms as function objects, as follows: 6167 </p> 6168<pre class="programlisting"><span class="keyword">double</span> <span class="identifier">a1</span><span class="special">[</span><span class="number">4</span><span class="special">]</span> <span class="special">=</span> <span class="special">{</span> <span class="number">56</span><span class="special">,</span> <span class="number">84</span><span class="special">,</span> <span class="number">37</span><span class="special">,</span> <span class="number">69</span> <span class="special">};</span> 6169<span class="keyword">double</span> <span class="identifier">a2</span><span class="special">[</span><span class="number">4</span><span class="special">]</span> <span class="special">=</span> <span class="special">{</span> <span class="number">65</span><span class="special">,</span> <span class="number">120</span><span class="special">,</span> <span class="number">60</span><span class="special">,</span> <span class="number">70</span> <span class="special">};</span> 6170<span class="keyword">double</span> <span class="identifier">a3</span><span class="special">[</span><span class="number">4</span><span class="special">]</span> <span class="special">=</span> <span class="special">{</span> <span class="number">0</span> <span class="special">};</span> 6171 6172<span class="comment">// Use std::transform() and a calculator expression</span> 6173<span class="comment">// to calculate percentages given two input sequences:</span> 6174<span class="identifier">std</span><span class="special">::</span><span class="identifier">transform</span><span class="special">(</span><span class="identifier">a1</span><span class="special">,</span> <span class="identifier">a1</span><span class="special">+</span><span class="number">4</span><span class="special">,</span> <span class="identifier">a2</span><span class="special">,</span> <span class="identifier">a3</span><span class="special">,</span> <span class="special">(</span><span class="identifier">_2</span> <span class="special">-</span> <span class="identifier">_1</span><span class="special">)</span> <span class="special">/</span> <span class="identifier">_2</span> <span class="special">*</span> <span class="number">100</span><span class="special">);</span> 6175</pre> 6176<p> 6177 This works because we gave calculator expressions an <code class="computeroutput"><span class="keyword">operator</span><span class="special">()</span></code> that evaluates the expression, replacing 6178 the placeholders with the arguments to <code class="computeroutput"><span class="keyword">operator</span><span class="special">()</span></code>. The overloaded <code class="computeroutput"><span class="identifier">calculator</span><span class="special"><>::</span><span class="keyword">operator</span><span class="special">()</span></code> looked like this: 6179 </p> 6180<pre class="programlisting"><span class="comment">// Overload operator() to invoke proto::eval() with</span> 6181<span class="comment">// our calculator_context.</span> 6182<span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">></span> 6183<span class="keyword">double</span> 6184<span class="identifier">calculator</span><span class="special"><</span><span class="identifier">Expr</span><span class="special">>::</span><span class="keyword">operator</span><span class="special">()(</span><span class="keyword">double</span> <span class="identifier">a1</span> <span class="special">=</span> <span class="number">0</span><span class="special">,</span> <span class="keyword">double</span> <span class="identifier">a2</span> <span class="special">=</span> <span class="number">0</span><span class="special">)</span> <span class="keyword">const</span> 6185<span class="special">{</span> 6186 <span class="identifier">calculator_context</span> <span class="identifier">ctx</span><span class="special">;</span> 6187 <span class="identifier">ctx</span><span class="special">.</span><span class="identifier">args</span><span class="special">.</span><span class="identifier">push_back</span><span class="special">(</span><span class="identifier">a1</span><span class="special">);</span> 6188 <span class="identifier">ctx</span><span class="special">.</span><span class="identifier">args</span><span class="special">.</span><span class="identifier">push_back</span><span class="special">(</span><span class="identifier">a2</span><span class="special">);</span> 6189 6190 <span class="keyword">return</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">eval</span><span class="special">(*</span><span class="keyword">this</span><span class="special">,</span> <span class="identifier">ctx</span><span class="special">);</span> 6191<span class="special">}</span> 6192</pre> 6193<p> 6194 Although this works, it's not ideal because it doesn't warn users if 6195 they supply too many or too few arguments to a calculator expression. 6196 Consider the following mistakes: 6197 </p> 6198<pre class="programlisting"><span class="special">(</span><span class="identifier">_1</span> <span class="special">*</span> <span class="identifier">_1</span><span class="special">)(</span><span class="number">4</span><span class="special">,</span> <span class="number">2</span><span class="special">);</span> <span class="comment">// Oops, too many arguments!</span> 6199<span class="special">(</span><span class="identifier">_2</span> <span class="special">*</span> <span class="identifier">_2</span><span class="special">)(</span><span class="number">42</span><span class="special">);</span> <span class="comment">// Oops, too few arguments!</span> 6200</pre> 6201<p> 6202 The expression <code class="computeroutput"><span class="identifier">_1</span> <span class="special">*</span> 6203 <span class="identifier">_1</span></code> defines a unary calculator 6204 expression; it takes one argument and squares it. If we pass more than 6205 one argument, the extra arguments will be silently ignored, which might 6206 be surprising to users. The next expression, <code class="computeroutput"><span class="identifier">_2</span> 6207 <span class="special">*</span> <span class="identifier">_2</span></code> 6208 defines a binary calculator expression; it takes two arguments, ignores 6209 the first and squares the second. If we only pass one argument, the code 6210 silently fills in <code class="computeroutput"><span class="number">0.0</span></code> for 6211 the second argument, which is also probably not what users expect. What 6212 can be done? 6213 </p> 6214<p> 6215 We can say that the <span class="emphasis"><em>arity</em></span> of a calculator expression 6216 is the number of arguments it expects, and it is equal to the largest 6217 placeholder in the expression. So, the arity of <code class="computeroutput"><span class="identifier">_1</span> 6218 <span class="special">*</span> <span class="identifier">_1</span></code> 6219 is one, and the arity of <code class="computeroutput"><span class="identifier">_2</span> 6220 <span class="special">*</span> <span class="identifier">_2</span></code> 6221 is two. We can increase the type-safety of our calculator EDSL by making 6222 sure the arity of an expression equals the actual number of arguments 6223 supplied. Computing the arity of an expression is simple with the help 6224 of Proto transforms. 6225 </p> 6226<p> 6227 It's straightforward to describe in words how the arity of an expression 6228 should be calculated. Consider that calculator expressions can be made 6229 of <code class="computeroutput"><span class="identifier">_1</span></code>, <code class="computeroutput"><span class="identifier">_2</span></code>, literals, unary expressions and 6230 binary expressions. The following table shows the arities for each of 6231 these 5 constituents. 6232 </p> 6233<div class="table"> 6234<a name="boost_proto.users_guide.back_end.expression_transformation.example__calculator_arity.t0"></a><p class="title"><b>Table 32.8. Calculator Sub-Expression Arities</b></p> 6235<div class="table-contents"><table class="table" summary="Calculator Sub-Expression Arities"> 6236<colgroup> 6237<col> 6238<col> 6239</colgroup> 6240<thead><tr> 6241<th> 6242 <p> 6243 Sub-Expression 6244 </p> 6245 </th> 6246<th> 6247 <p> 6248 Arity 6249 </p> 6250 </th> 6251</tr></thead> 6252<tbody> 6253<tr> 6254<td> 6255 <p> 6256 Placeholder 1 6257 </p> 6258 </td> 6259<td> 6260 <p> 6261 <code class="computeroutput"><span class="number">1</span></code> 6262 </p> 6263 </td> 6264</tr> 6265<tr> 6266<td> 6267 <p> 6268 Placeholder 2 6269 </p> 6270 </td> 6271<td> 6272 <p> 6273 <code class="computeroutput"><span class="number">2</span></code> 6274 </p> 6275 </td> 6276</tr> 6277<tr> 6278<td> 6279 <p> 6280 Literal 6281 </p> 6282 </td> 6283<td> 6284 <p> 6285 <code class="computeroutput"><span class="number">0</span></code> 6286 </p> 6287 </td> 6288</tr> 6289<tr> 6290<td> 6291 <p> 6292 Unary Expression 6293 </p> 6294 </td> 6295<td> 6296 <p> 6297 <span class="emphasis"><em>arity of the operand</em></span> 6298 </p> 6299 </td> 6300</tr> 6301<tr> 6302<td> 6303 <p> 6304 Binary Expression 6305 </p> 6306 </td> 6307<td> 6308 <p> 6309 <span class="emphasis"><em>max arity of the two operands</em></span> 6310 </p> 6311 </td> 6312</tr> 6313</tbody> 6314</table></div> 6315</div> 6316<br class="table-break"><p> 6317 Using this information, we can write the grammar for calculator expressions 6318 and attach transforms for computing the arity of each constituent. The 6319 code below computes the expression arity as a compile-time integer, using 6320 integral wrappers and metafunctions from the Boost MPL Library. The grammar 6321 is described below. 6322 </p> 6323<p> 6324</p> 6325<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">CalcArity</span> 6326 <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">or_</span><span class="special"><</span> 6327 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special"><</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special"><</span> <span class="identifier">placeholder</span><span class="special"><</span><span class="number">0</span><span class="special">></span> <span class="special">>,</span> 6328 <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">int_</span><span class="special"><</span><span class="number">1</span><span class="special">>()</span> 6329 <span class="special">></span> 6330 <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special"><</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special"><</span> <span class="identifier">placeholder</span><span class="special"><</span><span class="number">1</span><span class="special">></span> <span class="special">>,</span> 6331 <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">int_</span><span class="special"><</span><span class="number">2</span><span class="special">>()</span> 6332 <span class="special">></span> 6333 <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special"><</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special"><</span><span class="identifier">_</span><span class="special">>,</span> 6334 <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">int_</span><span class="special"><</span><span class="number">0</span><span class="special">>()</span> 6335 <span class="special">></span> 6336 <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special"><</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">unary_expr</span><span class="special"><</span><span class="identifier">_</span><span class="special">,</span> <span class="identifier">CalcArity</span><span class="special">>,</span> 6337 <span class="identifier">CalcArity</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_child</span><span class="special">)</span> 6338 <span class="special">></span> 6339 <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special"><</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">binary_expr</span><span class="special"><</span><span class="identifier">_</span><span class="special">,</span> <span class="identifier">CalcArity</span><span class="special">,</span> <span class="identifier">CalcArity</span><span class="special">>,</span> 6340 <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">max</span><span class="special"><</span><span class="identifier">CalcArity</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_left</span><span class="special">),</span> 6341 <span class="identifier">CalcArity</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_right</span><span class="special">)>()</span> 6342 <span class="special">></span> 6343 <span class="special">></span> 6344<span class="special">{};</span> 6345</pre> 6346<p> 6347 </p> 6348<p> 6349 When we find a placeholder terminal or a literal, we use an <span class="emphasis"><em>object 6350 transform</em></span> such as <code class="computeroutput"><span class="identifier">mpl</span><span class="special">::</span><span class="identifier">int_</span><span class="special"><</span><span class="number">1</span><span class="special">>()</span></code> 6351 to create a (default-constructed) compile-time integer representing the 6352 arity of that terminal. 6353 </p> 6354<p> 6355 For unary expressions, we use <code class="computeroutput"><span class="identifier">CalcArity</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_child</span><span class="special">)</span></code> which is a <span class="emphasis"><em>callable transform</em></span> 6356 that computes the arity of the expression's child. 6357 </p> 6358<p> 6359 The transform for binary expressions has a few new tricks. Let's look 6360 more closely: 6361 </p> 6362<pre class="programlisting"><span class="comment">// Compute the left and right arities and</span> 6363<span class="comment">// take the larger of the two.</span> 6364<span class="identifier">mpl</span><span class="special">::</span><span class="identifier">max</span><span class="special"><</span><span class="identifier">CalcArity</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_left</span><span class="special">),</span> 6365 <span class="identifier">CalcArity</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_right</span><span class="special">)>()</span> 6366</pre> 6367<p> 6368 This is an object transform; it default-constructs ... what exactly? 6369 The <code class="computeroutput"><span class="identifier">mpl</span><span class="special">::</span><span class="identifier">max</span><span class="special"><></span></code> 6370 template is an MPL metafunction that accepts two compile-time integers. 6371 It has a nested <code class="computeroutput"><span class="special">::</span><span class="identifier">type</span></code> 6372 typedef (not shown) that is the maximum of the two. But here, we appear 6373 to be passing it two things that are <span class="emphasis"><em>not</em></span> compile-time 6374 integers; they're Proto callable transforms. Proto is smart enough to 6375 recognize that fact. It first evaluates the two nested callable transforms, 6376 computing the arities of the left and right child expressions. Then it 6377 puts the resulting integers into <code class="computeroutput"><span class="identifier">mpl</span><span class="special">::</span><span class="identifier">max</span><span class="special"><></span></code> and evaluates the metafunction 6378 by asking for the nested <code class="computeroutput"><span class="special">::</span><span class="identifier">type</span></code>. That is the type of the object 6379 that gets default-constructed and returned. 6380 </p> 6381<p> 6382 More generally, when evaluating object transforms, Proto looks at the 6383 object type and checks whether it is a template specialization, like 6384 <code class="computeroutput"><span class="identifier">mpl</span><span class="special">::</span><span class="identifier">max</span><span class="special"><></span></code>. 6385 If it is, Proto looks for nested transforms that it can evaluate. After 6386 any nested transforms have been evaluated and substituted back into the 6387 template, the new template specialization is the result type, unless 6388 that type has a nested <code class="computeroutput"><span class="special">::</span><span class="identifier">type</span></code>, in which case that becomes the 6389 result. 6390 </p> 6391<p> 6392 Now that we can calculate the arity of a calculator expression, let's 6393 redefine the <code class="computeroutput"><span class="identifier">calculator</span><span class="special"><></span></code> expression wrapper we wrote in 6394 the Getting Started guide to use the <code class="computeroutput"><span class="identifier">CalcArity</span></code> 6395 grammar and some macros from Boost.MPL to issue compile-time errors when 6396 users specify too many or too few arguments. 6397 </p> 6398<pre class="programlisting"><span class="comment">// The calculator expression wrapper, as defined in the Hello</span> 6399<span class="comment">// Calculator example in the Getting Started guide. It behaves</span> 6400<span class="comment">// just like the expression it wraps, but with extra operator()</span> 6401<span class="comment">// member functions that evaluate the expression.</span> 6402<span class="comment">// NEW: Use the CalcArity grammar to ensure that the correct</span> 6403<span class="comment">// number of arguments are supplied.</span> 6404<span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">></span> 6405<span class="keyword">struct</span> <span class="identifier">calculator</span> 6406 <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">extends</span><span class="special"><</span><span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">calculator</span><span class="special"><</span><span class="identifier">Expr</span><span class="special">>,</span> <span class="identifier">calculator_domain</span><span class="special">></span> 6407<span class="special">{</span> 6408 <span class="keyword">typedef</span> 6409 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">extends</span><span class="special"><</span><span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">calculator</span><span class="special"><</span><span class="identifier">Expr</span><span class="special">>,</span> <span class="identifier">calculator_domain</span><span class="special">></span> 6410 <span class="identifier">base_type</span><span class="special">;</span> 6411 6412 <span class="identifier">calculator</span><span class="special">(</span><span class="identifier">Expr</span> <span class="keyword">const</span> <span class="special">&</span><span class="identifier">expr</span> <span class="special">=</span> <span class="identifier">Expr</span><span class="special">())</span> 6413 <span class="special">:</span> <span class="identifier">base_type</span><span class="special">(</span><span class="identifier">expr</span><span class="special">)</span> 6414 <span class="special">{}</span> 6415 6416 <span class="keyword">typedef</span> <span class="keyword">double</span> <span class="identifier">result_type</span><span class="special">;</span> 6417 6418 <span class="comment">// Use CalcArity to compute the arity of Expr: </span> 6419 <span class="keyword">static</span> <span class="keyword">int</span> <span class="keyword">const</span> <span class="identifier">arity</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">result_of</span><span class="special"><</span><span class="identifier">CalcArity</span><span class="special">(</span><span class="identifier">Expr</span><span class="special">)>::</span><span class="identifier">type</span><span class="special">::</span><span class="identifier">value</span><span class="special">;</span> 6420 6421 <span class="keyword">double</span> <span class="keyword">operator</span><span class="special">()()</span> <span class="keyword">const</span> 6422 <span class="special">{</span> 6423 <span class="identifier">BOOST_MPL_ASSERT_RELATION</span><span class="special">(</span><span class="number">0</span><span class="special">,</span> <span class="special">==,</span> <span class="identifier">arity</span><span class="special">);</span> 6424 <span class="identifier">calculator_context</span> <span class="identifier">ctx</span><span class="special">;</span> 6425 <span class="keyword">return</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">eval</span><span class="special">(*</span><span class="keyword">this</span><span class="special">,</span> <span class="identifier">ctx</span><span class="special">);</span> 6426 <span class="special">}</span> 6427 6428 <span class="keyword">double</span> <span class="keyword">operator</span><span class="special">()(</span><span class="keyword">double</span> <span class="identifier">a1</span><span class="special">)</span> <span class="keyword">const</span> 6429 <span class="special">{</span> 6430 <span class="identifier">BOOST_MPL_ASSERT_RELATION</span><span class="special">(</span><span class="number">1</span><span class="special">,</span> <span class="special">==,</span> <span class="identifier">arity</span><span class="special">);</span> 6431 <span class="identifier">calculator_context</span> <span class="identifier">ctx</span><span class="special">;</span> 6432 <span class="identifier">ctx</span><span class="special">.</span><span class="identifier">args</span><span class="special">.</span><span class="identifier">push_back</span><span class="special">(</span><span class="identifier">a1</span><span class="special">);</span> 6433 <span class="keyword">return</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">eval</span><span class="special">(*</span><span class="keyword">this</span><span class="special">,</span> <span class="identifier">ctx</span><span class="special">);</span> 6434 <span class="special">}</span> 6435 6436 <span class="keyword">double</span> <span class="keyword">operator</span><span class="special">()(</span><span class="keyword">double</span> <span class="identifier">a1</span><span class="special">,</span> <span class="keyword">double</span> <span class="identifier">a2</span><span class="special">)</span> <span class="keyword">const</span> 6437 <span class="special">{</span> 6438 <span class="identifier">BOOST_MPL_ASSERT_RELATION</span><span class="special">(</span><span class="number">2</span><span class="special">,</span> <span class="special">==,</span> <span class="identifier">arity</span><span class="special">);</span> 6439 <span class="identifier">calculator_context</span> <span class="identifier">ctx</span><span class="special">;</span> 6440 <span class="identifier">ctx</span><span class="special">.</span><span class="identifier">args</span><span class="special">.</span><span class="identifier">push_back</span><span class="special">(</span><span class="identifier">a1</span><span class="special">);</span> 6441 <span class="identifier">ctx</span><span class="special">.</span><span class="identifier">args</span><span class="special">.</span><span class="identifier">push_back</span><span class="special">(</span><span class="identifier">a2</span><span class="special">);</span> 6442 <span class="keyword">return</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">eval</span><span class="special">(*</span><span class="keyword">this</span><span class="special">,</span> <span class="identifier">ctx</span><span class="special">);</span> 6443 <span class="special">}</span> 6444<span class="special">};</span> 6445</pre> 6446<p> 6447 Note the use of <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">result_of</span><span class="special"><></span></code> to access the return type of 6448 the <code class="computeroutput"><span class="identifier">CalcArity</span></code> function 6449 object. Since we used compile-time integers in our transforms, the arity 6450 of the expression is encoded in the return type of the <code class="computeroutput"><span class="identifier">CalcArity</span></code> function object. Proto grammars 6451 are valid TR1-style function objects, so you can use <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">result_of</span><span class="special"><></span></code> to figure out their return types. 6452 </p> 6453<p> 6454 With our compile-time assertions in place, when users provide too many 6455 or too few arguments to a calculator expression, as in: 6456 </p> 6457<pre class="programlisting"><span class="special">(</span><span class="identifier">_2</span> <span class="special">*</span> <span class="identifier">_2</span><span class="special">)(</span><span class="number">42</span><span class="special">);</span> <span class="comment">// Oops, too few arguments!</span> 6458</pre> 6459<p> 6460 ... they will get a compile-time error message on the line with the assertion 6461 that reads something like this<a href="#ftn.boost_proto.users_guide.back_end.expression_transformation.example__calculator_arity.f0" class="footnote" name="boost_proto.users_guide.back_end.expression_transformation.example__calculator_arity.f0"><sup class="footnote">[35]</sup></a>: 6462 </p> 6463<pre class="programlisting">c:\boost\org\trunk\libs\proto\scratch\main.cpp(97) : error C2664: 'boost::mpl::asse 6464rtion_failed' : cannot convert parameter 1 from 'boost::mpl::failed ************boo 6465st::mpl::assert_relation<x,y,__formal>::************' to 'boost::mpl::assert<false> 6466::type' 6467 with 6468 [ 6469 x=1, 6470 y=2, 6471 __formal=bool boost::mpl::operator==(boost::mpl::failed,boost::mpl::failed) 6472 ] 6473</pre> 6474<p> 6475 The point of this exercise was to show that we can write a fairly simple 6476 Proto grammar with embedded transforms that is declarative and readable 6477 and can compute interesting properties of arbitrarily complicated expressions. 6478 But transforms can do more than that. Boost.Xpressive uses transforms 6479 to turn expressions into finite state automata for matching regular expressions, 6480 and Boost.Spirit uses transforms to build recursive descent parser generators. 6481 Proto comes with a collection of built-in transforms that you can use 6482 to perform very sophisticated expression manipulations like these. In 6483 the next few sections we'll see some of them in action. 6484 </p> 6485</div> 6486<div class="section"> 6487<div class="titlepage"><div><div><h5 class="title"> 6488<a name="boost_proto.users_guide.back_end.expression_transformation.state"></a><a class="link" href="users_guide.html#boost_proto.users_guide.back_end.expression_transformation.state" title="Transforms With State Accumulation">Transforms 6489 With State Accumulation</a> 6490</h5></div></div></div> 6491<p> 6492 So far, we've only seen examples of grammars with transforms that accept 6493 one argument: the expression to transform. But consider for a moment 6494 how, in ordinary procedural code, you would turn a binary tree into a 6495 linked list. You would start with an empty list. Then, you would recursively 6496 convert the right branch to a list, and use the result as the initial 6497 state while converting the left branch to a list. That is, you would 6498 need a function that takes two parameters: the current node and the list 6499 so far. These sorts of <span class="emphasis"><em>accumulation</em></span> problems are 6500 quite common when processing trees. The linked list is an example of 6501 an accumulation variable or <span class="emphasis"><em>state</em></span>. Each iteration 6502 of the algorithm takes the current element and state, applies some binary 6503 function to the two and creates a new state. In the STL, this algorithm 6504 is called <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">accumulate</span><span class="special">()</span></code>. 6505 In many other languages, it is called <span class="emphasis"><em>fold</em></span>. Let's 6506 see how to implement a fold algorithm with Proto transforms. 6507 </p> 6508<p> 6509 All Proto grammars can optionally accept a state parameter in addition 6510 to the expression to transform. If you want to fold a tree to a list, 6511 you'll need to make use of the state parameter to pass around the list 6512 you've built so far. As for the list, the Boost.Fusion library provides 6513 a <code class="computeroutput"><span class="identifier">fusion</span><span class="special">::</span><span class="identifier">cons</span><span class="special"><></span></code> 6514 type from which you can build heterogeneous lists. The type <code class="computeroutput"><span class="identifier">fusion</span><span class="special">::</span><span class="identifier">nil</span></code> represents an empty list. 6515 </p> 6516<p> 6517 Below is a grammar that recognizes output expressions like <code class="computeroutput"><span class="identifier">cout_</span> <span class="special"><<</span> 6518 <span class="number">42</span> <span class="special"><<</span> 6519 <span class="char">'\n'</span></code> and puts the arguments into 6520 a Fusion list. It is explained below. 6521 </p> 6522<pre class="programlisting"><span class="comment">// Fold the terminals in output statements like</span> 6523<span class="comment">// "cout_ << 42 << '\n'" into a Fusion cons-list.</span> 6524<span class="keyword">struct</span> <span class="identifier">FoldToList</span> 6525 <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">or_</span><span class="special"><</span> 6526 <span class="comment">// Don't add the ostream terminal to the list</span> 6527 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special"><</span> 6528 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special"><</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">ostream</span> <span class="special">&</span> <span class="special">></span> 6529 <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_state</span> 6530 <span class="special">></span> 6531 <span class="comment">// Put all other terminals at the head of the</span> 6532 <span class="comment">// list that we're building in the "state" parameter</span> 6533 <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special"><</span> 6534 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special"><</span><span class="identifier">_</span><span class="special">></span> 6535 <span class="special">,</span> <span class="identifier">fusion</span><span class="special">::</span><span class="identifier">cons</span><span class="special"><</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_value</span><span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_state</span><span class="special">>(</span> 6536 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_value</span><span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_state</span> 6537 <span class="special">)</span> 6538 <span class="special">></span> 6539 <span class="comment">// For left-shift operations, first fold the right</span> 6540 <span class="comment">// child to a list using the current state. Use</span> 6541 <span class="comment">// the result as the state parameter when folding</span> 6542 <span class="comment">// the left child to a list.</span> 6543 <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special"><</span> 6544 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">shift_left</span><span class="special"><</span><span class="identifier">FoldToList</span><span class="special">,</span> <span class="identifier">FoldToList</span><span class="special">></span> 6545 <span class="special">,</span> <span class="identifier">FoldToList</span><span class="special">(</span> 6546 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_left</span> 6547 <span class="special">,</span> <span class="identifier">FoldToList</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_right</span><span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_state</span><span class="special">)</span> 6548 <span class="special">)</span> 6549 <span class="special">></span> 6550 <span class="special">></span> 6551<span class="special">{};</span> 6552</pre> 6553<p> 6554 Before reading on, see if you can apply what you know already about object, 6555 callable and primitive transforms to figure out how this grammar works. 6556 </p> 6557<p> 6558 When you use the <code class="computeroutput"><span class="identifier">FoldToList</span></code> 6559 function, you'll need to pass two arguments: the expression to fold, 6560 and the initial state: an empty list. Those two arguments get passed 6561 around to each transform. We learned previously that <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_value</span></code> 6562 is a primitive transform that accepts a terminal expression and extracts 6563 its value. What we didn't know until now was that it also accepts the 6564 current state <span class="emphasis"><em>and ignores it</em></span>. <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_state</span></code> 6565 is also a primitive transform. It accepts the current expression, which 6566 it ignores, and the current state, which it returns. 6567 </p> 6568<p> 6569 When we find a terminal, we stick it at the head of the cons list, using 6570 the current state as the tail of the list. (The first alternate causes 6571 the <code class="computeroutput"><span class="identifier">ostream</span></code> to be skipped. 6572 We don't want <code class="computeroutput"><span class="identifier">cout</span></code> in 6573 the list.) When we find a shift-left node, we apply the following transform: 6574 </p> 6575<pre class="programlisting"><span class="comment">// Fold the right child and use the result as</span> 6576<span class="comment">// state while folding the right.</span> 6577<span class="identifier">FoldToList</span><span class="special">(</span> 6578 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_left</span> 6579 <span class="special">,</span> <span class="identifier">FoldToList</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_right</span><span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_state</span><span class="special">)</span> 6580<span class="special">)</span> 6581</pre> 6582<p> 6583 You can read this transform as follows: using the current state, fold 6584 the right child to a list. Use the new list as the state while folding 6585 the left child to a list. 6586 </p> 6587<div class="tip"><table border="0" summary="Tip"> 6588<tr> 6589<td rowspan="2" align="center" valign="top" width="25"><img alt="[Tip]" src="../../../doc/src/images/tip.png"></td> 6590<th align="left">Tip</th> 6591</tr> 6592<tr><td align="left" valign="top"> 6593<p> 6594 If your compiler is Microsoft Visual C++, you'll find that the above 6595 transform does not compile. The compiler has bugs with its handling 6596 of nested function types. You can work around the bug by wrapping the 6597 inner transform in <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">call</span><span class="special"><></span></code> as follows: 6598 </p> 6599<p> 6600</p> 6601<pre class="programlisting"><span class="identifier">FoldToList</span><span class="special">(</span> 6602 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_left</span> 6603 <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">call</span><span class="special"><</span><span class="identifier">FoldToList</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_right</span><span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_state</span><span class="special">)></span> 6604<span class="special">)</span> 6605</pre> 6606<p> 6607 </p> 6608<p> 6609 <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">call</span><span class="special"><></span></code> 6610 turns a callable transform into a primitive transform, but more on 6611 that later. 6612 </p> 6613</td></tr> 6614</table></div> 6615<p> 6616 Now that we have defined the <code class="computeroutput"><span class="identifier">FoldToList</span></code> 6617 function object, we can use it to turn output expressions into lists 6618 as follows: 6619 </p> 6620<pre class="programlisting"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special"><</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">ostream</span> <span class="special">&>::</span><span class="identifier">type</span> <span class="keyword">const</span> <span class="identifier">cout_</span> <span class="special">=</span> <span class="special">{</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span><span class="special">};</span> 6621 6622<span class="comment">// This is the type of the list we build below</span> 6623<span class="keyword">typedef</span> 6624 <span class="identifier">fusion</span><span class="special">::</span><span class="identifier">cons</span><span class="special"><</span> 6625 <span class="keyword">int</span> 6626 <span class="special">,</span> <span class="identifier">fusion</span><span class="special">::</span><span class="identifier">cons</span><span class="special"><</span> 6627 <span class="keyword">double</span> 6628 <span class="special">,</span> <span class="identifier">fusion</span><span class="special">::</span><span class="identifier">cons</span><span class="special"><</span> 6629 <span class="keyword">char</span> 6630 <span class="special">,</span> <span class="identifier">fusion</span><span class="special">::</span><span class="identifier">nil</span> 6631 <span class="special">></span> 6632 <span class="special">></span> 6633 <span class="special">></span> 6634<span class="identifier">result_type</span><span class="special">;</span> 6635 6636<span class="comment">// Fold an output expression into a Fusion list, using</span> 6637<span class="comment">// fusion::nil as the initial state of the transformation.</span> 6638<span class="identifier">FoldToList</span> <span class="identifier">to_list</span><span class="special">;</span> 6639<span class="identifier">result_type</span> <span class="identifier">args</span> <span class="special">=</span> <span class="identifier">to_list</span><span class="special">(</span><span class="identifier">cout_</span> <span class="special"><<</span> <span class="number">1</span> <span class="special"><<</span> <span class="number">3.14</span> <span class="special"><<</span> <span class="char">'\n'</span><span class="special">,</span> <span class="identifier">fusion</span><span class="special">::</span><span class="identifier">nil</span><span class="special">());</span> 6640 6641<span class="comment">// Now "args" is the list: {1, 3.14, '\n'}</span> 6642</pre> 6643<p> 6644 When writing transforms, "fold" is such a basic operation that 6645 Proto provides a number of built-in fold transforms. We'll get to them 6646 later. For now, rest assured that you won't always have to stretch your 6647 brain so far to do such basic things. 6648 </p> 6649</div> 6650<div class="section"> 6651<div class="titlepage"><div><div><h5 class="title"> 6652<a name="boost_proto.users_guide.back_end.expression_transformation.data"></a><a class="link" href="users_guide.html#boost_proto.users_guide.back_end.expression_transformation.data" title="Passing Auxiliary Data to Transforms">Passing 6653 Auxiliary Data to Transforms</a> 6654</h5></div></div></div> 6655<p> 6656 In the last section, we saw that we can pass a second parameter to grammars 6657 with transforms: an accumulation variable or <span class="emphasis"><em>state</em></span> 6658 that gets updated as your transform executes. There are times when your 6659 transforms will need to access auxiliary data that does <span class="emphasis"><em>not</em></span> 6660 accumulate, so bundling it with the state parameter is impractical. Instead, 6661 you can pass auxiliary data as a third parameter, known as the <span class="emphasis"><em>data</em></span> 6662 parameter. 6663 </p> 6664<p> 6665 Let's modify our previous example so that it writes each terminal to 6666 <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span></code> before it puts it into a list. 6667 This could be handy for debugging your transforms, for instance. We can 6668 make it general by passing a <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">ostream</span></code> 6669 into the transform in the data parameter. Within the transform itself, 6670 we can retrieve the <code class="computeroutput"><span class="identifier">ostream</span></code> 6671 with the <code class="computeroutput"><a class="link" href="../boost/proto/_data.html" title="Struct _data">proto::_data</a></code> 6672 transform. The strategy is as follows: use the <code class="computeroutput"><a class="link" href="../boost/proto/and_.html" title="Struct template and_">proto::and_<></a></code> 6673 transform to chain two actions. The second action will create the <code class="computeroutput"><span class="identifier">fusion</span><span class="special">::</span><span class="identifier">cons</span><span class="special"><></span></code> 6674 node as before. The first action, however, will display the current expression. 6675 For that, we first construct an instance of <code class="computeroutput"><a class="link" href="../boost/proto/functional/display_expr.html" title="Struct display_expr">proto::functional::display_expr</a></code> and then call 6676 it. 6677 </p> 6678<pre class="programlisting"><span class="comment">// Fold the terminals in output statements like</span> 6679<span class="comment">// "cout_ << 42 << '\n'" into a Fusion cons-list.</span> 6680<span class="keyword">struct</span> <span class="identifier">FoldToList</span> 6681 <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">or_</span><span class="special"><</span> 6682 <span class="comment">// Don't add the ostream terminal to the list</span> 6683 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special"><</span> 6684 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special"><</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">ostream</span> <span class="special">&</span> <span class="special">></span> 6685 <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_state</span> 6686 <span class="special">></span> 6687 <span class="comment">// Put all other terminals at the head of the</span> 6688 <span class="comment">// list that we're building in the "state" parameter</span> 6689 <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special"><</span> 6690 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special"><</span><span class="identifier">_</span><span class="special">></span> 6691 <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">and_</span><span class="special"><</span> 6692 <span class="comment">// First, write the terminal to an ostream passed</span> 6693 <span class="comment">// in the data parameter</span> 6694 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">lazy</span><span class="special"><</span> 6695 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">make</span><span class="special"><</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">functional</span><span class="special">::</span><span class="identifier">display_expr</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_data</span><span class="special">)>(</span><span class="identifier">_</span><span class="special">)</span> 6696 <span class="special">></span> 6697 <span class="comment">// Then, constuct the new cons list.</span> 6698 <span class="special">,</span> <span class="identifier">fusion</span><span class="special">::</span><span class="identifier">cons</span><span class="special"><</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_value</span><span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_state</span><span class="special">>(</span> 6699 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_value</span><span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_state</span> 6700 <span class="special">)</span> 6701 <span class="special">></span> 6702 <span class="special">></span> 6703 <span class="comment">// For left-shift operations, first fold the right</span> 6704 <span class="comment">// child to a list using the current state. Use</span> 6705 <span class="comment">// the result as the state parameter when folding</span> 6706 <span class="comment">// the left child to a list.</span> 6707 <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special"><</span> 6708 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">shift_left</span><span class="special"><</span><span class="identifier">FoldToList</span><span class="special">,</span> <span class="identifier">FoldToList</span><span class="special">></span> 6709 <span class="special">,</span> <span class="identifier">FoldToList</span><span class="special">(</span> 6710 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_left</span> 6711 <span class="special">,</span> <span class="identifier">FoldToList</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_right</span><span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_state</span><span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_data</span><span class="special">)</span> 6712 <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_data</span> 6713 <span class="special">)</span> 6714 <span class="special">></span> 6715 <span class="special">></span> 6716<span class="special">{};</span> 6717</pre> 6718<p> 6719 This is a lot to take in, no doubt. But focus on the second <code class="computeroutput"><span class="identifier">when</span></code> clause above. It says: when you 6720 find a terminal, first display the terminal using the <code class="computeroutput"><span class="identifier">ostream</span></code> 6721 you find in the data parameter, then take the value of the terminal and 6722 the current state to build a new <code class="computeroutput"><span class="identifier">cons</span></code> 6723 list. The function object <code class="computeroutput"><span class="identifier">display_expr</span></code> 6724 does the job of printing the terminal, and <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">and_</span><span class="special"><></span></code> chains the actions together and 6725 executes them in sequence, returning the result of the last one. 6726 </p> 6727<div class="note"><table border="0" summary="Note"> 6728<tr> 6729<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../doc/src/images/note.png"></td> 6730<th align="left">Note</th> 6731</tr> 6732<tr><td align="left" valign="top"><p> 6733 Also new is <code class="computeroutput"><a class="link" href="../boost/proto/lazy.html" title="Struct template lazy">proto::lazy<></a></code>. Sometimes you 6734 don't have a ready-made callable object to execute. Instead, you want 6735 to first make one and <span class="emphasis"><em>then</em></span> execute it. Above, 6736 we need to create a <code class="computeroutput"><span class="identifier">display_expr</span></code>, 6737 initializing it with our <code class="computeroutput"><span class="identifier">ostream</span></code>. 6738 After that, we want to invoke it by passing it the current expression. 6739 It's as if we were doing <code class="computeroutput"><span class="identifier">display_expr</span><span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span><span class="special">)(</span><span class="emphasis"><em>the-expr</em></span><span class="special">)</span></code>. 6740 We achieve this two-phase evaluation using <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">lazy</span><span class="special"><></span></code>. If this doesn't make sense 6741 yet, don't worry about it. 6742 </p></td></tr> 6743</table></div> 6744<p> 6745 We can use the above transform as before, but now we can pass an <code class="computeroutput"><span class="identifier">ostream</span></code> as the third parameter and 6746 get to watch the transform in action. Here's a sample usage: 6747 </p> 6748<pre class="programlisting"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special"><</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">ostream</span> <span class="special">&>::</span><span class="identifier">type</span> <span class="keyword">const</span> <span class="identifier">cout_</span> <span class="special">=</span> <span class="special">{</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span><span class="special">};</span> 6749 6750<span class="comment">// This is the type of the list we build below</span> 6751<span class="keyword">typedef</span> 6752 <span class="identifier">fusion</span><span class="special">::</span><span class="identifier">cons</span><span class="special"><</span> 6753 <span class="keyword">int</span> 6754 <span class="special">,</span> <span class="identifier">fusion</span><span class="special">::</span><span class="identifier">cons</span><span class="special"><</span> 6755 <span class="keyword">double</span> 6756 <span class="special">,</span> <span class="identifier">fusion</span><span class="special">::</span><span class="identifier">cons</span><span class="special"><</span> 6757 <span class="keyword">char</span> 6758 <span class="special">,</span> <span class="identifier">fusion</span><span class="special">::</span><span class="identifier">nil</span> 6759 <span class="special">></span> 6760 <span class="special">></span> 6761 <span class="special">></span> 6762<span class="identifier">result_type</span><span class="special">;</span> 6763 6764<span class="comment">// Fold an output expression into a Fusion list, using</span> 6765<span class="comment">// fusion::nil as the initial state of the transformation.</span> 6766<span class="comment">// Pass std::cout as the data parameter so that we can track</span> 6767<span class="comment">// the progress of the transform on the console.</span> 6768<span class="identifier">FoldToList</span> <span class="identifier">to_list</span><span class="special">;</span> 6769<span class="identifier">result_type</span> <span class="identifier">args</span> <span class="special">=</span> <span class="identifier">to_list</span><span class="special">(</span><span class="identifier">cout_</span> <span class="special"><<</span> <span class="number">1</span> <span class="special"><<</span> <span class="number">3.14</span> <span class="special"><<</span> <span class="char">'\n'</span><span class="special">,</span> <span class="identifier">fusion</span><span class="special">::</span><span class="identifier">nil</span><span class="special">(),</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span><span class="special">);</span> 6770 6771<span class="comment">// Now "args" is the list: {1, 3.14, '\n'}</span> 6772</pre> 6773<p> 6774 This code displays the following: 6775 </p> 6776<pre class="programlisting">terminal( 6777) 6778terminal(3.14) 6779terminal(1)</pre> 6780<p> 6781 This is a rather round-about way of demonstrating that you can pass extra 6782 data to a transform as a third parameter. There are no restrictions on 6783 what this parameter can be, and, unlike the state parameter, Proto will 6784 never mess with it. 6785 </p> 6786<h6> 6787<a name="boost_proto.users_guide.back_end.expression_transformation.data.h0"></a> 6788 <span class="phrase"><a name="boost_proto.users_guide.back_end.expression_transformation.data.transform_environment_variables"></a></span><a class="link" href="users_guide.html#boost_proto.users_guide.back_end.expression_transformation.data.transform_environment_variables">Transform 6789 Environment Variables</a> 6790 </h6> 6791<div class="note"><table border="0" summary="Note"> 6792<tr> 6793<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../doc/src/images/note.png"></td> 6794<th align="left">Note</th> 6795</tr> 6796<tr><td align="left" valign="top"><p> 6797 <span class="emphasis"><em>This is an advanced topic. Feel free to skip if you are new 6798 to Proto.</em></span> 6799 </p></td></tr> 6800</table></div> 6801<p> 6802 The example above uses the data parameter as a transport mechanism for 6803 an unstructured blob of data; in this case, a reference to an <code class="computeroutput"><span class="identifier">ostream</span></code>. As your Proto algorithms become 6804 more sophisticated, you may find that an unstructured blob of data isn't 6805 terribly convenient to work with. Different parts of your algorithm may 6806 be interested in different bits of data. What you want, instead, is a 6807 way to pass in a collection of <span class="emphasis"><em>environment variables</em></span> 6808 to a transform, like a collection of key/value pairs. Then, you can easily 6809 get at the piece of data you want by asking the data parameter for the 6810 value associated with a particular key. Proto's <span class="emphasis"><em>transform environments</em></span> 6811 give you just that. 6812 </p> 6813<p> 6814 Let's start by defining a key. 6815 </p> 6816<pre class="programlisting"><span class="identifier">BOOST_PROTO_DEFINE_ENV_VAR</span><span class="special">(</span><span class="identifier">mykey_type</span><span class="special">,</span> <span class="identifier">mykey</span><span class="special">);</span> 6817</pre> 6818<p> 6819 This defines a global constant <code class="computeroutput"><span class="identifier">mykey</span></code> 6820 with the type <code class="computeroutput"><span class="identifier">mykey_type</span></code>. 6821 We can use <code class="computeroutput"><span class="identifier">mykey</span></code> to store 6822 a piece of assiciated data in a transform environment, as so: 6823 </p> 6824<pre class="programlisting"><span class="comment">// Call the MyEval algorithm with a transform environment containing</span> 6825<span class="comment">// two key/value pairs: one for proto::data and one for mykey</span> 6826<span class="identifier">MyEval</span><span class="special">()(</span> <span class="identifier">expr</span><span class="special">,</span> <span class="identifier">state</span><span class="special">,</span> <span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">data</span> <span class="special">=</span> <span class="number">42</span><span class="special">,</span> <span class="identifier">mykey</span> <span class="special">=</span> <span class="string">"hello world"</span><span class="special">)</span> <span class="special">);</span> 6827</pre> 6828<p> 6829 The above means to invoke the <code class="computeroutput"><span class="identifier">MyEval</span></code> 6830 algorithm with three parameters: an expression, an initial state, and 6831 a transform environment containing two key/value pairs. 6832 </p> 6833<p> 6834 From within a Proto algorithm, you can access the values associated with 6835 different keys using the <code class="computeroutput"><a class="link" href="../boost/proto/_env_var.html" title="Struct template _env_var">proto::_env_var<></a></code> 6836 transform. For instance, <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_env_var</span><span class="special"><</span><span class="identifier">mykey_type</span><span class="special">></span></code> would fetch the value <code class="computeroutput"><span class="string">"hello world"</span></code> from the transform 6837 environment created above. 6838 </p> 6839<p> 6840 The <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_data</span></code> transform has some additional 6841 smarts. Rather than always returning the third parameter regarless of 6842 whether it is a blob or a transform environment, it checks first to see 6843 if it's a blob or not. If so, that's what gets returned. If not, it returns 6844 the value associated with the <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">data</span></code> 6845 key. In the above example, that would be the value <code class="computeroutput"><span class="number">42</span></code>. 6846 </p> 6847<p> 6848 There's a small host of functions, metafunction, and classes that you 6849 can use to create and manipulate transform environments, some for testing 6850 whether an object is a transform environment, some for coercing an object 6851 to be a transform environment, and some for querying a transform environment 6852 whether or not is has a value for a particular key. For an exhaustive 6853 treatment of the topic, check out the reference for the <code class="computeroutput"><a class="link" href="reference.html#header.boost.proto.transform.env_hpp" title="Header <boost/proto/transform/env.hpp>">boost/proto/transform/env.hpp</a></code> 6854 header. 6855 </p> 6856</div> 6857<div class="section"> 6858<div class="titlepage"><div><div><h5 class="title"> 6859<a name="boost_proto.users_guide.back_end.expression_transformation.implicit_params"></a><a class="link" href="users_guide.html#boost_proto.users_guide.back_end.expression_transformation.implicit_params" title="Implicit Parameters to Primitive Transforms">Implicit 6860 Parameters to Primitive Transforms</a> 6861</h5></div></div></div> 6862<p> 6863 Let's use <code class="computeroutput"><span class="identifier">FoldToList</span></code> 6864 example from the previous two sections to illustrate some other niceties 6865 of Proto transforms. We've seen that grammars, when used as function 6866 objects, can accept up to 3 parameters, and that when using these grammars 6867 in callable transforms, you can also specify up to 3 parameters. Let's 6868 take another look at the transform associated with non-terminals from 6869 the last section: 6870 </p> 6871<pre class="programlisting"><span class="identifier">FoldToList</span><span class="special">(</span> 6872 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_left</span> 6873 <span class="special">,</span> <span class="identifier">FoldToList</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_right</span><span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_state</span><span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_data</span><span class="special">)</span> 6874 <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_data</span> 6875<span class="special">)</span> 6876</pre> 6877<p> 6878 Here we specify all three parameters to both invocations of the <code class="computeroutput"><span class="identifier">FoldToList</span></code> grammar. But we don't have 6879 to specify all three. If we don't specify a third parameter, <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_data</span></code> is assumed. Likewise for the 6880 second parameter and <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_state</span></code>. 6881 So the above transform could have been written more simply as: 6882 </p> 6883<pre class="programlisting"><span class="identifier">FoldToList</span><span class="special">(</span> 6884 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_left</span> 6885 <span class="special">,</span> <span class="identifier">StringCopy</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_right</span><span class="special">)</span> 6886<span class="special">)</span> 6887</pre> 6888<p> 6889 The same is true for any primitive transform. The following are all equivalent: 6890 </p> 6891<div class="table"> 6892<a name="boost_proto.users_guide.back_end.expression_transformation.implicit_params.t0"></a><p class="title"><b>Table 32.9. Implicit Parameters to Primitive Transforms</b></p> 6893<div class="table-contents"><table class="table" summary="Implicit Parameters to Primitive Transforms"> 6894<colgroup><col></colgroup> 6895<thead><tr><th> 6896 <p> 6897 Equivalent Transforms 6898 </p> 6899 </th></tr></thead> 6900<tbody> 6901<tr><td> 6902 <p> 6903 <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special"><</span><span class="identifier">_</span><span class="special">,</span> 6904 <span class="identifier">FoldToList</span><span class="special">></span></code> 6905 </p> 6906 </td></tr> 6907<tr><td> 6908 <p> 6909 <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special"><</span><span class="identifier">_</span><span class="special">,</span> 6910 <span class="identifier">FoldToList</span><span class="special">()></span></code> 6911 </p> 6912 </td></tr> 6913<tr><td> 6914 <p> 6915 <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special"><</span><span class="identifier">_</span><span class="special">,</span> 6916 <span class="identifier">FoldToList</span><span class="special">(</span><span class="identifier">_</span><span class="special">)></span></code> 6917 </p> 6918 </td></tr> 6919<tr><td> 6920 <p> 6921 <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special"><</span><span class="identifier">_</span><span class="special">,</span> 6922 <span class="identifier">FoldToList</span><span class="special">(</span><span class="identifier">_</span><span class="special">,</span> 6923 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_state</span><span class="special">)></span></code> 6924 </p> 6925 </td></tr> 6926<tr><td> 6927 <p> 6928 <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special"><</span><span class="identifier">_</span><span class="special">,</span> 6929 <span class="identifier">FoldToList</span><span class="special">(</span><span class="identifier">_</span><span class="special">,</span> 6930 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_state</span><span class="special">,</span> 6931 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_data</span><span class="special">)></span></code> 6932 </p> 6933 </td></tr> 6934</tbody> 6935</table></div> 6936</div> 6937<br class="table-break"><div class="note"><table border="0" summary="Note"> 6938<tr> 6939<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../doc/src/images/note.png"></td> 6940<th align="left">Note</th> 6941</tr> 6942<tr><td align="left" valign="top"> 6943<p> 6944 <span class="bold"><strong>Grammars Are Primitive Transforms Are Function 6945 Objects</strong></span> 6946 </p> 6947<p> 6948 So far, we've said that all Proto grammars are function objects. But 6949 it's more accurate to say that Proto grammars are primitive transforms 6950 -- a special kind of function object that takes between 1 and 3 arguments, 6951 and that Proto knows to treat specially when used in a callable transform, 6952 as in the table above. 6953 </p> 6954</td></tr> 6955</table></div> 6956<div class="note"><table border="0" summary="Note"> 6957<tr> 6958<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../doc/src/images/note.png"></td> 6959<th align="left">Note</th> 6960</tr> 6961<tr><td align="left" valign="top"> 6962<p> 6963 <span class="bold"><strong>Not All Function Objects Are Primitive Transforms</strong></span> 6964 </p> 6965<p> 6966 You might be tempted now to drop the <code class="computeroutput"><span class="identifier">_state</span></code> 6967 and <code class="computeroutput"><span class="identifier">_data</span></code> parameters 6968 for all your callable transforms. That would be an error. You can only 6969 do that for primitive transforms, and not all callables are primitive 6970 transforms. Later on, we'll see what distinguishes ordinary callables 6971 from their more powerful primitive transfor cousins, but the short 6972 version is this: primitive transforms inherit from <code class="computeroutput"><a class="link" href="../boost/proto/transform.html" title="Struct template transform">proto::transform<></a></code>. 6973 </p> 6974</td></tr> 6975</table></div> 6976<p> 6977 Once you know that primitive transforms will always receive all three 6978 parameters -- expression, state, and data -- it makes things possible 6979 that wouldn't be otherwise. For instance, consider that for binary expressions, 6980 these two transforms are equivalent. Can you see why? 6981 </p> 6982<div class="table"> 6983<a name="boost_proto.users_guide.back_end.expression_transformation.implicit_params.t1"></a><p class="title"><b>Table 32.10. Two Equivalent Transforms</b></p> 6984<div class="table-contents"><table class="table" summary="Two Equivalent Transforms"> 6985<colgroup> 6986<col> 6987<col> 6988</colgroup> 6989<thead><tr> 6990<th> 6991 <p> 6992 Without <code class="literal">proto::reverse_fold<></code> 6993 </p> 6994 </th> 6995<th> 6996 <p> 6997 With <code class="literal">proto::reverse_fold<></code> 6998 </p> 6999 </th> 7000</tr></thead> 7001<tbody><tr> 7002<td> 7003 <p> 7004</p> 7005<pre class="programlisting"><span class="identifier">FoldToList</span><span class="special">(</span> 7006 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_left</span> 7007 <span class="special">,</span> <span class="identifier">FoldToList</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_right</span><span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_state</span><span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_data</span><span class="special">)</span> 7008 <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_data</span> 7009<span class="special">)</span></pre> 7010<p> 7011 </p> 7012 </td> 7013<td> 7014 <p> 7015</p> 7016<pre class="programlisting"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">reverse_fold</span><span class="special"><</span><span class="identifier">_</span><span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_state</span><span class="special">,</span> <span class="identifier">FoldToList</span><span class="special">></span></pre> 7017<p> 7018 </p> 7019 </td> 7020</tr></tbody> 7021</table></div> 7022</div> 7023<br class="table-break"> 7024</div> 7025<div class="section"> 7026<div class="titlepage"><div><div><h5 class="title"> 7027<a name="boost_proto.users_guide.back_end.expression_transformation.unpacking_expressions"></a><a class="link" href="users_guide.html#boost_proto.users_guide.back_end.expression_transformation.unpacking_expressions" title="Unpacking Expressions">Unpacking 7028 Expressions</a> 7029</h5></div></div></div> 7030<p> 7031 Processing expressions with an arbitrary number of children can be a 7032 pain. What if you want to do something to each child, then pass the results 7033 as arguments to some other function? Can you do it just once without 7034 worrying about how many children an expression has? Yes. This is where 7035 Proto's <span class="emphasis"><em>unpacking expressions</em></span> come in handy. Unpacking 7036 expressions give you a way to write callable and object transforms that 7037 handle <span class="emphasis"><em>n</em></span>-ary expressions. 7038 </p> 7039<div class="note"><table border="0" summary="Note"> 7040<tr> 7041<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../doc/src/images/note.png"></td> 7042<th align="left">Note</th> 7043</tr> 7044<tr><td align="left" valign="top"> 7045<p> 7046 <span class="bold"><strong>Inspired by C++11 Variadic Templates</strong></span> 7047 </p> 7048<p> 7049 Proto's unpacking expressions take inspiration from the C++11 feature 7050 of the same name. If you are familiar with variadic functions, and 7051 in particular how to expand a function parameter pack, this discussion 7052 should seem very familiar. However, this feature doesn't actually use 7053 any C++11 features, so the code describe here will work with any compliant 7054 C++98 compiler. 7055 </p> 7056</td></tr> 7057</table></div> 7058<h6> 7059<a name="boost_proto.users_guide.back_end.expression_transformation.unpacking_expressions.h0"></a> 7060 <span class="phrase"><a name="boost_proto.users_guide.back_end.expression_transformation.unpacking_expressions.example__a_c___expression_evaluator"></a></span><a class="link" href="users_guide.html#boost_proto.users_guide.back_end.expression_transformation.unpacking_expressions.example__a_c___expression_evaluator">Example: 7061 A C++ Expression Evaluator</a> 7062 </h6> 7063<p> 7064 Proto has the built-in <code class="computeroutput"><a class="link" href="../boost/proto/_default.html" title="Struct template _default">proto::_default<></a></code> 7065 transform for evaluating Proto expressions in a C++-ish way. But if it 7066 didn't, it wouldn't be too hard to implement one from scratch using Proto's 7067 unpacking patterns. The transform <code class="computeroutput"><span class="identifier">eval</span></code> 7068 below does just that. 7069 </p> 7070<pre class="programlisting"><span class="comment">// A callable polymorphic function object that takes an unpacked expression</span> 7071<span class="comment">// and a tag, and evaluates the expression. A plus tag and two operands adds</span> 7072<span class="comment">// them with operator +, for instance.</span> 7073<span class="keyword">struct</span> <span class="identifier">do_eval</span> <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">callable</span> 7074<span class="special">{</span> 7075 <span class="keyword">typedef</span> <span class="keyword">double</span> <span class="identifier">result_type</span><span class="special">;</span> 7076 7077<span class="preprocessor">#define</span> <span class="identifier">UNARY_OP</span><span class="special">(</span><span class="identifier">TAG</span><span class="special">,</span> <span class="identifier">OP</span><span class="special">)</span> <span class="special">\</span> 7078 <span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">Arg</span><span class="special">></span> <span class="special">\</span> 7079 <span class="keyword">double</span> <span class="keyword">operator</span><span class="special">()(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">TAG</span><span class="special">,</span> <span class="identifier">Arg</span> <span class="identifier">arg</span><span class="special">)</span> <span class="keyword">const</span> <span class="special">\</span> 7080 <span class="special">{</span> <span class="special">\</span> 7081 <span class="keyword">return</span> <span class="identifier">OP</span> <span class="identifier">arg</span><span class="special">;</span> <span class="special">\</span> 7082 <span class="special">}</span> <span class="special">\</span> 7083 <span class="comment">/**/</span> 7084 7085<span class="preprocessor">#define</span> <span class="identifier">BINARY_OP</span><span class="special">(</span><span class="identifier">TAG</span><span class="special">,</span> <span class="identifier">OP</span><span class="special">)</span> <span class="special">\</span> 7086 <span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">Left</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Right</span><span class="special">></span> <span class="special">\</span> 7087 <span class="keyword">double</span> <span class="keyword">operator</span><span class="special">()(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">TAG</span><span class="special">,</span> <span class="identifier">Left</span> <span class="identifier">left</span><span class="special">,</span> <span class="identifier">Right</span> <span class="identifier">right</span><span class="special">)</span> <span class="keyword">const</span> <span class="special">\</span> 7088 <span class="special">{</span> <span class="special">\</span> 7089 <span class="keyword">return</span> <span class="identifier">left</span> <span class="identifier">OP</span> <span class="identifier">right</span><span class="special">;</span> <span class="special">\</span> 7090 <span class="special">}</span> <span class="special">\</span> 7091 <span class="comment">/**/</span> 7092 7093 <span class="identifier">UNARY_OP</span><span class="special">(</span><span class="identifier">negate</span><span class="special">,</span> <span class="special">-)</span> 7094 <span class="identifier">BINARY_OP</span><span class="special">(</span><span class="identifier">plus</span><span class="special">,</span> <span class="special">+)</span> 7095 <span class="identifier">BINARY_OP</span><span class="special">(</span><span class="identifier">minus</span><span class="special">,</span> <span class="special">-)</span> 7096 <span class="identifier">BINARY_OP</span><span class="special">(</span><span class="identifier">multiplies</span><span class="special">,</span> <span class="special">*)</span> 7097 <span class="identifier">BINARY_OP</span><span class="special">(</span><span class="identifier">divides</span><span class="special">,</span> <span class="special">/)</span> 7098 <span class="comment">/*... others ...*/</span> 7099<span class="special">};</span> 7100 7101<span class="keyword">struct</span> <span class="identifier">eval</span> 7102 <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">or_</span><span class="special"><</span> 7103 <span class="comment">// Evaluate terminals by simply returning their value</span> 7104 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special"><</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special"><</span><span class="identifier">_</span><span class="special">>,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_value</span><span class="special">></span> 7105 7106 <span class="comment">// Non-terminals are handled by unpacking the expression,</span> 7107 <span class="comment">// recursively calling eval on each child, and passing</span> 7108 <span class="comment">// the results along with the expression's tag to do_eval</span> 7109 <span class="comment">// defined above.</span> 7110 <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">otherwise</span><span class="special"><</span><span class="identifier">do_eval</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag_of</span><span class="special"><</span><span class="identifier">_</span><span class="special">>(),</span> <span class="identifier">eval</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">pack</span><span class="special">(</span><span class="identifier">_</span><span class="special">))...)></span> 7111 <span class="comment">// UNPACKING PATTERN HERE -------------------^^^^^^^^^^^^^^^^^^^^^^^^</span> 7112 <span class="special">></span> 7113<span class="special">{};</span> 7114</pre> 7115<p> 7116 The bulk of the above code is devoted to the <code class="computeroutput"><span class="identifier">do_eval</span></code> 7117 function object that maps tag types to behaviors, but the interesting 7118 bit is the definition of the <code class="computeroutput"><span class="identifier">eval</span></code> 7119 algorithm at the bottom. Terminals are handled quite simply, but non-terminals 7120 could be unary, binary, ternary, even <span class="emphasis"><em>n</em></span>-ary if we 7121 consider function call expressions. The <code class="computeroutput"><span class="identifier">eval</span></code> 7122 algorithm handles this uniformly with the help of an unpacking pattern. 7123 </p> 7124<p> 7125 Non-terminals are evaluated with this callable transform: 7126 </p> 7127<pre class="programlisting"><span class="identifier">do_eval</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag_of</span><span class="special"><</span><span class="identifier">_</span><span class="special">>(),</span> <span class="identifier">eval</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">pack</span><span class="special">(</span><span class="identifier">_</span><span class="special">))...)</span> 7128</pre> 7129<p> 7130 You can read this as: call the <code class="computeroutput"><span class="identifier">do_eval</span></code> 7131 function object with the tag of the current expression and all its children 7132 after they have each been evaluated with <code class="computeroutput"><span class="identifier">eval</span></code>. 7133 The unpacking pattern is the bit just before the ellipsis: <code class="computeroutput"><span class="identifier">eval</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">pack</span><span class="special">(</span><span class="identifier">_</span><span class="special">))</span></code>. 7134 </p> 7135<p> 7136 What's going on here is this. The unpacking expression gets repeated 7137 once for each child in the expression currently being evaluated. In each 7138 repetition, the type <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">pack</span><span class="special">(</span><span class="identifier">_</span><span class="special">)</span></code> gets replaced with <code class="literal">proto::_child_c<<span class="emphasis"><em>N</em></span>></code>. 7139 So, if a unary expression is passed to <code class="computeroutput"><span class="identifier">eval</span></code>, 7140 it actually gets evaluated like this: 7141 </p> 7142<pre class="programlisting"><span class="comment">// After the unpacking pattern is expanded for a unary expression</span> 7143<span class="identifier">do_eval</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag_of</span><span class="special"><</span><span class="identifier">_</span><span class="special">>(),</span> <span class="identifier">eval</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_child_c</span><span class="special"><</span><span class="number">0</span><span class="special">>))</span> 7144</pre> 7145<p> 7146 And when passed a binary expression, the unpacking pattern expands like 7147 this: 7148 </p> 7149<pre class="programlisting"><span class="comment">// After the unpacking pattern is expanded for a binary expression</span> 7150<span class="identifier">do_eval</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag_of</span><span class="special"><</span><span class="identifier">_</span><span class="special">>(),</span> <span class="identifier">eval</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_child_c</span><span class="special"><</span><span class="number">0</span><span class="special">>),</span> <span class="identifier">eval</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_child_c</span><span class="special"><</span><span class="number">1</span><span class="special">>))</span> 7151</pre> 7152<p> 7153 Although it can't happen in our example, when passed a terminal, the 7154 unpacking pattern expands such that it extracts the value from the terminal 7155 instead of the children. So it gets handled like this: 7156 </p> 7157<pre class="programlisting"><span class="comment">// If a terminal were passed to this transform, Proto would try</span> 7158<span class="comment">// to evaluate it like this, which would fail:</span> 7159<span class="identifier">do_eval</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag_of</span><span class="special"><</span><span class="identifier">_</span><span class="special">>(),</span> <span class="identifier">eval</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_value</span><span class="special">))</span> 7160</pre> 7161<p> 7162 That doesn't make sense. <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_value</span></code> 7163 would return something that isn't a Proto expression, and <code class="computeroutput"><span class="identifier">eval</span></code> wouldn't be able to evaluate it. 7164 Proto algorithms don't work unless you pass them Proto expressions. 7165 </p> 7166<div class="note"><table border="0" summary="Note"> 7167<tr> 7168<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../doc/src/images/note.png"></td> 7169<th align="left">Note</th> 7170</tr> 7171<tr><td align="left" valign="top"> 7172<p> 7173 <span class="bold"><strong>Kickin' It Old School</strong></span> 7174 </p> 7175<p> 7176 You may be thinking, my compiler doesn't support C++11 variadic templates! 7177 How can this possibly work? The answer is simple: The <code class="computeroutput"><span class="special">...</span></code> above isn't a C++11 pack expansion. 7178 It's actually an old-school C-style vararg. Remember that callable 7179 and object transforms are <span class="emphasis"><em>function types</em></span>. A transform 7180 with one of these pseudo-pack expansions is really just the type of 7181 a boring, old vararg function. Proto just interprets it differently. 7182 </p> 7183</td></tr> 7184</table></div> 7185<p> 7186 Unpacking patterns are very expressive. Any callable or object transform 7187 can be used as an unpacking pattern, so long as <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">pack</span><span class="special">(</span><span class="identifier">_</span><span class="special">)</span></code> appears exactly once somewhere within 7188 it. This gives you a lot of flexibility in how you want to process the 7189 children of an expression before passing them on to some function object 7190 or object constructor. 7191 </p> 7192</div> 7193<div class="section"> 7194<div class="titlepage"><div><div><h5 class="title"> 7195<a name="boost_proto.users_guide.back_end.expression_transformation.external_transforms"></a><a class="link" href="users_guide.html#boost_proto.users_guide.back_end.expression_transformation.external_transforms" title="Separating Grammars And Transforms">Separating 7196 Grammars And Transforms</a> 7197</h5></div></div></div> 7198<div class="note"><table border="0" summary="Note"> 7199<tr> 7200<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../doc/src/images/note.png"></td> 7201<th align="left">Note</th> 7202</tr> 7203<tr><td align="left" valign="top"><p> 7204 This is an advanced topic that is only necessary for people defining 7205 large EDSLs. Feel free to skip this if you're just getting started 7206 with Proto. 7207 </p></td></tr> 7208</table></div> 7209<p> 7210 So far, we've seen examples of grammars with embedded transforms. In 7211 practice, grammars can get pretty large, and you may want to use them 7212 to drive several different computations. For instance, you may have a 7213 grammar for a linear algebra domain, and you may want to use it to compute 7214 the shape of the result (vector or matrix?) and also to compute the result 7215 optimally. You don't want to have to copy and paste the whole shebang 7216 just to tweak one of the embedded transforms. What you want instead is 7217 to define the grammar once, and specify the transforms later when you're 7218 ready to evaluate an expression. For that, you use <span class="emphasis"><em>external 7219 transforms</em></span>. The pattern you'll use is this: replace one or 7220 more of the transforms in your grammar with the special placeholder 7221 <code class="computeroutput"><a class="link" href="../boost/proto/external_transform.html" title="Struct external_transform">proto::external_transform</a></code>. 7222 Then, you'll create a bundle of transforms that you will pass to the 7223 grammar in the data parameter (the 3rd parameter after the expression 7224 and state) when evaluating it. 7225 </p> 7226<p> 7227 To illustrate external transforms, we'll build a calculator evaluator 7228 that can be configured to throw an exception on division by zero. Here 7229 is a bare-bones front end that defines a domain, a grammar, an expression 7230 wrapper, and some placeholder terminals. 7231 </p> 7232<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">assert</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> 7233<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">mpl</span><span class="special">/</span><span class="keyword">int</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> 7234<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">fusion</span><span class="special">/</span><span class="identifier">container</span><span class="special">/</span><span class="identifier">vector</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> 7235<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">fusion</span><span class="special">/</span><span class="identifier">container</span><span class="special">/</span><span class="identifier">generation</span><span class="special">/</span><span class="identifier">make_vector</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> 7236<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">proto</span><span class="special">/</span><span class="identifier">proto</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> 7237<span class="keyword">namespace</span> <span class="identifier">mpl</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">mpl</span><span class="special">;</span> 7238<span class="keyword">namespace</span> <span class="identifier">proto</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">proto</span><span class="special">;</span> 7239<span class="keyword">namespace</span> <span class="identifier">fusion</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">fusion</span><span class="special">;</span> 7240 7241<span class="comment">// The argument placeholder type</span> 7242<span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">I</span><span class="special">></span> <span class="keyword">struct</span> <span class="identifier">placeholder</span> <span class="special">:</span> <span class="identifier">I</span> <span class="special">{};</span> 7243 7244<span class="comment">// The grammar for valid calculator expressions</span> 7245<span class="keyword">struct</span> <span class="identifier">calc_grammar</span> 7246 <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">or_</span><span class="special"><</span> 7247 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special"><</span><span class="identifier">placeholder</span><span class="special"><</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_</span><span class="special">></span> <span class="special">></span> 7248 <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> 7249 <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">plus</span><span class="special"><</span><span class="identifier">calc_grammar</span><span class="special">,</span> <span class="identifier">calc_grammar</span><span class="special">></span> 7250 <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">minus</span><span class="special"><</span><span class="identifier">calc_grammar</span><span class="special">,</span> <span class="identifier">calc_grammar</span><span class="special">></span> 7251 <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">multiplies</span><span class="special"><</span><span class="identifier">calc_grammar</span><span class="special">,</span> <span class="identifier">calc_grammar</span><span class="special">></span> 7252 <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">divides</span><span class="special"><</span><span class="identifier">calc_grammar</span><span class="special">,</span> <span class="identifier">calc_grammar</span><span class="special">></span> 7253 <span class="special">></span> 7254<span class="special">{};</span> 7255 7256<span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">E</span><span class="special">></span> <span class="keyword">struct</span> <span class="identifier">calc_expr</span><span class="special">;</span> 7257<span class="keyword">struct</span> <span class="identifier">calc_domain</span> <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">domain</span><span class="special"><</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">generator</span><span class="special"><</span><span class="identifier">calc_expr</span><span class="special">></span> <span class="special">></span> <span class="special">{};</span> 7258 7259<span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">E</span><span class="special">></span> 7260<span class="keyword">struct</span> <span class="identifier">calc_expr</span> 7261 <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">extends</span><span class="special"><</span><span class="identifier">E</span><span class="special">,</span> <span class="identifier">calc_expr</span><span class="special"><</span><span class="identifier">E</span><span class="special">>,</span> <span class="identifier">calc_domain</span><span class="special">></span> 7262<span class="special">{</span> 7263 <span class="identifier">calc_expr</span><span class="special">(</span><span class="identifier">E</span> <span class="keyword">const</span> <span class="special">&</span><span class="identifier">e</span> <span class="special">=</span> <span class="identifier">E</span><span class="special">())</span> <span class="special">:</span> <span class="identifier">calc_expr</span><span class="special">::</span><span class="identifier">proto_extends</span><span class="special">(</span><span class="identifier">e</span><span class="special">)</span> <span class="special">{}</span> 7264<span class="special">};</span> 7265 7266<span class="identifier">calc_expr</span><span class="special"><</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special"><</span><span class="identifier">placeholder</span><span class="special"><</span><span class="identifier">mpl</span><span class="special">::</span><span class="identifier">int_</span><span class="special"><</span><span class="number">0</span><span class="special">></span> <span class="special">></span> <span class="special">>::</span><span class="identifier">type</span><span class="special">></span> <span class="identifier">_1</span><span class="special">;</span> 7267<span class="identifier">calc_expr</span><span class="special"><</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special"><</span><span class="identifier">placeholder</span><span class="special"><</span><span class="identifier">mpl</span><span class="special">::</span><span class="identifier">int_</span><span class="special"><</span><span class="number">1</span><span class="special">></span> <span class="special">></span> <span class="special">>::</span><span class="identifier">type</span><span class="special">></span> <span class="identifier">_2</span><span class="special">;</span> 7268 7269<span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span> 7270<span class="special">{</span> 7271 <span class="comment">// Build a calculator expression, and do nothing with it.</span> 7272 <span class="special">(</span><span class="identifier">_1</span> <span class="special">+</span> <span class="identifier">_2</span><span class="special">);</span> 7273<span class="special">}</span> 7274</pre> 7275<p> 7276 Now, let's embed transforms into <code class="computeroutput"><span class="identifier">calc_grammar</span></code> 7277 so that we can use it to evaluate calculator expressions: 7278 </p> 7279<pre class="programlisting"><span class="comment">// The calculator grammar with embedded transforms for evaluating expression.</span> 7280<span class="keyword">struct</span> <span class="identifier">calc_grammar</span> 7281 <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">or_</span><span class="special"><</span> 7282 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special"><</span> 7283 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special"><</span><span class="identifier">placeholder</span><span class="special"><</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_</span><span class="special">></span> <span class="special">></span> 7284 <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">functional</span><span class="special">::</span><span class="identifier">at</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_state</span><span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_value</span><span class="special">)</span> 7285 <span class="special">></span> 7286 <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special"><</span> 7287 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> 7288 <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_value</span> 7289 <span class="special">></span> 7290 <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special"><</span> 7291 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">plus</span><span class="special"><</span><span class="identifier">calc_grammar</span><span class="special">,</span> <span class="identifier">calc_grammar</span><span class="special">></span> 7292 <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_default</span><span class="special"><</span><span class="identifier">calc_grammar</span><span class="special">></span> 7293 <span class="special">></span> 7294 <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special"><</span> 7295 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">minus</span><span class="special"><</span><span class="identifier">calc_grammar</span><span class="special">,</span> <span class="identifier">calc_grammar</span><span class="special">></span> 7296 <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_default</span><span class="special"><</span><span class="identifier">calc_grammar</span><span class="special">></span> 7297 <span class="special">></span> 7298 <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special"><</span> 7299 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">multiplies</span><span class="special"><</span><span class="identifier">calc_grammar</span><span class="special">,</span> <span class="identifier">calc_grammar</span><span class="special">></span> 7300 <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_default</span><span class="special"><</span><span class="identifier">calc_grammar</span><span class="special">></span> 7301 <span class="special">></span> 7302 <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special"><</span> 7303 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">divides</span><span class="special"><</span><span class="identifier">calc_grammar</span><span class="special">,</span> <span class="identifier">calc_grammar</span><span class="special">></span> 7304 <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_default</span><span class="special"><</span><span class="identifier">calc_grammar</span><span class="special">></span> 7305 <span class="special">></span> 7306 <span class="special">></span> 7307<span class="special">{};</span> 7308</pre> 7309<p> 7310 With this definition of <code class="computeroutput"><span class="identifier">calc_grammar</span></code> 7311 we can evaluate expressions by passing along a Fusion vector containing 7312 the values to use for the <code class="computeroutput"><span class="identifier">_1</span></code> 7313 and <code class="computeroutput"><span class="identifier">_2</span></code> placeholders: 7314 </p> 7315<pre class="programlisting"><span class="keyword">int</span> <span class="identifier">result</span> <span class="special">=</span> <span class="identifier">calc_grammar</span><span class="special">()(</span><span class="identifier">_1</span> <span class="special">+</span> <span class="identifier">_2</span><span class="special">,</span> <span class="identifier">fusion</span><span class="special">::</span><span class="identifier">make_vector</span><span class="special">(</span><span class="number">3</span><span class="special">,</span> <span class="number">4</span><span class="special">));</span> 7316<span class="identifier">BOOST_ASSERT</span><span class="special">(</span><span class="identifier">result</span> <span class="special">==</span> <span class="number">7</span><span class="special">);</span> 7317</pre> 7318<p> 7319 We also want an alternative evaluation strategy that checks for division 7320 by zero and throws an exception. Just how ridiculous would it be to copy 7321 the entire <code class="computeroutput"><span class="identifier">calc_grammar</span></code> 7322 just to change the one line that transforms division expressions?! External 7323 transforms are ideally suited to this problem. 7324 </p> 7325<p> 7326 First, we give the division rule in our grammar a "name"; that 7327 is, we make it a struct. We'll use this unique type later to dispatch 7328 to the right transforms. 7329 </p> 7330<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">calc_grammar</span><span class="special">;</span> 7331<span class="keyword">struct</span> <span class="identifier">divides_rule</span> <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">divides</span><span class="special"><</span><span class="identifier">calc_grammar</span><span class="special">,</span> <span class="identifier">calc_grammar</span><span class="special">></span> <span class="special">{};</span> 7332</pre> 7333<p> 7334 Next, we change <code class="computeroutput"><span class="identifier">calc_grammar</span></code> 7335 to make the handling of division expressions external. 7336 </p> 7337<pre class="programlisting"><span class="comment">// The calculator grammar with an external transform for evaluating</span> 7338<span class="comment">// division expressions.</span> 7339<span class="keyword">struct</span> <span class="identifier">calc_grammar</span> 7340 <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">or_</span><span class="special"><</span> 7341 <span class="comment">/* ... as before ... */</span> 7342 <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special"><</span> 7343 <span class="identifier">divides_rule</span> 7344 <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">external_transform</span> 7345 <span class="special">></span> 7346 <span class="special">></span> 7347<span class="special">{};</span> 7348</pre> 7349<p> 7350 The use of <code class="computeroutput"><a class="link" href="../boost/proto/external_transform.html" title="Struct external_transform">proto::external_transform</a></code> above 7351 makes the handling of division expressions externally parameterizeable. 7352 </p> 7353<p> 7354 Next, we use <code class="computeroutput"><a class="link" href="../boost/proto/external_transforms.html" title="Struct template external_transforms">proto::external_transforms<></a></code> 7355 (note the trailing 's') to capture our evaluation strategy in a bundle 7356 that we can pass along to the transform in the data parameter. Read on 7357 for the explanation. 7358 </p> 7359<pre class="programlisting"><span class="comment">// Evaluate division nodes as before</span> 7360<span class="keyword">struct</span> <span class="identifier">non_checked_division</span> 7361 <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">external_transforms</span><span class="special"><</span> 7362 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special"><</span> <span class="identifier">divides_rule</span><span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_default</span><span class="special"><</span><span class="identifier">calc_grammar</span><span class="special">></span> <span class="special">></span> 7363 <span class="special">></span> 7364<span class="special">{};</span> 7365 7366<span class="comment">/* ... */</span> 7367 7368<span class="identifier">non_checked_division</span> <span class="identifier">non_checked</span><span class="special">;</span> 7369<span class="keyword">int</span> <span class="identifier">result2</span> <span class="special">=</span> <span class="identifier">calc_grammar</span><span class="special">()(</span><span class="identifier">_1</span> <span class="special">/</span> <span class="identifier">_2</span><span class="special">,</span> <span class="identifier">fusion</span><span class="special">::</span><span class="identifier">make_vector</span><span class="special">(</span><span class="number">6</span><span class="special">,</span> <span class="number">2</span><span class="special">),</span> <span class="identifier">non_checked</span><span class="special">);</span> 7370</pre> 7371<p> 7372 The struct <code class="computeroutput"><span class="identifier">non_cecked_division</span></code> 7373 associates the transform <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_default</span><span class="special"><</span><span class="identifier">calc_grammar</span><span class="special">></span></code> with the <code class="computeroutput"><span class="identifier">divides_rule</span></code> 7374 grammar rule. An instance of that struct is passed along as the third 7375 parameter when invoking <code class="computeroutput"><span class="identifier">calc_grammar</span></code>. 7376 </p> 7377<p> 7378 Now, let's implement checked division. The rest should be unsurprising. 7379 </p> 7380<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">division_by_zero</span> <span class="special">:</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">exception</span> <span class="special">{};</span> 7381 7382<span class="keyword">struct</span> <span class="identifier">do_checked_divide</span> <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">callable</span> 7383<span class="special">{</span> 7384 <span class="keyword">typedef</span> <span class="keyword">int</span> <span class="identifier">result_type</span><span class="special">;</span> 7385 <span class="keyword">int</span> <span class="keyword">operator</span><span class="special">()(</span><span class="keyword">int</span> <span class="identifier">left</span><span class="special">,</span> <span class="keyword">int</span> <span class="identifier">right</span><span class="special">)</span> <span class="keyword">const</span> 7386 <span class="special">{</span> 7387 <span class="keyword">if</span> <span class="special">(</span><span class="identifier">right</span> <span class="special">==</span> <span class="number">0</span><span class="special">)</span> <span class="keyword">throw</span> <span class="identifier">division_by_zero</span><span class="special">();</span> 7388 <span class="keyword">return</span> <span class="identifier">left</span> <span class="special">/</span> <span class="identifier">right</span><span class="special">;</span> 7389 <span class="special">}</span> 7390<span class="special">};</span> 7391 7392<span class="keyword">struct</span> <span class="identifier">checked_division</span> 7393 <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">external_transforms</span><span class="special"><</span> 7394 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special"><</span> 7395 <span class="identifier">divides_rule</span> 7396 <span class="special">,</span> <span class="identifier">do_checked_divide</span><span class="special">(</span><span class="identifier">calc_grammar</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_left</span><span class="special">),</span> <span class="identifier">calc_grammar</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_right</span><span class="special">))</span> 7397 <span class="special">></span> 7398 <span class="special">></span> 7399<span class="special">{};</span> 7400 7401<span class="comment">/* ... */</span> 7402 7403<span class="keyword">try</span> 7404<span class="special">{</span> 7405 <span class="identifier">checked_division</span> <span class="identifier">checked</span><span class="special">;</span> 7406 <span class="keyword">int</span> <span class="identifier">result3</span> <span class="special">=</span> <span class="identifier">calc_grammar_extern</span><span class="special">()(</span><span class="identifier">_1</span> <span class="special">/</span> <span class="identifier">_2</span><span class="special">,</span> <span class="identifier">fusion</span><span class="special">::</span><span class="identifier">make_vector</span><span class="special">(</span><span class="number">6</span><span class="special">,</span> <span class="number">0</span><span class="special">),</span> <span class="identifier">checked</span><span class="special">);</span> 7407<span class="special">}</span> 7408<span class="keyword">catch</span><span class="special">(</span><span class="identifier">division_by_zero</span><span class="special">)</span> 7409<span class="special">{</span> 7410 <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="string">"caught division by zero!\n"</span><span class="special">;</span> 7411<span class="special">}</span> 7412</pre> 7413<p> 7414 The above code demonstrates how a single grammar can be used with different 7415 transforms specified externally. This makes it possible to reuse a grammar 7416 to drive several different computations. 7417 </p> 7418<h6> 7419<a name="boost_proto.users_guide.back_end.expression_transformation.external_transforms.h0"></a> 7420 <span class="phrase"><a name="boost_proto.users_guide.back_end.expression_transformation.external_transforms.separating_data_from_external_transforms"></a></span><a class="link" href="users_guide.html#boost_proto.users_guide.back_end.expression_transformation.external_transforms.separating_data_from_external_transforms">Separating 7421 Data From External Transforms</a> 7422 </h6> 7423<p> 7424 As described above, the external transforms feature usurps the data parameter, 7425 which is intended to be a place where you can pass arbitrary data, and 7426 gives it a specific meaning. But what if you are already using the data 7427 parameter for something else? The answer is to use a transform environment. 7428 By associating your external transforms with the <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">transforms</span></code> 7429 key, you are free to pass arbitrary data in other slots. 7430 </p> 7431<p> 7432 To continue the above example, what if we also needed to pass a piece 7433 of data into our transform along with the external transforms? It would 7434 look like this: 7435 </p> 7436<pre class="programlisting"><span class="keyword">int</span> <span class="identifier">result3</span> <span class="special">=</span> <span class="identifier">calc_grammar_extern</span><span class="special">()(</span> 7437 <span class="identifier">_1</span> <span class="special">/</span> <span class="identifier">_2</span> 7438 <span class="special">,</span> <span class="identifier">fusion</span><span class="special">::</span><span class="identifier">make_vector</span><span class="special">(</span><span class="number">6</span><span class="special">,</span> <span class="number">0</span><span class="special">)</span> 7439 <span class="special">,</span> <span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">data</span> <span class="special">=</span> <span class="number">42</span><span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">transforms</span> <span class="special">=</span> <span class="identifier">checked</span><span class="special">)</span> 7440<span class="special">);</span> 7441</pre> 7442<p> 7443 In the above invocation of the <code class="computeroutput"><span class="identifier">calc_grammar_extern</span></code> 7444 algorithm, the map of external transforms is associated with the <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">transforms</span></code> key and passed to the algorithm 7445 in a transform environment. Also in the transform environment is a key/value 7446 pair that associates the value <code class="computeroutput"><span class="number">42</span></code> 7447 with the <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">data</span></code> key. 7448 </p> 7449</div> 7450<div class="section"> 7451<div class="titlepage"><div><div><h5 class="title"> 7452<a name="boost_proto.users_guide.back_end.expression_transformation.canned_transforms"></a><a class="link" href="users_guide.html#boost_proto.users_guide.back_end.expression_transformation.canned_transforms" title="Proto's Built-In Transforms">Proto's 7453 Built-In Transforms</a> 7454</h5></div></div></div> 7455<p> 7456 Primitive transforms are the building blocks for more interesting composite 7457 transforms. Proto defines a bunch of generally useful primitive transforms. 7458 They are summarized below. 7459 </p> 7460<div class="variablelist"> 7461<p class="title"><b></b></p> 7462<dl class="variablelist"> 7463<dt><span class="term"> <code class="computeroutput"><a class="link" href="../boost/proto/_value.html" title="Struct _value">proto::_value</a></code></span></dt> 7464<dd><p> 7465 Given a terminal expression, return the value of the terminal. 7466 </p></dd> 7467<dt><span class="term"> <code class="computeroutput"><a class="link" href="../boost/proto/_child_c.html" title="Struct template _child_c">proto::_child_c<></a></code></span></dt> 7468<dd><p> 7469 Given a non-terminal expression, <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_child_c</span><span class="special"><</span><em class="replaceable"><code>N</code></em><span class="special">></span></code> returns the <em class="replaceable"><code>N</code></em>-th 7470 child. 7471 </p></dd> 7472<dt><span class="term"> <code class="computeroutput"><a class="link" href="reference.html#boost.proto._child">proto::_child</a></code></span></dt> 7473<dd><p> 7474 A synonym for <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_child_c</span><span class="special"><</span><span class="number">0</span><span class="special">></span></code>. 7475 </p></dd> 7476<dt><span class="term"> <code class="computeroutput"><a class="link" href="reference.html#boost.proto._left">proto::_left</a></code></span></dt> 7477<dd><p> 7478 A synonym for <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_child_c</span><span class="special"><</span><span class="number">0</span><span class="special">></span></code>. 7479 </p></dd> 7480<dt><span class="term"> <code class="computeroutput"><a class="link" href="reference.html#boost.proto._right">proto::_right</a></code></span></dt> 7481<dd><p> 7482 A synonym for <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_child_c</span><span class="special"><</span><span class="number">1</span><span class="special">></span></code>. 7483 </p></dd> 7484<dt><span class="term"> <code class="computeroutput"><a class="link" href="../boost/proto/_expr.html" title="Struct _expr">proto::_expr</a></code></span></dt> 7485<dd><p> 7486 Returns the current expression unmodified. 7487 </p></dd> 7488<dt><span class="term"> <code class="computeroutput"><a class="link" href="../boost/proto/_state.html" title="Struct _state">proto::_state</a></code></span></dt> 7489<dd><p> 7490 Returns the current state unmodified. 7491 </p></dd> 7492<dt><span class="term"> <code class="computeroutput"><a class="link" href="../boost/proto/_data.html" title="Struct _data">proto::_data</a></code></span></dt> 7493<dd><p> 7494 Returns the current data unmodified. 7495 </p></dd> 7496<dt><span class="term"> <code class="computeroutput"><a class="link" href="../boost/proto/call.html" title="Struct template call">proto::call<></a></code></span></dt> 7497<dd><p> 7498 For a given callable transform <code class="computeroutput"><em class="replaceable"><code>CT</code></em></code>, 7499 <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">call</span><span class="special"><</span><em class="replaceable"><code>CT</code></em><span class="special">></span></code> turns the callable transform 7500 into a primitive transform. This is useful for disambiguating callable 7501 transforms from object transforms, and also for working around 7502 compiler bugs with nested function types. 7503 </p></dd> 7504<dt><span class="term"> <code class="computeroutput"><a class="link" href="../boost/proto/make.html" title="Struct template make">proto::make<></a></code></span></dt> 7505<dd><p> 7506 For a given object transform <code class="computeroutput"><em class="replaceable"><code>OT</code></em></code>, 7507 <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">make</span><span class="special"><</span><em class="replaceable"><code>OT</code></em><span class="special">></span></code> turns the object transform 7508 into a primitive transform. This is useful for disambiguating object 7509 transforms from callable transforms, and also for working around 7510 compiler bugs with nested function types. 7511 </p></dd> 7512<dt><span class="term"> <code class="computeroutput"><a class="link" href="../boost/proto/_default.html" title="Struct template _default">proto::_default<></a></code></span></dt> 7513<dd><p> 7514 Given a grammar <em class="replaceable"><code>G</code></em>, <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_default</span><span class="special"><</span><em class="replaceable"><code>G</code></em><span class="special">></span></code> evaluates the current node 7515 according to the standard C++ meaning of the operation the node 7516 represents. For instance, if the current node is a binary plus 7517 node, the two children will both be evaluated according to <code class="computeroutput"><em class="replaceable"><code>G</code></em></code> 7518 and the results will be added and returned. The return type is 7519 deduced with the help of the Boost.Typeof library. 7520 </p></dd> 7521<dt><span class="term"> <code class="computeroutput"><a class="link" href="../boost/proto/fold.html" title="Struct template fold">proto::fold<></a></code></span></dt> 7522<dd><p> 7523 Given three transforms <code class="computeroutput"><em class="replaceable"><code>ET</code></em></code>, 7524 <code class="computeroutput"><em class="replaceable"><code>ST</code></em></code>, and <code class="computeroutput"><em class="replaceable"><code>FT</code></em></code>, 7525 <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">fold</span><span class="special"><</span><em class="replaceable"><code>ET</code></em><span class="special">,</span> <em class="replaceable"><code>ST</code></em><span class="special">,</span> <em class="replaceable"><code>FT</code></em><span class="special">></span></code> first evaluates <code class="computeroutput"><em class="replaceable"><code>ET</code></em></code> 7526 to obtain a Fusion sequence and <code class="computeroutput"><em class="replaceable"><code>ST</code></em></code> 7527 to obtain an initial state for the fold, and then evaluates <code class="computeroutput"><em class="replaceable"><code>FT</code></em></code> 7528 for each element in the sequence to generate the next state from 7529 the previous. 7530 </p></dd> 7531<dt><span class="term"> <code class="computeroutput"><a class="link" href="../boost/proto/reverse_fold.html" title="Struct template reverse_fold">proto::reverse_fold<></a></code></span></dt> 7532<dd><p> 7533 Like <code class="computeroutput"><a class="link" href="../boost/proto/fold.html" title="Struct template fold">proto::fold<></a></code>, except the 7534 elements in the Fusion sequence are iterated in reverse order. 7535 </p></dd> 7536<dt><span class="term"> <code class="computeroutput"><a class="link" href="../boost/proto/fold_tree.html" title="Struct template fold_tree">proto::fold_tree<></a></code></span></dt> 7537<dd><p> 7538 Like <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">fold</span><span class="special"><</span><em class="replaceable"><code>ET</code></em><span class="special">,</span> <em class="replaceable"><code>ST</code></em><span class="special">,</span> <em class="replaceable"><code>FT</code></em><span class="special">></span></code>, except that the result of 7539 the <code class="computeroutput"><em class="replaceable"><code>ET</code></em></code> transform is treated 7540 as an expression tree that is <span class="emphasis"><em>flattened</em></span> to 7541 generate the sequence to be folded. Flattening an expression tree 7542 causes child nodes with the same tag type as the parent to be put 7543 into sequence. For instance, <code class="computeroutput"><span class="identifier">a</span> 7544 <span class="special">>></span> <span class="identifier">b</span> 7545 <span class="special">>></span> <span class="identifier">c</span></code> 7546 would be flattened to the sequence [<code class="computeroutput"><span class="identifier">a</span></code>, 7547 <code class="computeroutput"><span class="identifier">b</span></code>, <code class="computeroutput"><span class="identifier">c</span></code>], and this is the sequence 7548 that would be folded. 7549 </p></dd> 7550<dt><span class="term"> <code class="computeroutput"><a class="link" href="../boost/proto/reverse_fold_tree.html" title="Struct template reverse_fold_tree">proto::reverse_fold_tree<></a></code></span></dt> 7551<dd><p> 7552 Like <code class="computeroutput"><a class="link" href="../boost/proto/fold_tree.html" title="Struct template fold_tree">proto::fold_tree<></a></code>, except that 7553 the flattened sequence is iterated in reverse order. 7554 </p></dd> 7555<dt><span class="term"> <code class="computeroutput"><a class="link" href="../boost/proto/lazy.html" title="Struct template lazy">proto::lazy<></a></code></span></dt> 7556<dd><p> 7557 A combination of <code class="computeroutput"><a class="link" href="../boost/proto/make.html" title="Struct template make">proto::make<></a></code> 7558 and <code class="computeroutput"><a class="link" href="../boost/proto/call.html" title="Struct template call">proto::call<></a></code> that is useful 7559 when the nature of the transform depends on the expression, state 7560 and/or data parameters. <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">lazy</span><span class="special"><</span><span class="identifier">R</span><span class="special">(</span><span class="identifier">A0</span><span class="special">,</span><span class="identifier">A1</span><span class="special">...</span><span class="identifier">An</span><span class="special">)></span></code> first evaluates <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">make</span><span class="special"><</span><span class="identifier">R</span><span class="special">()></span></code> 7561 to compute a callable type <code class="computeroutput"><span class="identifier">R2</span></code>. 7562 Then, it evaluates <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">call</span><span class="special"><</span><span class="identifier">R2</span><span class="special">(</span><span class="identifier">A0</span><span class="special">,</span><span class="identifier">A1</span><span class="special">...</span><span class="identifier">An</span><span class="special">)></span></code>. 7563 </p></dd> 7564</dl> 7565</div> 7566<h6> 7567<a name="boost_proto.users_guide.back_end.expression_transformation.canned_transforms.h0"></a> 7568 <span class="phrase"><a name="boost_proto.users_guide.back_end.expression_transformation.canned_transforms.all_grammars_are_primitive_transforms"></a></span><a class="link" href="users_guide.html#boost_proto.users_guide.back_end.expression_transformation.canned_transforms.all_grammars_are_primitive_transforms">All 7569 Grammars Are Primitive Transforms</a> 7570 </h6> 7571<p> 7572 In addition to the above primitive transforms, all of Proto's grammar 7573 elements are also primitive transforms. Their behaviors are described 7574 below. 7575 </p> 7576<div class="variablelist"> 7577<p class="title"><b></b></p> 7578<dl class="variablelist"> 7579<dt><span class="term"> <code class="computeroutput"><a class="link" href="../boost/proto/_.html" title="Struct _">proto::_</a></code></span></dt> 7580<dd><p> 7581 Return the current expression unmodified. 7582 </p></dd> 7583<dt><span class="term"> <code class="computeroutput"><a class="link" href="../boost/proto/or_.html" title="Struct template or_">proto::or_<></a></code></span></dt> 7584<dd><p> 7585 For the specified set of alternate sub-grammars, find the one that 7586 matches the given expression and apply its associated transform. 7587 </p></dd> 7588<dt><span class="term"> <code class="computeroutput"><a class="link" href="../boost/proto/and_.html" title="Struct template and_">proto::and_<></a></code></span></dt> 7589<dd><p> 7590 For the given set of sub-grammars, apply all the associated transforms 7591 and return the result of the last. 7592 </p></dd> 7593<dt><span class="term"> <code class="computeroutput"><a class="link" href="../boost/proto/not_.html" title="Struct template not_">proto::not_<></a></code></span></dt> 7594<dd><p> 7595 Return the current expression unmodified. 7596 </p></dd> 7597<dt><span class="term"> <code class="computeroutput"><a class="link" href="../boost/proto/if_.html" title="Struct template if_">proto::if_<></a></code></span></dt> 7598<dd><p> 7599 Given three transforms, evaluate the first and treat the result 7600 as a compile-time Boolean value. If it is true, evaluate the second 7601 transform. Otherwise, evaluate the third. 7602 </p></dd> 7603<dt><span class="term"> <code class="computeroutput"><a class="link" href="../boost/proto/switch_.html" title="Struct template switch_">proto::switch_<></a></code></span></dt> 7604<dd><p> 7605 As with <code class="computeroutput"><a class="link" href="../boost/proto/or_.html" title="Struct template or_">proto::or_<></a></code>, find the sub-grammar 7606 that matches the given expression and apply its associated transform. 7607 </p></dd> 7608<dt><span class="term"> <code class="computeroutput"><a class="link" href="../boost/proto/terminal.html" title="Struct template terminal">proto::terminal<></a></code></span></dt> 7609<dd><p> 7610 Return the current terminal expression unmodified. 7611 </p></dd> 7612<dt><span class="term"> <code class="computeroutput"><a class="link" href="../boost/proto/plus.html" title="Struct template plus">proto::plus<></a></code>, <code class="computeroutput"><a class="link" href="../boost/proto/nary_expr.html" title="Struct template nary_expr">proto::nary_expr<></a></code>, 7613 et. al.</span></dt> 7614<dd><p> 7615 A Proto grammar that matches a non-terminal such as <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">plus</span><span class="special"><</span><em class="replaceable"><code>G0</code></em><span class="special">,</span> <em class="replaceable"><code>G1</code></em><span class="special">></span></code>, when used as a primitive transform, 7616 creates a new plus node where the left child is transformed according 7617 to <code class="computeroutput"><em class="replaceable"><code>G0</code></em></code> and the right child 7618 with <code class="computeroutput"><em class="replaceable"><code>G1</code></em></code>. 7619 </p></dd> 7620</dl> 7621</div> 7622<h6> 7623<a name="boost_proto.users_guide.back_end.expression_transformation.canned_transforms.h1"></a> 7624 <span class="phrase"><a name="boost_proto.users_guide.back_end.expression_transformation.canned_transforms.the_pass_through_transform"></a></span><a class="link" href="users_guide.html#boost_proto.users_guide.back_end.expression_transformation.canned_transforms.the_pass_through_transform">The 7625 Pass-Through Transform</a> 7626 </h6> 7627<p> 7628 Note the primitive transform associated with grammar elements such as 7629 <code class="computeroutput"><a class="link" href="../boost/proto/plus.html" title="Struct template plus">proto::plus<></a></code> described above. 7630 They possess a so-called <span class="emphasis"><em>pass-through</em></span> transform. 7631 The pass-through transform accepts an expression of a certain tag type 7632 (say, <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">plus</span></code>) and creates a new expression 7633 of the same tag type, where each child expression is transformed according 7634 to the corresponding child grammar of the pass-through transform. So 7635 for instance this grammar ... 7636 </p> 7637<pre class="programlisting"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">function</span><span class="special"><</span> <span class="identifier">X</span><span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">vararg</span><span class="special"><</span><span class="identifier">Y</span><span class="special">></span> <span class="special">></span> 7638</pre> 7639<p> 7640 ... matches function expressions where the first child matches the <code class="computeroutput"><span class="identifier">X</span></code> grammar and the rest match the <code class="computeroutput"><span class="identifier">Y</span></code> grammar. When used as a transform, 7641 the above grammar will create a new function expression where the first 7642 child is transformed according to <code class="computeroutput"><span class="identifier">X</span></code> 7643 and the rest are transformed according to <code class="computeroutput"><span class="identifier">Y</span></code>. 7644 </p> 7645<p> 7646 The following class templates in Proto can be used as grammars with pass-through 7647 transforms: 7648 </p> 7649<div class="table"> 7650<a name="boost_proto.users_guide.back_end.expression_transformation.canned_transforms.t0"></a><p class="title"><b>Table 32.11. Class Templates With Pass-Through Transforms</b></p> 7651<div class="table-contents"><table class="table" summary="Class Templates With Pass-Through Transforms"> 7652<colgroup><col></colgroup> 7653<thead><tr><th> 7654 <p> 7655 Templates with Pass-Through Transforms 7656 </p> 7657 </th></tr></thead> 7658<tbody> 7659<tr><td> 7660 <p> 7661 <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">unary_plus</span><span class="special"><></span></code> 7662 </p> 7663 </td></tr> 7664<tr><td> 7665 <p> 7666 <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">negate</span><span class="special"><></span></code> 7667 </p> 7668 </td></tr> 7669<tr><td> 7670 <p> 7671 <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">dereference</span><span class="special"><></span></code> 7672 </p> 7673 </td></tr> 7674<tr><td> 7675 <p> 7676 <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">complement</span><span class="special"><></span></code> 7677 </p> 7678 </td></tr> 7679<tr><td> 7680 <p> 7681 <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">address_of</span><span class="special"><></span></code> 7682 </p> 7683 </td></tr> 7684<tr><td> 7685 <p> 7686 <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">logical_not</span><span class="special"><></span></code> 7687 </p> 7688 </td></tr> 7689<tr><td> 7690 <p> 7691 <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">pre_inc</span><span class="special"><></span></code> 7692 </p> 7693 </td></tr> 7694<tr><td> 7695 <p> 7696 <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">pre_dec</span><span class="special"><></span></code> 7697 </p> 7698 </td></tr> 7699<tr><td> 7700 <p> 7701 <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">post_inc</span><span class="special"><></span></code> 7702 </p> 7703 </td></tr> 7704<tr><td> 7705 <p> 7706 <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">post_dec</span><span class="special"><></span></code> 7707 </p> 7708 </td></tr> 7709<tr><td> 7710 <p> 7711 <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">shift_left</span><span class="special"><></span></code> 7712 </p> 7713 </td></tr> 7714<tr><td> 7715 <p> 7716 <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">shift_right</span><span class="special"><></span></code> 7717 </p> 7718 </td></tr> 7719<tr><td> 7720 <p> 7721 <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">multiplies</span><span class="special"><></span></code> 7722 </p> 7723 </td></tr> 7724<tr><td> 7725 <p> 7726 <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">divides</span><span class="special"><></span></code> 7727 </p> 7728 </td></tr> 7729<tr><td> 7730 <p> 7731 <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">modulus</span><span class="special"><></span></code> 7732 </p> 7733 </td></tr> 7734<tr><td> 7735 <p> 7736 <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">plus</span><span class="special"><></span></code> 7737 </p> 7738 </td></tr> 7739<tr><td> 7740 <p> 7741 <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">minus</span><span class="special"><></span></code> 7742 </p> 7743 </td></tr> 7744<tr><td> 7745 <p> 7746 <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">less</span><span class="special"><></span></code> 7747 </p> 7748 </td></tr> 7749<tr><td> 7750 <p> 7751 <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">greater</span><span class="special"><></span></code> 7752 </p> 7753 </td></tr> 7754<tr><td> 7755 <p> 7756 <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">less_equal</span><span class="special"><></span></code> 7757 </p> 7758 </td></tr> 7759<tr><td> 7760 <p> 7761 <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">greater_equal</span><span class="special"><></span></code> 7762 </p> 7763 </td></tr> 7764<tr><td> 7765 <p> 7766 <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">equal_to</span><span class="special"><></span></code> 7767 </p> 7768 </td></tr> 7769<tr><td> 7770 <p> 7771 <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">not_equal_to</span><span class="special"><></span></code> 7772 </p> 7773 </td></tr> 7774<tr><td> 7775 <p> 7776 <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">logical_or</span><span class="special"><></span></code> 7777 </p> 7778 </td></tr> 7779<tr><td> 7780 <p> 7781 <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">logical_and</span><span class="special"><></span></code> 7782 </p> 7783 </td></tr> 7784<tr><td> 7785 <p> 7786 <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">bitwise_and</span><span class="special"><></span></code> 7787 </p> 7788 </td></tr> 7789<tr><td> 7790 <p> 7791 <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">bitwise_or</span><span class="special"><></span></code> 7792 </p> 7793 </td></tr> 7794<tr><td> 7795 <p> 7796 <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">bitwise_xor</span><span class="special"><></span></code> 7797 </p> 7798 </td></tr> 7799<tr><td> 7800 <p> 7801 <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">comma</span><span class="special"><></span></code> 7802 </p> 7803 </td></tr> 7804<tr><td> 7805 <p> 7806 <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">mem_ptr</span><span class="special"><></span></code> 7807 </p> 7808 </td></tr> 7809<tr><td> 7810 <p> 7811 <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">assign</span><span class="special"><></span></code> 7812 </p> 7813 </td></tr> 7814<tr><td> 7815 <p> 7816 <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">shift_left_assign</span><span class="special"><></span></code> 7817 </p> 7818 </td></tr> 7819<tr><td> 7820 <p> 7821 <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">shift_right_assign</span><span class="special"><></span></code> 7822 </p> 7823 </td></tr> 7824<tr><td> 7825 <p> 7826 <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">multiplies_assign</span><span class="special"><></span></code> 7827 </p> 7828 </td></tr> 7829<tr><td> 7830 <p> 7831 <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">divides_assign</span><span class="special"><></span></code> 7832 </p> 7833 </td></tr> 7834<tr><td> 7835 <p> 7836 <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">modulus_assign</span><span class="special"><></span></code> 7837 </p> 7838 </td></tr> 7839<tr><td> 7840 <p> 7841 <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">plus_assign</span><span class="special"><></span></code> 7842 </p> 7843 </td></tr> 7844<tr><td> 7845 <p> 7846 <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">minus_assign</span><span class="special"><></span></code> 7847 </p> 7848 </td></tr> 7849<tr><td> 7850 <p> 7851 <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">bitwise_and_assign</span><span class="special"><></span></code> 7852 </p> 7853 </td></tr> 7854<tr><td> 7855 <p> 7856 <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">bitwise_or_assign</span><span class="special"><></span></code> 7857 </p> 7858 </td></tr> 7859<tr><td> 7860 <p> 7861 <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">bitwise_xor_assign</span><span class="special"><></span></code> 7862 </p> 7863 </td></tr> 7864<tr><td> 7865 <p> 7866 <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">subscript</span><span class="special"><></span></code> 7867 </p> 7868 </td></tr> 7869<tr><td> 7870 <p> 7871 <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">if_else_</span><span class="special"><></span></code> 7872 </p> 7873 </td></tr> 7874<tr><td> 7875 <p> 7876 <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">function</span><span class="special"><></span></code> 7877 </p> 7878 </td></tr> 7879<tr><td> 7880 <p> 7881 <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">unary_expr</span><span class="special"><></span></code> 7882 </p> 7883 </td></tr> 7884<tr><td> 7885 <p> 7886 <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">binary_expr</span><span class="special"><></span></code> 7887 </p> 7888 </td></tr> 7889<tr><td> 7890 <p> 7891 <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">nary_expr</span><span class="special"><></span></code> 7892 </p> 7893 </td></tr> 7894</tbody> 7895</table></div> 7896</div> 7897<br class="table-break"><h6> 7898<a name="boost_proto.users_guide.back_end.expression_transformation.canned_transforms.h2"></a> 7899 <span class="phrase"><a name="boost_proto.users_guide.back_end.expression_transformation.canned_transforms.the_many_roles_of_proto_operator_metafunctions"></a></span><a class="link" href="users_guide.html#boost_proto.users_guide.back_end.expression_transformation.canned_transforms.the_many_roles_of_proto_operator_metafunctions">The 7900 Many Roles of Proto Operator Metafunctions</a> 7901 </h6> 7902<p> 7903 We've seen templates such as <code class="computeroutput"><a class="link" href="../boost/proto/terminal.html" title="Struct template terminal">proto::terminal<></a></code>, 7904 <code class="computeroutput"><a class="link" href="../boost/proto/plus.html" title="Struct template plus">proto::plus<></a></code> and <code class="computeroutput"><a class="link" href="../boost/proto/nary_expr.html" title="Struct template nary_expr">proto::nary_expr<></a></code> 7905 fill many roles. They are metafunction that generate expression types. 7906 They are grammars that match expression types. And they are primitive 7907 transforms. The following code samples show examples of each. 7908 </p> 7909<p> 7910 <span class="bold"><strong>As Metafunctions ...</strong></span> 7911 </p> 7912<pre class="programlisting"><span class="comment">// proto::terminal<> and proto::plus<> are metafunctions</span> 7913<span class="comment">// that generate expression types:</span> 7914<span class="keyword">typedef</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special"><</span><span class="keyword">int</span><span class="special">>::</span><span class="identifier">type</span> <span class="identifier">int_</span><span class="special">;</span> 7915<span class="keyword">typedef</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">plus</span><span class="special"><</span><span class="identifier">int_</span><span class="special">,</span> <span class="identifier">int_</span><span class="special">>::</span><span class="identifier">type</span> <span class="identifier">plus_</span><span class="special">;</span> 7916 7917<span class="identifier">int_</span> <span class="identifier">i</span> <span class="special">=</span> <span class="special">{</span><span class="number">42</span><span class="special">},</span> <span class="identifier">j</span> <span class="special">=</span> <span class="special">{</span><span class="number">24</span><span class="special">};</span> 7918<span class="identifier">plus_</span> <span class="identifier">p</span> <span class="special">=</span> <span class="special">{</span><span class="identifier">i</span><span class="special">,</span> <span class="identifier">j</span><span class="special">};</span> 7919</pre> 7920<p> 7921 <span class="bold"><strong>As Grammars ...</strong></span> 7922 </p> 7923<pre class="programlisting"><span class="comment">// proto::terminal<> and proto::plus<> are grammars that</span> 7924<span class="comment">// match expression types</span> 7925<span class="keyword">struct</span> <span class="identifier">Int</span> <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> <span class="special">{};</span> 7926<span class="keyword">struct</span> <span class="identifier">Plus</span> <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">plus</span><span class="special"><</span><span class="identifier">Int</span><span class="special">,</span> <span class="identifier">Int</span><span class="special">></span> <span class="special">{};</span> 7927 7928<span class="identifier">BOOST_MPL_ASSERT</span><span class="special">((</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">matches</span><span class="special"><</span> <span class="identifier">int_</span><span class="special">,</span> <span class="identifier">Int</span> <span class="special">></span> <span class="special">));</span> 7929<span class="identifier">BOOST_MPL_ASSERT</span><span class="special">((</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">matches</span><span class="special"><</span> <span class="identifier">plus_</span><span class="special">,</span> <span class="identifier">Plus</span> <span class="special">></span> <span class="special">));</span> 7930</pre> 7931<p> 7932 <span class="bold"><strong>As Primitive Transforms ...</strong></span> 7933 </p> 7934<pre class="programlisting"><span class="comment">// A transform that removes all unary_plus nodes in an expression</span> 7935<span class="keyword">struct</span> <span class="identifier">RemoveUnaryPlus</span> 7936 <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">or_</span><span class="special"><</span> 7937 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special"><</span> 7938 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">unary_plus</span><span class="special"><</span><span class="identifier">RemoveUnaryPlus</span><span class="special">></span> 7939 <span class="special">,</span> <span class="identifier">RemoveUnaryPlus</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_child</span><span class="special">)</span> 7940 <span class="special">></span> 7941 <span class="comment">// Use proto::terminal<> and proto::nary_expr<></span> 7942 <span class="comment">// both as grammars and as primitive transforms.</span> 7943 <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special"><</span><span class="identifier">_</span><span class="special">></span> 7944 <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">nary_expr</span><span class="special"><</span><span class="identifier">_</span><span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">vararg</span><span class="special"><</span><span class="identifier">RemoveUnaryPlus</span><span class="special">></span> <span class="special">></span> 7945 <span class="special">></span> 7946<span class="special">{};</span> 7947 7948<span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span> 7949<span class="special">{</span> 7950 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">literal</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> <span class="identifier">i</span><span class="special">(</span><span class="number">0</span><span class="special">);</span> 7951 7952 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">display_expr</span><span class="special">(</span> 7953 <span class="special">+</span><span class="identifier">i</span> <span class="special">-</span> <span class="special">+(</span><span class="identifier">i</span> <span class="special">-</span> <span class="special">+</span><span class="identifier">i</span><span class="special">)</span> 7954 <span class="special">);</span> 7955 7956 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">display_expr</span><span class="special">(</span> 7957 <span class="identifier">RemoveUnaryPlus</span><span class="special">()(</span> <span class="special">+</span><span class="identifier">i</span> <span class="special">-</span> <span class="special">+(</span><span class="identifier">i</span> <span class="special">-</span> <span class="special">+</span><span class="identifier">i</span><span class="special">)</span> <span class="special">)</span> 7958 <span class="special">);</span> 7959<span class="special">}</span> 7960</pre> 7961<p> 7962 The above code displays the following, which shows that unary plus nodes 7963 have been stripped from the expression: 7964 </p> 7965<pre class="programlisting">minus( 7966 unary_plus( 7967 terminal(0) 7968 ) 7969 , unary_plus( 7970 minus( 7971 terminal(0) 7972 , unary_plus( 7973 terminal(0) 7974 ) 7975 ) 7976 ) 7977) 7978minus( 7979 terminal(0) 7980 , minus( 7981 terminal(0) 7982 , terminal(0) 7983 ) 7984) 7985</pre> 7986</div> 7987<div class="section"> 7988<div class="titlepage"><div><div><h5 class="title"> 7989<a name="boost_proto.users_guide.back_end.expression_transformation.primitives"></a><a class="link" href="users_guide.html#boost_proto.users_guide.back_end.expression_transformation.primitives" title="Building Custom Primitive Transforms">Building 7990 Custom Primitive Transforms</a> 7991</h5></div></div></div> 7992<p> 7993 In previous sections, we've seen how to compose larger transforms out 7994 of smaller transforms using function types. The smaller transforms from 7995 which larger transforms are composed are <span class="emphasis"><em>primitive transforms</em></span>, 7996 and Proto provides a bunch of common ones such as <code class="computeroutput"><span class="identifier">_child0</span></code> 7997 and <code class="computeroutput"><span class="identifier">_value</span></code>. In this section 7998 we'll see how to author your own primitive transforms. 7999 </p> 8000<div class="note"><table border="0" summary="Note"> 8001<tr> 8002<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../doc/src/images/note.png"></td> 8003<th align="left">Note</th> 8004</tr> 8005<tr><td align="left" valign="top"><p> 8006 There are a few reasons why you might want to write your own primitive 8007 transforms. For instance, your transform may be complicated, and composing 8008 it out of primitives becomes unwieldy. You might also need to work 8009 around compiler bugs on legacy compilers that make composing transforms 8010 using function types problematic. Finally, you might also decide to 8011 define your own primitive transforms to improve compile times. Since 8012 Proto can simply invoke a primitive transform directly without having 8013 to process arguments or differentiate callable transforms from object 8014 transforms, primitive transforms are more efficient. 8015 </p></td></tr> 8016</table></div> 8017<p> 8018 Primitive transforms inherit from <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">transform</span><span class="special"><></span></code> and have a nested <code class="computeroutput"><span class="identifier">impl</span><span class="special"><></span></code> 8019 template that inherits from <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">transform_impl</span><span class="special"><></span></code>. For example, this is how Proto 8020 defines the <code class="computeroutput"><span class="identifier">_child_c</span><span class="special"><</span><em class="replaceable"><code>N</code></em><span class="special">></span></code> 8021 transform, which returns the <em class="replaceable"><code>N</code></em>-th child of 8022 the current expression: 8023 </p> 8024<pre class="programlisting"><span class="keyword">namespace</span> <span class="identifier">boost</span> <span class="special">{</span> <span class="keyword">namespace</span> <span class="identifier">proto</span> 8025<span class="special">{</span> 8026 <span class="comment">// A primitive transform that returns N-th child</span> 8027 <span class="comment">// of the current expression.</span> 8028 <span class="keyword">template</span><span class="special"><</span><span class="keyword">int</span> <span class="identifier">N</span><span class="special">></span> 8029 <span class="keyword">struct</span> <span class="identifier">_child_c</span> <span class="special">:</span> <span class="identifier">transform</span><span class="special"><</span><span class="identifier">_child_c</span><span class="special"><</span><span class="identifier">N</span><span class="special">></span> <span class="special">></span> 8030 <span class="special">{</span> 8031 <span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">State</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Data</span><span class="special">></span> 8032 <span class="keyword">struct</span> <span class="identifier">impl</span> <span class="special">:</span> <span class="identifier">transform_impl</span><span class="special"><</span><span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">State</span><span class="special">,</span> <span class="identifier">Data</span><span class="special">></span> 8033 <span class="special">{</span> 8034 <span class="keyword">typedef</span> 8035 <span class="keyword">typename</span> <span class="identifier">result_of</span><span class="special">::</span><span class="identifier">child_c</span><span class="special"><</span><span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">N</span><span class="special">>::</span><span class="identifier">type</span> 8036 <span class="identifier">result_type</span><span class="special">;</span> 8037 8038 <span class="identifier">result_type</span> <span class="keyword">operator</span> <span class="special">()(</span> 8039 <span class="keyword">typename</span> <span class="identifier">impl</span><span class="special">::</span><span class="identifier">expr_param</span> <span class="identifier">expr</span> 8040 <span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">impl</span><span class="special">::</span><span class="identifier">state_param</span> <span class="identifier">state</span> 8041 <span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">impl</span><span class="special">::</span><span class="identifier">data_param</span> <span class="identifier">data</span> 8042 <span class="special">)</span> <span class="keyword">const</span> 8043 <span class="special">{</span> 8044 <span class="keyword">return</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">child_c</span><span class="special"><</span><span class="identifier">N</span><span class="special">>(</span><span class="identifier">expr</span><span class="special">);</span> 8045 <span class="special">}</span> 8046 <span class="special">};</span> 8047 <span class="special">};</span> 8048 8049 <span class="comment">// Note that _child_c<N> is callable, so that</span> 8050 <span class="comment">// it can be used in callable transforms, as:</span> 8051 <span class="comment">// _child_c<0>(_child_c<1>)</span> 8052 <span class="keyword">template</span><span class="special"><</span><span class="keyword">int</span> <span class="identifier">N</span><span class="special">></span> 8053 <span class="keyword">struct</span> <span class="identifier">is_callable</span><span class="special"><</span><span class="identifier">_child_c</span><span class="special"><</span><span class="identifier">N</span><span class="special">></span> <span class="special">></span> 8054 <span class="special">:</span> <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">true_</span> 8055 <span class="special">{};</span> 8056<span class="special">}}</span> 8057</pre> 8058<p> 8059 The <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">transform</span><span class="special"><></span></code> 8060 base class provides the <code class="computeroutput"><span class="keyword">operator</span><span class="special">()</span></code> overloads and the nested <code class="computeroutput"><span class="identifier">result</span><span class="special"><></span></code> 8061 template that make your transform a valid function object. These are 8062 implemented in terms of the nested <code class="computeroutput"><span class="identifier">impl</span><span class="special"><></span></code> template you define. 8063 </p> 8064<p> 8065 The <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">transform_impl</span><span class="special"><></span></code> 8066 base class is a convenience. It provides some nested typedefs that are 8067 generally useful. They are specified in the table below: 8068 </p> 8069<div class="table"> 8070<a name="boost_proto.users_guide.back_end.expression_transformation.primitives.t0"></a><p class="title"><b>Table 32.12. proto::transform_impl<Expr, State, Data> typedefs</b></p> 8071<div class="table-contents"><table class="table" summary="proto::transform_impl<Expr, State, Data> typedefs"> 8072<colgroup> 8073<col> 8074<col> 8075</colgroup> 8076<thead><tr> 8077<th> 8078 <p> 8079 typedef 8080 </p> 8081 </th> 8082<th> 8083 <p> 8084 Equivalent To 8085 </p> 8086 </th> 8087</tr></thead> 8088<tbody> 8089<tr> 8090<td> 8091 <p> 8092 <code class="computeroutput"><span class="identifier">expr</span></code> 8093 </p> 8094 </td> 8095<td> 8096 <p> 8097 <code class="computeroutput"><span class="keyword">typename</span> <span class="identifier">remove_reference</span><span class="special"><</span><span class="identifier">Expr</span><span class="special">>::</span><span class="identifier">type</span></code> 8098 </p> 8099 </td> 8100</tr> 8101<tr> 8102<td> 8103 <p> 8104 <code class="computeroutput"><span class="identifier">state</span></code> 8105 </p> 8106 </td> 8107<td> 8108 <p> 8109 <code class="computeroutput"><span class="keyword">typename</span> <span class="identifier">remove_reference</span><span class="special"><</span><span class="identifier">State</span><span class="special">>::</span><span class="identifier">type</span></code> 8110 </p> 8111 </td> 8112</tr> 8113<tr> 8114<td> 8115 <p> 8116 <code class="computeroutput"><span class="identifier">data</span></code> 8117 </p> 8118 </td> 8119<td> 8120 <p> 8121 <code class="computeroutput"><span class="keyword">typename</span> <span class="identifier">remove_reference</span><span class="special"><</span><span class="identifier">Data</span><span class="special">>::</span><span class="identifier">type</span></code> 8122 </p> 8123 </td> 8124</tr> 8125<tr> 8126<td> 8127 <p> 8128 <code class="computeroutput"><span class="identifier">expr_param</span></code> 8129 </p> 8130 </td> 8131<td> 8132 <p> 8133 <code class="computeroutput"><span class="keyword">typename</span> <span class="identifier">add_reference</span><span class="special"><</span><span class="keyword">typename</span> 8134 <span class="identifier">add_const</span><span class="special"><</span><span class="identifier">Expr</span><span class="special">>::</span><span class="identifier">type</span><span class="special">>::</span><span class="identifier">type</span></code> 8135 </p> 8136 </td> 8137</tr> 8138<tr> 8139<td> 8140 <p> 8141 <code class="computeroutput"><span class="identifier">state_param</span></code> 8142 </p> 8143 </td> 8144<td> 8145 <p> 8146 <code class="computeroutput"><span class="keyword">typename</span> <span class="identifier">add_reference</span><span class="special"><</span><span class="keyword">typename</span> 8147 <span class="identifier">add_const</span><span class="special"><</span><span class="identifier">State</span><span class="special">>::</span><span class="identifier">type</span><span class="special">>::</span><span class="identifier">type</span></code> 8148 </p> 8149 </td> 8150</tr> 8151<tr> 8152<td> 8153 <p> 8154 <code class="computeroutput"><span class="identifier">data_param</span></code> 8155 </p> 8156 </td> 8157<td> 8158 <p> 8159 <code class="computeroutput"><span class="keyword">typename</span> <span class="identifier">add_reference</span><span class="special"><</span><span class="keyword">typename</span> 8160 <span class="identifier">add_const</span><span class="special"><</span><span class="identifier">Data</span><span class="special">>::</span><span class="identifier">type</span><span class="special">>::</span><span class="identifier">type</span></code> 8161 </p> 8162 </td> 8163</tr> 8164</tbody> 8165</table></div> 8166</div> 8167<br class="table-break"><p> 8168 You'll notice that <code class="computeroutput"><span class="identifier">_child_c</span><span class="special">::</span><span class="identifier">impl</span><span class="special">::</span><span class="keyword">operator</span><span class="special">()</span></code> takes arguments of types <code class="computeroutput"><span class="identifier">expr_param</span></code>, <code class="computeroutput"><span class="identifier">state_param</span></code>, 8169 and <code class="computeroutput"><span class="identifier">data_param</span></code>. The typedefs 8170 make it easy to accept arguments by reference or const reference accordingly. 8171 </p> 8172<p> 8173 The only other interesting bit is the <code class="computeroutput"><span class="identifier">is_callable</span><span class="special"><></span></code> specialization, which will be 8174 described in the <a class="link" href="users_guide.html#boost_proto.users_guide.back_end.expression_transformation.is_callable" title="Making Your Transform Callable">next 8175 section</a>. 8176 </p> 8177</div> 8178<div class="section"> 8179<div class="titlepage"><div><div><h5 class="title"> 8180<a name="boost_proto.users_guide.back_end.expression_transformation.is_callable"></a><a class="link" href="users_guide.html#boost_proto.users_guide.back_end.expression_transformation.is_callable" title="Making Your Transform Callable">Making 8181 Your Transform Callable</a> 8182</h5></div></div></div> 8183<p> 8184 Transforms are typically of the form <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special"><</span> <span class="identifier">Something</span><span class="special">,</span> <span class="identifier">R</span><span class="special">(</span><span class="identifier">A0</span><span class="special">,</span><span class="identifier">A1</span><span class="special">,...)</span> <span class="special">></span></code>. 8185 The question is whether <code class="computeroutput"><span class="identifier">R</span></code> 8186 represents a function to call or an object to construct, and the answer 8187 determines how <code class="computeroutput"><a class="link" href="../boost/proto/when.html" title="Struct template when">proto::when<></a></code> evaluates the transform. 8188 <code class="computeroutput"><a class="link" href="../boost/proto/when.html" title="Struct template when">proto::when<></a></code> uses the <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">is_callable</span><span class="special"><></span></code> 8189 trait to disambiguate between the two. Proto does its best to guess whether 8190 a type is callable or not, but it doesn't always get it right. It's best 8191 to know the rules Proto uses, so that you know when you need to be more 8192 explicit. 8193 </p> 8194<p> 8195 For most types <code class="computeroutput"><span class="identifier">R</span></code>, <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">is_callable</span><span class="special"><</span><span class="identifier">R</span><span class="special">></span></code> 8196 checks for inheritance from <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">callable</span></code>. 8197 However, if the type <code class="computeroutput"><span class="identifier">R</span></code> 8198 is a template specialization, Proto assumes that it is <span class="emphasis"><em>not</em></span> 8199 callable <span class="emphasis"><em>even if the template inherits from <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">callable</span></code></em></span>. 8200 We'll see why in a minute. Consider the following erroneous callable 8201 object: 8202 </p> 8203<pre class="programlisting"><span class="comment">// Proto can't tell this defines something callable!</span> 8204<span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">></span> 8205<span class="keyword">struct</span> <span class="identifier">times2</span> <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">callable</span> 8206<span class="special">{</span> 8207 <span class="keyword">typedef</span> <span class="identifier">T</span> <span class="identifier">result_type</span><span class="special">;</span> 8208 8209 <span class="identifier">T</span> <span class="keyword">operator</span><span class="special">()(</span><span class="identifier">T</span> <span class="identifier">i</span><span class="special">)</span> <span class="keyword">const</span> 8210 <span class="special">{</span> 8211 <span class="keyword">return</span> <span class="identifier">i</span> <span class="special">*</span> <span class="number">2</span><span class="special">;</span> 8212 <span class="special">}</span> 8213<span class="special">};</span> 8214 8215<span class="comment">// ERROR! This is not going to multiply the int by 2:</span> 8216<span class="keyword">struct</span> <span class="identifier">IntTimes2</span> 8217 <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special"><</span> 8218 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> 8219 <span class="special">,</span> <span class="identifier">times2</span><span class="special"><</span><span class="keyword">int</span><span class="special">>(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_value</span><span class="special">)</span> 8220 <span class="special">></span> 8221<span class="special">{};</span> 8222</pre> 8223<p> 8224 The problem is that Proto doesn't know that <code class="computeroutput"><span class="identifier">times2</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span></code> is callable, so rather that invoking 8225 the <code class="computeroutput"><span class="identifier">times2</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span></code> 8226 function object, Proto will try to construct a <code class="computeroutput"><span class="identifier">times2</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span></code> object and initialize it will an 8227 <code class="computeroutput"><span class="keyword">int</span></code>. That will not compile. 8228 </p> 8229<div class="note"><table border="0" summary="Note"> 8230<tr> 8231<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../doc/src/images/note.png"></td> 8232<th align="left">Note</th> 8233</tr> 8234<tr><td align="left" valign="top"><p> 8235 Why can't Proto tell that <code class="computeroutput"><span class="identifier">times2</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span></code> is callable? After all, it inherits 8236 from <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">callable</span></code>, and that is detectable, 8237 right? The problem is that merely asking whether some type <code class="computeroutput"><span class="identifier">X</span><span class="special"><</span><span class="identifier">Y</span><span class="special">></span></code> 8238 inherits from <code class="computeroutput"><span class="identifier">callable</span></code> 8239 will cause the template <code class="computeroutput"><span class="identifier">X</span><span class="special"><</span><span class="identifier">Y</span><span class="special">></span></code> to be instantiated. That's a problem 8240 for a type like <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special"><</span><span class="identifier">_value</span><span class="special">(</span><span class="identifier">_child1</span><span class="special">)></span></code>. <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special"><></span></code> will not suffer to be instantiated 8241 with <code class="computeroutput"><span class="identifier">_value</span><span class="special">(</span><span class="identifier">_child1</span><span class="special">)</span></code> 8242 as a template parameter. Since merely asking the question will sometimes 8243 result in a hard error, Proto can't ask; it has to assume that <code class="computeroutput"><span class="identifier">X</span><span class="special"><</span><span class="identifier">Y</span><span class="special">></span></code> 8244 represents an object to construct and not a function to call. 8245 </p></td></tr> 8246</table></div> 8247<p> 8248 There are a couple of solutions to the <code class="computeroutput"><span class="identifier">times2</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span></code> problem. One solution is to wrap 8249 the transform in <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">call</span><span class="special"><></span></code>. This forces Proto to treat 8250 <code class="computeroutput"><span class="identifier">times2</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span></code> 8251 as callable: 8252 </p> 8253<pre class="programlisting"><span class="comment">// OK, calls times2<int></span> 8254<span class="keyword">struct</span> <span class="identifier">IntTimes2</span> 8255 <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special"><</span> 8256 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> 8257 <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">call</span><span class="special"><</span><span class="identifier">times2</span><span class="special"><</span><span class="keyword">int</span><span class="special">>(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_value</span><span class="special">)></span> 8258 <span class="special">></span> 8259<span class="special">{};</span> 8260</pre> 8261<p> 8262 This can be a bit of a pain, because we need to wrap every use of <code class="computeroutput"><span class="identifier">times2</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span></code>, 8263 which can be tedious and error prone, and makes our grammar cluttered 8264 and harder to read. 8265 </p> 8266<p> 8267 Another solution is to specialize <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">is_callable</span><span class="special"><></span></code> on our <code class="computeroutput"><span class="identifier">times2</span><span class="special"><></span></code> template: 8268 </p> 8269<pre class="programlisting"><span class="keyword">namespace</span> <span class="identifier">boost</span> <span class="special">{</span> <span class="keyword">namespace</span> <span class="identifier">proto</span> 8270<span class="special">{</span> 8271 <span class="comment">// Tell Proto that times2<> is callable</span> 8272 <span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">></span> 8273 <span class="keyword">struct</span> <span class="identifier">is_callable</span><span class="special"><</span><span class="identifier">times2</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="special">></span> 8274 <span class="special">:</span> <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">true_</span> 8275 <span class="special">{};</span> 8276<span class="special">}}</span> 8277 8278<span class="comment">// OK, times2<> is callable</span> 8279<span class="keyword">struct</span> <span class="identifier">IntTimes2</span> 8280 <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special"><</span> 8281 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> 8282 <span class="special">,</span> <span class="identifier">times2</span><span class="special"><</span><span class="keyword">int</span><span class="special">>(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_value</span><span class="special">)</span> 8283 <span class="special">></span> 8284<span class="special">{};</span> 8285</pre> 8286<p> 8287 This is better, but still a pain because of the need to open Proto's 8288 namespace. 8289 </p> 8290<p> 8291 You could simply make sure that the callable type is not a template specialization. 8292 Consider the following: 8293 </p> 8294<pre class="programlisting"><span class="comment">// No longer a template specialization!</span> 8295<span class="keyword">struct</span> <span class="identifier">times2int</span> <span class="special">:</span> <span class="identifier">times2</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> <span class="special">{};</span> 8296 8297<span class="comment">// OK, times2int is callable</span> 8298<span class="keyword">struct</span> <span class="identifier">IntTimes2</span> 8299 <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special"><</span> 8300 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> 8301 <span class="special">,</span> <span class="identifier">times2int</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_value</span><span class="special">)</span> 8302 <span class="special">></span> 8303<span class="special">{};</span> 8304</pre> 8305<p> 8306 This works because now Proto can tell that <code class="computeroutput"><span class="identifier">times2int</span></code> 8307 inherits (indirectly) from <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">callable</span></code>. 8308 Any non-template types can be safely checked for inheritance because, 8309 as they are not templates, there is no worry about instantiation errors. 8310 </p> 8311<p> 8312 There is one last way to tell Proto that <code class="computeroutput"><span class="identifier">times2</span><span class="special"><></span></code> is callable. You could add an 8313 extra dummy template parameter that defaults to <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">callable</span></code>: 8314 </p> 8315<pre class="programlisting"><span class="comment">// Proto will recognize this as callable</span> 8316<span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Callable</span> <span class="special">=</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">callable</span><span class="special">></span> 8317<span class="keyword">struct</span> <span class="identifier">times2</span> <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">callable</span> 8318<span class="special">{</span> 8319 <span class="keyword">typedef</span> <span class="identifier">T</span> <span class="identifier">result_type</span><span class="special">;</span> 8320 8321 <span class="identifier">T</span> <span class="keyword">operator</span><span class="special">()(</span><span class="identifier">T</span> <span class="identifier">i</span><span class="special">)</span> <span class="keyword">const</span> 8322 <span class="special">{</span> 8323 <span class="keyword">return</span> <span class="identifier">i</span> <span class="special">*</span> <span class="number">2</span><span class="special">;</span> 8324 <span class="special">}</span> 8325<span class="special">};</span> 8326 8327<span class="comment">// OK, this works!</span> 8328<span class="keyword">struct</span> <span class="identifier">IntTimes2</span> 8329 <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special"><</span> 8330 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> 8331 <span class="special">,</span> <span class="identifier">times2</span><span class="special"><</span><span class="keyword">int</span><span class="special">>(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_value</span><span class="special">)</span> 8332 <span class="special">></span> 8333<span class="special">{};</span> 8334</pre> 8335<p> 8336 Note that in addition to the extra template parameter, <code class="computeroutput"><span class="identifier">times2</span><span class="special"><></span></code> 8337 still inherits from <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">callable</span></code>. 8338 That's not necessary in this example but it is good style because any 8339 types derived from <code class="computeroutput"><span class="identifier">times2</span><span class="special"><></span></code> (as <code class="computeroutput"><span class="identifier">times2int</span></code> 8340 defined above) will still be considered callable. 8341 </p> 8342</div> 8343</div> 8344</div> 8345<div class="section"> 8346<div class="titlepage"><div><div><h3 class="title"> 8347<a name="boost_proto.users_guide.examples"></a><a class="link" href="users_guide.html#boost_proto.users_guide.examples" title="Examples">Examples</a> 8348</h3></div></div></div> 8349<div class="toc"><dl class="toc"> 8350<dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.examples.hello_world">Hello 8351 World: Building an Expression Template and Evaluating It</a></span></dt> 8352<dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.examples.calc1">Calc1: Defining 8353 an Evaluation Context</a></span></dt> 8354<dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.examples.calc2">Calc2: Adding 8355 Members Using <code class="literal">proto::extends<></code></a></span></dt> 8356<dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.examples.calc3">Calc3: Defining 8357 a Simple Transform</a></span></dt> 8358<dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.examples.lazy_vector">Lazy 8359 Vector: Controlling Operator Overloads</a></span></dt> 8360<dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.examples.rgb">RGB: Type Manipulations 8361 with Proto Transforms</a></span></dt> 8362<dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.examples.tarray">TArray: A 8363 Simple Linear Algebra Library</a></span></dt> 8364<dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.examples.vec3">Vec3: Computing 8365 With Transforms and Contexts</a></span></dt> 8366<dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.examples.vector">Vector: Adapting 8367 a Non-Proto Terminal Type</a></span></dt> 8368<dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.examples.mixed">Mixed: Adapting 8369 Several Non-Proto Terminal Types</a></span></dt> 8370<dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.examples.map_assign">Map Assign: 8371 An Intermediate Transform</a></span></dt> 8372<dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.examples.future_group">Future 8373 Group: A More Advanced Transform</a></span></dt> 8374<dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.examples.lambda">Lambda: A 8375 Simple Lambda Library with Proto</a></span></dt> 8376<dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.examples.checked_calc">Checked 8377 Calculator: A Simple Example of External Transforms</a></span></dt> 8378</dl></div> 8379<p> 8380 A code example is worth a thousand words ... 8381 </p> 8382<div class="section"> 8383<div class="titlepage"><div><div><h4 class="title"> 8384<a name="boost_proto.users_guide.examples.hello_world"></a><a class="link" href="users_guide.html#boost_proto.users_guide.examples.hello_world" title="Hello World: Building an Expression Template and Evaluating It">Hello 8385 World: Building an Expression Template and Evaluating It</a> 8386</h4></div></div></div> 8387<p> 8388 A trivial example which builds and expression template and evaluates it. 8389 </p> 8390<p> 8391</p> 8392<pre class="programlisting"><span class="comment">////////////////////////////////////////////////////////////////////</span> 8393<span class="comment">// Copyright 2008 Eric Niebler. Distributed under the Boost</span> 8394<span class="comment">// Software License, Version 1.0. (See accompanying file</span> 8395<span class="comment">// LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)</span> 8396 8397<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">iostream</span><span class="special">></span> 8398<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">proto</span><span class="special">/</span><span class="identifier">core</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> 8399<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">proto</span><span class="special">/</span><span class="identifier">context</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> 8400<span class="comment">// This #include is only needed for compilers that use typeof emulation:</span> 8401<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">typeof</span><span class="special">/</span><span class="identifier">std</span><span class="special">/</span><span class="identifier">ostream</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> 8402<span class="keyword">namespace</span> <span class="identifier">proto</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">proto</span><span class="special">;</span> 8403 8404<span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special"><</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">ostream</span> <span class="special">&</span> <span class="special">>::</span><span class="identifier">type</span> <span class="identifier">cout_</span> <span class="special">=</span> <span class="special">{</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span><span class="special">};</span> 8405 8406<span class="keyword">template</span><span class="special"><</span> <span class="keyword">typename</span> <span class="identifier">Expr</span> <span class="special">></span> 8407<span class="keyword">void</span> <span class="identifier">evaluate</span><span class="special">(</span> <span class="identifier">Expr</span> <span class="keyword">const</span> <span class="special">&</span> <span class="identifier">expr</span> <span class="special">)</span> 8408<span class="special">{</span> 8409 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">default_context</span> <span class="identifier">ctx</span><span class="special">;</span> 8410 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">eval</span><span class="special">(</span><span class="identifier">expr</span><span class="special">,</span> <span class="identifier">ctx</span><span class="special">);</span> 8411<span class="special">}</span> 8412 8413<span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span> 8414<span class="special">{</span> 8415 <span class="identifier">evaluate</span><span class="special">(</span> <span class="identifier">cout_</span> <span class="special"><<</span> <span class="string">"hello"</span> <span class="special"><<</span> <span class="char">','</span> <span class="special"><<</span> <span class="string">" world"</span> <span class="special">);</span> 8416 <span class="keyword">return</span> <span class="number">0</span><span class="special">;</span> 8417<span class="special">}</span> 8418</pre> 8419<p> 8420 </p> 8421</div> 8422<div class="section"> 8423<div class="titlepage"><div><div><h4 class="title"> 8424<a name="boost_proto.users_guide.examples.calc1"></a><a class="link" href="users_guide.html#boost_proto.users_guide.examples.calc1" title="Calc1: Defining an Evaluation Context">Calc1: Defining 8425 an Evaluation Context</a> 8426</h4></div></div></div> 8427<p> 8428 A simple example that builds a miniature embedded domain-specific language 8429 for lazy arithmetic expressions, with TR1 bind-style argument placeholders. 8430 </p> 8431<p> 8432</p> 8433<pre class="programlisting"><span class="comment">// Copyright 2008 Eric Niebler. Distributed under the Boost</span> 8434<span class="comment">// Software License, Version 1.0. (See accompanying file</span> 8435<span class="comment">// LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)</span> 8436<span class="comment">//</span> 8437<span class="comment">// This is a simple example of how to build an arithmetic expression</span> 8438<span class="comment">// evaluator with placeholders.</span> 8439 8440<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">iostream</span><span class="special">></span> 8441<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">proto</span><span class="special">/</span><span class="identifier">core</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> 8442<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">proto</span><span class="special">/</span><span class="identifier">context</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> 8443<span class="keyword">namespace</span> <span class="identifier">proto</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">proto</span><span class="special">;</span> 8444<span class="keyword">using</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_</span><span class="special">;</span> 8445 8446<span class="keyword">template</span><span class="special"><</span><span class="keyword">int</span> <span class="identifier">I</span><span class="special">></span> <span class="keyword">struct</span> <span class="identifier">placeholder</span> <span class="special">{};</span> 8447 8448<span class="comment">// Define some placeholders</span> 8449<span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special"><</span> <span class="identifier">placeholder</span><span class="special"><</span> <span class="number">1</span> <span class="special">></span> <span class="special">>::</span><span class="identifier">type</span> <span class="keyword">const</span> <span class="identifier">_1</span> <span class="special">=</span> <span class="special">{{}};</span> 8450<span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special"><</span> <span class="identifier">placeholder</span><span class="special"><</span> <span class="number">2</span> <span class="special">></span> <span class="special">>::</span><span class="identifier">type</span> <span class="keyword">const</span> <span class="identifier">_2</span> <span class="special">=</span> <span class="special">{{}};</span> 8451 8452<span class="comment">// Define a calculator context, for evaluating arithmetic expressions</span> 8453<span class="keyword">struct</span> <span class="identifier">calculator_context</span> 8454 <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">callable_context</span><span class="special"><</span> <span class="identifier">calculator_context</span> <span class="keyword">const</span> <span class="special">></span> 8455<span class="special">{</span> 8456 <span class="comment">// The values bound to the placeholders</span> 8457 <span class="keyword">double</span> <span class="identifier">d</span><span class="special">[</span><span class="number">2</span><span class="special">];</span> 8458 8459 <span class="comment">// The result of evaluating arithmetic expressions</span> 8460 <span class="keyword">typedef</span> <span class="keyword">double</span> <span class="identifier">result_type</span><span class="special">;</span> 8461 8462 <span class="keyword">explicit</span> <span class="identifier">calculator_context</span><span class="special">(</span><span class="keyword">double</span> <span class="identifier">d1</span> <span class="special">=</span> <span class="number">0.</span><span class="special">,</span> <span class="keyword">double</span> <span class="identifier">d2</span> <span class="special">=</span> <span class="number">0.</span><span class="special">)</span> 8463 <span class="special">{</span> 8464 <span class="identifier">d</span><span class="special">[</span><span class="number">0</span><span class="special">]</span> <span class="special">=</span> <span class="identifier">d1</span><span class="special">;</span> 8465 <span class="identifier">d</span><span class="special">[</span><span class="number">1</span><span class="special">]</span> <span class="special">=</span> <span class="identifier">d2</span><span class="special">;</span> 8466 <span class="special">}</span> 8467 8468 <span class="comment">// Handle the evaluation of the placeholder terminals</span> 8469 <span class="keyword">template</span><span class="special"><</span><span class="keyword">int</span> <span class="identifier">I</span><span class="special">></span> 8470 <span class="keyword">double</span> <span class="keyword">operator</span> <span class="special">()(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">,</span> <span class="identifier">placeholder</span><span class="special"><</span><span class="identifier">I</span><span class="special">>)</span> <span class="keyword">const</span> 8471 <span class="special">{</span> 8472 <span class="keyword">return</span> <span class="identifier">d</span><span class="special">[</span> <span class="identifier">I</span> <span class="special">-</span> <span class="number">1</span> <span class="special">];</span> 8473 <span class="special">}</span> 8474<span class="special">};</span> 8475 8476<span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">></span> 8477<span class="keyword">double</span> <span class="identifier">evaluate</span><span class="special">(</span> <span class="identifier">Expr</span> <span class="keyword">const</span> <span class="special">&</span><span class="identifier">expr</span><span class="special">,</span> <span class="keyword">double</span> <span class="identifier">d1</span> <span class="special">=</span> <span class="number">0.</span><span class="special">,</span> <span class="keyword">double</span> <span class="identifier">d2</span> <span class="special">=</span> <span class="number">0.</span> <span class="special">)</span> 8478<span class="special">{</span> 8479 <span class="comment">// Create a calculator context with d1 and d2 substituted for _1 and _2</span> 8480 <span class="identifier">calculator_context</span> <span class="keyword">const</span> <span class="identifier">ctx</span><span class="special">(</span><span class="identifier">d1</span><span class="special">,</span> <span class="identifier">d2</span><span class="special">);</span> 8481 8482 <span class="comment">// Evaluate the calculator expression with the calculator_context</span> 8483 <span class="keyword">return</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">eval</span><span class="special">(</span><span class="identifier">expr</span><span class="special">,</span> <span class="identifier">ctx</span><span class="special">);</span> 8484<span class="special">}</span> 8485 8486<span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span> 8487<span class="special">{</span> 8488 <span class="comment">// Displays "5"</span> 8489 <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="identifier">evaluate</span><span class="special">(</span> <span class="identifier">_1</span> <span class="special">+</span> <span class="number">2.0</span><span class="special">,</span> <span class="number">3.0</span> <span class="special">)</span> <span class="special"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span> 8490 8491 <span class="comment">// Displays "6"</span> 8492 <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="identifier">evaluate</span><span class="special">(</span> <span class="identifier">_1</span> <span class="special">*</span> <span class="identifier">_2</span><span class="special">,</span> <span class="number">3.0</span><span class="special">,</span> <span class="number">2.0</span> <span class="special">)</span> <span class="special"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span> 8493 8494 <span class="comment">// Displays "0.5"</span> 8495 <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="identifier">evaluate</span><span class="special">(</span> <span class="special">(</span><span class="identifier">_1</span> <span class="special">-</span> <span class="identifier">_2</span><span class="special">)</span> <span class="special">/</span> <span class="identifier">_2</span><span class="special">,</span> <span class="number">3.0</span><span class="special">,</span> <span class="number">2.0</span> <span class="special">)</span> <span class="special"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span> 8496 8497 <span class="keyword">return</span> <span class="number">0</span><span class="special">;</span> 8498<span class="special">}</span> 8499</pre> 8500<p> 8501 </p> 8502</div> 8503<div class="section"> 8504<div class="titlepage"><div><div><h4 class="title"> 8505<a name="boost_proto.users_guide.examples.calc2"></a><a class="link" href="users_guide.html#boost_proto.users_guide.examples.calc2" title="Calc2: Adding Members Using proto::extends<>">Calc2: Adding 8506 Members Using <code class="literal">proto::extends<></code></a> 8507</h4></div></div></div> 8508<p> 8509 An extension of the Calc1 example that uses <code class="computeroutput"><a class="link" href="../boost/proto/extends.html" title="Struct template extends">proto::extends<></a></code> 8510 to make calculator expressions valid function objects that can be used 8511 with STL algorithms. 8512 </p> 8513<p> 8514</p> 8515<pre class="programlisting"><span class="comment">// Copyright 2008 Eric Niebler. Distributed under the Boost</span> 8516<span class="comment">// Software License, Version 1.0. (See accompanying file</span> 8517<span class="comment">// LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)</span> 8518<span class="comment">//</span> 8519<span class="comment">// This example enhances the simple arithmetic expression evaluator</span> 8520<span class="comment">// in calc1.cpp by using proto::extends to make arithmetic</span> 8521<span class="comment">// expressions immediately evaluable with operator (), a-la a</span> 8522<span class="comment">// function object</span> 8523 8524<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">iostream</span><span class="special">></span> 8525<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">proto</span><span class="special">/</span><span class="identifier">core</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> 8526<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">proto</span><span class="special">/</span><span class="identifier">context</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> 8527<span class="keyword">namespace</span> <span class="identifier">proto</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">proto</span><span class="special">;</span> 8528<span class="keyword">using</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_</span><span class="special">;</span> 8529 8530<span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">></span> 8531<span class="keyword">struct</span> <span class="identifier">calculator_expression</span><span class="special">;</span> 8532 8533<span class="comment">// Tell proto how to generate expressions in the calculator_domain</span> 8534<span class="keyword">struct</span> <span class="identifier">calculator_domain</span> 8535 <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">domain</span><span class="special"><</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">generator</span><span class="special"><</span><span class="identifier">calculator_expression</span><span class="special">></span> <span class="special">></span> 8536<span class="special">{};</span> 8537 8538<span class="comment">// Will be used to define the placeholders _1 and _2</span> 8539<span class="keyword">template</span><span class="special"><</span><span class="keyword">int</span> <span class="identifier">I</span><span class="special">></span> <span class="keyword">struct</span> <span class="identifier">placeholder</span> <span class="special">{};</span> 8540 8541<span class="comment">// Define a calculator context, for evaluating arithmetic expressions</span> 8542<span class="comment">// (This is as before, in calc1.cpp)</span> 8543<span class="keyword">struct</span> <span class="identifier">calculator_context</span> 8544 <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">callable_context</span><span class="special"><</span> <span class="identifier">calculator_context</span> <span class="keyword">const</span> <span class="special">></span> 8545<span class="special">{</span> 8546 <span class="comment">// The values bound to the placeholders</span> 8547 <span class="keyword">double</span> <span class="identifier">d</span><span class="special">[</span><span class="number">2</span><span class="special">];</span> 8548 8549 <span class="comment">// The result of evaluating arithmetic expressions</span> 8550 <span class="keyword">typedef</span> <span class="keyword">double</span> <span class="identifier">result_type</span><span class="special">;</span> 8551 8552 <span class="keyword">explicit</span> <span class="identifier">calculator_context</span><span class="special">(</span><span class="keyword">double</span> <span class="identifier">d1</span> <span class="special">=</span> <span class="number">0.</span><span class="special">,</span> <span class="keyword">double</span> <span class="identifier">d2</span> <span class="special">=</span> <span class="number">0.</span><span class="special">)</span> 8553 <span class="special">{</span> 8554 <span class="identifier">d</span><span class="special">[</span><span class="number">0</span><span class="special">]</span> <span class="special">=</span> <span class="identifier">d1</span><span class="special">;</span> 8555 <span class="identifier">d</span><span class="special">[</span><span class="number">1</span><span class="special">]</span> <span class="special">=</span> <span class="identifier">d2</span><span class="special">;</span> 8556 <span class="special">}</span> 8557 8558 <span class="comment">// Handle the evaluation of the placeholder terminals</span> 8559 <span class="keyword">template</span><span class="special"><</span><span class="keyword">int</span> <span class="identifier">I</span><span class="special">></span> 8560 <span class="keyword">double</span> <span class="keyword">operator</span> <span class="special">()(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">,</span> <span class="identifier">placeholder</span><span class="special"><</span><span class="identifier">I</span><span class="special">>)</span> <span class="keyword">const</span> 8561 <span class="special">{</span> 8562 <span class="keyword">return</span> <span class="identifier">d</span><span class="special">[</span> <span class="identifier">I</span> <span class="special">-</span> <span class="number">1</span> <span class="special">];</span> 8563 <span class="special">}</span> 8564<span class="special">};</span> 8565 8566<span class="comment">// Wrap all calculator expressions in this type, which defines</span> 8567<span class="comment">// operator () to evaluate the expression.</span> 8568<span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">></span> 8569<span class="keyword">struct</span> <span class="identifier">calculator_expression</span> 8570 <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">extends</span><span class="special"><</span><span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">calculator_expression</span><span class="special"><</span><span class="identifier">Expr</span><span class="special">>,</span> <span class="identifier">calculator_domain</span><span class="special">></span> 8571<span class="special">{</span> 8572 <span class="keyword">explicit</span> <span class="identifier">calculator_expression</span><span class="special">(</span><span class="identifier">Expr</span> <span class="keyword">const</span> <span class="special">&</span><span class="identifier">expr</span> <span class="special">=</span> <span class="identifier">Expr</span><span class="special">())</span> 8573 <span class="special">:</span> <span class="identifier">calculator_expression</span><span class="special">::</span><span class="identifier">proto_extends</span><span class="special">(</span><span class="identifier">expr</span><span class="special">)</span> 8574 <span class="special">{}</span> 8575 8576 <span class="identifier">BOOST_PROTO_EXTENDS_USING_ASSIGN</span><span class="special">(</span><span class="identifier">calculator_expression</span><span class="special"><</span><span class="identifier">Expr</span><span class="special">>)</span> 8577 8578 <span class="comment">// Override operator () to evaluate the expression</span> 8579 <span class="keyword">double</span> <span class="keyword">operator</span> <span class="special">()()</span> <span class="keyword">const</span> 8580 <span class="special">{</span> 8581 <span class="identifier">calculator_context</span> <span class="keyword">const</span> <span class="identifier">ctx</span><span class="special">;</span> 8582 <span class="keyword">return</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">eval</span><span class="special">(*</span><span class="keyword">this</span><span class="special">,</span> <span class="identifier">ctx</span><span class="special">);</span> 8583 <span class="special">}</span> 8584 8585 <span class="keyword">double</span> <span class="keyword">operator</span> <span class="special">()(</span><span class="keyword">double</span> <span class="identifier">d1</span><span class="special">)</span> <span class="keyword">const</span> 8586 <span class="special">{</span> 8587 <span class="identifier">calculator_context</span> <span class="keyword">const</span> <span class="identifier">ctx</span><span class="special">(</span><span class="identifier">d1</span><span class="special">);</span> 8588 <span class="keyword">return</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">eval</span><span class="special">(*</span><span class="keyword">this</span><span class="special">,</span> <span class="identifier">ctx</span><span class="special">);</span> 8589 <span class="special">}</span> 8590 8591 <span class="keyword">double</span> <span class="keyword">operator</span> <span class="special">()(</span><span class="keyword">double</span> <span class="identifier">d1</span><span class="special">,</span> <span class="keyword">double</span> <span class="identifier">d2</span><span class="special">)</span> <span class="keyword">const</span> 8592 <span class="special">{</span> 8593 <span class="identifier">calculator_context</span> <span class="keyword">const</span> <span class="identifier">ctx</span><span class="special">(</span><span class="identifier">d1</span><span class="special">,</span> <span class="identifier">d2</span><span class="special">);</span> 8594 <span class="keyword">return</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">eval</span><span class="special">(*</span><span class="keyword">this</span><span class="special">,</span> <span class="identifier">ctx</span><span class="special">);</span> 8595 <span class="special">}</span> 8596<span class="special">};</span> 8597 8598<span class="comment">// Define some placeholders (notice they're wrapped in calculator_expression<>)</span> 8599<span class="identifier">calculator_expression</span><span class="special"><</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special"><</span> <span class="identifier">placeholder</span><span class="special"><</span> <span class="number">1</span> <span class="special">></span> <span class="special">>::</span><span class="identifier">type</span><span class="special">></span> <span class="keyword">const</span> <span class="identifier">_1</span><span class="special">;</span> 8600<span class="identifier">calculator_expression</span><span class="special"><</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special"><</span> <span class="identifier">placeholder</span><span class="special"><</span> <span class="number">2</span> <span class="special">></span> <span class="special">>::</span><span class="identifier">type</span><span class="special">></span> <span class="keyword">const</span> <span class="identifier">_2</span><span class="special">;</span> 8601 8602<span class="comment">// Now, our arithmetic expressions are immediately executable function objects:</span> 8603<span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span> 8604<span class="special">{</span> 8605 <span class="comment">// Displays "5"</span> 8606 <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="special">(</span><span class="identifier">_1</span> <span class="special">+</span> <span class="number">2.0</span><span class="special">)(</span> <span class="number">3.0</span> <span class="special">)</span> <span class="special"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span> 8607 8608 <span class="comment">// Displays "6"</span> 8609 <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="special">(</span> <span class="identifier">_1</span> <span class="special">*</span> <span class="identifier">_2</span> <span class="special">)(</span> <span class="number">3.0</span><span class="special">,</span> <span class="number">2.0</span> <span class="special">)</span> <span class="special"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span> 8610 8611 <span class="comment">// Displays "0.5"</span> 8612 <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="special">(</span> <span class="special">(</span><span class="identifier">_1</span> <span class="special">-</span> <span class="identifier">_2</span><span class="special">)</span> <span class="special">/</span> <span class="identifier">_2</span> <span class="special">)(</span> <span class="number">3.0</span><span class="special">,</span> <span class="number">2.0</span> <span class="special">)</span> <span class="special"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span> 8613 8614 <span class="keyword">return</span> <span class="number">0</span><span class="special">;</span> 8615<span class="special">}</span> 8616</pre> 8617<p> 8618 </p> 8619</div> 8620<div class="section"> 8621<div class="titlepage"><div><div><h4 class="title"> 8622<a name="boost_proto.users_guide.examples.calc3"></a><a class="link" href="users_guide.html#boost_proto.users_guide.examples.calc3" title="Calc3: Defining a Simple Transform">Calc3: Defining 8623 a Simple Transform</a> 8624</h4></div></div></div> 8625<p> 8626 An extension of the Calc2 example that uses a Proto transform to calculate 8627 the arity of a calculator expression and statically assert that the correct 8628 number of arguments are passed. 8629 </p> 8630<p> 8631</p> 8632<pre class="programlisting"><span class="comment">// Copyright 2008 Eric Niebler. Distributed under the Boost</span> 8633<span class="comment">// Software License, Version 1.0. (See accompanying file</span> 8634<span class="comment">// LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)</span> 8635<span class="comment">//</span> 8636<span class="comment">// This example enhances the arithmetic expression evaluator</span> 8637<span class="comment">// in calc2.cpp by using a proto transform to calculate the</span> 8638<span class="comment">// number of arguments an expression requires and using a</span> 8639<span class="comment">// compile-time assert to guarantee that the right number of</span> 8640<span class="comment">// arguments are actually specified.</span> 8641 8642<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">iostream</span><span class="special">></span> 8643<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">mpl</span><span class="special">/</span><span class="keyword">int</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> 8644<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">mpl</span><span class="special">/</span><span class="identifier">assert</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> 8645<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">mpl</span><span class="special">/</span><span class="identifier">min_max</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> 8646<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">proto</span><span class="special">/</span><span class="identifier">core</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> 8647<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">proto</span><span class="special">/</span><span class="identifier">context</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> 8648<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">proto</span><span class="special">/</span><span class="identifier">transform</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> 8649<span class="keyword">namespace</span> <span class="identifier">mpl</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">mpl</span><span class="special">;</span> 8650<span class="keyword">namespace</span> <span class="identifier">proto</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">proto</span><span class="special">;</span> 8651<span class="keyword">using</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_</span><span class="special">;</span> 8652 8653<span class="comment">// Will be used to define the placeholders _1 and _2</span> 8654<span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">I</span><span class="special">></span> <span class="keyword">struct</span> <span class="identifier">placeholder</span> <span class="special">:</span> <span class="identifier">I</span> <span class="special">{};</span> 8655 8656<span class="comment">// This grammar basically says that a calculator expression is one of:</span> 8657<span class="comment">// - A placeholder terminal</span> 8658<span class="comment">// - Some other terminal</span> 8659<span class="comment">// - Some non-terminal whose children are calculator expressions</span> 8660<span class="comment">// In addition, it has transforms that say how to calculate the</span> 8661<span class="comment">// expression arity for each of the three cases.</span> 8662<span class="keyword">struct</span> <span class="identifier">CalculatorGrammar</span> 8663 <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">or_</span><span class="special"><</span> 8664 8665 <span class="comment">// placeholders have a non-zero arity ...</span> 8666 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special"><</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special"><</span> <span class="identifier">placeholder</span><span class="special"><</span><span class="identifier">_</span><span class="special">></span> <span class="special">>,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_value</span> <span class="special">></span> 8667 8668 <span class="comment">// Any other terminals have arity 0 ...</span> 8669 <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special"><</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special"><</span><span class="identifier">_</span><span class="special">>,</span> <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">int_</span><span class="special"><</span><span class="number">0</span><span class="special">>()</span> <span class="special">></span> 8670 8671 <span class="comment">// For any non-terminals, find the arity of the children and</span> 8672 <span class="comment">// take the maximum. This is recursive.</span> 8673 <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special"><</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">nary_expr</span><span class="special"><</span><span class="identifier">_</span><span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">vararg</span><span class="special"><</span><span class="identifier">_</span><span class="special">></span> <span class="special">></span> 8674 <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">fold</span><span class="special"><</span><span class="identifier">_</span><span class="special">,</span> <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">int_</span><span class="special"><</span><span class="number">0</span><span class="special">>(),</span> <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">max</span><span class="special"><</span><span class="identifier">CalculatorGrammar</span><span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_state</span><span class="special">>()</span> <span class="special">></span> <span class="special">></span> 8675 8676 <span class="special">></span> 8677<span class="special">{};</span> 8678 8679<span class="comment">// Simple wrapper for calculating a calculator expression's arity.</span> 8680<span class="comment">// It specifies mpl::int_<0> as the initial state. The data, which</span> 8681<span class="comment">// is not used, is mpl::void_.</span> 8682<span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">></span> 8683<span class="keyword">struct</span> <span class="identifier">calculator_arity</span> 8684 <span class="special">:</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">result_of</span><span class="special"><</span><span class="identifier">CalculatorGrammar</span><span class="special">(</span><span class="identifier">Expr</span><span class="special">)></span> 8685<span class="special">{};</span> 8686 8687<span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">></span> 8688<span class="keyword">struct</span> <span class="identifier">calculator_expression</span><span class="special">;</span> 8689 8690<span class="comment">// Tell proto how to generate expressions in the calculator_domain</span> 8691<span class="keyword">struct</span> <span class="identifier">calculator_domain</span> 8692 <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">domain</span><span class="special"><</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">generator</span><span class="special"><</span><span class="identifier">calculator_expression</span><span class="special">></span> <span class="special">></span> 8693<span class="special">{};</span> 8694 8695<span class="comment">// Define a calculator context, for evaluating arithmetic expressions</span> 8696<span class="comment">// (This is as before, in calc1.cpp and calc2.cpp)</span> 8697<span class="keyword">struct</span> <span class="identifier">calculator_context</span> 8698 <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">callable_context</span><span class="special"><</span> <span class="identifier">calculator_context</span> <span class="keyword">const</span> <span class="special">></span> 8699<span class="special">{</span> 8700 <span class="comment">// The values bound to the placeholders</span> 8701 <span class="keyword">double</span> <span class="identifier">d</span><span class="special">[</span><span class="number">2</span><span class="special">];</span> 8702 8703 <span class="comment">// The result of evaluating arithmetic expressions</span> 8704 <span class="keyword">typedef</span> <span class="keyword">double</span> <span class="identifier">result_type</span><span class="special">;</span> 8705 8706 <span class="keyword">explicit</span> <span class="identifier">calculator_context</span><span class="special">(</span><span class="keyword">double</span> <span class="identifier">d1</span> <span class="special">=</span> <span class="number">0.</span><span class="special">,</span> <span class="keyword">double</span> <span class="identifier">d2</span> <span class="special">=</span> <span class="number">0.</span><span class="special">)</span> 8707 <span class="special">{</span> 8708 <span class="identifier">d</span><span class="special">[</span><span class="number">0</span><span class="special">]</span> <span class="special">=</span> <span class="identifier">d1</span><span class="special">;</span> 8709 <span class="identifier">d</span><span class="special">[</span><span class="number">1</span><span class="special">]</span> <span class="special">=</span> <span class="identifier">d2</span><span class="special">;</span> 8710 <span class="special">}</span> 8711 8712 <span class="comment">// Handle the evaluation of the placeholder terminals</span> 8713 <span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">I</span><span class="special">></span> 8714 <span class="keyword">double</span> <span class="keyword">operator</span> <span class="special">()(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">,</span> <span class="identifier">placeholder</span><span class="special"><</span><span class="identifier">I</span><span class="special">>)</span> <span class="keyword">const</span> 8715 <span class="special">{</span> 8716 <span class="keyword">return</span> <span class="identifier">d</span><span class="special">[</span> <span class="identifier">I</span><span class="special">()</span> <span class="special">-</span> <span class="number">1</span> <span class="special">];</span> 8717 <span class="special">}</span> 8718<span class="special">};</span> 8719 8720<span class="comment">// Wrap all calculator expressions in this type, which defines</span> 8721<span class="comment">// operator () to evaluate the expression.</span> 8722<span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">></span> 8723<span class="keyword">struct</span> <span class="identifier">calculator_expression</span> 8724 <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">extends</span><span class="special"><</span><span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">calculator_expression</span><span class="special"><</span><span class="identifier">Expr</span><span class="special">>,</span> <span class="identifier">calculator_domain</span><span class="special">></span> 8725<span class="special">{</span> 8726 <span class="keyword">typedef</span> 8727 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">extends</span><span class="special"><</span><span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">calculator_expression</span><span class="special"><</span><span class="identifier">Expr</span><span class="special">>,</span> <span class="identifier">calculator_domain</span><span class="special">></span> 8728 <span class="identifier">base_type</span><span class="special">;</span> 8729 8730 <span class="keyword">explicit</span> <span class="identifier">calculator_expression</span><span class="special">(</span><span class="identifier">Expr</span> <span class="keyword">const</span> <span class="special">&</span><span class="identifier">expr</span> <span class="special">=</span> <span class="identifier">Expr</span><span class="special">())</span> 8731 <span class="special">:</span> <span class="identifier">base_type</span><span class="special">(</span><span class="identifier">expr</span><span class="special">)</span> 8732 <span class="special">{}</span> 8733 8734 <span class="identifier">BOOST_PROTO_EXTENDS_USING_ASSIGN</span><span class="special">(</span><span class="identifier">calculator_expression</span><span class="special"><</span><span class="identifier">Expr</span><span class="special">>)</span> 8735 8736 <span class="comment">// Override operator () to evaluate the expression</span> 8737 <span class="keyword">double</span> <span class="keyword">operator</span> <span class="special">()()</span> <span class="keyword">const</span> 8738 <span class="special">{</span> 8739 <span class="comment">// Assert that the expression has arity 0</span> 8740 <span class="identifier">BOOST_MPL_ASSERT_RELATION</span><span class="special">(</span><span class="number">0</span><span class="special">,</span> <span class="special">==,</span> <span class="identifier">calculator_arity</span><span class="special"><</span><span class="identifier">Expr</span><span class="special">>::</span><span class="identifier">type</span><span class="special">::</span><span class="identifier">value</span><span class="special">);</span> 8741 <span class="identifier">calculator_context</span> <span class="keyword">const</span> <span class="identifier">ctx</span><span class="special">;</span> 8742 <span class="keyword">return</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">eval</span><span class="special">(*</span><span class="keyword">this</span><span class="special">,</span> <span class="identifier">ctx</span><span class="special">);</span> 8743 <span class="special">}</span> 8744 8745 <span class="keyword">double</span> <span class="keyword">operator</span> <span class="special">()(</span><span class="keyword">double</span> <span class="identifier">d1</span><span class="special">)</span> <span class="keyword">const</span> 8746 <span class="special">{</span> 8747 <span class="comment">// Assert that the expression has arity 1</span> 8748 <span class="identifier">BOOST_MPL_ASSERT_RELATION</span><span class="special">(</span><span class="number">1</span><span class="special">,</span> <span class="special">==,</span> <span class="identifier">calculator_arity</span><span class="special"><</span><span class="identifier">Expr</span><span class="special">>::</span><span class="identifier">type</span><span class="special">::</span><span class="identifier">value</span><span class="special">);</span> 8749 <span class="identifier">calculator_context</span> <span class="keyword">const</span> <span class="identifier">ctx</span><span class="special">(</span><span class="identifier">d1</span><span class="special">);</span> 8750 <span class="keyword">return</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">eval</span><span class="special">(*</span><span class="keyword">this</span><span class="special">,</span> <span class="identifier">ctx</span><span class="special">);</span> 8751 <span class="special">}</span> 8752 8753 <span class="keyword">double</span> <span class="keyword">operator</span> <span class="special">()(</span><span class="keyword">double</span> <span class="identifier">d1</span><span class="special">,</span> <span class="keyword">double</span> <span class="identifier">d2</span><span class="special">)</span> <span class="keyword">const</span> 8754 <span class="special">{</span> 8755 <span class="comment">// Assert that the expression has arity 2</span> 8756 <span class="identifier">BOOST_MPL_ASSERT_RELATION</span><span class="special">(</span><span class="number">2</span><span class="special">,</span> <span class="special">==,</span> <span class="identifier">calculator_arity</span><span class="special"><</span><span class="identifier">Expr</span><span class="special">>::</span><span class="identifier">type</span><span class="special">::</span><span class="identifier">value</span><span class="special">);</span> 8757 <span class="identifier">calculator_context</span> <span class="keyword">const</span> <span class="identifier">ctx</span><span class="special">(</span><span class="identifier">d1</span><span class="special">,</span> <span class="identifier">d2</span><span class="special">);</span> 8758 <span class="keyword">return</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">eval</span><span class="special">(*</span><span class="keyword">this</span><span class="special">,</span> <span class="identifier">ctx</span><span class="special">);</span> 8759 <span class="special">}</span> 8760<span class="special">};</span> 8761 8762<span class="comment">// Define some placeholders (notice they're wrapped in calculator_expression<>)</span> 8763<span class="identifier">calculator_expression</span><span class="special"><</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special"><</span> <span class="identifier">placeholder</span><span class="special"><</span> <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">int_</span><span class="special"><</span><span class="number">1</span><span class="special">></span> <span class="special">></span> <span class="special">>::</span><span class="identifier">type</span><span class="special">></span> <span class="keyword">const</span> <span class="identifier">_1</span><span class="special">;</span> 8764<span class="identifier">calculator_expression</span><span class="special"><</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special"><</span> <span class="identifier">placeholder</span><span class="special"><</span> <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">int_</span><span class="special"><</span><span class="number">2</span><span class="special">></span> <span class="special">></span> <span class="special">>::</span><span class="identifier">type</span><span class="special">></span> <span class="keyword">const</span> <span class="identifier">_2</span><span class="special">;</span> 8765 8766<span class="comment">// Now, our arithmetic expressions are immediately executable function objects:</span> 8767<span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span> 8768<span class="special">{</span> 8769 <span class="comment">// Displays "5"</span> 8770 <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="special">(</span><span class="identifier">_1</span> <span class="special">+</span> <span class="number">2.0</span><span class="special">)(</span> <span class="number">3.0</span> <span class="special">)</span> <span class="special"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span> 8771 8772 <span class="comment">// Displays "6"</span> 8773 <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="special">(</span> <span class="identifier">_1</span> <span class="special">*</span> <span class="identifier">_2</span> <span class="special">)(</span> <span class="number">3.0</span><span class="special">,</span> <span class="number">2.0</span> <span class="special">)</span> <span class="special"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span> 8774 8775 <span class="comment">// Displays "0.5"</span> 8776 <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="special">(</span> <span class="special">(</span><span class="identifier">_1</span> <span class="special">-</span> <span class="identifier">_2</span><span class="special">)</span> <span class="special">/</span> <span class="identifier">_2</span> <span class="special">)(</span> <span class="number">3.0</span><span class="special">,</span> <span class="number">2.0</span> <span class="special">)</span> <span class="special"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span> 8777 8778 <span class="comment">// This won't compile because the arity of the</span> 8779 <span class="comment">// expression doesn't match the number of arguments</span> 8780 <span class="comment">// ( (_1 - _2) / _2 )( 3.0 );</span> 8781 8782 <span class="keyword">return</span> <span class="number">0</span><span class="special">;</span> 8783<span class="special">}</span> 8784</pre> 8785<p> 8786 </p> 8787</div> 8788<div class="section"> 8789<div class="titlepage"><div><div><h4 class="title"> 8790<a name="boost_proto.users_guide.examples.lazy_vector"></a><a class="link" href="users_guide.html#boost_proto.users_guide.examples.lazy_vector" title="Lazy Vector: Controlling Operator Overloads">Lazy 8791 Vector: Controlling Operator Overloads</a> 8792</h4></div></div></div> 8793<p> 8794 This example constructs a mini-library for linear algebra, using expression 8795 templates to eliminate the need for temporaries when adding vectors of 8796 numbers. 8797 </p> 8798<p> 8799 This example uses a domain with a grammar to prune the set of overloaded 8800 operators. Only those operators that produce valid lazy vector expressions 8801 are allowed. 8802 </p> 8803<p> 8804</p> 8805<pre class="programlisting"><span class="comment">///////////////////////////////////////////////////////////////////////////////</span> 8806<span class="comment">// Copyright 2008 Eric Niebler. Distributed under the Boost</span> 8807<span class="comment">// Software License, Version 1.0. (See accompanying file</span> 8808<span class="comment">// LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)</span> 8809<span class="comment">//</span> 8810<span class="comment">// This example constructs a mini-library for linear algebra, using</span> 8811<span class="comment">// expression templates to eliminate the need for temporaries when</span> 8812<span class="comment">// adding vectors of numbers.</span> 8813<span class="comment">//</span> 8814<span class="comment">// This example uses a domain with a grammar to prune the set</span> 8815<span class="comment">// of overloaded operators. Only those operators that produce</span> 8816<span class="comment">// valid lazy vector expressions are allowed.</span> 8817 8818<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">vector</span><span class="special">></span> 8819<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">iostream</span><span class="special">></span> 8820<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">mpl</span><span class="special">/</span><span class="keyword">int</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> 8821<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">proto</span><span class="special">/</span><span class="identifier">core</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> 8822<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">proto</span><span class="special">/</span><span class="identifier">context</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> 8823<span class="keyword">namespace</span> <span class="identifier">mpl</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">mpl</span><span class="special">;</span> 8824<span class="keyword">namespace</span> <span class="identifier">proto</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">proto</span><span class="special">;</span> 8825<span class="keyword">using</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_</span><span class="special">;</span> 8826 8827<span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">></span> 8828<span class="keyword">struct</span> <span class="identifier">lazy_vector_expr</span><span class="special">;</span> 8829 8830<span class="comment">// This grammar describes which lazy vector expressions</span> 8831<span class="comment">// are allowed; namely, vector terminals and addition</span> 8832<span class="comment">// and subtraction of lazy vector expressions.</span> 8833<span class="keyword">struct</span> <span class="identifier">LazyVectorGrammar</span> 8834 <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">or_</span><span class="special"><</span> 8835 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special"><</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special"><</span><span class="identifier">_</span><span class="special">></span> <span class="special">></span> 8836 <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">plus</span><span class="special"><</span> <span class="identifier">LazyVectorGrammar</span><span class="special">,</span> <span class="identifier">LazyVectorGrammar</span> <span class="special">></span> 8837 <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">minus</span><span class="special"><</span> <span class="identifier">LazyVectorGrammar</span><span class="special">,</span> <span class="identifier">LazyVectorGrammar</span> <span class="special">></span> 8838 <span class="special">></span> 8839<span class="special">{};</span> 8840 8841<span class="comment">// Tell proto that in the lazy_vector_domain, all</span> 8842<span class="comment">// expressions should be wrapped in laxy_vector_expr<></span> 8843<span class="comment">// and must conform to the lazy vector grammar.</span> 8844<span class="keyword">struct</span> <span class="identifier">lazy_vector_domain</span> 8845 <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">domain</span><span class="special"><</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">generator</span><span class="special"><</span><span class="identifier">lazy_vector_expr</span><span class="special">>,</span> <span class="identifier">LazyVectorGrammar</span><span class="special">></span> 8846<span class="special">{};</span> 8847 8848<span class="comment">// Here is an evaluation context that indexes into a lazy vector</span> 8849<span class="comment">// expression, and combines the result.</span> 8850<span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">Size</span> <span class="special">=</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span><span class="special">></span> 8851<span class="keyword">struct</span> <span class="identifier">lazy_subscript_context</span> 8852<span class="special">{</span> 8853 <span class="identifier">lazy_subscript_context</span><span class="special">(</span><span class="identifier">Size</span> <span class="identifier">subscript</span><span class="special">)</span> 8854 <span class="special">:</span> <span class="identifier">subscript_</span><span class="special">(</span><span class="identifier">subscript</span><span class="special">)</span> 8855 <span class="special">{}</span> 8856 8857 <span class="comment">// Use default_eval for all the operations ...</span> 8858 <span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Tag</span> <span class="special">=</span> <span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">::</span><span class="identifier">proto_tag</span><span class="special">></span> 8859 <span class="keyword">struct</span> <span class="identifier">eval</span> 8860 <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">default_eval</span><span class="special"><</span><span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">lazy_subscript_context</span><span class="special">></span> 8861 <span class="special">{};</span> 8862 8863 <span class="comment">// ... except for terminals, which we index with our subscript</span> 8864 <span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">></span> 8865 <span class="keyword">struct</span> <span class="identifier">eval</span><span class="special"><</span><span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">></span> 8866 <span class="special">{</span> 8867 <span class="keyword">typedef</span> <span class="keyword">typename</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">value</span><span class="special"><</span><span class="identifier">Expr</span><span class="special">>::</span><span class="identifier">type</span><span class="special">::</span><span class="identifier">value_type</span> <span class="identifier">result_type</span><span class="special">;</span> 8868 8869 <span class="identifier">result_type</span> <span class="keyword">operator</span> <span class="special">()(</span> <span class="identifier">Expr</span> <span class="keyword">const</span> <span class="special">&</span> <span class="identifier">expr</span><span class="special">,</span> <span class="identifier">lazy_subscript_context</span> <span class="special">&</span> <span class="identifier">ctx</span> <span class="special">)</span> <span class="keyword">const</span> 8870 <span class="special">{</span> 8871 <span class="keyword">return</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">value</span><span class="special">(</span> <span class="identifier">expr</span> <span class="special">)[</span> <span class="identifier">ctx</span><span class="special">.</span><span class="identifier">subscript_</span> <span class="special">];</span> 8872 <span class="special">}</span> 8873 <span class="special">};</span> 8874 8875 <span class="identifier">Size</span> <span class="identifier">subscript_</span><span class="special">;</span> 8876<span class="special">};</span> 8877 8878<span class="comment">// Here is the domain-specific expression wrapper, which overrides</span> 8879<span class="comment">// operator [] to evaluate the expression using the lazy_subscript_context.</span> 8880<span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">></span> 8881<span class="keyword">struct</span> <span class="identifier">lazy_vector_expr</span> 8882 <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">extends</span><span class="special"><</span><span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">lazy_vector_expr</span><span class="special"><</span><span class="identifier">Expr</span><span class="special">>,</span> <span class="identifier">lazy_vector_domain</span><span class="special">></span> 8883<span class="special">{</span> 8884 <span class="identifier">lazy_vector_expr</span><span class="special">(</span> <span class="identifier">Expr</span> <span class="keyword">const</span> <span class="special">&</span> <span class="identifier">expr</span> <span class="special">=</span> <span class="identifier">Expr</span><span class="special">()</span> <span class="special">)</span> 8885 <span class="special">:</span> <span class="identifier">lazy_vector_expr</span><span class="special">::</span><span class="identifier">proto_extends</span><span class="special">(</span> <span class="identifier">expr</span> <span class="special">)</span> 8886 <span class="special">{}</span> 8887 8888 <span class="comment">// Use the lazy_subscript_context<> to implement subscripting</span> 8889 <span class="comment">// of a lazy vector expression tree.</span> 8890 <span class="keyword">template</span><span class="special"><</span> <span class="keyword">typename</span> <span class="identifier">Size</span> <span class="special">></span> 8891 <span class="keyword">typename</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">eval</span><span class="special"><</span> <span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">lazy_subscript_context</span><span class="special"><</span><span class="identifier">Size</span><span class="special">></span> <span class="special">>::</span><span class="identifier">type</span> 8892 <span class="keyword">operator</span> <span class="special">[](</span> <span class="identifier">Size</span> <span class="identifier">subscript</span> <span class="special">)</span> <span class="keyword">const</span> 8893 <span class="special">{</span> 8894 <span class="identifier">lazy_subscript_context</span><span class="special"><</span><span class="identifier">Size</span><span class="special">></span> <span class="identifier">ctx</span><span class="special">(</span><span class="identifier">subscript</span><span class="special">);</span> 8895 <span class="keyword">return</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">eval</span><span class="special">(*</span><span class="keyword">this</span><span class="special">,</span> <span class="identifier">ctx</span><span class="special">);</span> 8896 <span class="special">}</span> 8897<span class="special">};</span> 8898 8899<span class="comment">// Here is our lazy_vector terminal, implemented in terms of lazy_vector_expr</span> 8900<span class="keyword">template</span><span class="special"><</span> <span class="keyword">typename</span> <span class="identifier">T</span> <span class="special">></span> 8901<span class="keyword">struct</span> <span class="identifier">lazy_vector</span> 8902 <span class="special">:</span> <span class="identifier">lazy_vector_expr</span><span class="special"><</span> <span class="keyword">typename</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special"><</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="special">>::</span><span class="identifier">type</span> <span class="special">></span> 8903<span class="special">{</span> 8904 <span class="keyword">typedef</span> <span class="keyword">typename</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special"><</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="special">>::</span><span class="identifier">type</span> <span class="identifier">expr_type</span><span class="special">;</span> 8905 8906 <span class="identifier">lazy_vector</span><span class="special">(</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="identifier">size</span> <span class="special">=</span> <span class="number">0</span><span class="special">,</span> <span class="identifier">T</span> <span class="keyword">const</span> <span class="special">&</span> <span class="identifier">value</span> <span class="special">=</span> <span class="identifier">T</span><span class="special">()</span> <span class="special">)</span> 8907 <span class="special">:</span> <span class="identifier">lazy_vector_expr</span><span class="special"><</span><span class="identifier">expr_type</span><span class="special">>(</span> <span class="identifier">expr_type</span><span class="special">::</span><span class="identifier">make</span><span class="special">(</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special"><</span><span class="identifier">T</span><span class="special">>(</span> <span class="identifier">size</span><span class="special">,</span> <span class="identifier">value</span> <span class="special">)</span> <span class="special">)</span> <span class="special">)</span> 8908 <span class="special">{}</span> 8909 8910 <span class="comment">// Here we define a += operator for lazy vector terminals that</span> 8911 <span class="comment">// takes a lazy vector expression and indexes it. expr[i] here</span> 8912 <span class="comment">// uses lazy_subscript_context<> under the covers.</span> 8913 <span class="keyword">template</span><span class="special"><</span> <span class="keyword">typename</span> <span class="identifier">Expr</span> <span class="special">></span> 8914 <span class="identifier">lazy_vector</span> <span class="special">&</span><span class="keyword">operator</span> <span class="special">+=</span> <span class="special">(</span><span class="identifier">Expr</span> <span class="keyword">const</span> <span class="special">&</span> <span class="identifier">expr</span><span class="special">)</span> 8915 <span class="special">{</span> 8916 <span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="identifier">size</span> <span class="special">=</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">value</span><span class="special">(*</span><span class="keyword">this</span><span class="special">).</span><span class="identifier">size</span><span class="special">();</span> 8917 <span class="keyword">for</span><span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="identifier">i</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</span> <span class="identifier">i</span> <span class="special"><</span> <span class="identifier">size</span><span class="special">;</span> <span class="special">++</span><span class="identifier">i</span><span class="special">)</span> 8918 <span class="special">{</span> 8919 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">value</span><span class="special">(*</span><span class="keyword">this</span><span class="special">)[</span><span class="identifier">i</span><span class="special">]</span> <span class="special">+=</span> <span class="identifier">expr</span><span class="special">[</span><span class="identifier">i</span><span class="special">];</span> 8920 <span class="special">}</span> 8921 <span class="keyword">return</span> <span class="special">*</span><span class="keyword">this</span><span class="special">;</span> 8922 <span class="special">}</span> 8923<span class="special">};</span> 8924 8925<span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span> 8926<span class="special">{</span> 8927 <span class="comment">// lazy_vectors with 4 elements each.</span> 8928 <span class="identifier">lazy_vector</span><span class="special"><</span> <span class="keyword">double</span> <span class="special">></span> <span class="identifier">v1</span><span class="special">(</span> <span class="number">4</span><span class="special">,</span> <span class="number">1.0</span> <span class="special">),</span> <span class="identifier">v2</span><span class="special">(</span> <span class="number">4</span><span class="special">,</span> <span class="number">2.0</span> <span class="special">),</span> <span class="identifier">v3</span><span class="special">(</span> <span class="number">4</span><span class="special">,</span> <span class="number">3.0</span> <span class="special">);</span> 8929 8930 <span class="comment">// Add two vectors lazily and get the 2nd element.</span> 8931 <span class="keyword">double</span> <span class="identifier">d1</span> <span class="special">=</span> <span class="special">(</span> <span class="identifier">v2</span> <span class="special">+</span> <span class="identifier">v3</span> <span class="special">)[</span> <span class="number">2</span> <span class="special">];</span> <span class="comment">// Look ma, no temporaries!</span> 8932 <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="identifier">d1</span> <span class="special"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span> 8933 8934 <span class="comment">// Subtract two vectors and add the result to a third vector.</span> 8935 <span class="identifier">v1</span> <span class="special">+=</span> <span class="identifier">v2</span> <span class="special">-</span> <span class="identifier">v3</span><span class="special">;</span> <span class="comment">// Still no temporaries!</span> 8936 <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="char">'{'</span> <span class="special"><<</span> <span class="identifier">v1</span><span class="special">[</span><span class="number">0</span><span class="special">]</span> <span class="special"><<</span> <span class="char">','</span> <span class="special"><<</span> <span class="identifier">v1</span><span class="special">[</span><span class="number">1</span><span class="special">]</span> 8937 <span class="special"><<</span> <span class="char">','</span> <span class="special"><<</span> <span class="identifier">v1</span><span class="special">[</span><span class="number">2</span><span class="special">]</span> <span class="special"><<</span> <span class="char">','</span> <span class="special"><<</span> <span class="identifier">v1</span><span class="special">[</span><span class="number">3</span><span class="special">]</span> <span class="special"><<</span> <span class="char">'}'</span> <span class="special"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span> 8938 8939 <span class="comment">// This expression is disallowed because it does not conform</span> 8940 <span class="comment">// to the LazyVectorGrammar</span> 8941 <span class="comment">//(v2 + v3) += v1;</span> 8942 8943 <span class="keyword">return</span> <span class="number">0</span><span class="special">;</span> 8944<span class="special">}</span> 8945</pre> 8946<p> 8947 </p> 8948</div> 8949<div class="section"> 8950<div class="titlepage"><div><div><h4 class="title"> 8951<a name="boost_proto.users_guide.examples.rgb"></a><a class="link" href="users_guide.html#boost_proto.users_guide.examples.rgb" title="RGB: Type Manipulations with Proto Transforms">RGB: Type Manipulations 8952 with Proto Transforms</a> 8953</h4></div></div></div> 8954<p> 8955 This is a simple example of doing arbitrary type manipulations with Proto 8956 transforms. It takes some expression involving primary colors and combines 8957 the colors according to arbitrary rules. It is a port of the RGB example 8958 from <a href="http://acts.nersc.gov/formertools/pete/index.html" target="_top">PETE</a>. 8959 </p> 8960<p> 8961</p> 8962<pre class="programlisting"><span class="comment">///////////////////////////////////////////////////////////////////////////////</span> 8963<span class="comment">// Copyright 2008 Eric Niebler. Distributed under the Boost</span> 8964<span class="comment">// Software License, Version 1.0. (See accompanying file</span> 8965<span class="comment">// LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)</span> 8966<span class="comment">//</span> 8967<span class="comment">// This is a simple example of doing arbitrary type manipulations with proto</span> 8968<span class="comment">// transforms. It takes some expression involving primary colors and combines</span> 8969<span class="comment">// the colors according to arbitrary rules. It is a port of the RGB example</span> 8970<span class="comment">// from PETE (http://www.codesourcery.com/pooma/download.html).</span> 8971 8972<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">iostream</span><span class="special">></span> 8973<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">proto</span><span class="special">/</span><span class="identifier">core</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> 8974<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">proto</span><span class="special">/</span><span class="identifier">transform</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> 8975<span class="keyword">namespace</span> <span class="identifier">proto</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">proto</span><span class="special">;</span> 8976 8977<span class="keyword">struct</span> <span class="identifier">RedTag</span> 8978<span class="special">{</span> 8979 <span class="keyword">friend</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">ostream</span> <span class="special">&</span><span class="keyword">operator</span> <span class="special"><<(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">ostream</span> <span class="special">&</span><span class="identifier">sout</span><span class="special">,</span> <span class="identifier">RedTag</span><span class="special">)</span> 8980 <span class="special">{</span> 8981 <span class="keyword">return</span> <span class="identifier">sout</span> <span class="special"><<</span> <span class="string">"This expression is red."</span><span class="special">;</span> 8982 <span class="special">}</span> 8983<span class="special">};</span> 8984 8985<span class="keyword">struct</span> <span class="identifier">BlueTag</span> 8986<span class="special">{</span> 8987 <span class="keyword">friend</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">ostream</span> <span class="special">&</span><span class="keyword">operator</span> <span class="special"><<(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">ostream</span> <span class="special">&</span><span class="identifier">sout</span><span class="special">,</span> <span class="identifier">BlueTag</span><span class="special">)</span> 8988 <span class="special">{</span> 8989 <span class="keyword">return</span> <span class="identifier">sout</span> <span class="special"><<</span> <span class="string">"This expression is blue."</span><span class="special">;</span> 8990 <span class="special">}</span> 8991<span class="special">};</span> 8992 8993<span class="keyword">struct</span> <span class="identifier">GreenTag</span> 8994<span class="special">{</span> 8995 <span class="keyword">friend</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">ostream</span> <span class="special">&</span><span class="keyword">operator</span> <span class="special"><<(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">ostream</span> <span class="special">&</span><span class="identifier">sout</span><span class="special">,</span> <span class="identifier">GreenTag</span><span class="special">)</span> 8996 <span class="special">{</span> 8997 <span class="keyword">return</span> <span class="identifier">sout</span> <span class="special"><<</span> <span class="string">"This expression is green."</span><span class="special">;</span> 8998 <span class="special">}</span> 8999<span class="special">};</span> 9000 9001<span class="keyword">typedef</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special"><</span><span class="identifier">RedTag</span><span class="special">>::</span><span class="identifier">type</span> <span class="identifier">RedT</span><span class="special">;</span> 9002<span class="keyword">typedef</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special"><</span><span class="identifier">BlueTag</span><span class="special">>::</span><span class="identifier">type</span> <span class="identifier">BlueT</span><span class="special">;</span> 9003<span class="keyword">typedef</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special"><</span><span class="identifier">GreenTag</span><span class="special">>::</span><span class="identifier">type</span> <span class="identifier">GreenT</span><span class="special">;</span> 9004 9005<span class="keyword">struct</span> <span class="identifier">Red</span><span class="special">;</span> 9006<span class="keyword">struct</span> <span class="identifier">Blue</span><span class="special">;</span> 9007<span class="keyword">struct</span> <span class="identifier">Green</span><span class="special">;</span> 9008 9009<span class="comment">///////////////////////////////////////////////////////////////////////////////</span> 9010<span class="comment">// A transform that produces new colors according to some arbitrary rules:</span> 9011<span class="comment">// red & green give blue, red & blue give green, blue and green give red.</span> 9012<span class="keyword">struct</span> <span class="identifier">Red</span> 9013 <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">or_</span><span class="special"><</span> 9014 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">plus</span><span class="special"><</span><span class="identifier">Green</span><span class="special">,</span> <span class="identifier">Blue</span><span class="special">></span> 9015 <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">plus</span><span class="special"><</span><span class="identifier">Blue</span><span class="special">,</span> <span class="identifier">Green</span><span class="special">></span> 9016 <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">plus</span><span class="special"><</span><span class="identifier">Red</span><span class="special">,</span> <span class="identifier">Red</span><span class="special">></span> 9017 <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special"><</span><span class="identifier">RedTag</span><span class="special">></span> 9018 <span class="special">></span> 9019<span class="special">{};</span> 9020 9021<span class="keyword">struct</span> <span class="identifier">Green</span> 9022 <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">or_</span><span class="special"><</span> 9023 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">plus</span><span class="special"><</span><span class="identifier">Red</span><span class="special">,</span> <span class="identifier">Blue</span><span class="special">></span> 9024 <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">plus</span><span class="special"><</span><span class="identifier">Blue</span><span class="special">,</span> <span class="identifier">Red</span><span class="special">></span> 9025 <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">plus</span><span class="special"><</span><span class="identifier">Green</span><span class="special">,</span> <span class="identifier">Green</span><span class="special">></span> 9026 <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special"><</span><span class="identifier">GreenTag</span><span class="special">></span> 9027 <span class="special">></span> 9028<span class="special">{};</span> 9029 9030<span class="keyword">struct</span> <span class="identifier">Blue</span> 9031 <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">or_</span><span class="special"><</span> 9032 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">plus</span><span class="special"><</span><span class="identifier">Red</span><span class="special">,</span> <span class="identifier">Green</span><span class="special">></span> 9033 <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">plus</span><span class="special"><</span><span class="identifier">Green</span><span class="special">,</span> <span class="identifier">Red</span><span class="special">></span> 9034 <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">plus</span><span class="special"><</span><span class="identifier">Blue</span><span class="special">,</span> <span class="identifier">Blue</span><span class="special">></span> 9035 <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special"><</span><span class="identifier">BlueTag</span><span class="special">></span> 9036 <span class="special">></span> 9037<span class="special">{};</span> 9038 9039<span class="keyword">struct</span> <span class="identifier">RGB</span> 9040 <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">or_</span><span class="special"><</span> 9041 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special"><</span> <span class="identifier">Red</span><span class="special">,</span> <span class="identifier">RedTag</span><span class="special">()</span> <span class="special">></span> 9042 <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special"><</span> <span class="identifier">Blue</span><span class="special">,</span> <span class="identifier">BlueTag</span><span class="special">()</span> <span class="special">></span> 9043 <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special"><</span> <span class="identifier">Green</span><span class="special">,</span> <span class="identifier">GreenTag</span><span class="special">()</span> <span class="special">></span> 9044 <span class="special">></span> 9045<span class="special">{};</span> 9046 9047<span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">></span> 9048<span class="keyword">void</span> <span class="identifier">printColor</span><span class="special">(</span><span class="identifier">Expr</span> <span class="keyword">const</span> <span class="special">&</span> <span class="identifier">expr</span><span class="special">)</span> 9049<span class="special">{</span> 9050 <span class="keyword">int</span> <span class="identifier">i</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</span> <span class="comment">// dummy state and data parameter, not used</span> 9051 <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="identifier">RGB</span><span class="special">()(</span><span class="identifier">expr</span><span class="special">,</span> <span class="identifier">i</span><span class="special">,</span> <span class="identifier">i</span><span class="special">)</span> <span class="special"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span> 9052<span class="special">}</span> 9053 9054<span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span> 9055<span class="special">{</span> 9056 <span class="identifier">printColor</span><span class="special">(</span><span class="identifier">RedT</span><span class="special">()</span> <span class="special">+</span> <span class="identifier">GreenT</span><span class="special">());</span> 9057 <span class="identifier">printColor</span><span class="special">(</span><span class="identifier">RedT</span><span class="special">()</span> <span class="special">+</span> <span class="identifier">GreenT</span><span class="special">()</span> <span class="special">+</span> <span class="identifier">BlueT</span><span class="special">());</span> 9058 <span class="identifier">printColor</span><span class="special">(</span><span class="identifier">RedT</span><span class="special">()</span> <span class="special">+</span> <span class="special">(</span><span class="identifier">GreenT</span><span class="special">()</span> <span class="special">+</span> <span class="identifier">BlueT</span><span class="special">()));</span> 9059 9060 <span class="keyword">return</span> <span class="number">0</span><span class="special">;</span> 9061<span class="special">}</span> 9062</pre> 9063<p> 9064 </p> 9065</div> 9066<div class="section"> 9067<div class="titlepage"><div><div><h4 class="title"> 9068<a name="boost_proto.users_guide.examples.tarray"></a><a class="link" href="users_guide.html#boost_proto.users_guide.examples.tarray" title="TArray: A Simple Linear Algebra Library">TArray: A 9069 Simple Linear Algebra Library</a> 9070</h4></div></div></div> 9071<p> 9072 This example constructs a mini-library for linear algebra, using expression 9073 templates to eliminate the need for temporaries when adding arrays of numbers. 9074 It duplicates the TArray example from <a href="http://acts.nersc.gov/formertools/pete/index.html" target="_top">PETE</a>. 9075 </p> 9076<p> 9077</p> 9078<pre class="programlisting"><span class="comment">///////////////////////////////////////////////////////////////////////////////</span> 9079<span class="comment">// Copyright 2008 Eric Niebler. Distributed under the Boost</span> 9080<span class="comment">// Software License, Version 1.0. (See accompanying file</span> 9081<span class="comment">// LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)</span> 9082<span class="comment">//</span> 9083<span class="comment">// This example constructs a mini-library for linear algebra, using</span> 9084<span class="comment">// expression templates to eliminate the need for temporaries when</span> 9085<span class="comment">// adding arrays of numbers. It duplicates the TArray example from</span> 9086<span class="comment">// PETE (http://www.codesourcery.com/pooma/download.html)</span> 9087 9088<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">iostream</span><span class="special">></span> 9089<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">mpl</span><span class="special">/</span><span class="keyword">int</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> 9090<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">proto</span><span class="special">/</span><span class="identifier">core</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> 9091<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">proto</span><span class="special">/</span><span class="identifier">context</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> 9092<span class="keyword">namespace</span> <span class="identifier">mpl</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">mpl</span><span class="special">;</span> 9093<span class="keyword">namespace</span> <span class="identifier">proto</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">proto</span><span class="special">;</span> 9094<span class="keyword">using</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_</span><span class="special">;</span> 9095 9096<span class="comment">// This grammar describes which TArray expressions</span> 9097<span class="comment">// are allowed; namely, int and array terminals</span> 9098<span class="comment">// plus, minus, multiplies and divides of TArray expressions.</span> 9099<span class="keyword">struct</span> <span class="identifier">TArrayGrammar</span> 9100 <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">or_</span><span class="special"><</span> 9101 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special"><</span> <span class="keyword">int</span> <span class="special">></span> 9102 <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special"><</span> <span class="keyword">int</span><span class="special">[</span><span class="number">3</span><span class="special">]</span> <span class="special">></span> 9103 <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">plus</span><span class="special"><</span> <span class="identifier">TArrayGrammar</span><span class="special">,</span> <span class="identifier">TArrayGrammar</span> <span class="special">></span> 9104 <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">minus</span><span class="special"><</span> <span class="identifier">TArrayGrammar</span><span class="special">,</span> <span class="identifier">TArrayGrammar</span> <span class="special">></span> 9105 <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">multiplies</span><span class="special"><</span> <span class="identifier">TArrayGrammar</span><span class="special">,</span> <span class="identifier">TArrayGrammar</span> <span class="special">></span> 9106 <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">divides</span><span class="special"><</span> <span class="identifier">TArrayGrammar</span><span class="special">,</span> <span class="identifier">TArrayGrammar</span> <span class="special">></span> 9107 <span class="special">></span> 9108<span class="special">{};</span> 9109 9110<span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">></span> 9111<span class="keyword">struct</span> <span class="identifier">TArrayExpr</span><span class="special">;</span> 9112 9113<span class="comment">// Tell proto that in the TArrayDomain, all</span> 9114<span class="comment">// expressions should be wrapped in TArrayExpr<> and</span> 9115<span class="comment">// must conform to the TArrayGrammar</span> 9116<span class="keyword">struct</span> <span class="identifier">TArrayDomain</span> 9117 <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">domain</span><span class="special"><</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">generator</span><span class="special"><</span><span class="identifier">TArrayExpr</span><span class="special">>,</span> <span class="identifier">TArrayGrammar</span><span class="special">></span> 9118<span class="special">{};</span> 9119 9120<span class="comment">// Here is an evaluation context that indexes into a TArray</span> 9121<span class="comment">// expression, and combines the result.</span> 9122<span class="keyword">struct</span> <span class="identifier">TArraySubscriptCtx</span> 9123 <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">callable_context</span><span class="special"><</span> <span class="identifier">TArraySubscriptCtx</span> <span class="keyword">const</span> <span class="special">></span> 9124<span class="special">{</span> 9125 <span class="keyword">typedef</span> <span class="keyword">int</span> <span class="identifier">result_type</span><span class="special">;</span> 9126 9127 <span class="identifier">TArraySubscriptCtx</span><span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">ptrdiff_t</span> <span class="identifier">i</span><span class="special">)</span> 9128 <span class="special">:</span> <span class="identifier">i_</span><span class="special">(</span><span class="identifier">i</span><span class="special">)</span> 9129 <span class="special">{}</span> 9130 9131 <span class="comment">// Index array terminals with our subscript. Everything</span> 9132 <span class="comment">// else will be handled by the default evaluation context.</span> 9133 <span class="keyword">int</span> <span class="keyword">operator</span> <span class="special">()(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">,</span> <span class="keyword">int</span> <span class="keyword">const</span> <span class="special">(&</span><span class="identifier">data</span><span class="special">)[</span><span class="number">3</span><span class="special">])</span> <span class="keyword">const</span> 9134 <span class="special">{</span> 9135 <span class="keyword">return</span> <span class="identifier">data</span><span class="special">[</span><span class="keyword">this</span><span class="special">-></span><span class="identifier">i_</span><span class="special">];</span> 9136 <span class="special">}</span> 9137 9138 <span class="identifier">std</span><span class="special">::</span><span class="identifier">ptrdiff_t</span> <span class="identifier">i_</span><span class="special">;</span> 9139<span class="special">};</span> 9140 9141<span class="comment">// Here is an evaluation context that prints a TArray expression.</span> 9142<span class="keyword">struct</span> <span class="identifier">TArrayPrintCtx</span> 9143 <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">callable_context</span><span class="special"><</span> <span class="identifier">TArrayPrintCtx</span> <span class="keyword">const</span> <span class="special">></span> 9144<span class="special">{</span> 9145 <span class="keyword">typedef</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">ostream</span> <span class="special">&</span><span class="identifier">result_type</span><span class="special">;</span> 9146 9147 <span class="identifier">TArrayPrintCtx</span><span class="special">()</span> <span class="special">{}</span> 9148 9149 <span class="identifier">std</span><span class="special">::</span><span class="identifier">ostream</span> <span class="special">&</span><span class="keyword">operator</span> <span class="special">()(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">,</span> <span class="keyword">int</span> <span class="identifier">i</span><span class="special">)</span> <span class="keyword">const</span> 9150 <span class="special">{</span> 9151 <span class="keyword">return</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="identifier">i</span><span class="special">;</span> 9152 <span class="special">}</span> 9153 9154 <span class="identifier">std</span><span class="special">::</span><span class="identifier">ostream</span> <span class="special">&</span><span class="keyword">operator</span> <span class="special">()(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">,</span> <span class="keyword">int</span> <span class="keyword">const</span> <span class="special">(&</span><span class="identifier">arr</span><span class="special">)[</span><span class="number">3</span><span class="special">])</span> <span class="keyword">const</span> 9155 <span class="special">{</span> 9156 <span class="keyword">return</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="char">'{'</span> <span class="special"><<</span> <span class="identifier">arr</span><span class="special">[</span><span class="number">0</span><span class="special">]</span> <span class="special"><<</span> <span class="string">", "</span> <span class="special"><<</span> <span class="identifier">arr</span><span class="special">[</span><span class="number">1</span><span class="special">]</span> <span class="special"><<</span> <span class="string">", "</span> <span class="special"><<</span> <span class="identifier">arr</span><span class="special">[</span><span class="number">2</span><span class="special">]</span> <span class="special"><<</span> <span class="char">'}'</span><span class="special">;</span> 9157 <span class="special">}</span> 9158 9159 <span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">L</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">R</span><span class="special">></span> 9160 <span class="identifier">std</span><span class="special">::</span><span class="identifier">ostream</span> <span class="special">&</span><span class="keyword">operator</span> <span class="special">()(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">plus</span><span class="special">,</span> <span class="identifier">L</span> <span class="keyword">const</span> <span class="special">&</span><span class="identifier">l</span><span class="special">,</span> <span class="identifier">R</span> <span class="keyword">const</span> <span class="special">&</span><span class="identifier">r</span><span class="special">)</span> <span class="keyword">const</span> 9161 <span class="special">{</span> 9162 <span class="keyword">return</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="char">'('</span> <span class="special"><<</span> <span class="identifier">l</span> <span class="special"><<</span> <span class="string">" + "</span> <span class="special"><<</span> <span class="identifier">r</span> <span class="special"><<</span> <span class="char">')'</span><span class="special">;</span> 9163 <span class="special">}</span> 9164 9165 <span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">L</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">R</span><span class="special">></span> 9166 <span class="identifier">std</span><span class="special">::</span><span class="identifier">ostream</span> <span class="special">&</span><span class="keyword">operator</span> <span class="special">()(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">minus</span><span class="special">,</span> <span class="identifier">L</span> <span class="keyword">const</span> <span class="special">&</span><span class="identifier">l</span><span class="special">,</span> <span class="identifier">R</span> <span class="keyword">const</span> <span class="special">&</span><span class="identifier">r</span><span class="special">)</span> <span class="keyword">const</span> 9167 <span class="special">{</span> 9168 <span class="keyword">return</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="char">'('</span> <span class="special"><<</span> <span class="identifier">l</span> <span class="special"><<</span> <span class="string">" - "</span> <span class="special"><<</span> <span class="identifier">r</span> <span class="special"><<</span> <span class="char">')'</span><span class="special">;</span> 9169 <span class="special">}</span> 9170 9171 <span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">L</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">R</span><span class="special">></span> 9172 <span class="identifier">std</span><span class="special">::</span><span class="identifier">ostream</span> <span class="special">&</span><span class="keyword">operator</span> <span class="special">()(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">multiplies</span><span class="special">,</span> <span class="identifier">L</span> <span class="keyword">const</span> <span class="special">&</span><span class="identifier">l</span><span class="special">,</span> <span class="identifier">R</span> <span class="keyword">const</span> <span class="special">&</span><span class="identifier">r</span><span class="special">)</span> <span class="keyword">const</span> 9173 <span class="special">{</span> 9174 <span class="keyword">return</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="identifier">l</span> <span class="special"><<</span> <span class="string">" * "</span> <span class="special"><<</span> <span class="identifier">r</span><span class="special">;</span> 9175 <span class="special">}</span> 9176 9177 <span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">L</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">R</span><span class="special">></span> 9178 <span class="identifier">std</span><span class="special">::</span><span class="identifier">ostream</span> <span class="special">&</span><span class="keyword">operator</span> <span class="special">()(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">divides</span><span class="special">,</span> <span class="identifier">L</span> <span class="keyword">const</span> <span class="special">&</span><span class="identifier">l</span><span class="special">,</span> <span class="identifier">R</span> <span class="keyword">const</span> <span class="special">&</span><span class="identifier">r</span><span class="special">)</span> <span class="keyword">const</span> 9179 <span class="special">{</span> 9180 <span class="keyword">return</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="identifier">l</span> <span class="special"><<</span> <span class="string">" / "</span> <span class="special"><<</span> <span class="identifier">r</span><span class="special">;</span> 9181 <span class="special">}</span> 9182<span class="special">};</span> 9183 9184<span class="comment">// Here is the domain-specific expression wrapper, which overrides</span> 9185<span class="comment">// operator [] to evaluate the expression using the TArraySubscriptCtx.</span> 9186<span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">></span> 9187<span class="keyword">struct</span> <span class="identifier">TArrayExpr</span> 9188 <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">extends</span><span class="special"><</span><span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">TArrayExpr</span><span class="special"><</span><span class="identifier">Expr</span><span class="special">>,</span> <span class="identifier">TArrayDomain</span><span class="special">></span> 9189<span class="special">{</span> 9190 <span class="keyword">typedef</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">extends</span><span class="special"><</span><span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">TArrayExpr</span><span class="special"><</span><span class="identifier">Expr</span><span class="special">>,</span> <span class="identifier">TArrayDomain</span><span class="special">></span> <span class="identifier">base_type</span><span class="special">;</span> 9191 9192 <span class="identifier">TArrayExpr</span><span class="special">(</span> <span class="identifier">Expr</span> <span class="keyword">const</span> <span class="special">&</span> <span class="identifier">expr</span> <span class="special">=</span> <span class="identifier">Expr</span><span class="special">()</span> <span class="special">)</span> 9193 <span class="special">:</span> <span class="identifier">base_type</span><span class="special">(</span> <span class="identifier">expr</span> <span class="special">)</span> 9194 <span class="special">{}</span> 9195 9196 <span class="comment">// Use the TArraySubscriptCtx to implement subscripting</span> 9197 <span class="comment">// of a TArray expression tree.</span> 9198 <span class="keyword">int</span> <span class="keyword">operator</span> <span class="special">[](</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">ptrdiff_t</span> <span class="identifier">i</span> <span class="special">)</span> <span class="keyword">const</span> 9199 <span class="special">{</span> 9200 <span class="identifier">TArraySubscriptCtx</span> <span class="keyword">const</span> <span class="identifier">ctx</span><span class="special">(</span><span class="identifier">i</span><span class="special">);</span> 9201 <span class="keyword">return</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">eval</span><span class="special">(*</span><span class="keyword">this</span><span class="special">,</span> <span class="identifier">ctx</span><span class="special">);</span> 9202 <span class="special">}</span> 9203 9204 <span class="comment">// Use the TArrayPrintCtx to display a TArray expression tree.</span> 9205 <span class="keyword">friend</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">ostream</span> <span class="special">&</span><span class="keyword">operator</span> <span class="special"><<(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">ostream</span> <span class="special">&</span><span class="identifier">sout</span><span class="special">,</span> <span class="identifier">TArrayExpr</span><span class="special"><</span><span class="identifier">Expr</span><span class="special">></span> <span class="keyword">const</span> <span class="special">&</span><span class="identifier">expr</span><span class="special">)</span> 9206 <span class="special">{</span> 9207 <span class="identifier">TArrayPrintCtx</span> <span class="keyword">const</span> <span class="identifier">ctx</span><span class="special">;</span> 9208 <span class="keyword">return</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">eval</span><span class="special">(</span><span class="identifier">expr</span><span class="special">,</span> <span class="identifier">ctx</span><span class="special">);</span> 9209 <span class="special">}</span> 9210<span class="special">};</span> 9211 9212<span class="comment">// Here is our TArray terminal, implemented in terms of TArrayExpr</span> 9213<span class="comment">// It is basically just an array of 3 integers.</span> 9214<span class="keyword">struct</span> <span class="identifier">TArray</span> 9215 <span class="special">:</span> <span class="identifier">TArrayExpr</span><span class="special"><</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special"><</span> <span class="keyword">int</span><span class="special">[</span><span class="number">3</span><span class="special">]</span> <span class="special">>::</span><span class="identifier">type</span> <span class="special">></span> 9216<span class="special">{</span> 9217 <span class="keyword">explicit</span> <span class="identifier">TArray</span><span class="special">(</span> <span class="keyword">int</span> <span class="identifier">i</span> <span class="special">=</span> <span class="number">0</span><span class="special">,</span> <span class="keyword">int</span> <span class="identifier">j</span> <span class="special">=</span> <span class="number">0</span><span class="special">,</span> <span class="keyword">int</span> <span class="identifier">k</span> <span class="special">=</span> <span class="number">0</span> <span class="special">)</span> 9218 <span class="special">{</span> 9219 <span class="special">(*</span><span class="keyword">this</span><span class="special">)[</span><span class="number">0</span><span class="special">]</span> <span class="special">=</span> <span class="identifier">i</span><span class="special">;</span> 9220 <span class="special">(*</span><span class="keyword">this</span><span class="special">)[</span><span class="number">1</span><span class="special">]</span> <span class="special">=</span> <span class="identifier">j</span><span class="special">;</span> 9221 <span class="special">(*</span><span class="keyword">this</span><span class="special">)[</span><span class="number">2</span><span class="special">]</span> <span class="special">=</span> <span class="identifier">k</span><span class="special">;</span> 9222 <span class="special">}</span> 9223 9224 <span class="comment">// Here we override operator [] to give read/write access to</span> 9225 <span class="comment">// the elements of the array. (We could use the TArrayExpr</span> 9226 <span class="comment">// operator [] if we made the subscript context smarter about</span> 9227 <span class="comment">// returning non-const reference when appropriate.)</span> 9228 <span class="keyword">int</span> <span class="special">&</span><span class="keyword">operator</span> <span class="special">[](</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">ptrdiff_t</span> <span class="identifier">i</span><span class="special">)</span> 9229 <span class="special">{</span> 9230 <span class="keyword">return</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">value</span><span class="special">(*</span><span class="keyword">this</span><span class="special">)[</span><span class="identifier">i</span><span class="special">];</span> 9231 <span class="special">}</span> 9232 9233 <span class="keyword">int</span> <span class="keyword">const</span> <span class="special">&</span><span class="keyword">operator</span> <span class="special">[](</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">ptrdiff_t</span> <span class="identifier">i</span><span class="special">)</span> <span class="keyword">const</span> 9234 <span class="special">{</span> 9235 <span class="keyword">return</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">value</span><span class="special">(*</span><span class="keyword">this</span><span class="special">)[</span><span class="identifier">i</span><span class="special">];</span> 9236 <span class="special">}</span> 9237 9238 <span class="comment">// Here we define a operator = for TArray terminals that</span> 9239 <span class="comment">// takes a TArray expression.</span> 9240 <span class="keyword">template</span><span class="special"><</span> <span class="keyword">typename</span> <span class="identifier">Expr</span> <span class="special">></span> 9241 <span class="identifier">TArray</span> <span class="special">&</span><span class="keyword">operator</span> <span class="special">=(</span><span class="identifier">Expr</span> <span class="keyword">const</span> <span class="special">&</span> <span class="identifier">expr</span><span class="special">)</span> 9242 <span class="special">{</span> 9243 <span class="comment">// proto::as_expr<TArrayDomain>(expr) is the same as</span> 9244 <span class="comment">// expr unless expr is an integer, in which case it</span> 9245 <span class="comment">// is made into a TArrayExpr terminal first.</span> 9246 <span class="keyword">return</span> <span class="keyword">this</span><span class="special">-></span><span class="identifier">assign</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">as_expr</span><span class="special"><</span><span class="identifier">TArrayDomain</span><span class="special">>(</span><span class="identifier">expr</span><span class="special">));</span> 9247 <span class="special">}</span> 9248 9249 <span class="keyword">template</span><span class="special"><</span> <span class="keyword">typename</span> <span class="identifier">Expr</span> <span class="special">></span> 9250 <span class="identifier">TArray</span> <span class="special">&</span><span class="identifier">printAssign</span><span class="special">(</span><span class="identifier">Expr</span> <span class="keyword">const</span> <span class="special">&</span> <span class="identifier">expr</span><span class="special">)</span> 9251 <span class="special">{</span> 9252 <span class="special">*</span><span class="keyword">this</span> <span class="special">=</span> <span class="identifier">expr</span><span class="special">;</span> 9253 <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="special">*</span><span class="keyword">this</span> <span class="special"><<</span> <span class="string">" = "</span> <span class="special"><<</span> <span class="identifier">expr</span> <span class="special"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span> 9254 <span class="keyword">return</span> <span class="special">*</span><span class="keyword">this</span><span class="special">;</span> 9255 <span class="special">}</span> 9256 9257<span class="keyword">private</span><span class="special">:</span> 9258 <span class="keyword">template</span><span class="special"><</span> <span class="keyword">typename</span> <span class="identifier">Expr</span> <span class="special">></span> 9259 <span class="identifier">TArray</span> <span class="special">&</span><span class="identifier">assign</span><span class="special">(</span><span class="identifier">Expr</span> <span class="keyword">const</span> <span class="special">&</span> <span class="identifier">expr</span><span class="special">)</span> 9260 <span class="special">{</span> 9261 <span class="comment">// expr[i] here uses TArraySubscriptCtx under the covers.</span> 9262 <span class="special">(*</span><span class="keyword">this</span><span class="special">)[</span><span class="number">0</span><span class="special">]</span> <span class="special">=</span> <span class="identifier">expr</span><span class="special">[</span><span class="number">0</span><span class="special">];</span> 9263 <span class="special">(*</span><span class="keyword">this</span><span class="special">)[</span><span class="number">1</span><span class="special">]</span> <span class="special">=</span> <span class="identifier">expr</span><span class="special">[</span><span class="number">1</span><span class="special">];</span> 9264 <span class="special">(*</span><span class="keyword">this</span><span class="special">)[</span><span class="number">2</span><span class="special">]</span> <span class="special">=</span> <span class="identifier">expr</span><span class="special">[</span><span class="number">2</span><span class="special">];</span> 9265 <span class="keyword">return</span> <span class="special">*</span><span class="keyword">this</span><span class="special">;</span> 9266 <span class="special">}</span> 9267<span class="special">};</span> 9268 9269<span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span> 9270<span class="special">{</span> 9271 <span class="identifier">TArray</span> <span class="identifier">a</span><span class="special">(</span><span class="number">3</span><span class="special">,</span><span class="number">1</span><span class="special">,</span><span class="number">2</span><span class="special">);</span> 9272 9273 <span class="identifier">TArray</span> <span class="identifier">b</span><span class="special">;</span> 9274 9275 <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="identifier">a</span> <span class="special"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span> 9276 <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="identifier">b</span> <span class="special"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span> 9277 9278 <span class="identifier">b</span><span class="special">[</span><span class="number">0</span><span class="special">]</span> <span class="special">=</span> <span class="number">7</span><span class="special">;</span> <span class="identifier">b</span><span class="special">[</span><span class="number">1</span><span class="special">]</span> <span class="special">=</span> <span class="number">33</span><span class="special">;</span> <span class="identifier">b</span><span class="special">[</span><span class="number">2</span><span class="special">]</span> <span class="special">=</span> <span class="special">-</span><span class="number">99</span><span class="special">;</span> 9279 9280 <span class="identifier">TArray</span> <span class="identifier">c</span><span class="special">(</span><span class="identifier">a</span><span class="special">);</span> 9281 9282 <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="identifier">c</span> <span class="special"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span> 9283 9284 <span class="identifier">a</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</span> 9285 9286 <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="identifier">a</span> <span class="special"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span> 9287 <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="identifier">b</span> <span class="special"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span> 9288 <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="identifier">c</span> <span class="special"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span> 9289 9290 <span class="identifier">a</span> <span class="special">=</span> <span class="identifier">b</span> <span class="special">+</span> <span class="identifier">c</span><span class="special">;</span> 9291 9292 <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="identifier">a</span> <span class="special"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span> 9293 9294 <span class="identifier">a</span><span class="special">.</span><span class="identifier">printAssign</span><span class="special">(</span><span class="identifier">b</span><span class="special">+</span><span class="identifier">c</span><span class="special">*(</span><span class="identifier">b</span> <span class="special">+</span> <span class="number">3</span><span class="special">*</span><span class="identifier">c</span><span class="special">));</span> 9295 9296 <span class="keyword">return</span> <span class="number">0</span><span class="special">;</span> 9297<span class="special">}</span> 9298</pre> 9299<p> 9300 </p> 9301</div> 9302<div class="section"> 9303<div class="titlepage"><div><div><h4 class="title"> 9304<a name="boost_proto.users_guide.examples.vec3"></a><a class="link" href="users_guide.html#boost_proto.users_guide.examples.vec3" title="Vec3: Computing With Transforms and Contexts">Vec3: Computing 9305 With Transforms and Contexts</a> 9306</h4></div></div></div> 9307<p> 9308 This is a simple example using <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">extends</span><span class="special"><></span></code> to extend a terminal type with 9309 additional behaviors, and using custom contexts and <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">eval</span><span class="special">()</span></code> for evaluating expressions. It is a port 9310 of the Vec3 example from <a href="http://acts.nersc.gov/formertools/pete/index.html" target="_top">PETE</a>. 9311 </p> 9312<p> 9313</p> 9314<pre class="programlisting"><span class="comment">///////////////////////////////////////////////////////////////////////////////</span> 9315<span class="comment">// Copyright 2008 Eric Niebler. Distributed under the Boost</span> 9316<span class="comment">// Software License, Version 1.0. (See accompanying file</span> 9317<span class="comment">// LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)</span> 9318<span class="comment">//</span> 9319<span class="comment">// This is a simple example using proto::extends to extend a terminal type with</span> 9320<span class="comment">// additional behaviors, and using custom contexts and proto::eval for</span> 9321<span class="comment">// evaluating expressions. It is a port of the Vec3 example</span> 9322<span class="comment">// from PETE (http://www.codesourcery.com/pooma/download.html).</span> 9323 9324<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">iostream</span><span class="special">></span> 9325<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">functional</span><span class="special">></span> 9326<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">assert</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> 9327<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">mpl</span><span class="special">/</span><span class="keyword">int</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> 9328<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">proto</span><span class="special">/</span><span class="identifier">core</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> 9329<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">proto</span><span class="special">/</span><span class="identifier">context</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> 9330<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">proto</span><span class="special">/</span><span class="identifier">proto_typeof</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> 9331<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">proto</span><span class="special">/</span><span class="identifier">transform</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> 9332<span class="keyword">namespace</span> <span class="identifier">mpl</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">mpl</span><span class="special">;</span> 9333<span class="keyword">namespace</span> <span class="identifier">proto</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">proto</span><span class="special">;</span> 9334<span class="keyword">using</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_</span><span class="special">;</span> 9335 9336<span class="comment">// Here is an evaluation context that indexes into a Vec3</span> 9337<span class="comment">// expression, and combines the result.</span> 9338<span class="keyword">struct</span> <span class="identifier">Vec3SubscriptCtx</span> 9339 <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">callable_context</span><span class="special"><</span> <span class="identifier">Vec3SubscriptCtx</span> <span class="keyword">const</span> <span class="special">></span> 9340<span class="special">{</span> 9341 <span class="keyword">typedef</span> <span class="keyword">int</span> <span class="identifier">result_type</span><span class="special">;</span> 9342 9343 <span class="identifier">Vec3SubscriptCtx</span><span class="special">(</span><span class="keyword">int</span> <span class="identifier">i</span><span class="special">)</span> 9344 <span class="special">:</span> <span class="identifier">i_</span><span class="special">(</span><span class="identifier">i</span><span class="special">)</span> 9345 <span class="special">{}</span> 9346 9347 <span class="comment">// Index array terminals with our subscript. Everything</span> 9348 <span class="comment">// else will be handled by the default evaluation context.</span> 9349 <span class="keyword">int</span> <span class="keyword">operator</span> <span class="special">()(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">,</span> <span class="keyword">int</span> <span class="keyword">const</span> <span class="special">(&</span><span class="identifier">arr</span><span class="special">)[</span><span class="number">3</span><span class="special">])</span> <span class="keyword">const</span> 9350 <span class="special">{</span> 9351 <span class="keyword">return</span> <span class="identifier">arr</span><span class="special">[</span><span class="keyword">this</span><span class="special">-></span><span class="identifier">i_</span><span class="special">];</span> 9352 <span class="special">}</span> 9353 9354 <span class="keyword">int</span> <span class="identifier">i_</span><span class="special">;</span> 9355<span class="special">};</span> 9356 9357<span class="comment">// Here is an evaluation context that counts the number</span> 9358<span class="comment">// of Vec3 terminals in an expression.</span> 9359<span class="keyword">struct</span> <span class="identifier">CountLeavesCtx</span> 9360 <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">callable_context</span><span class="special"><</span> <span class="identifier">CountLeavesCtx</span><span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">null_context</span> <span class="special">></span> 9361<span class="special">{</span> 9362 <span class="identifier">CountLeavesCtx</span><span class="special">()</span> 9363 <span class="special">:</span> <span class="identifier">count</span><span class="special">(</span><span class="number">0</span><span class="special">)</span> 9364 <span class="special">{}</span> 9365 9366 <span class="keyword">typedef</span> <span class="keyword">void</span> <span class="identifier">result_type</span><span class="special">;</span> 9367 9368 <span class="keyword">void</span> <span class="keyword">operator</span> <span class="special">()(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">,</span> <span class="keyword">int</span> <span class="keyword">const</span><span class="special">(&)[</span><span class="number">3</span><span class="special">])</span> 9369 <span class="special">{</span> 9370 <span class="special">++</span><span class="keyword">this</span><span class="special">-></span><span class="identifier">count</span><span class="special">;</span> 9371 <span class="special">}</span> 9372 9373 <span class="keyword">int</span> <span class="identifier">count</span><span class="special">;</span> 9374<span class="special">};</span> 9375 9376<span class="keyword">struct</span> <span class="identifier">iplus</span> <span class="special">:</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">plus</span><span class="special"><</span><span class="keyword">int</span><span class="special">>,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">callable</span> <span class="special">{};</span> 9377 9378<span class="comment">// Here is a transform that does the same thing as the above context.</span> 9379<span class="comment">// It demonstrates the use of the std::plus<> function object</span> 9380<span class="comment">// with the fold transform. With minor modifications, this</span> 9381<span class="comment">// transform could be used to calculate the leaf count at compile</span> 9382<span class="comment">// time, rather than at runtime.</span> 9383<span class="keyword">struct</span> <span class="identifier">CountLeaves</span> 9384 <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">or_</span><span class="special"><</span> 9385 <span class="comment">// match a Vec3 terminal, return 1</span> 9386 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special"><</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special"><</span><span class="keyword">int</span><span class="special">[</span><span class="number">3</span><span class="special">]>,</span> <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">int_</span><span class="special"><</span><span class="number">1</span><span class="special">>()</span> <span class="special">></span> 9387 <span class="comment">// match a terminal, return int() (which is 0)</span> 9388 <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special"><</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special"><</span><span class="identifier">_</span><span class="special">>,</span> <span class="keyword">int</span><span class="special">()</span> <span class="special">></span> 9389 <span class="comment">// fold everything else, using std::plus<> to add</span> 9390 <span class="comment">// the leaf count of each child to the accumulated state.</span> 9391 <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">otherwise</span><span class="special"><</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">fold</span><span class="special"><</span><span class="identifier">_</span><span class="special">,</span> <span class="keyword">int</span><span class="special">(),</span> <span class="identifier">iplus</span><span class="special">(</span><span class="identifier">CountLeaves</span><span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_state</span><span class="special">)</span> <span class="special">></span> <span class="special">></span> 9392 <span class="special">></span> 9393<span class="special">{};</span> 9394 9395<span class="comment">// Here is the Vec3 struct, which is a vector of 3 integers.</span> 9396<span class="keyword">struct</span> <span class="identifier">Vec3</span> 9397 <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">extends</span><span class="special"><</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special"><</span><span class="keyword">int</span><span class="special">[</span><span class="number">3</span><span class="special">]>::</span><span class="identifier">type</span><span class="special">,</span> <span class="identifier">Vec3</span><span class="special">></span> 9398<span class="special">{</span> 9399 <span class="keyword">explicit</span> <span class="identifier">Vec3</span><span class="special">(</span><span class="keyword">int</span> <span class="identifier">i</span><span class="special">=</span><span class="number">0</span><span class="special">,</span> <span class="keyword">int</span> <span class="identifier">j</span><span class="special">=</span><span class="number">0</span><span class="special">,</span> <span class="keyword">int</span> <span class="identifier">k</span><span class="special">=</span><span class="number">0</span><span class="special">)</span> 9400 <span class="special">{</span> 9401 <span class="special">(*</span><span class="keyword">this</span><span class="special">)[</span><span class="number">0</span><span class="special">]</span> <span class="special">=</span> <span class="identifier">i</span><span class="special">;</span> 9402 <span class="special">(*</span><span class="keyword">this</span><span class="special">)[</span><span class="number">1</span><span class="special">]</span> <span class="special">=</span> <span class="identifier">j</span><span class="special">;</span> 9403 <span class="special">(*</span><span class="keyword">this</span><span class="special">)[</span><span class="number">2</span><span class="special">]</span> <span class="special">=</span> <span class="identifier">k</span><span class="special">;</span> 9404 <span class="special">}</span> 9405 9406 <span class="keyword">int</span> <span class="special">&</span><span class="keyword">operator</span> <span class="special">[](</span><span class="keyword">int</span> <span class="identifier">i</span><span class="special">)</span> 9407 <span class="special">{</span> 9408 <span class="keyword">return</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">value</span><span class="special">(*</span><span class="keyword">this</span><span class="special">)[</span><span class="identifier">i</span><span class="special">];</span> 9409 <span class="special">}</span> 9410 9411 <span class="keyword">int</span> <span class="keyword">const</span> <span class="special">&</span><span class="keyword">operator</span> <span class="special">[](</span><span class="keyword">int</span> <span class="identifier">i</span><span class="special">)</span> <span class="keyword">const</span> 9412 <span class="special">{</span> 9413 <span class="keyword">return</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">value</span><span class="special">(*</span><span class="keyword">this</span><span class="special">)[</span><span class="identifier">i</span><span class="special">];</span> 9414 <span class="special">}</span> 9415 9416 <span class="comment">// Here we define a operator = for Vec3 terminals that</span> 9417 <span class="comment">// takes a Vec3 expression.</span> 9418 <span class="keyword">template</span><span class="special"><</span> <span class="keyword">typename</span> <span class="identifier">Expr</span> <span class="special">></span> 9419 <span class="identifier">Vec3</span> <span class="special">&</span><span class="keyword">operator</span> <span class="special">=(</span><span class="identifier">Expr</span> <span class="keyword">const</span> <span class="special">&</span> <span class="identifier">expr</span><span class="special">)</span> 9420 <span class="special">{</span> 9421 <span class="keyword">typedef</span> <span class="identifier">Vec3SubscriptCtx</span> <span class="keyword">const</span> <span class="identifier">CVec3SubscriptCtx</span><span class="special">;</span> 9422 <span class="special">(*</span><span class="keyword">this</span><span class="special">)[</span><span class="number">0</span><span class="special">]</span> <span class="special">=</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">eval</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">as_expr</span><span class="special">(</span><span class="identifier">expr</span><span class="special">),</span> <span class="identifier">CVec3SubscriptCtx</span><span class="special">(</span><span class="number">0</span><span class="special">));</span> 9423 <span class="special">(*</span><span class="keyword">this</span><span class="special">)[</span><span class="number">1</span><span class="special">]</span> <span class="special">=</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">eval</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">as_expr</span><span class="special">(</span><span class="identifier">expr</span><span class="special">),</span> <span class="identifier">CVec3SubscriptCtx</span><span class="special">(</span><span class="number">1</span><span class="special">));</span> 9424 <span class="special">(*</span><span class="keyword">this</span><span class="special">)[</span><span class="number">2</span><span class="special">]</span> <span class="special">=</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">eval</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">as_expr</span><span class="special">(</span><span class="identifier">expr</span><span class="special">),</span> <span class="identifier">CVec3SubscriptCtx</span><span class="special">(</span><span class="number">2</span><span class="special">));</span> 9425 <span class="keyword">return</span> <span class="special">*</span><span class="keyword">this</span><span class="special">;</span> 9426 <span class="special">}</span> 9427 9428 <span class="comment">// This copy-assign is needed because a template is never</span> 9429 <span class="comment">// considered for copy assignment.</span> 9430 <span class="identifier">Vec3</span> <span class="special">&</span><span class="keyword">operator</span><span class="special">=(</span><span class="identifier">Vec3</span> <span class="keyword">const</span> <span class="special">&</span><span class="identifier">that</span><span class="special">)</span> 9431 <span class="special">{</span> 9432 <span class="special">(*</span><span class="keyword">this</span><span class="special">)[</span><span class="number">0</span><span class="special">]</span> <span class="special">=</span> <span class="identifier">that</span><span class="special">[</span><span class="number">0</span><span class="special">];</span> 9433 <span class="special">(*</span><span class="keyword">this</span><span class="special">)[</span><span class="number">1</span><span class="special">]</span> <span class="special">=</span> <span class="identifier">that</span><span class="special">[</span><span class="number">1</span><span class="special">];</span> 9434 <span class="special">(*</span><span class="keyword">this</span><span class="special">)[</span><span class="number">2</span><span class="special">]</span> <span class="special">=</span> <span class="identifier">that</span><span class="special">[</span><span class="number">2</span><span class="special">];</span> 9435 <span class="keyword">return</span> <span class="special">*</span><span class="keyword">this</span><span class="special">;</span> 9436 <span class="special">}</span> 9437 9438 <span class="keyword">void</span> <span class="identifier">print</span><span class="special">()</span> <span class="keyword">const</span> 9439 <span class="special">{</span> 9440 <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="char">'{'</span> <span class="special"><<</span> <span class="special">(*</span><span class="keyword">this</span><span class="special">)[</span><span class="number">0</span><span class="special">]</span> 9441 <span class="special"><<</span> <span class="string">", "</span> <span class="special"><<</span> <span class="special">(*</span><span class="keyword">this</span><span class="special">)[</span><span class="number">1</span><span class="special">]</span> 9442 <span class="special"><<</span> <span class="string">", "</span> <span class="special"><<</span> <span class="special">(*</span><span class="keyword">this</span><span class="special">)[</span><span class="number">2</span><span class="special">]</span> 9443 <span class="special"><<</span> <span class="char">'}'</span> <span class="special"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span> 9444 <span class="special">}</span> 9445<span class="special">};</span> 9446 9447<span class="comment">// The count_leaves() function uses the CountLeaves transform and</span> 9448<span class="comment">// to count the number of leaves in an expression.</span> 9449<span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">></span> 9450<span class="keyword">int</span> <span class="identifier">count_leaves</span><span class="special">(</span><span class="identifier">Expr</span> <span class="keyword">const</span> <span class="special">&</span><span class="identifier">expr</span><span class="special">)</span> 9451<span class="special">{</span> 9452 <span class="comment">// Count the number of Vec3 terminals using the</span> 9453 <span class="comment">// CountLeavesCtx evaluation context.</span> 9454 <span class="identifier">CountLeavesCtx</span> <span class="identifier">ctx</span><span class="special">;</span> 9455 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">eval</span><span class="special">(</span><span class="identifier">expr</span><span class="special">,</span> <span class="identifier">ctx</span><span class="special">);</span> 9456 9457 <span class="comment">// This is another way to count the leaves using a transform.</span> 9458 <span class="keyword">int</span> <span class="identifier">i</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</span> 9459 <span class="identifier">BOOST_ASSERT</span><span class="special">(</span> <span class="identifier">CountLeaves</span><span class="special">()(</span><span class="identifier">expr</span><span class="special">,</span> <span class="identifier">i</span><span class="special">,</span> <span class="identifier">i</span><span class="special">)</span> <span class="special">==</span> <span class="identifier">ctx</span><span class="special">.</span><span class="identifier">count</span> <span class="special">);</span> 9460 9461 <span class="keyword">return</span> <span class="identifier">ctx</span><span class="special">.</span><span class="identifier">count</span><span class="special">;</span> 9462<span class="special">}</span> 9463 9464<span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span> 9465<span class="special">{</span> 9466 <span class="identifier">Vec3</span> <span class="identifier">a</span><span class="special">,</span> <span class="identifier">b</span><span class="special">,</span> <span class="identifier">c</span><span class="special">;</span> 9467 9468 <span class="identifier">c</span> <span class="special">=</span> <span class="number">4</span><span class="special">;</span> 9469 9470 <span class="identifier">b</span><span class="special">[</span><span class="number">0</span><span class="special">]</span> <span class="special">=</span> <span class="special">-</span><span class="number">1</span><span class="special">;</span> 9471 <span class="identifier">b</span><span class="special">[</span><span class="number">1</span><span class="special">]</span> <span class="special">=</span> <span class="special">-</span><span class="number">2</span><span class="special">;</span> 9472 <span class="identifier">b</span><span class="special">[</span><span class="number">2</span><span class="special">]</span> <span class="special">=</span> <span class="special">-</span><span class="number">3</span><span class="special">;</span> 9473 9474 <span class="identifier">a</span> <span class="special">=</span> <span class="identifier">b</span> <span class="special">+</span> <span class="identifier">c</span><span class="special">;</span> 9475 9476 <span class="identifier">a</span><span class="special">.</span><span class="identifier">print</span><span class="special">();</span> 9477 9478 <span class="identifier">Vec3</span> <span class="identifier">d</span><span class="special">;</span> 9479 <span class="identifier">BOOST_PROTO_AUTO</span><span class="special">(</span><span class="identifier">expr1</span><span class="special">,</span> <span class="identifier">b</span> <span class="special">+</span> <span class="identifier">c</span><span class="special">);</span> 9480 <span class="identifier">d</span> <span class="special">=</span> <span class="identifier">expr1</span><span class="special">;</span> 9481 <span class="identifier">d</span><span class="special">.</span><span class="identifier">print</span><span class="special">();</span> 9482 9483 <span class="keyword">int</span> <span class="identifier">num</span> <span class="special">=</span> <span class="identifier">count_leaves</span><span class="special">(</span><span class="identifier">expr1</span><span class="special">);</span> 9484 <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="identifier">num</span> <span class="special"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span> 9485 9486 <span class="identifier">BOOST_PROTO_AUTO</span><span class="special">(</span><span class="identifier">expr2</span><span class="special">,</span> <span class="identifier">b</span> <span class="special">+</span> <span class="number">3</span> <span class="special">*</span> <span class="identifier">c</span><span class="special">);</span> 9487 <span class="identifier">num</span> <span class="special">=</span> <span class="identifier">count_leaves</span><span class="special">(</span><span class="identifier">expr2</span><span class="special">);</span> 9488 <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="identifier">num</span> <span class="special"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span> 9489 9490 <span class="identifier">BOOST_PROTO_AUTO</span><span class="special">(</span><span class="identifier">expr3</span><span class="special">,</span> <span class="identifier">b</span> <span class="special">+</span> <span class="identifier">c</span> <span class="special">*</span> <span class="identifier">d</span><span class="special">);</span> 9491 <span class="identifier">num</span> <span class="special">=</span> <span class="identifier">count_leaves</span><span class="special">(</span><span class="identifier">expr3</span><span class="special">);</span> 9492 <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="identifier">num</span> <span class="special"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span> 9493 9494 <span class="keyword">return</span> <span class="number">0</span><span class="special">;</span> 9495<span class="special">}</span> 9496</pre> 9497<p> 9498 </p> 9499</div> 9500<div class="section"> 9501<div class="titlepage"><div><div><h4 class="title"> 9502<a name="boost_proto.users_guide.examples.vector"></a><a class="link" href="users_guide.html#boost_proto.users_guide.examples.vector" title="Vector: Adapting a Non-Proto Terminal Type">Vector: Adapting 9503 a Non-Proto Terminal Type</a> 9504</h4></div></div></div> 9505<p> 9506 This is an example of using <code class="computeroutput"><span class="identifier">BOOST_PROTO_DEFINE_OPERATORS</span><span class="special">()</span></code> to Protofy expressions using <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special"><></span></code>, 9507 a non-Proto type. It is a port of the Vector example from <a href="http://acts.nersc.gov/formertools/pete/index.html" target="_top">PETE</a>. 9508 </p> 9509<p> 9510</p> 9511<pre class="programlisting"><span class="comment">///////////////////////////////////////////////////////////////////////////////</span> 9512<span class="comment">// Copyright 2008 Eric Niebler. Distributed under the Boost</span> 9513<span class="comment">// Software License, Version 1.0. (See accompanying file</span> 9514<span class="comment">// LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)</span> 9515<span class="comment">//</span> 9516<span class="comment">// This is an example of using BOOST_PROTO_DEFINE_OPERATORS to Protofy</span> 9517<span class="comment">// expressions using std::vector<>, a non-proto type. It is a port of the</span> 9518<span class="comment">// Vector example from PETE (http://www.codesourcery.com/pooma/download.html).</span> 9519 9520<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">vector</span><span class="special">></span> 9521<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">iostream</span><span class="special">></span> 9522<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">stdexcept</span><span class="special">></span> 9523<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">mpl</span><span class="special">/</span><span class="keyword">bool</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> 9524<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">proto</span><span class="special">/</span><span class="identifier">core</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> 9525<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">proto</span><span class="special">/</span><span class="identifier">debug</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> 9526<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">proto</span><span class="special">/</span><span class="identifier">context</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> 9527<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">utility</span><span class="special">/</span><span class="identifier">enable_if</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> 9528<span class="keyword">namespace</span> <span class="identifier">mpl</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">mpl</span><span class="special">;</span> 9529<span class="keyword">namespace</span> <span class="identifier">proto</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">proto</span><span class="special">;</span> 9530<span class="keyword">using</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_</span><span class="special">;</span> 9531 9532<span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">></span> 9533<span class="keyword">struct</span> <span class="identifier">VectorExpr</span><span class="special">;</span> 9534 9535<span class="comment">// Here is an evaluation context that indexes into a std::vector</span> 9536<span class="comment">// expression and combines the result.</span> 9537<span class="keyword">struct</span> <span class="identifier">VectorSubscriptCtx</span> 9538<span class="special">{</span> 9539 <span class="identifier">VectorSubscriptCtx</span><span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="identifier">i</span><span class="special">)</span> 9540 <span class="special">:</span> <span class="identifier">i_</span><span class="special">(</span><span class="identifier">i</span><span class="special">)</span> 9541 <span class="special">{}</span> 9542 9543 <span class="comment">// Unless this is a vector terminal, use the</span> 9544 <span class="comment">// default evaluation context</span> 9545 <span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">EnableIf</span> <span class="special">=</span> <span class="keyword">void</span><span class="special">></span> 9546 <span class="keyword">struct</span> <span class="identifier">eval</span> 9547 <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">default_eval</span><span class="special"><</span><span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">VectorSubscriptCtx</span> <span class="keyword">const</span><span class="special">></span> 9548 <span class="special">{};</span> 9549 9550 <span class="comment">// Index vector terminals with our subscript.</span> 9551 <span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">></span> 9552 <span class="keyword">struct</span> <span class="identifier">eval</span><span class="special"><</span> 9553 <span class="identifier">Expr</span> 9554 <span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">enable_if</span><span class="special"><</span> 9555 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">matches</span><span class="special"><</span><span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special"><</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special"><</span><span class="identifier">_</span><span class="special">,</span> <span class="identifier">_</span><span class="special">></span> <span class="special">></span> <span class="special">></span> 9556 <span class="special">>::</span><span class="identifier">type</span> 9557 <span class="special">></span> 9558 <span class="special">{</span> 9559 <span class="keyword">typedef</span> <span class="keyword">typename</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">value</span><span class="special"><</span><span class="identifier">Expr</span><span class="special">>::</span><span class="identifier">type</span><span class="special">::</span><span class="identifier">value_type</span> <span class="identifier">result_type</span><span class="special">;</span> 9560 9561 <span class="identifier">result_type</span> <span class="keyword">operator</span> <span class="special">()(</span><span class="identifier">Expr</span> <span class="special">&</span><span class="identifier">expr</span><span class="special">,</span> <span class="identifier">VectorSubscriptCtx</span> <span class="keyword">const</span> <span class="special">&</span><span class="identifier">ctx</span><span class="special">)</span> <span class="keyword">const</span> 9562 <span class="special">{</span> 9563 <span class="keyword">return</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">value</span><span class="special">(</span><span class="identifier">expr</span><span class="special">)[</span><span class="identifier">ctx</span><span class="special">.</span><span class="identifier">i_</span><span class="special">];</span> 9564 <span class="special">}</span> 9565 <span class="special">};</span> 9566 9567 <span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="identifier">i_</span><span class="special">;</span> 9568<span class="special">};</span> 9569 9570<span class="comment">// Here is an evaluation context that verifies that all the</span> 9571<span class="comment">// vectors in an expression have the same size.</span> 9572<span class="keyword">struct</span> <span class="identifier">VectorSizeCtx</span> 9573<span class="special">{</span> 9574 <span class="identifier">VectorSizeCtx</span><span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="identifier">size</span><span class="special">)</span> 9575 <span class="special">:</span> <span class="identifier">size_</span><span class="special">(</span><span class="identifier">size</span><span class="special">)</span> 9576 <span class="special">{}</span> 9577 9578 <span class="comment">// Unless this is a vector terminal, use the</span> 9579 <span class="comment">// null evaluation context</span> 9580 <span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">EnableIf</span> <span class="special">=</span> <span class="keyword">void</span><span class="special">></span> 9581 <span class="keyword">struct</span> <span class="identifier">eval</span> 9582 <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">null_eval</span><span class="special"><</span><span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">VectorSizeCtx</span> <span class="keyword">const</span><span class="special">></span> 9583 <span class="special">{};</span> 9584 9585 <span class="comment">// Index array terminals with our subscript. Everything</span> 9586 <span class="comment">// else will be handled by the default evaluation context.</span> 9587 <span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">></span> 9588 <span class="keyword">struct</span> <span class="identifier">eval</span><span class="special"><</span> 9589 <span class="identifier">Expr</span> 9590 <span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">enable_if</span><span class="special"><</span> 9591 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">matches</span><span class="special"><</span><span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special"><</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special"><</span><span class="identifier">_</span><span class="special">,</span> <span class="identifier">_</span><span class="special">></span> <span class="special">></span> <span class="special">></span> 9592 <span class="special">>::</span><span class="identifier">type</span> 9593 <span class="special">></span> 9594 <span class="special">{</span> 9595 <span class="keyword">typedef</span> <span class="keyword">void</span> <span class="identifier">result_type</span><span class="special">;</span> 9596 9597 <span class="identifier">result_type</span> <span class="keyword">operator</span> <span class="special">()(</span><span class="identifier">Expr</span> <span class="special">&</span><span class="identifier">expr</span><span class="special">,</span> <span class="identifier">VectorSizeCtx</span> <span class="keyword">const</span> <span class="special">&</span><span class="identifier">ctx</span><span class="special">)</span> <span class="keyword">const</span> 9598 <span class="special">{</span> 9599 <span class="keyword">if</span><span class="special">(</span><span class="identifier">ctx</span><span class="special">.</span><span class="identifier">size_</span> <span class="special">!=</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">value</span><span class="special">(</span><span class="identifier">expr</span><span class="special">).</span><span class="identifier">size</span><span class="special">())</span> 9600 <span class="special">{</span> 9601 <span class="keyword">throw</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">runtime_error</span><span class="special">(</span><span class="string">"LHS and RHS are not compatible"</span><span class="special">);</span> 9602 <span class="special">}</span> 9603 <span class="special">}</span> 9604 <span class="special">};</span> 9605 9606 <span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="identifier">size_</span><span class="special">;</span> 9607<span class="special">};</span> 9608 9609<span class="comment">// A grammar which matches all the assignment operators,</span> 9610<span class="comment">// so we can easily disable them.</span> 9611<span class="keyword">struct</span> <span class="identifier">AssignOps</span> 9612 <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">switch_</span><span class="special"><</span><span class="keyword">struct</span> <span class="identifier">AssignOpsCases</span><span class="special">></span> 9613<span class="special">{};</span> 9614 9615<span class="comment">// Here are the cases used by the switch_ above.</span> 9616<span class="keyword">struct</span> <span class="identifier">AssignOpsCases</span> 9617<span class="special">{</span> 9618 <span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">Tag</span><span class="special">,</span> <span class="keyword">int</span> <span class="identifier">D</span> <span class="special">=</span> <span class="number">0</span><span class="special">></span> <span class="keyword">struct</span> <span class="identifier">case_</span> <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">not_</span><span class="special"><</span><span class="identifier">_</span><span class="special">></span> <span class="special">{};</span> 9619 9620 <span class="keyword">template</span><span class="special"><</span><span class="keyword">int</span> <span class="identifier">D</span><span class="special">></span> <span class="keyword">struct</span> <span class="identifier">case_</span><span class="special"><</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">plus_assign</span><span class="special">,</span> <span class="identifier">D</span> <span class="special">></span> <span class="special">:</span> <span class="identifier">_</span> <span class="special">{};</span> 9621 <span class="keyword">template</span><span class="special"><</span><span class="keyword">int</span> <span class="identifier">D</span><span class="special">></span> <span class="keyword">struct</span> <span class="identifier">case_</span><span class="special"><</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">minus_assign</span><span class="special">,</span> <span class="identifier">D</span> <span class="special">></span> <span class="special">:</span> <span class="identifier">_</span> <span class="special">{};</span> 9622 <span class="keyword">template</span><span class="special"><</span><span class="keyword">int</span> <span class="identifier">D</span><span class="special">></span> <span class="keyword">struct</span> <span class="identifier">case_</span><span class="special"><</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">multiplies_assign</span><span class="special">,</span> <span class="identifier">D</span> <span class="special">></span> <span class="special">:</span> <span class="identifier">_</span> <span class="special">{};</span> 9623 <span class="keyword">template</span><span class="special"><</span><span class="keyword">int</span> <span class="identifier">D</span><span class="special">></span> <span class="keyword">struct</span> <span class="identifier">case_</span><span class="special"><</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">divides_assign</span><span class="special">,</span> <span class="identifier">D</span> <span class="special">></span> <span class="special">:</span> <span class="identifier">_</span> <span class="special">{};</span> 9624 <span class="keyword">template</span><span class="special"><</span><span class="keyword">int</span> <span class="identifier">D</span><span class="special">></span> <span class="keyword">struct</span> <span class="identifier">case_</span><span class="special"><</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">modulus_assign</span><span class="special">,</span> <span class="identifier">D</span> <span class="special">></span> <span class="special">:</span> <span class="identifier">_</span> <span class="special">{};</span> 9625 <span class="keyword">template</span><span class="special"><</span><span class="keyword">int</span> <span class="identifier">D</span><span class="special">></span> <span class="keyword">struct</span> <span class="identifier">case_</span><span class="special"><</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">shift_left_assign</span><span class="special">,</span> <span class="identifier">D</span> <span class="special">></span> <span class="special">:</span> <span class="identifier">_</span> <span class="special">{};</span> 9626 <span class="keyword">template</span><span class="special"><</span><span class="keyword">int</span> <span class="identifier">D</span><span class="special">></span> <span class="keyword">struct</span> <span class="identifier">case_</span><span class="special"><</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">shift_right_assign</span><span class="special">,</span> <span class="identifier">D</span> <span class="special">></span> <span class="special">:</span> <span class="identifier">_</span> <span class="special">{};</span> 9627 <span class="keyword">template</span><span class="special"><</span><span class="keyword">int</span> <span class="identifier">D</span><span class="special">></span> <span class="keyword">struct</span> <span class="identifier">case_</span><span class="special"><</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">bitwise_and_assign</span><span class="special">,</span> <span class="identifier">D</span> <span class="special">></span> <span class="special">:</span> <span class="identifier">_</span> <span class="special">{};</span> 9628 <span class="keyword">template</span><span class="special"><</span><span class="keyword">int</span> <span class="identifier">D</span><span class="special">></span> <span class="keyword">struct</span> <span class="identifier">case_</span><span class="special"><</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">bitwise_or_assign</span><span class="special">,</span> <span class="identifier">D</span> <span class="special">></span> <span class="special">:</span> <span class="identifier">_</span> <span class="special">{};</span> 9629 <span class="keyword">template</span><span class="special"><</span><span class="keyword">int</span> <span class="identifier">D</span><span class="special">></span> <span class="keyword">struct</span> <span class="identifier">case_</span><span class="special"><</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">bitwise_xor_assign</span><span class="special">,</span> <span class="identifier">D</span> <span class="special">></span> <span class="special">:</span> <span class="identifier">_</span> <span class="special">{};</span> 9630<span class="special">};</span> 9631 9632<span class="comment">// A vector grammar is a terminal or some op that is not an</span> 9633<span class="comment">// assignment op. (Assignment will be handled specially.)</span> 9634<span class="keyword">struct</span> <span class="identifier">VectorGrammar</span> 9635 <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">or_</span><span class="special"><</span> 9636 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special"><</span><span class="identifier">_</span><span class="special">></span> 9637 <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">and_</span><span class="special"><</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">nary_expr</span><span class="special"><</span><span class="identifier">_</span><span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">vararg</span><span class="special"><</span><span class="identifier">VectorGrammar</span><span class="special">></span> <span class="special">>,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">not_</span><span class="special"><</span><span class="identifier">AssignOps</span><span class="special">></span> <span class="special">></span> 9638 <span class="special">></span> 9639<span class="special">{};</span> 9640 9641<span class="comment">// Expressions in the vector domain will be wrapped in VectorExpr<></span> 9642<span class="comment">// and must conform to the VectorGrammar</span> 9643<span class="keyword">struct</span> <span class="identifier">VectorDomain</span> 9644 <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">domain</span><span class="special"><</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">generator</span><span class="special"><</span><span class="identifier">VectorExpr</span><span class="special">>,</span> <span class="identifier">VectorGrammar</span><span class="special">></span> 9645<span class="special">{};</span> 9646 9647<span class="comment">// Here is VectorExpr, which extends a proto expr type by</span> 9648<span class="comment">// giving it an operator [] which uses the VectorSubscriptCtx</span> 9649<span class="comment">// to evaluate an expression with a given index.</span> 9650<span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">></span> 9651<span class="keyword">struct</span> <span class="identifier">VectorExpr</span> 9652 <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">extends</span><span class="special"><</span><span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">VectorExpr</span><span class="special"><</span><span class="identifier">Expr</span><span class="special">>,</span> <span class="identifier">VectorDomain</span><span class="special">></span> 9653<span class="special">{</span> 9654 <span class="keyword">explicit</span> <span class="identifier">VectorExpr</span><span class="special">(</span><span class="identifier">Expr</span> <span class="keyword">const</span> <span class="special">&</span><span class="identifier">expr</span><span class="special">)</span> 9655 <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">extends</span><span class="special"><</span><span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">VectorExpr</span><span class="special"><</span><span class="identifier">Expr</span><span class="special">>,</span> <span class="identifier">VectorDomain</span><span class="special">>(</span><span class="identifier">expr</span><span class="special">)</span> 9656 <span class="special">{}</span> 9657 9658 <span class="comment">// Use the VectorSubscriptCtx to implement subscripting</span> 9659 <span class="comment">// of a Vector expression tree.</span> 9660 <span class="keyword">typename</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">eval</span><span class="special"><</span><span class="identifier">Expr</span> <span class="keyword">const</span><span class="special">,</span> <span class="identifier">VectorSubscriptCtx</span> <span class="keyword">const</span><span class="special">>::</span><span class="identifier">type</span> 9661 <span class="keyword">operator</span> <span class="special">[](</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="identifier">i</span> <span class="special">)</span> <span class="keyword">const</span> 9662 <span class="special">{</span> 9663 <span class="identifier">VectorSubscriptCtx</span> <span class="keyword">const</span> <span class="identifier">ctx</span><span class="special">(</span><span class="identifier">i</span><span class="special">);</span> 9664 <span class="keyword">return</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">eval</span><span class="special">(*</span><span class="keyword">this</span><span class="special">,</span> <span class="identifier">ctx</span><span class="special">);</span> 9665 <span class="special">}</span> 9666<span class="special">};</span> 9667 9668<span class="comment">// Define a trait type for detecting vector terminals, to</span> 9669<span class="comment">// be used by the BOOST_PROTO_DEFINE_OPERATORS macro below.</span> 9670<span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">></span> 9671<span class="keyword">struct</span> <span class="identifier">IsVector</span> 9672 <span class="special">:</span> <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">false_</span> 9673<span class="special">{};</span> 9674 9675<span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">A</span><span class="special">></span> 9676<span class="keyword">struct</span> <span class="identifier">IsVector</span><span class="special"><</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special"><</span><span class="identifier">T</span><span class="special">,</span> <span class="identifier">A</span><span class="special">></span> <span class="special">></span> 9677 <span class="special">:</span> <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">true_</span> 9678<span class="special">{};</span> 9679 9680<span class="keyword">namespace</span> <span class="identifier">VectorOps</span> 9681<span class="special">{</span> 9682 <span class="comment">// This defines all the overloads to make expressions involving</span> 9683 <span class="comment">// std::vector to build expression templates.</span> 9684 <span class="identifier">BOOST_PROTO_DEFINE_OPERATORS</span><span class="special">(</span><span class="identifier">IsVector</span><span class="special">,</span> <span class="identifier">VectorDomain</span><span class="special">)</span> 9685 9686 <span class="keyword">typedef</span> <span class="identifier">VectorSubscriptCtx</span> <span class="keyword">const</span> <span class="identifier">CVectorSubscriptCtx</span><span class="special">;</span> 9687 9688 <span class="comment">// Assign to a vector from some expression.</span> 9689 <span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">A</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">></span> 9690 <span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special"><</span><span class="identifier">T</span><span class="special">,</span> <span class="identifier">A</span><span class="special">></span> <span class="special">&</span><span class="identifier">assign</span><span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special"><</span><span class="identifier">T</span><span class="special">,</span> <span class="identifier">A</span><span class="special">></span> <span class="special">&</span><span class="identifier">arr</span><span class="special">,</span> <span class="identifier">Expr</span> <span class="keyword">const</span> <span class="special">&</span><span class="identifier">expr</span><span class="special">)</span> 9691 <span class="special">{</span> 9692 <span class="identifier">VectorSizeCtx</span> <span class="keyword">const</span> <span class="identifier">size</span><span class="special">(</span><span class="identifier">arr</span><span class="special">.</span><span class="identifier">size</span><span class="special">());</span> 9693 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">eval</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">as_expr</span><span class="special"><</span><span class="identifier">VectorDomain</span><span class="special">>(</span><span class="identifier">expr</span><span class="special">),</span> <span class="identifier">size</span><span class="special">);</span> <span class="comment">// will throw if the sizes don't match</span> 9694 <span class="keyword">for</span><span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="identifier">i</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</span> <span class="identifier">i</span> <span class="special"><</span> <span class="identifier">arr</span><span class="special">.</span><span class="identifier">size</span><span class="special">();</span> <span class="special">++</span><span class="identifier">i</span><span class="special">)</span> 9695 <span class="special">{</span> 9696 <span class="identifier">arr</span><span class="special">[</span><span class="identifier">i</span><span class="special">]</span> <span class="special">=</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">as_expr</span><span class="special"><</span><span class="identifier">VectorDomain</span><span class="special">>(</span><span class="identifier">expr</span><span class="special">)[</span><span class="identifier">i</span><span class="special">];</span> 9697 <span class="special">}</span> 9698 <span class="keyword">return</span> <span class="identifier">arr</span><span class="special">;</span> 9699 <span class="special">}</span> 9700 9701 <span class="comment">// Add-assign to a vector from some expression.</span> 9702 <span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">A</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">></span> 9703 <span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special"><</span><span class="identifier">T</span><span class="special">,</span> <span class="identifier">A</span><span class="special">></span> <span class="special">&</span><span class="keyword">operator</span> <span class="special">+=(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special"><</span><span class="identifier">T</span><span class="special">,</span> <span class="identifier">A</span><span class="special">></span> <span class="special">&</span><span class="identifier">arr</span><span class="special">,</span> <span class="identifier">Expr</span> <span class="keyword">const</span> <span class="special">&</span><span class="identifier">expr</span><span class="special">)</span> 9704 <span class="special">{</span> 9705 <span class="identifier">VectorSizeCtx</span> <span class="keyword">const</span> <span class="identifier">size</span><span class="special">(</span><span class="identifier">arr</span><span class="special">.</span><span class="identifier">size</span><span class="special">());</span> 9706 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">eval</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">as_expr</span><span class="special"><</span><span class="identifier">VectorDomain</span><span class="special">>(</span><span class="identifier">expr</span><span class="special">),</span> <span class="identifier">size</span><span class="special">);</span> <span class="comment">// will throw if the sizes don't match</span> 9707 <span class="keyword">for</span><span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="identifier">i</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</span> <span class="identifier">i</span> <span class="special"><</span> <span class="identifier">arr</span><span class="special">.</span><span class="identifier">size</span><span class="special">();</span> <span class="special">++</span><span class="identifier">i</span><span class="special">)</span> 9708 <span class="special">{</span> 9709 <span class="identifier">arr</span><span class="special">[</span><span class="identifier">i</span><span class="special">]</span> <span class="special">+=</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">as_expr</span><span class="special"><</span><span class="identifier">VectorDomain</span><span class="special">>(</span><span class="identifier">expr</span><span class="special">)[</span><span class="identifier">i</span><span class="special">];</span> 9710 <span class="special">}</span> 9711 <span class="keyword">return</span> <span class="identifier">arr</span><span class="special">;</span> 9712 <span class="special">}</span> 9713<span class="special">}</span> 9714 9715<span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span> 9716<span class="special">{</span> 9717 <span class="keyword">using</span> <span class="keyword">namespace</span> <span class="identifier">VectorOps</span><span class="special">;</span> 9718 9719 <span class="keyword">int</span> <span class="identifier">i</span><span class="special">;</span> 9720 <span class="keyword">const</span> <span class="keyword">int</span> <span class="identifier">n</span> <span class="special">=</span> <span class="number">10</span><span class="special">;</span> 9721 <span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> <span class="identifier">a</span><span class="special">,</span><span class="identifier">b</span><span class="special">,</span><span class="identifier">c</span><span class="special">,</span><span class="identifier">d</span><span class="special">;</span> 9722 <span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special"><</span><span class="keyword">double</span><span class="special">></span> <span class="identifier">e</span><span class="special">(</span><span class="identifier">n</span><span class="special">);</span> 9723 9724 <span class="keyword">for</span> <span class="special">(</span><span class="identifier">i</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</span> <span class="identifier">i</span> <span class="special"><</span> <span class="identifier">n</span><span class="special">;</span> <span class="special">++</span><span class="identifier">i</span><span class="special">)</span> 9725 <span class="special">{</span> 9726 <span class="identifier">a</span><span class="special">.</span><span class="identifier">push_back</span><span class="special">(</span><span class="identifier">i</span><span class="special">);</span> 9727 <span class="identifier">b</span><span class="special">.</span><span class="identifier">push_back</span><span class="special">(</span><span class="number">2</span><span class="special">*</span><span class="identifier">i</span><span class="special">);</span> 9728 <span class="identifier">c</span><span class="special">.</span><span class="identifier">push_back</span><span class="special">(</span><span class="number">3</span><span class="special">*</span><span class="identifier">i</span><span class="special">);</span> 9729 <span class="identifier">d</span><span class="special">.</span><span class="identifier">push_back</span><span class="special">(</span><span class="identifier">i</span><span class="special">);</span> 9730 <span class="special">}</span> 9731 9732 <span class="identifier">VectorOps</span><span class="special">::</span><span class="identifier">assign</span><span class="special">(</span><span class="identifier">b</span><span class="special">,</span> <span class="number">2</span><span class="special">);</span> 9733 <span class="identifier">VectorOps</span><span class="special">::</span><span class="identifier">assign</span><span class="special">(</span><span class="identifier">d</span><span class="special">,</span> <span class="identifier">a</span> <span class="special">+</span> <span class="identifier">b</span> <span class="special">*</span> <span class="identifier">c</span><span class="special">);</span> 9734 <span class="identifier">a</span> <span class="special">+=</span> <span class="identifier">if_else</span><span class="special">(</span><span class="identifier">d</span> <span class="special"><</span> <span class="number">30</span><span class="special">,</span> <span class="identifier">b</span><span class="special">,</span> <span class="identifier">c</span><span class="special">);</span> 9735 9736 <span class="identifier">VectorOps</span><span class="special">::</span><span class="identifier">assign</span><span class="special">(</span><span class="identifier">e</span><span class="special">,</span> <span class="identifier">c</span><span class="special">);</span> 9737 <span class="identifier">e</span> <span class="special">+=</span> <span class="identifier">e</span> <span class="special">-</span> <span class="number">4</span> <span class="special">/</span> <span class="special">(</span><span class="identifier">c</span> <span class="special">+</span> <span class="number">1</span><span class="special">);</span> 9738 9739 <span class="keyword">for</span> <span class="special">(</span><span class="identifier">i</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</span> <span class="identifier">i</span> <span class="special"><</span> <span class="identifier">n</span><span class="special">;</span> <span class="special">++</span><span class="identifier">i</span><span class="special">)</span> 9740 <span class="special">{</span> 9741 <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> 9742 <span class="special"><<</span> <span class="string">" a("</span> <span class="special"><<</span> <span class="identifier">i</span> <span class="special"><<</span> <span class="string">") = "</span> <span class="special"><<</span> <span class="identifier">a</span><span class="special">[</span><span class="identifier">i</span><span class="special">]</span> 9743 <span class="special"><<</span> <span class="string">" b("</span> <span class="special"><<</span> <span class="identifier">i</span> <span class="special"><<</span> <span class="string">") = "</span> <span class="special"><<</span> <span class="identifier">b</span><span class="special">[</span><span class="identifier">i</span><span class="special">]</span> 9744 <span class="special"><<</span> <span class="string">" c("</span> <span class="special"><<</span> <span class="identifier">i</span> <span class="special"><<</span> <span class="string">") = "</span> <span class="special"><<</span> <span class="identifier">c</span><span class="special">[</span><span class="identifier">i</span><span class="special">]</span> 9745 <span class="special"><<</span> <span class="string">" d("</span> <span class="special"><<</span> <span class="identifier">i</span> <span class="special"><<</span> <span class="string">") = "</span> <span class="special"><<</span> <span class="identifier">d</span><span class="special">[</span><span class="identifier">i</span><span class="special">]</span> 9746 <span class="special"><<</span> <span class="string">" e("</span> <span class="special"><<</span> <span class="identifier">i</span> <span class="special"><<</span> <span class="string">") = "</span> <span class="special"><<</span> <span class="identifier">e</span><span class="special">[</span><span class="identifier">i</span><span class="special">]</span> 9747 <span class="special"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span> 9748 <span class="special">}</span> 9749<span class="special">}</span> 9750</pre> 9751<p> 9752 </p> 9753</div> 9754<div class="section"> 9755<div class="titlepage"><div><div><h4 class="title"> 9756<a name="boost_proto.users_guide.examples.mixed"></a><a class="link" href="users_guide.html#boost_proto.users_guide.examples.mixed" title="Mixed: Adapting Several Non-Proto Terminal Types">Mixed: Adapting 9757 Several Non-Proto Terminal Types</a> 9758</h4></div></div></div> 9759<p> 9760 This is an example of using <code class="computeroutput"><span class="identifier">BOOST_PROTO_DEFINE_OPERATORS</span><span class="special">()</span></code> to Protofy expressions using <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special"><></span></code> 9761 and <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">list</span><span class="special"><></span></code>, 9762 non-Proto types. It is a port of the Mixed example from <a href="http://acts.nersc.gov/formertools/pete/index.html" target="_top">PETE</a>. 9763 </p> 9764<p> 9765</p> 9766<pre class="programlisting"><span class="comment">///////////////////////////////////////////////////////////////////////////////</span> 9767<span class="comment">// Copyright 2008 Eric Niebler. Distributed under the Boost</span> 9768<span class="comment">// Software License, Version 1.0. (See accompanying file</span> 9769<span class="comment">// LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)</span> 9770<span class="comment">//</span> 9771<span class="comment">// This is an example of using BOOST_PROTO_DEFINE_OPERATORS to Protofy</span> 9772<span class="comment">// expressions using std::vector<> and std::list, non-proto types. It is a port</span> 9773<span class="comment">// of the Mixed example from PETE.</span> 9774<span class="comment">// (http://www.codesourcery.com/pooma/download.html).</span> 9775 9776<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">list</span><span class="special">></span> 9777<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">cmath</span><span class="special">></span> 9778<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">vector</span><span class="special">></span> 9779<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">complex</span><span class="special">></span> 9780<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">iostream</span><span class="special">></span> 9781<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">stdexcept</span><span class="special">></span> 9782<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">proto</span><span class="special">/</span><span class="identifier">core</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> 9783<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">proto</span><span class="special">/</span><span class="identifier">debug</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> 9784<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">proto</span><span class="special">/</span><span class="identifier">context</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> 9785<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">proto</span><span class="special">/</span><span class="identifier">transform</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> 9786<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">utility</span><span class="special">/</span><span class="identifier">enable_if</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> 9787<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">typeof</span><span class="special">/</span><span class="identifier">std</span><span class="special">/</span><span class="identifier">list</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> 9788<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">typeof</span><span class="special">/</span><span class="identifier">std</span><span class="special">/</span><span class="identifier">vector</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> 9789<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">typeof</span><span class="special">/</span><span class="identifier">std</span><span class="special">/</span><span class="identifier">complex</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> 9790<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">type_traits</span><span class="special">/</span><span class="identifier">remove_reference</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> 9791<span class="keyword">namespace</span> <span class="identifier">proto</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">proto</span><span class="special">;</span> 9792<span class="keyword">namespace</span> <span class="identifier">mpl</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">mpl</span><span class="special">;</span> 9793<span class="keyword">using</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_</span><span class="special">;</span> 9794 9795<span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">></span> 9796<span class="keyword">struct</span> <span class="identifier">MixedExpr</span><span class="special">;</span> 9797 9798<span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">Iter</span><span class="special">></span> 9799<span class="keyword">struct</span> <span class="identifier">iterator_wrapper</span> 9800<span class="special">{</span> 9801 <span class="keyword">typedef</span> <span class="identifier">Iter</span> <span class="identifier">iterator</span><span class="special">;</span> 9802 9803 <span class="keyword">explicit</span> <span class="identifier">iterator_wrapper</span><span class="special">(</span><span class="identifier">Iter</span> <span class="identifier">iter</span><span class="special">)</span> 9804 <span class="special">:</span> <span class="identifier">it</span><span class="special">(</span><span class="identifier">iter</span><span class="special">)</span> 9805 <span class="special">{}</span> 9806 9807 <span class="keyword">mutable</span> <span class="identifier">Iter</span> <span class="identifier">it</span><span class="special">;</span> 9808<span class="special">};</span> 9809 9810<span class="keyword">struct</span> <span class="identifier">begin</span> <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">callable</span> 9811<span class="special">{</span> 9812 <span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">Sig</span><span class="special">></span> 9813 <span class="keyword">struct</span> <span class="identifier">result</span><span class="special">;</span> 9814 9815 <span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">This</span><span class="special">,</span> <span class="keyword">class</span> <span class="identifier">Cont</span><span class="special">></span> 9816 <span class="keyword">struct</span> <span class="identifier">result</span><span class="special"><</span><span class="identifier">This</span><span class="special">(</span><span class="identifier">Cont</span><span class="special">)></span> 9817 <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">as_expr</span><span class="special"><</span> 9818 <span class="identifier">iterator_wrapper</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">remove_reference</span><span class="special"><</span><span class="identifier">Cont</span><span class="special">>::</span><span class="identifier">type</span><span class="special">::</span><span class="identifier">const_iterator</span><span class="special">></span> 9819 <span class="special">></span> 9820 <span class="special">{};</span> 9821 9822 <span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">Cont</span><span class="special">></span> 9823 <span class="keyword">typename</span> <span class="identifier">result</span><span class="special"><</span><span class="identifier">begin</span><span class="special">(</span><span class="identifier">Cont</span> <span class="keyword">const</span> <span class="special">&)>::</span><span class="identifier">type</span> 9824 <span class="keyword">operator</span> <span class="special">()(</span><span class="identifier">Cont</span> <span class="keyword">const</span> <span class="special">&</span><span class="identifier">cont</span><span class="special">)</span> <span class="keyword">const</span> 9825 <span class="special">{</span> 9826 <span class="identifier">iterator_wrapper</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">Cont</span><span class="special">::</span><span class="identifier">const_iterator</span><span class="special">></span> <span class="identifier">it</span><span class="special">(</span><span class="identifier">cont</span><span class="special">.</span><span class="identifier">begin</span><span class="special">());</span> 9827 <span class="keyword">return</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">as_expr</span><span class="special">(</span><span class="identifier">it</span><span class="special">);</span> 9828 <span class="special">}</span> 9829<span class="special">};</span> 9830 9831<span class="comment">// Here is a grammar that replaces vector and list terminals with their</span> 9832<span class="comment">// begin iterators</span> 9833<span class="keyword">struct</span> <span class="identifier">Begin</span> 9834 <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">or_</span><span class="special"><</span> 9835 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special"><</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special"><</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special"><</span><span class="identifier">_</span><span class="special">,</span> <span class="identifier">_</span><span class="special">></span> <span class="special">>,</span> <span class="identifier">begin</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_value</span><span class="special">)</span> <span class="special">></span> 9836 <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special"><</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special"><</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">list</span><span class="special"><</span><span class="identifier">_</span><span class="special">,</span> <span class="identifier">_</span><span class="special">></span> <span class="special">>,</span> <span class="identifier">begin</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_value</span><span class="special">)</span> <span class="special">></span> 9837 <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special"><</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special"><</span><span class="identifier">_</span><span class="special">></span> <span class="special">></span> 9838 <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special"><</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">nary_expr</span><span class="special"><</span><span class="identifier">_</span><span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">vararg</span><span class="special"><</span><span class="identifier">Begin</span><span class="special">></span> <span class="special">></span> <span class="special">></span> 9839 <span class="special">></span> 9840<span class="special">{};</span> 9841 9842<span class="comment">// Here is an evaluation context that dereferences iterator</span> 9843<span class="comment">// terminals.</span> 9844<span class="keyword">struct</span> <span class="identifier">DereferenceCtx</span> 9845<span class="special">{</span> 9846 <span class="comment">// Unless this is an iterator terminal, use the</span> 9847 <span class="comment">// default evaluation context</span> 9848 <span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">EnableIf</span> <span class="special">=</span> <span class="keyword">void</span><span class="special">></span> 9849 <span class="keyword">struct</span> <span class="identifier">eval</span> 9850 <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">default_eval</span><span class="special"><</span><span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">DereferenceCtx</span> <span class="keyword">const</span><span class="special">></span> 9851 <span class="special">{};</span> 9852 9853 <span class="comment">// Dereference iterator terminals.</span> 9854 <span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">></span> 9855 <span class="keyword">struct</span> <span class="identifier">eval</span><span class="special"><</span> 9856 <span class="identifier">Expr</span> 9857 <span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">enable_if</span><span class="special"><</span> 9858 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">matches</span><span class="special"><</span><span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special"><</span><span class="identifier">iterator_wrapper</span><span class="special"><</span><span class="identifier">_</span><span class="special">></span> <span class="special">></span> <span class="special">></span> 9859 <span class="special">>::</span><span class="identifier">type</span> 9860 <span class="special">></span> 9861 <span class="special">{</span> 9862 <span class="keyword">typedef</span> <span class="keyword">typename</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">value</span><span class="special"><</span><span class="identifier">Expr</span><span class="special">>::</span><span class="identifier">type</span> <span class="identifier">IteratorWrapper</span><span class="special">;</span> 9863 <span class="keyword">typedef</span> <span class="keyword">typename</span> <span class="identifier">IteratorWrapper</span><span class="special">::</span><span class="identifier">iterator</span> <span class="identifier">iterator</span><span class="special">;</span> 9864 <span class="keyword">typedef</span> <span class="keyword">typename</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">iterator_traits</span><span class="special"><</span><span class="identifier">iterator</span><span class="special">>::</span><span class="identifier">reference</span> <span class="identifier">result_type</span><span class="special">;</span> 9865 9866 <span class="identifier">result_type</span> <span class="keyword">operator</span> <span class="special">()(</span><span class="identifier">Expr</span> <span class="special">&</span><span class="identifier">expr</span><span class="special">,</span> <span class="identifier">DereferenceCtx</span> <span class="keyword">const</span> <span class="special">&)</span> <span class="keyword">const</span> 9867 <span class="special">{</span> 9868 <span class="keyword">return</span> <span class="special">*</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">value</span><span class="special">(</span><span class="identifier">expr</span><span class="special">).</span><span class="identifier">it</span><span class="special">;</span> 9869 <span class="special">}</span> 9870 <span class="special">};</span> 9871<span class="special">};</span> 9872 9873<span class="comment">// Here is an evaluation context that increments iterator</span> 9874<span class="comment">// terminals.</span> 9875<span class="keyword">struct</span> <span class="identifier">IncrementCtx</span> 9876<span class="special">{</span> 9877 <span class="comment">// Unless this is an iterator terminal, use the</span> 9878 <span class="comment">// default evaluation context</span> 9879 <span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">EnableIf</span> <span class="special">=</span> <span class="keyword">void</span><span class="special">></span> 9880 <span class="keyword">struct</span> <span class="identifier">eval</span> 9881 <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">null_eval</span><span class="special"><</span><span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">IncrementCtx</span> <span class="keyword">const</span><span class="special">></span> 9882 <span class="special">{};</span> 9883 9884 <span class="comment">// advance iterator terminals.</span> 9885 <span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">></span> 9886 <span class="keyword">struct</span> <span class="identifier">eval</span><span class="special"><</span> 9887 <span class="identifier">Expr</span> 9888 <span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">enable_if</span><span class="special"><</span> 9889 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">matches</span><span class="special"><</span><span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special"><</span><span class="identifier">iterator_wrapper</span><span class="special"><</span><span class="identifier">_</span><span class="special">></span> <span class="special">></span> <span class="special">></span> 9890 <span class="special">>::</span><span class="identifier">type</span> 9891 <span class="special">></span> 9892 <span class="special">{</span> 9893 <span class="keyword">typedef</span> <span class="keyword">void</span> <span class="identifier">result_type</span><span class="special">;</span> 9894 9895 <span class="identifier">result_type</span> <span class="keyword">operator</span> <span class="special">()(</span><span class="identifier">Expr</span> <span class="special">&</span><span class="identifier">expr</span><span class="special">,</span> <span class="identifier">IncrementCtx</span> <span class="keyword">const</span> <span class="special">&)</span> <span class="keyword">const</span> 9896 <span class="special">{</span> 9897 <span class="special">++</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">value</span><span class="special">(</span><span class="identifier">expr</span><span class="special">).</span><span class="identifier">it</span><span class="special">;</span> 9898 <span class="special">}</span> 9899 <span class="special">};</span> 9900<span class="special">};</span> 9901 9902<span class="comment">// A grammar which matches all the assignment operators,</span> 9903<span class="comment">// so we can easily disable them.</span> 9904<span class="keyword">struct</span> <span class="identifier">AssignOps</span> 9905 <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">switch_</span><span class="special"><</span><span class="keyword">struct</span> <span class="identifier">AssignOpsCases</span><span class="special">></span> 9906<span class="special">{};</span> 9907 9908<span class="comment">// Here are the cases used by the switch_ above.</span> 9909<span class="keyword">struct</span> <span class="identifier">AssignOpsCases</span> 9910<span class="special">{</span> 9911 <span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">Tag</span><span class="special">,</span> <span class="keyword">int</span> <span class="identifier">D</span> <span class="special">=</span> <span class="number">0</span><span class="special">></span> <span class="keyword">struct</span> <span class="identifier">case_</span> <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">not_</span><span class="special"><</span><span class="identifier">_</span><span class="special">></span> <span class="special">{};</span> 9912 9913 <span class="keyword">template</span><span class="special"><</span><span class="keyword">int</span> <span class="identifier">D</span><span class="special">></span> <span class="keyword">struct</span> <span class="identifier">case_</span><span class="special"><</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">plus_assign</span><span class="special">,</span> <span class="identifier">D</span> <span class="special">></span> <span class="special">:</span> <span class="identifier">_</span> <span class="special">{};</span> 9914 <span class="keyword">template</span><span class="special"><</span><span class="keyword">int</span> <span class="identifier">D</span><span class="special">></span> <span class="keyword">struct</span> <span class="identifier">case_</span><span class="special"><</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">minus_assign</span><span class="special">,</span> <span class="identifier">D</span> <span class="special">></span> <span class="special">:</span> <span class="identifier">_</span> <span class="special">{};</span> 9915 <span class="keyword">template</span><span class="special"><</span><span class="keyword">int</span> <span class="identifier">D</span><span class="special">></span> <span class="keyword">struct</span> <span class="identifier">case_</span><span class="special"><</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">multiplies_assign</span><span class="special">,</span> <span class="identifier">D</span> <span class="special">></span> <span class="special">:</span> <span class="identifier">_</span> <span class="special">{};</span> 9916 <span class="keyword">template</span><span class="special"><</span><span class="keyword">int</span> <span class="identifier">D</span><span class="special">></span> <span class="keyword">struct</span> <span class="identifier">case_</span><span class="special"><</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">divides_assign</span><span class="special">,</span> <span class="identifier">D</span> <span class="special">></span> <span class="special">:</span> <span class="identifier">_</span> <span class="special">{};</span> 9917 <span class="keyword">template</span><span class="special"><</span><span class="keyword">int</span> <span class="identifier">D</span><span class="special">></span> <span class="keyword">struct</span> <span class="identifier">case_</span><span class="special"><</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">modulus_assign</span><span class="special">,</span> <span class="identifier">D</span> <span class="special">></span> <span class="special">:</span> <span class="identifier">_</span> <span class="special">{};</span> 9918 <span class="keyword">template</span><span class="special"><</span><span class="keyword">int</span> <span class="identifier">D</span><span class="special">></span> <span class="keyword">struct</span> <span class="identifier">case_</span><span class="special"><</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">shift_left_assign</span><span class="special">,</span> <span class="identifier">D</span> <span class="special">></span> <span class="special">:</span> <span class="identifier">_</span> <span class="special">{};</span> 9919 <span class="keyword">template</span><span class="special"><</span><span class="keyword">int</span> <span class="identifier">D</span><span class="special">></span> <span class="keyword">struct</span> <span class="identifier">case_</span><span class="special"><</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">shift_right_assign</span><span class="special">,</span> <span class="identifier">D</span> <span class="special">></span> <span class="special">:</span> <span class="identifier">_</span> <span class="special">{};</span> 9920 <span class="keyword">template</span><span class="special"><</span><span class="keyword">int</span> <span class="identifier">D</span><span class="special">></span> <span class="keyword">struct</span> <span class="identifier">case_</span><span class="special"><</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">bitwise_and_assign</span><span class="special">,</span> <span class="identifier">D</span> <span class="special">></span> <span class="special">:</span> <span class="identifier">_</span> <span class="special">{};</span> 9921 <span class="keyword">template</span><span class="special"><</span><span class="keyword">int</span> <span class="identifier">D</span><span class="special">></span> <span class="keyword">struct</span> <span class="identifier">case_</span><span class="special"><</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">bitwise_or_assign</span><span class="special">,</span> <span class="identifier">D</span> <span class="special">></span> <span class="special">:</span> <span class="identifier">_</span> <span class="special">{};</span> 9922 <span class="keyword">template</span><span class="special"><</span><span class="keyword">int</span> <span class="identifier">D</span><span class="special">></span> <span class="keyword">struct</span> <span class="identifier">case_</span><span class="special"><</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">bitwise_xor_assign</span><span class="special">,</span> <span class="identifier">D</span> <span class="special">></span> <span class="special">:</span> <span class="identifier">_</span> <span class="special">{};</span> 9923<span class="special">};</span> 9924 9925<span class="comment">// An expression conforms to the MixedGrammar if it is a terminal or some</span> 9926<span class="comment">// op that is not an assignment op. (Assignment will be handled specially.)</span> 9927<span class="keyword">struct</span> <span class="identifier">MixedGrammar</span> 9928 <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">or_</span><span class="special"><</span> 9929 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special"><</span><span class="identifier">_</span><span class="special">></span> 9930 <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">and_</span><span class="special"><</span> 9931 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">nary_expr</span><span class="special"><</span><span class="identifier">_</span><span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">vararg</span><span class="special"><</span><span class="identifier">MixedGrammar</span><span class="special">></span> <span class="special">></span> 9932 <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">not_</span><span class="special"><</span><span class="identifier">AssignOps</span><span class="special">></span> 9933 <span class="special">></span> 9934 <span class="special">></span> 9935<span class="special">{};</span> 9936 9937<span class="comment">// Expressions in the MixedDomain will be wrapped in MixedExpr<></span> 9938<span class="comment">// and must conform to the MixedGrammar</span> 9939<span class="keyword">struct</span> <span class="identifier">MixedDomain</span> 9940 <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">domain</span><span class="special"><</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">generator</span><span class="special"><</span><span class="identifier">MixedExpr</span><span class="special">>,</span> <span class="identifier">MixedGrammar</span><span class="special">></span> 9941<span class="special">{};</span> 9942 9943<span class="comment">// Here is MixedExpr, a wrapper for expression types in the MixedDomain.</span> 9944<span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">></span> 9945<span class="keyword">struct</span> <span class="identifier">MixedExpr</span> 9946 <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">extends</span><span class="special"><</span><span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">MixedExpr</span><span class="special"><</span><span class="identifier">Expr</span><span class="special">>,</span> <span class="identifier">MixedDomain</span><span class="special">></span> 9947<span class="special">{</span> 9948 <span class="keyword">explicit</span> <span class="identifier">MixedExpr</span><span class="special">(</span><span class="identifier">Expr</span> <span class="keyword">const</span> <span class="special">&</span><span class="identifier">expr</span><span class="special">)</span> 9949 <span class="special">:</span> <span class="identifier">MixedExpr</span><span class="special">::</span><span class="identifier">proto_extends</span><span class="special">(</span><span class="identifier">expr</span><span class="special">)</span> 9950 <span class="special">{}</span> 9951<span class="keyword">private</span><span class="special">:</span> 9952 <span class="comment">// hide this:</span> 9953 <span class="keyword">using</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">extends</span><span class="special"><</span><span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">MixedExpr</span><span class="special"><</span><span class="identifier">Expr</span><span class="special">>,</span> <span class="identifier">MixedDomain</span><span class="special">>::</span><span class="keyword">operator</span> <span class="special">[];</span> 9954<span class="special">};</span> 9955 9956<span class="comment">// Define a trait type for detecting vector and list terminals, to</span> 9957<span class="comment">// be used by the BOOST_PROTO_DEFINE_OPERATORS macro below.</span> 9958<span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">></span> 9959<span class="keyword">struct</span> <span class="identifier">IsMixed</span> 9960 <span class="special">:</span> <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">false_</span> 9961<span class="special">{};</span> 9962 9963<span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">A</span><span class="special">></span> 9964<span class="keyword">struct</span> <span class="identifier">IsMixed</span><span class="special"><</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">list</span><span class="special"><</span><span class="identifier">T</span><span class="special">,</span> <span class="identifier">A</span><span class="special">></span> <span class="special">></span> 9965 <span class="special">:</span> <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">true_</span> 9966<span class="special">{};</span> 9967 9968<span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">A</span><span class="special">></span> 9969<span class="keyword">struct</span> <span class="identifier">IsMixed</span><span class="special"><</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special"><</span><span class="identifier">T</span><span class="special">,</span> <span class="identifier">A</span><span class="special">></span> <span class="special">></span> 9970 <span class="special">:</span> <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">true_</span> 9971<span class="special">{};</span> 9972 9973<span class="keyword">namespace</span> <span class="identifier">MixedOps</span> 9974<span class="special">{</span> 9975 <span class="comment">// This defines all the overloads to make expressions involving</span> 9976 <span class="comment">// std::vector to build expression templates.</span> 9977 <span class="identifier">BOOST_PROTO_DEFINE_OPERATORS</span><span class="special">(</span><span class="identifier">IsMixed</span><span class="special">,</span> <span class="identifier">MixedDomain</span><span class="special">)</span> 9978 9979 <span class="keyword">struct</span> <span class="identifier">assign_op</span> 9980 <span class="special">{</span> 9981 <span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">U</span><span class="special">></span> 9982 <span class="keyword">void</span> <span class="keyword">operator</span> <span class="special">()(</span><span class="identifier">T</span> <span class="special">&</span><span class="identifier">t</span><span class="special">,</span> <span class="identifier">U</span> <span class="keyword">const</span> <span class="special">&</span><span class="identifier">u</span><span class="special">)</span> <span class="keyword">const</span> 9983 <span class="special">{</span> 9984 <span class="identifier">t</span> <span class="special">=</span> <span class="identifier">u</span><span class="special">;</span> 9985 <span class="special">}</span> 9986 <span class="special">};</span> 9987 9988 <span class="keyword">struct</span> <span class="identifier">plus_assign_op</span> 9989 <span class="special">{</span> 9990 <span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">U</span><span class="special">></span> 9991 <span class="keyword">void</span> <span class="keyword">operator</span> <span class="special">()(</span><span class="identifier">T</span> <span class="special">&</span><span class="identifier">t</span><span class="special">,</span> <span class="identifier">U</span> <span class="keyword">const</span> <span class="special">&</span><span class="identifier">u</span><span class="special">)</span> <span class="keyword">const</span> 9992 <span class="special">{</span> 9993 <span class="identifier">t</span> <span class="special">+=</span> <span class="identifier">u</span><span class="special">;</span> 9994 <span class="special">}</span> 9995 <span class="special">};</span> 9996 9997 <span class="keyword">struct</span> <span class="identifier">minus_assign_op</span> 9998 <span class="special">{</span> 9999 <span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">U</span><span class="special">></span> 10000 <span class="keyword">void</span> <span class="keyword">operator</span> <span class="special">()(</span><span class="identifier">T</span> <span class="special">&</span><span class="identifier">t</span><span class="special">,</span> <span class="identifier">U</span> <span class="keyword">const</span> <span class="special">&</span><span class="identifier">u</span><span class="special">)</span> <span class="keyword">const</span> 10001 <span class="special">{</span> 10002 <span class="identifier">t</span> <span class="special">-=</span> <span class="identifier">u</span><span class="special">;</span> 10003 <span class="special">}</span> 10004 <span class="special">};</span> 10005 10006 <span class="keyword">struct</span> <span class="identifier">sin_</span> 10007 <span class="special">{</span> 10008 <span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">Sig</span><span class="special">></span> 10009 <span class="keyword">struct</span> <span class="identifier">result</span><span class="special">;</span> 10010 10011 <span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">This</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Arg</span><span class="special">></span> 10012 <span class="keyword">struct</span> <span class="identifier">result</span><span class="special"><</span><span class="identifier">This</span><span class="special">(</span><span class="identifier">Arg</span><span class="special">)></span> 10013 <span class="special">:</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">remove_const</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">remove_reference</span><span class="special"><</span><span class="identifier">Arg</span><span class="special">>::</span><span class="identifier">type</span><span class="special">></span> 10014 <span class="special">{};</span> 10015 10016 <span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">Arg</span><span class="special">></span> 10017 <span class="identifier">Arg</span> <span class="keyword">operator</span> <span class="special">()(</span><span class="identifier">Arg</span> <span class="keyword">const</span> <span class="special">&</span><span class="identifier">a</span><span class="special">)</span> <span class="keyword">const</span> 10018 <span class="special">{</span> 10019 <span class="keyword">return</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">sin</span><span class="special">(</span><span class="identifier">a</span><span class="special">);</span> 10020 <span class="special">}</span> 10021 <span class="special">};</span> 10022 10023 <span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">A</span><span class="special">></span> 10024 <span class="keyword">typename</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">make_expr</span><span class="special"><</span> 10025 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">function</span> 10026 <span class="special">,</span> <span class="identifier">MixedDomain</span> 10027 <span class="special">,</span> <span class="identifier">sin_</span> <span class="keyword">const</span> 10028 <span class="special">,</span> <span class="identifier">A</span> <span class="keyword">const</span> <span class="special">&</span> 10029 <span class="special">>::</span><span class="identifier">type</span> <span class="identifier">sin</span><span class="special">(</span><span class="identifier">A</span> <span class="keyword">const</span> <span class="special">&</span><span class="identifier">a</span><span class="special">)</span> 10030 <span class="special">{</span> 10031 <span class="keyword">return</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">make_expr</span><span class="special"><</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">function</span><span class="special">,</span> <span class="identifier">MixedDomain</span><span class="special">>(</span><span class="identifier">sin_</span><span class="special">(),</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">ref</span><span class="special">(</span><span class="identifier">a</span><span class="special">));</span> 10032 <span class="special">}</span> 10033 10034 <span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">FwdIter</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Op</span><span class="special">></span> 10035 <span class="keyword">void</span> <span class="identifier">evaluate</span><span class="special">(</span><span class="identifier">FwdIter</span> <span class="identifier">begin</span><span class="special">,</span> <span class="identifier">FwdIter</span> <span class="identifier">end</span><span class="special">,</span> <span class="identifier">Expr</span> <span class="keyword">const</span> <span class="special">&</span><span class="identifier">expr</span><span class="special">,</span> <span class="identifier">Op</span> <span class="identifier">op</span><span class="special">)</span> 10036 <span class="special">{</span> 10037 <span class="identifier">IncrementCtx</span> <span class="keyword">const</span> <span class="identifier">inc</span> <span class="special">=</span> <span class="special">{};</span> 10038 <span class="identifier">DereferenceCtx</span> <span class="keyword">const</span> <span class="identifier">deref</span> <span class="special">=</span> <span class="special">{};</span> 10039 <span class="keyword">typename</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">result_of</span><span class="special"><</span><span class="identifier">Begin</span><span class="special">(</span><span class="identifier">Expr</span> <span class="keyword">const</span> <span class="special">&)>::</span><span class="identifier">type</span> <span class="identifier">expr2</span> <span class="special">=</span> <span class="identifier">Begin</span><span class="special">()(</span><span class="identifier">expr</span><span class="special">);</span> 10040 <span class="keyword">for</span><span class="special">(;</span> <span class="identifier">begin</span> <span class="special">!=</span> <span class="identifier">end</span><span class="special">;</span> <span class="special">++</span><span class="identifier">begin</span><span class="special">)</span> 10041 <span class="special">{</span> 10042 <span class="identifier">op</span><span class="special">(*</span><span class="identifier">begin</span><span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">eval</span><span class="special">(</span><span class="identifier">expr2</span><span class="special">,</span> <span class="identifier">deref</span><span class="special">));</span> 10043 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">eval</span><span class="special">(</span><span class="identifier">expr2</span><span class="special">,</span> <span class="identifier">inc</span><span class="special">);</span> 10044 <span class="special">}</span> 10045 <span class="special">}</span> 10046 10047 <span class="comment">// Add-assign to a vector from some expression.</span> 10048 <span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">A</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">></span> 10049 <span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special"><</span><span class="identifier">T</span><span class="special">,</span> <span class="identifier">A</span><span class="special">></span> <span class="special">&</span><span class="identifier">assign</span><span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special"><</span><span class="identifier">T</span><span class="special">,</span> <span class="identifier">A</span><span class="special">></span> <span class="special">&</span><span class="identifier">arr</span><span class="special">,</span> <span class="identifier">Expr</span> <span class="keyword">const</span> <span class="special">&</span><span class="identifier">expr</span><span class="special">)</span> 10050 <span class="special">{</span> 10051 <span class="identifier">evaluate</span><span class="special">(</span><span class="identifier">arr</span><span class="special">.</span><span class="identifier">begin</span><span class="special">(),</span> <span class="identifier">arr</span><span class="special">.</span><span class="identifier">end</span><span class="special">(),</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">as_expr</span><span class="special"><</span><span class="identifier">MixedDomain</span><span class="special">>(</span><span class="identifier">expr</span><span class="special">),</span> <span class="identifier">assign_op</span><span class="special">());</span> 10052 <span class="keyword">return</span> <span class="identifier">arr</span><span class="special">;</span> 10053 <span class="special">}</span> 10054 10055 <span class="comment">// Add-assign to a list from some expression.</span> 10056 <span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">A</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">></span> 10057 <span class="identifier">std</span><span class="special">::</span><span class="identifier">list</span><span class="special"><</span><span class="identifier">T</span><span class="special">,</span> <span class="identifier">A</span><span class="special">></span> <span class="special">&</span><span class="identifier">assign</span><span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">list</span><span class="special"><</span><span class="identifier">T</span><span class="special">,</span> <span class="identifier">A</span><span class="special">></span> <span class="special">&</span><span class="identifier">arr</span><span class="special">,</span> <span class="identifier">Expr</span> <span class="keyword">const</span> <span class="special">&</span><span class="identifier">expr</span><span class="special">)</span> 10058 <span class="special">{</span> 10059 <span class="identifier">evaluate</span><span class="special">(</span><span class="identifier">arr</span><span class="special">.</span><span class="identifier">begin</span><span class="special">(),</span> <span class="identifier">arr</span><span class="special">.</span><span class="identifier">end</span><span class="special">(),</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">as_expr</span><span class="special"><</span><span class="identifier">MixedDomain</span><span class="special">>(</span><span class="identifier">expr</span><span class="special">),</span> <span class="identifier">assign_op</span><span class="special">());</span> 10060 <span class="keyword">return</span> <span class="identifier">arr</span><span class="special">;</span> 10061 <span class="special">}</span> 10062 10063 <span class="comment">// Add-assign to a vector from some expression.</span> 10064 <span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">A</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">></span> 10065 <span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special"><</span><span class="identifier">T</span><span class="special">,</span> <span class="identifier">A</span><span class="special">></span> <span class="special">&</span><span class="keyword">operator</span> <span class="special">+=(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special"><</span><span class="identifier">T</span><span class="special">,</span> <span class="identifier">A</span><span class="special">></span> <span class="special">&</span><span class="identifier">arr</span><span class="special">,</span> <span class="identifier">Expr</span> <span class="keyword">const</span> <span class="special">&</span><span class="identifier">expr</span><span class="special">)</span> 10066 <span class="special">{</span> 10067 <span class="identifier">evaluate</span><span class="special">(</span><span class="identifier">arr</span><span class="special">.</span><span class="identifier">begin</span><span class="special">(),</span> <span class="identifier">arr</span><span class="special">.</span><span class="identifier">end</span><span class="special">(),</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">as_expr</span><span class="special"><</span><span class="identifier">MixedDomain</span><span class="special">>(</span><span class="identifier">expr</span><span class="special">),</span> <span class="identifier">plus_assign_op</span><span class="special">());</span> 10068 <span class="keyword">return</span> <span class="identifier">arr</span><span class="special">;</span> 10069 <span class="special">}</span> 10070 10071 <span class="comment">// Add-assign to a list from some expression.</span> 10072 <span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">A</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">></span> 10073 <span class="identifier">std</span><span class="special">::</span><span class="identifier">list</span><span class="special"><</span><span class="identifier">T</span><span class="special">,</span> <span class="identifier">A</span><span class="special">></span> <span class="special">&</span><span class="keyword">operator</span> <span class="special">+=(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">list</span><span class="special"><</span><span class="identifier">T</span><span class="special">,</span> <span class="identifier">A</span><span class="special">></span> <span class="special">&</span><span class="identifier">arr</span><span class="special">,</span> <span class="identifier">Expr</span> <span class="keyword">const</span> <span class="special">&</span><span class="identifier">expr</span><span class="special">)</span> 10074 <span class="special">{</span> 10075 <span class="identifier">evaluate</span><span class="special">(</span><span class="identifier">arr</span><span class="special">.</span><span class="identifier">begin</span><span class="special">(),</span> <span class="identifier">arr</span><span class="special">.</span><span class="identifier">end</span><span class="special">(),</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">as_expr</span><span class="special"><</span><span class="identifier">MixedDomain</span><span class="special">>(</span><span class="identifier">expr</span><span class="special">),</span> <span class="identifier">plus_assign_op</span><span class="special">());</span> 10076 <span class="keyword">return</span> <span class="identifier">arr</span><span class="special">;</span> 10077 <span class="special">}</span> 10078 10079 <span class="comment">// Minus-assign to a vector from some expression.</span> 10080 <span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">A</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">></span> 10081 <span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special"><</span><span class="identifier">T</span><span class="special">,</span> <span class="identifier">A</span><span class="special">></span> <span class="special">&</span><span class="keyword">operator</span> <span class="special">-=(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special"><</span><span class="identifier">T</span><span class="special">,</span> <span class="identifier">A</span><span class="special">></span> <span class="special">&</span><span class="identifier">arr</span><span class="special">,</span> <span class="identifier">Expr</span> <span class="keyword">const</span> <span class="special">&</span><span class="identifier">expr</span><span class="special">)</span> 10082 <span class="special">{</span> 10083 <span class="identifier">evaluate</span><span class="special">(</span><span class="identifier">arr</span><span class="special">.</span><span class="identifier">begin</span><span class="special">(),</span> <span class="identifier">arr</span><span class="special">.</span><span class="identifier">end</span><span class="special">(),</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">as_expr</span><span class="special"><</span><span class="identifier">MixedDomain</span><span class="special">>(</span><span class="identifier">expr</span><span class="special">),</span> <span class="identifier">minus_assign_op</span><span class="special">());</span> 10084 <span class="keyword">return</span> <span class="identifier">arr</span><span class="special">;</span> 10085 <span class="special">}</span> 10086 10087 <span class="comment">// Minus-assign to a list from some expression.</span> 10088 <span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">A</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">></span> 10089 <span class="identifier">std</span><span class="special">::</span><span class="identifier">list</span><span class="special"><</span><span class="identifier">T</span><span class="special">,</span> <span class="identifier">A</span><span class="special">></span> <span class="special">&</span><span class="keyword">operator</span> <span class="special">-=(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">list</span><span class="special"><</span><span class="identifier">T</span><span class="special">,</span> <span class="identifier">A</span><span class="special">></span> <span class="special">&</span><span class="identifier">arr</span><span class="special">,</span> <span class="identifier">Expr</span> <span class="keyword">const</span> <span class="special">&</span><span class="identifier">expr</span><span class="special">)</span> 10090 <span class="special">{</span> 10091 <span class="identifier">evaluate</span><span class="special">(</span><span class="identifier">arr</span><span class="special">.</span><span class="identifier">begin</span><span class="special">(),</span> <span class="identifier">arr</span><span class="special">.</span><span class="identifier">end</span><span class="special">(),</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">as_expr</span><span class="special"><</span><span class="identifier">MixedDomain</span><span class="special">>(</span><span class="identifier">expr</span><span class="special">),</span> <span class="identifier">minus_assign_op</span><span class="special">());</span> 10092 <span class="keyword">return</span> <span class="identifier">arr</span><span class="special">;</span> 10093 <span class="special">}</span> 10094<span class="special">}</span> 10095 10096<span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span> 10097<span class="special">{</span> 10098 <span class="keyword">using</span> <span class="keyword">namespace</span> <span class="identifier">MixedOps</span><span class="special">;</span> 10099 10100 <span class="keyword">int</span> <span class="identifier">n</span> <span class="special">=</span> <span class="number">10</span><span class="special">;</span> 10101 <span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> <span class="identifier">a</span><span class="special">,</span><span class="identifier">b</span><span class="special">,</span><span class="identifier">c</span><span class="special">,</span><span class="identifier">d</span><span class="special">;</span> 10102 <span class="identifier">std</span><span class="special">::</span><span class="identifier">list</span><span class="special"><</span><span class="keyword">double</span><span class="special">></span> <span class="identifier">e</span><span class="special">;</span> 10103 <span class="identifier">std</span><span class="special">::</span><span class="identifier">list</span><span class="special"><</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">complex</span><span class="special"><</span><span class="keyword">double</span><span class="special">></span> <span class="special">></span> <span class="identifier">f</span><span class="special">;</span> 10104 10105 <span class="keyword">int</span> <span class="identifier">i</span><span class="special">;</span> 10106 <span class="keyword">for</span><span class="special">(</span><span class="identifier">i</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</span><span class="identifier">i</span> <span class="special"><</span> <span class="identifier">n</span><span class="special">;</span> <span class="special">++</span><span class="identifier">i</span><span class="special">)</span> 10107 <span class="special">{</span> 10108 <span class="identifier">a</span><span class="special">.</span><span class="identifier">push_back</span><span class="special">(</span><span class="identifier">i</span><span class="special">);</span> 10109 <span class="identifier">b</span><span class="special">.</span><span class="identifier">push_back</span><span class="special">(</span><span class="number">2</span><span class="special">*</span><span class="identifier">i</span><span class="special">);</span> 10110 <span class="identifier">c</span><span class="special">.</span><span class="identifier">push_back</span><span class="special">(</span><span class="number">3</span><span class="special">*</span><span class="identifier">i</span><span class="special">);</span> 10111 <span class="identifier">d</span><span class="special">.</span><span class="identifier">push_back</span><span class="special">(</span><span class="identifier">i</span><span class="special">);</span> 10112 <span class="identifier">e</span><span class="special">.</span><span class="identifier">push_back</span><span class="special">(</span><span class="number">0.0</span><span class="special">);</span> 10113 <span class="identifier">f</span><span class="special">.</span><span class="identifier">push_back</span><span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">complex</span><span class="special"><</span><span class="keyword">double</span><span class="special">>(</span><span class="number">1.0</span><span class="special">,</span> <span class="number">1.0</span><span class="special">));</span> 10114 <span class="special">}</span> 10115 10116 <span class="identifier">MixedOps</span><span class="special">::</span><span class="identifier">assign</span><span class="special">(</span><span class="identifier">b</span><span class="special">,</span> <span class="number">2</span><span class="special">);</span> 10117 <span class="identifier">MixedOps</span><span class="special">::</span><span class="identifier">assign</span><span class="special">(</span><span class="identifier">d</span><span class="special">,</span> <span class="identifier">a</span> <span class="special">+</span> <span class="identifier">b</span> <span class="special">*</span> <span class="identifier">c</span><span class="special">);</span> 10118 <span class="identifier">a</span> <span class="special">+=</span> <span class="identifier">if_else</span><span class="special">(</span><span class="identifier">d</span> <span class="special"><</span> <span class="number">30</span><span class="special">,</span> <span class="identifier">b</span><span class="special">,</span> <span class="identifier">c</span><span class="special">);</span> 10119 10120 <span class="identifier">MixedOps</span><span class="special">::</span><span class="identifier">assign</span><span class="special">(</span><span class="identifier">e</span><span class="special">,</span> <span class="identifier">c</span><span class="special">);</span> 10121 <span class="identifier">e</span> <span class="special">+=</span> <span class="identifier">e</span> <span class="special">-</span> <span class="number">4</span> <span class="special">/</span> <span class="special">(</span><span class="identifier">c</span> <span class="special">+</span> <span class="number">1</span><span class="special">);</span> 10122 10123 <span class="identifier">f</span> <span class="special">-=</span> <span class="identifier">sin</span><span class="special">(</span><span class="number">0.1</span> <span class="special">*</span> <span class="identifier">e</span> <span class="special">*</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">complex</span><span class="special"><</span><span class="keyword">double</span><span class="special">>(</span><span class="number">0.2</span><span class="special">,</span> <span class="number">1.2</span><span class="special">));</span> 10124 10125 <span class="identifier">std</span><span class="special">::</span><span class="identifier">list</span><span class="special"><</span><span class="keyword">double</span><span class="special">>::</span><span class="identifier">const_iterator</span> <span class="identifier">ei</span> <span class="special">=</span> <span class="identifier">e</span><span class="special">.</span><span class="identifier">begin</span><span class="special">();</span> 10126 <span class="identifier">std</span><span class="special">::</span><span class="identifier">list</span><span class="special"><</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">complex</span><span class="special"><</span><span class="keyword">double</span><span class="special">></span> <span class="special">>::</span><span class="identifier">const_iterator</span> <span class="identifier">fi</span> <span class="special">=</span> <span class="identifier">f</span><span class="special">.</span><span class="identifier">begin</span><span class="special">();</span> 10127 <span class="keyword">for</span> <span class="special">(</span><span class="identifier">i</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</span> <span class="identifier">i</span> <span class="special"><</span> <span class="identifier">n</span><span class="special">;</span> <span class="special">++</span><span class="identifier">i</span><span class="special">)</span> 10128 <span class="special">{</span> 10129 <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> 10130 <span class="special"><<</span> <span class="string">"a("</span> <span class="special"><<</span> <span class="identifier">i</span> <span class="special"><<</span> <span class="string">") = "</span> <span class="special"><<</span> <span class="identifier">a</span><span class="special">[</span><span class="identifier">i</span><span class="special">]</span> 10131 <span class="special"><<</span> <span class="string">" b("</span> <span class="special"><<</span> <span class="identifier">i</span> <span class="special"><<</span> <span class="string">") = "</span> <span class="special"><<</span> <span class="identifier">b</span><span class="special">[</span><span class="identifier">i</span><span class="special">]</span> 10132 <span class="special"><<</span> <span class="string">" c("</span> <span class="special"><<</span> <span class="identifier">i</span> <span class="special"><<</span> <span class="string">") = "</span> <span class="special"><<</span> <span class="identifier">c</span><span class="special">[</span><span class="identifier">i</span><span class="special">]</span> 10133 <span class="special"><<</span> <span class="string">" d("</span> <span class="special"><<</span> <span class="identifier">i</span> <span class="special"><<</span> <span class="string">") = "</span> <span class="special"><<</span> <span class="identifier">d</span><span class="special">[</span><span class="identifier">i</span><span class="special">]</span> 10134 <span class="special"><<</span> <span class="string">" e("</span> <span class="special"><<</span> <span class="identifier">i</span> <span class="special"><<</span> <span class="string">") = "</span> <span class="special"><<</span> <span class="special">*</span><span class="identifier">ei</span><span class="special">++</span> 10135 <span class="special"><<</span> <span class="string">" f("</span> <span class="special"><<</span> <span class="identifier">i</span> <span class="special"><<</span> <span class="string">") = "</span> <span class="special"><<</span> <span class="special">*</span><span class="identifier">fi</span><span class="special">++</span> 10136 <span class="special"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span> 10137 <span class="special">}</span> 10138<span class="special">}</span> 10139</pre> 10140<p> 10141 </p> 10142</div> 10143<div class="section"> 10144<div class="titlepage"><div><div><h4 class="title"> 10145<a name="boost_proto.users_guide.examples.map_assign"></a><a class="link" href="users_guide.html#boost_proto.users_guide.examples.map_assign" title="Map Assign: An Intermediate Transform">Map Assign: 10146 An Intermediate Transform</a> 10147</h4></div></div></div> 10148<p> 10149 A demonstration of how to implement <code class="computeroutput"><span class="identifier">map_list_of</span><span class="special">()</span></code> from the Boost.Assign library using Proto. 10150 <code class="computeroutput"><span class="identifier">map_list_assign</span><span class="special">()</span></code> 10151 is used to conveniently initialize a <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">map</span><span class="special"><></span></code>. By using Proto, we can avoid any 10152 dynamic allocation while building the intermediate representation. 10153 </p> 10154<p> 10155</p> 10156<pre class="programlisting"><span class="comment">// Copyright 2008 Eric Niebler. Distributed under the Boost</span> 10157<span class="comment">// Software License, Version 1.0. (See accompanying file</span> 10158<span class="comment">// LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)</span> 10159<span class="comment">//</span> 10160<span class="comment">// This is a port of map_list_of() from the Boost.Assign library.</span> 10161<span class="comment">// It has the advantage of being more efficient at runtime by not</span> 10162<span class="comment">// building any temporary container that requires dynamic allocation.</span> 10163 10164<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">map</span><span class="special">></span> 10165<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">string</span><span class="special">></span> 10166<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">iostream</span><span class="special">></span> 10167<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">proto</span><span class="special">/</span><span class="identifier">core</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> 10168<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">proto</span><span class="special">/</span><span class="identifier">transform</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> 10169<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">type_traits</span><span class="special">/</span><span class="identifier">add_reference</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> 10170<span class="keyword">namespace</span> <span class="identifier">proto</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">proto</span><span class="special">;</span> 10171<span class="keyword">using</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_</span><span class="special">;</span> 10172 10173<span class="keyword">struct</span> <span class="identifier">map_list_of_tag</span> 10174<span class="special">{};</span> 10175 10176<span class="comment">// A simple callable function object that inserts a</span> 10177<span class="comment">// (key,value) pair into a map.</span> 10178<span class="keyword">struct</span> <span class="identifier">insert</span> 10179 <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">callable</span> 10180<span class="special">{</span> 10181 <span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">Sig</span><span class="special">></span> 10182 <span class="keyword">struct</span> <span class="identifier">result</span><span class="special">;</span> 10183 10184 <span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">This</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Map</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Key</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Value</span><span class="special">></span> 10185 <span class="keyword">struct</span> <span class="identifier">result</span><span class="special"><</span><span class="identifier">This</span><span class="special">(</span><span class="identifier">Map</span><span class="special">,</span> <span class="identifier">Key</span><span class="special">,</span> <span class="identifier">Value</span><span class="special">)></span> 10186 <span class="special">:</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">add_reference</span><span class="special"><</span><span class="identifier">Map</span><span class="special">></span> 10187 <span class="special">{};</span> 10188 10189 <span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">Map</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Key</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Value</span><span class="special">></span> 10190 <span class="identifier">Map</span> <span class="special">&</span><span class="keyword">operator</span><span class="special">()(</span><span class="identifier">Map</span> <span class="special">&</span><span class="identifier">map</span><span class="special">,</span> <span class="identifier">Key</span> <span class="keyword">const</span> <span class="special">&</span><span class="identifier">key</span><span class="special">,</span> <span class="identifier">Value</span> <span class="keyword">const</span> <span class="special">&</span><span class="identifier">value</span><span class="special">)</span> <span class="keyword">const</span> 10191 <span class="special">{</span> 10192 <span class="identifier">map</span><span class="special">.</span><span class="identifier">insert</span><span class="special">(</span><span class="keyword">typename</span> <span class="identifier">Map</span><span class="special">::</span><span class="identifier">value_type</span><span class="special">(</span><span class="identifier">key</span><span class="special">,</span> <span class="identifier">value</span><span class="special">));</span> 10193 <span class="keyword">return</span> <span class="identifier">map</span><span class="special">;</span> 10194 <span class="special">}</span> 10195<span class="special">};</span> 10196 10197<span class="comment">// Work-arounds for Microsoft Visual C++ 7.1</span> 10198<span class="preprocessor">#if</span> <span class="identifier">BOOST_WORKAROUND</span><span class="special">(</span><span class="identifier">BOOST_MSVC</span><span class="special">,</span> <span class="special">==</span> <span class="number">1310</span><span class="special">)</span> 10199<span class="preprocessor">#define</span> <span class="identifier">MapListOf</span><span class="special">(</span><span class="identifier">x</span><span class="special">)</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">call</span><span class="special"><</span><span class="identifier">MapListOf</span><span class="special">(</span><span class="identifier">x</span><span class="special">)></span> 10200<span class="preprocessor">#define</span> <span class="identifier">_value</span><span class="special">(</span><span class="identifier">x</span><span class="special">)</span> <span class="identifier">call</span><span class="special"><</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_value</span><span class="special">(</span><span class="identifier">x</span><span class="special">)></span> 10201<span class="preprocessor">#endif</span> 10202 10203<span class="comment">// The grammar for valid map-list expressions, and a</span> 10204<span class="comment">// transform that populates the map.</span> 10205<span class="keyword">struct</span> <span class="identifier">MapListOf</span> 10206 <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">or_</span><span class="special"><</span> 10207 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special"><</span> 10208 <span class="comment">// map_list_of(a,b)</span> 10209 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">function</span><span class="special"><</span> 10210 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special"><</span><span class="identifier">map_list_of_tag</span><span class="special">></span> 10211 <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special"><</span><span class="identifier">_</span><span class="special">></span> 10212 <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special"><</span><span class="identifier">_</span><span class="special">></span> 10213 <span class="special">></span> 10214 <span class="special">,</span> <span class="identifier">insert</span><span class="special">(</span> 10215 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_data</span> 10216 <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_value</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_child1</span><span class="special">)</span> 10217 <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_value</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_child2</span><span class="special">)</span> 10218 <span class="special">)</span> 10219 <span class="special">></span> 10220 <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special"><</span> 10221 <span class="comment">// map_list_of(a,b)(c,d)...</span> 10222 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">function</span><span class="special"><</span> 10223 <span class="identifier">MapListOf</span> 10224 <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special"><</span><span class="identifier">_</span><span class="special">></span> 10225 <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special"><</span><span class="identifier">_</span><span class="special">></span> 10226 <span class="special">></span> 10227 <span class="special">,</span> <span class="identifier">insert</span><span class="special">(</span> 10228 <span class="identifier">MapListOf</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_child0</span><span class="special">)</span> 10229 <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_value</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_child1</span><span class="special">)</span> 10230 <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_value</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_child2</span><span class="special">)</span> 10231 <span class="special">)</span> 10232 <span class="special">></span> 10233 <span class="special">></span> 10234<span class="special">{};</span> 10235 10236<span class="preprocessor">#if</span> <span class="identifier">BOOST_WORKAROUND</span><span class="special">(</span><span class="identifier">BOOST_MSVC</span><span class="special">,</span> <span class="special">==</span> <span class="number">1310</span><span class="special">)</span> 10237<span class="preprocessor">#undef</span> <span class="identifier">MapListOf</span> 10238<span class="preprocessor">#undef</span> <span class="identifier">_value</span> 10239<span class="preprocessor">#endif</span> 10240 10241<span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">></span> 10242<span class="keyword">struct</span> <span class="identifier">map_list_of_expr</span><span class="special">;</span> 10243 10244<span class="keyword">struct</span> <span class="identifier">map_list_of_dom</span> 10245 <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">domain</span><span class="special"><</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">pod_generator</span><span class="special"><</span><span class="identifier">map_list_of_expr</span><span class="special">>,</span> <span class="identifier">MapListOf</span><span class="special">></span> 10246<span class="special">{};</span> 10247 10248<span class="comment">// An expression wrapper that provides a conversion to a</span> 10249<span class="comment">// map that uses the MapListOf</span> 10250<span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">></span> 10251<span class="keyword">struct</span> <span class="identifier">map_list_of_expr</span> 10252<span class="special">{</span> 10253 <span class="identifier">BOOST_PROTO_BASIC_EXTENDS</span><span class="special">(</span><span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">map_list_of_expr</span><span class="special">,</span> <span class="identifier">map_list_of_dom</span><span class="special">)</span> 10254 <span class="identifier">BOOST_PROTO_EXTENDS_FUNCTION</span><span class="special">()</span> 10255 10256 <span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">Key</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Value</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Cmp</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Al</span><span class="special">></span> 10257 <span class="keyword">operator</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">map</span><span class="special"><</span><span class="identifier">Key</span><span class="special">,</span> <span class="identifier">Value</span><span class="special">,</span> <span class="identifier">Cmp</span><span class="special">,</span> <span class="identifier">Al</span><span class="special">></span> <span class="special">()</span> <span class="keyword">const</span> 10258 <span class="special">{</span> 10259 <span class="identifier">BOOST_MPL_ASSERT</span><span class="special">((</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">matches</span><span class="special"><</span><span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">MapListOf</span><span class="special">>));</span> 10260 <span class="identifier">std</span><span class="special">::</span><span class="identifier">map</span><span class="special"><</span><span class="identifier">Key</span><span class="special">,</span> <span class="identifier">Value</span><span class="special">,</span> <span class="identifier">Cmp</span><span class="special">,</span> <span class="identifier">Al</span><span class="special">></span> <span class="identifier">map</span><span class="special">;</span> 10261 <span class="keyword">return</span> <span class="identifier">MapListOf</span><span class="special">()(*</span><span class="keyword">this</span><span class="special">,</span> <span class="number">0</span><span class="special">,</span> <span class="identifier">map</span><span class="special">);</span> 10262 <span class="special">}</span> 10263<span class="special">};</span> 10264 10265<span class="identifier">map_list_of_expr</span><span class="special"><</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special"><</span><span class="identifier">map_list_of_tag</span><span class="special">>::</span><span class="identifier">type</span><span class="special">></span> <span class="keyword">const</span> <span class="identifier">map_list_of</span> <span class="special">=</span> <span class="special">{{{}}};</span> 10266 10267<span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span> 10268<span class="special">{</span> 10269 <span class="comment">// Initialize a map:</span> 10270 <span class="identifier">std</span><span class="special">::</span><span class="identifier">map</span><span class="special"><</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">,</span> <span class="keyword">int</span><span class="special">></span> <span class="identifier">op</span> <span class="special">=</span> 10271 <span class="identifier">map_list_of</span> 10272 <span class="special">(</span><span class="string">"<"</span><span class="special">,</span> <span class="number">1</span><span class="special">)</span> 10273 <span class="special">(</span><span class="string">"<="</span><span class="special">,</span><span class="number">2</span><span class="special">)</span> 10274 <span class="special">(</span><span class="string">">"</span><span class="special">,</span> <span class="number">3</span><span class="special">)</span> 10275 <span class="special">(</span><span class="string">">="</span><span class="special">,</span><span class="number">4</span><span class="special">)</span> 10276 <span class="special">(</span><span class="string">"="</span><span class="special">,</span> <span class="number">5</span><span class="special">)</span> 10277 <span class="special">(</span><span class="string">"<>"</span><span class="special">,</span><span class="number">6</span><span class="special">)</span> 10278 <span class="special">;</span> 10279 10280 <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="string">"\"<\" --> "</span> <span class="special"><<</span> <span class="identifier">op</span><span class="special">[</span><span class="string">"<"</span><span class="special">]</span> <span class="special"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span> 10281 <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="string">"\"<=\" --> "</span> <span class="special"><<</span> <span class="identifier">op</span><span class="special">[</span><span class="string">"<="</span><span class="special">]</span> <span class="special"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span> 10282 <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="string">"\">\" --> "</span> <span class="special"><<</span> <span class="identifier">op</span><span class="special">[</span><span class="string">">"</span><span class="special">]</span> <span class="special"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span> 10283 <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="string">"\">=\" --> "</span> <span class="special"><<</span> <span class="identifier">op</span><span class="special">[</span><span class="string">">="</span><span class="special">]</span> <span class="special"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span> 10284 <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="string">"\"=\" --> "</span> <span class="special"><<</span> <span class="identifier">op</span><span class="special">[</span><span class="string">"="</span><span class="special">]</span> <span class="special"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span> 10285 <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="string">"\"<>\" --> "</span> <span class="special"><<</span> <span class="identifier">op</span><span class="special">[</span><span class="string">"<>"</span><span class="special">]</span> <span class="special"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span> 10286 10287 <span class="keyword">return</span> <span class="number">0</span><span class="special">;</span> 10288<span class="special">}</span> 10289</pre> 10290<p> 10291 </p> 10292</div> 10293<div class="section"> 10294<div class="titlepage"><div><div><h4 class="title"> 10295<a name="boost_proto.users_guide.examples.future_group"></a><a class="link" href="users_guide.html#boost_proto.users_guide.examples.future_group" title="Future Group: A More Advanced Transform">Future 10296 Group: A More Advanced Transform</a> 10297</h4></div></div></div> 10298<p> 10299 An advanced example of a Proto transform that implements Howard Hinnant's 10300 design for <span class="emphasis"><em>future groups</em></span> that block for all or some 10301 asynchronous operations to complete and returns their results in a tuple 10302 of the appropriate type. 10303 </p> 10304<p> 10305</p> 10306<pre class="programlisting"><span class="comment">// Copyright 2008 Eric Niebler. Distributed under the Boost</span> 10307<span class="comment">// Software License, Version 1.0. (See accompanying file</span> 10308<span class="comment">// LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)</span> 10309<span class="comment">//</span> 10310<span class="comment">// This is an example of using Proto transforms to implement</span> 10311<span class="comment">// Howard Hinnant's future group proposal.</span> 10312 10313<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">fusion</span><span class="special">/</span><span class="identifier">include</span><span class="special">/</span><span class="identifier">vector</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> 10314<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">fusion</span><span class="special">/</span><span class="identifier">include</span><span class="special">/</span><span class="identifier">as_vector</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> 10315<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">fusion</span><span class="special">/</span><span class="identifier">include</span><span class="special">/</span><span class="identifier">joint_view</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> 10316<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">fusion</span><span class="special">/</span><span class="identifier">include</span><span class="special">/</span><span class="identifier">single_view</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> 10317<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">proto</span><span class="special">/</span><span class="identifier">core</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> 10318<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">proto</span><span class="special">/</span><span class="identifier">transform</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> 10319<span class="keyword">namespace</span> <span class="identifier">mpl</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">mpl</span><span class="special">;</span> 10320<span class="keyword">namespace</span> <span class="identifier">proto</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">proto</span><span class="special">;</span> 10321<span class="keyword">namespace</span> <span class="identifier">fusion</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">fusion</span><span class="special">;</span> 10322<span class="keyword">using</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_</span><span class="special">;</span> 10323 10324<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">L</span><span class="special">,</span><span class="keyword">class</span> <span class="identifier">R</span><span class="special">></span> 10325<span class="keyword">struct</span> <span class="identifier">pick_left</span> 10326<span class="special">{</span> 10327 <span class="identifier">BOOST_MPL_ASSERT</span><span class="special">((</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">is_same</span><span class="special"><</span><span class="identifier">L</span><span class="special">,</span> <span class="identifier">R</span><span class="special">>));</span> 10328 <span class="keyword">typedef</span> <span class="identifier">L</span> <span class="identifier">type</span><span class="special">;</span> 10329<span class="special">};</span> 10330 10331<span class="comment">// Work-arounds for Microsoft Visual C++ 7.1</span> 10332<span class="preprocessor">#if</span> <span class="identifier">BOOST_WORKAROUND</span><span class="special">(</span><span class="identifier">BOOST_MSVC</span><span class="special">,</span> <span class="special">==</span> <span class="number">1310</span><span class="special">)</span> 10333<span class="preprocessor">#define</span> <span class="identifier">FutureGroup</span><span class="special">(</span><span class="identifier">x</span><span class="special">)</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">call</span><span class="special"><</span><span class="identifier">FutureGroup</span><span class="special">(</span><span class="identifier">x</span><span class="special">)></span> 10334<span class="preprocessor">#endif</span> 10335 10336<span class="comment">// Define the grammar of future group expression, as well as a</span> 10337<span class="comment">// transform to turn them into a Fusion sequence of the correct</span> 10338<span class="comment">// type.</span> 10339<span class="keyword">struct</span> <span class="identifier">FutureGroup</span> 10340 <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">or_</span><span class="special"><</span> 10341 <span class="comment">// terminals become a single-element Fusion sequence</span> 10342 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special"><</span> 10343 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special"><</span><span class="identifier">_</span><span class="special">></span> 10344 <span class="special">,</span> <span class="identifier">fusion</span><span class="special">::</span><span class="identifier">single_view</span><span class="special"><</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_value</span><span class="special">>(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_value</span><span class="special">)</span> 10345 <span class="special">></span> 10346 <span class="comment">// (a && b) becomes a concatenation of the sequence</span> 10347 <span class="comment">// from 'a' and the one from 'b':</span> 10348 <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special"><</span> 10349 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">logical_and</span><span class="special"><</span><span class="identifier">FutureGroup</span><span class="special">,</span> <span class="identifier">FutureGroup</span><span class="special">></span> 10350 <span class="special">,</span> <span class="identifier">fusion</span><span class="special">::</span><span class="identifier">joint_view</span><span class="special"><</span> 10351 <span class="identifier">boost</span><span class="special">::</span><span class="identifier">add_const</span><span class="special"><</span><span class="identifier">FutureGroup</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_left</span><span class="special">)</span> <span class="special">></span> 10352 <span class="special">,</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">add_const</span><span class="special"><</span><span class="identifier">FutureGroup</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_right</span><span class="special">)</span> <span class="special">></span> 10353 <span class="special">>(</span><span class="identifier">FutureGroup</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_left</span><span class="special">),</span> <span class="identifier">FutureGroup</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_right</span><span class="special">))</span> 10354 <span class="special">></span> 10355 <span class="comment">// (a || b) becomes the sequence for 'a', so long</span> 10356 <span class="comment">// as it is the same as the sequence for 'b'.</span> 10357 <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special"><</span> 10358 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">logical_or</span><span class="special"><</span><span class="identifier">FutureGroup</span><span class="special">,</span> <span class="identifier">FutureGroup</span><span class="special">></span> 10359 <span class="special">,</span> <span class="identifier">pick_left</span><span class="special"><</span> 10360 <span class="identifier">FutureGroup</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_left</span><span class="special">)</span> 10361 <span class="special">,</span> <span class="identifier">FutureGroup</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_right</span><span class="special">)</span> 10362 <span class="special">>(</span><span class="identifier">FutureGroup</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_left</span><span class="special">))</span> 10363 <span class="special">></span> 10364 <span class="special">></span> 10365<span class="special">{};</span> 10366 10367<span class="preprocessor">#if</span> <span class="identifier">BOOST_WORKAROUND</span><span class="special">(</span><span class="identifier">BOOST_MSVC</span><span class="special">,</span> <span class="special">==</span> <span class="number">1310</span><span class="special">)</span> 10368<span class="preprocessor">#undef</span> <span class="identifier">FutureGroup</span> 10369<span class="preprocessor">#endif</span> 10370 10371<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">E</span><span class="special">></span> 10372<span class="keyword">struct</span> <span class="identifier">future_expr</span><span class="special">;</span> 10373 10374<span class="keyword">struct</span> <span class="identifier">future_dom</span> 10375 <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">domain</span><span class="special"><</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">generator</span><span class="special"><</span><span class="identifier">future_expr</span><span class="special">>,</span> <span class="identifier">FutureGroup</span><span class="special">></span> 10376<span class="special">{};</span> 10377 10378<span class="comment">// Expressions in the future group domain have a .get()</span> 10379<span class="comment">// member function that (ostensibly) blocks for the futures</span> 10380<span class="comment">// to complete and returns the results in an appropriate</span> 10381<span class="comment">// tuple.</span> 10382<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">E</span><span class="special">></span> 10383<span class="keyword">struct</span> <span class="identifier">future_expr</span> 10384 <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">extends</span><span class="special"><</span><span class="identifier">E</span><span class="special">,</span> <span class="identifier">future_expr</span><span class="special"><</span><span class="identifier">E</span><span class="special">>,</span> <span class="identifier">future_dom</span><span class="special">></span> 10385<span class="special">{</span> 10386 <span class="keyword">explicit</span> <span class="identifier">future_expr</span><span class="special">(</span><span class="identifier">E</span> <span class="keyword">const</span> <span class="special">&</span><span class="identifier">e</span><span class="special">)</span> 10387 <span class="special">:</span> <span class="identifier">future_expr</span><span class="special">::</span><span class="identifier">proto_extends</span><span class="special">(</span><span class="identifier">e</span><span class="special">)</span> 10388 <span class="special">{}</span> 10389 10390 <span class="keyword">typename</span> <span class="identifier">fusion</span><span class="special">::</span><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">as_vector</span><span class="special"><</span> 10391 <span class="keyword">typename</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">result_of</span><span class="special"><</span><span class="identifier">FutureGroup</span><span class="special">(</span><span class="identifier">E</span><span class="special">)>::</span><span class="identifier">type</span> 10392 <span class="special">>::</span><span class="identifier">type</span> 10393 <span class="identifier">get</span><span class="special">()</span> <span class="keyword">const</span> 10394 <span class="special">{</span> 10395 <span class="keyword">return</span> <span class="identifier">fusion</span><span class="special">::</span><span class="identifier">as_vector</span><span class="special">(</span><span class="identifier">FutureGroup</span><span class="special">()(*</span><span class="keyword">this</span><span class="special">));</span> 10396 <span class="special">}</span> 10397<span class="special">};</span> 10398 10399<span class="comment">// The future<> type has an even simpler .get()</span> 10400<span class="comment">// member function.</span> 10401<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> 10402<span class="keyword">struct</span> <span class="identifier">future</span> 10403 <span class="special">:</span> <span class="identifier">future_expr</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="identifier">type</span><span class="special">></span> 10404<span class="special">{</span> 10405 <span class="identifier">future</span><span class="special">(</span><span class="identifier">T</span> <span class="keyword">const</span> <span class="special">&</span><span class="identifier">t</span> <span class="special">=</span> <span class="identifier">T</span><span class="special">())</span> 10406 <span class="special">:</span> <span class="identifier">future</span><span class="special">::</span><span class="identifier">proto_derived_expr</span><span class="special">(</span><span class="identifier">future</span><span class="special">::</span><span class="identifier">proto_base_expr</span><span class="special">::</span><span class="identifier">make</span><span class="special">(</span><span class="identifier">t</span><span class="special">))</span> 10407 <span class="special">{}</span> 10408 10409 <span class="identifier">T</span> <span class="identifier">get</span><span class="special">()</span> <span class="keyword">const</span> 10410 <span class="special">{</span> 10411 <span class="keyword">return</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">value</span><span class="special">(*</span><span class="keyword">this</span><span class="special">);</span> 10412 <span class="special">}</span> 10413<span class="special">};</span> 10414 10415<span class="comment">// TEST CASES</span> 10416<span class="keyword">struct</span> <span class="identifier">A</span> <span class="special">{};</span> 10417<span class="keyword">struct</span> <span class="identifier">B</span> <span class="special">{};</span> 10418<span class="keyword">struct</span> <span class="identifier">C</span> <span class="special">{};</span> 10419 10420<span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span> 10421<span class="special">{</span> 10422 <span class="keyword">using</span> <span class="identifier">fusion</span><span class="special">::</span><span class="identifier">vector</span><span class="special">;</span> 10423 <span class="identifier">future</span><span class="special"><</span><span class="identifier">A</span><span class="special">></span> <span class="identifier">a</span><span class="special">;</span> 10424 <span class="identifier">future</span><span class="special"><</span><span class="identifier">B</span><span class="special">></span> <span class="identifier">b</span><span class="special">;</span> 10425 <span class="identifier">future</span><span class="special"><</span><span class="identifier">C</span><span class="special">></span> <span class="identifier">c</span><span class="special">;</span> 10426 <span class="identifier">future</span><span class="special"><</span><span class="identifier">vector</span><span class="special"><</span><span class="identifier">A</span><span class="special">,</span><span class="identifier">B</span><span class="special">></span> <span class="special">></span> <span class="identifier">ab</span><span class="special">;</span> 10427 10428 <span class="comment">// Verify that various future groups have the</span> 10429 <span class="comment">// correct return types.</span> 10430 <span class="identifier">A</span> <span class="identifier">t0</span> <span class="special">=</span> <span class="identifier">a</span><span class="special">.</span><span class="identifier">get</span><span class="special">();</span> 10431 <span class="identifier">vector</span><span class="special"><</span><span class="identifier">A</span><span class="special">,</span> <span class="identifier">B</span><span class="special">,</span> <span class="identifier">C</span><span class="special">></span> <span class="identifier">t1</span> <span class="special">=</span> <span class="special">(</span><span class="identifier">a</span> <span class="special">&&</span> <span class="identifier">b</span> <span class="special">&&</span> <span class="identifier">c</span><span class="special">).</span><span class="identifier">get</span><span class="special">();</span> 10432 <span class="identifier">vector</span><span class="special"><</span><span class="identifier">A</span><span class="special">,</span> <span class="identifier">C</span><span class="special">></span> <span class="identifier">t2</span> <span class="special">=</span> <span class="special">((</span><span class="identifier">a</span> <span class="special">||</span> <span class="identifier">a</span><span class="special">)</span> <span class="special">&&</span> <span class="identifier">c</span><span class="special">).</span><span class="identifier">get</span><span class="special">();</span> 10433 <span class="identifier">vector</span><span class="special"><</span><span class="identifier">A</span><span class="special">,</span> <span class="identifier">B</span><span class="special">,</span> <span class="identifier">C</span><span class="special">></span> <span class="identifier">t3</span> <span class="special">=</span> <span class="special">((</span><span class="identifier">a</span> <span class="special">&&</span> <span class="identifier">b</span> <span class="special">||</span> <span class="identifier">a</span> <span class="special">&&</span> <span class="identifier">b</span><span class="special">)</span> <span class="special">&&</span> <span class="identifier">c</span><span class="special">).</span><span class="identifier">get</span><span class="special">();</span> 10434 <span class="identifier">vector</span><span class="special"><</span><span class="identifier">vector</span><span class="special"><</span><span class="identifier">A</span><span class="special">,</span> <span class="identifier">B</span><span class="special">>,</span> <span class="identifier">C</span><span class="special">></span> <span class="identifier">t4</span> <span class="special">=</span> <span class="special">((</span><span class="identifier">ab</span> <span class="special">||</span> <span class="identifier">ab</span><span class="special">)</span> <span class="special">&&</span> <span class="identifier">c</span><span class="special">).</span><span class="identifier">get</span><span class="special">();</span> 10435 10436 <span class="keyword">return</span> <span class="number">0</span><span class="special">;</span> 10437<span class="special">}</span> 10438</pre> 10439<p> 10440 </p> 10441</div> 10442<div class="section"> 10443<div class="titlepage"><div><div><h4 class="title"> 10444<a name="boost_proto.users_guide.examples.lambda"></a><a class="link" href="users_guide.html#boost_proto.users_guide.examples.lambda" title="Lambda: A Simple Lambda Library with Proto">Lambda: A 10445 Simple Lambda Library with Proto</a> 10446</h4></div></div></div> 10447<p> 10448 This is an advanced example that shows how to implement a simple lambda 10449 EDSL with Proto, like the Boost.Lambda_library. It uses contexts, transforms 10450 and expression extension. 10451 </p> 10452<p> 10453</p> 10454<pre class="programlisting"><span class="comment">///////////////////////////////////////////////////////////////////////////////</span> 10455<span class="comment">// Copyright 2008 Eric Niebler. Distributed under the Boost</span> 10456<span class="comment">// Software License, Version 1.0. (See accompanying file</span> 10457<span class="comment">// LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)</span> 10458<span class="comment">//</span> 10459<span class="comment">// This example builds a simple but functional lambda library using Proto.</span> 10460 10461<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">iostream</span><span class="special">></span> 10462<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">algorithm</span><span class="special">></span> 10463<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">mpl</span><span class="special">/</span><span class="keyword">int</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> 10464<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">mpl</span><span class="special">/</span><span class="identifier">min_max</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> 10465<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">mpl</span><span class="special">/</span><span class="identifier">eval_if</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> 10466<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">mpl</span><span class="special">/</span><span class="identifier">identity</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> 10467<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">mpl</span><span class="special">/</span><span class="identifier">next_prior</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> 10468<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">fusion</span><span class="special">/</span><span class="identifier">tuple</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> 10469<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">typeof</span><span class="special">/</span><span class="identifier">typeof</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> 10470<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">typeof</span><span class="special">/</span><span class="identifier">std</span><span class="special">/</span><span class="identifier">ostream</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> 10471<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">typeof</span><span class="special">/</span><span class="identifier">std</span><span class="special">/</span><span class="identifier">iostream</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> 10472<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">proto</span><span class="special">/</span><span class="identifier">core</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> 10473<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">proto</span><span class="special">/</span><span class="identifier">context</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> 10474<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">proto</span><span class="special">/</span><span class="identifier">transform</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> 10475<span class="keyword">namespace</span> <span class="identifier">mpl</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">mpl</span><span class="special">;</span> 10476<span class="keyword">namespace</span> <span class="identifier">proto</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">proto</span><span class="special">;</span> 10477<span class="keyword">namespace</span> <span class="identifier">fusion</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">fusion</span><span class="special">;</span> 10478<span class="keyword">using</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_</span><span class="special">;</span> 10479 10480<span class="comment">// Forward declaration of the lambda expression wrapper</span> 10481<span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">></span> 10482<span class="keyword">struct</span> <span class="identifier">lambda</span><span class="special">;</span> 10483 10484<span class="keyword">struct</span> <span class="identifier">lambda_domain</span> 10485 <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">domain</span><span class="special"><</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">pod_generator</span><span class="special"><</span><span class="identifier">lambda</span><span class="special">></span> <span class="special">></span> 10486<span class="special">{};</span> 10487 10488<span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">I</span><span class="special">></span> 10489<span class="keyword">struct</span> <span class="identifier">placeholder</span> 10490<span class="special">{</span> 10491 <span class="keyword">typedef</span> <span class="identifier">I</span> <span class="identifier">arity</span><span class="special">;</span> 10492<span class="special">};</span> 10493 10494<span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">></span> 10495<span class="keyword">struct</span> <span class="identifier">placeholder_arity</span> 10496<span class="special">{</span> 10497 <span class="keyword">typedef</span> <span class="keyword">typename</span> <span class="identifier">T</span><span class="special">::</span><span class="identifier">arity</span> <span class="identifier">type</span><span class="special">;</span> 10498<span class="special">};</span> 10499 10500<span class="comment">// The lambda grammar, with the transforms for calculating the max arity</span> 10501<span class="keyword">struct</span> <span class="identifier">lambda_arity</span> 10502 <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">or_</span><span class="special"><</span> 10503 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special"><</span> 10504 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special"><</span> <span class="identifier">placeholder</span><span class="special"><</span><span class="identifier">_</span><span class="special">></span> <span class="special">></span> 10505 <span class="special">,</span> <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">next</span><span class="special"><</span><span class="identifier">placeholder_arity</span><span class="special"><</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_value</span><span class="special">></span> <span class="special">>()</span> 10506 <span class="special">></span> 10507 <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special"><</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special"><</span><span class="identifier">_</span><span class="special">></span> 10508 <span class="special">,</span> <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">int_</span><span class="special"><</span><span class="number">0</span><span class="special">>()</span> 10509 <span class="special">></span> 10510 <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special"><</span> 10511 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">nary_expr</span><span class="special"><</span><span class="identifier">_</span><span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">vararg</span><span class="special"><</span><span class="identifier">_</span><span class="special">></span> <span class="special">></span> 10512 <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">fold</span><span class="special"><</span><span class="identifier">_</span><span class="special">,</span> <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">int_</span><span class="special"><</span><span class="number">0</span><span class="special">>(),</span> <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">max</span><span class="special"><</span><span class="identifier">lambda_arity</span><span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_state</span><span class="special">>()></span> 10513 <span class="special">></span> 10514 <span class="special">></span> 10515<span class="special">{};</span> 10516 10517<span class="comment">// The lambda context is the same as the default context</span> 10518<span class="comment">// with the addition of special handling for lambda placeholders</span> 10519<span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">Tuple</span><span class="special">></span> 10520<span class="keyword">struct</span> <span class="identifier">lambda_context</span> 10521 <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">callable_context</span><span class="special"><</span><span class="identifier">lambda_context</span><span class="special"><</span><span class="identifier">Tuple</span><span class="special">></span> <span class="keyword">const</span><span class="special">></span> 10522<span class="special">{</span> 10523 <span class="identifier">lambda_context</span><span class="special">(</span><span class="identifier">Tuple</span> <span class="keyword">const</span> <span class="special">&</span><span class="identifier">args</span><span class="special">)</span> 10524 <span class="special">:</span> <span class="identifier">args_</span><span class="special">(</span><span class="identifier">args</span><span class="special">)</span> 10525 <span class="special">{}</span> 10526 10527 <span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">Sig</span><span class="special">></span> 10528 <span class="keyword">struct</span> <span class="identifier">result</span><span class="special">;</span> 10529 10530 <span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">This</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">I</span><span class="special">></span> 10531 <span class="keyword">struct</span> <span class="identifier">result</span><span class="special"><</span><span class="identifier">This</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">,</span> <span class="identifier">placeholder</span><span class="special"><</span><span class="identifier">I</span><span class="special">></span> <span class="keyword">const</span> <span class="special">&)></span> 10532 <span class="special">:</span> <span class="identifier">fusion</span><span class="special">::</span><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">at</span><span class="special"><</span><span class="identifier">Tuple</span><span class="special">,</span> <span class="identifier">I</span><span class="special">></span> 10533 <span class="special">{};</span> 10534 10535 <span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">I</span><span class="special">></span> 10536 <span class="keyword">typename</span> <span class="identifier">fusion</span><span class="special">::</span><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">at</span><span class="special"><</span><span class="identifier">Tuple</span><span class="special">,</span> <span class="identifier">I</span><span class="special">>::</span><span class="identifier">type</span> 10537 <span class="keyword">operator</span> <span class="special">()(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">,</span> <span class="identifier">placeholder</span><span class="special"><</span><span class="identifier">I</span><span class="special">></span> <span class="keyword">const</span> <span class="special">&)</span> <span class="keyword">const</span> 10538 <span class="special">{</span> 10539 <span class="keyword">return</span> <span class="identifier">fusion</span><span class="special">::</span><span class="identifier">at</span><span class="special"><</span><span class="identifier">I</span><span class="special">>(</span><span class="keyword">this</span><span class="special">-></span><span class="identifier">args_</span><span class="special">);</span> 10540 <span class="special">}</span> 10541 10542 <span class="identifier">Tuple</span> <span class="identifier">args_</span><span class="special">;</span> 10543<span class="special">};</span> 10544 10545<span class="comment">// The lambda<> expression wrapper makes expressions polymorphic</span> 10546<span class="comment">// function objects</span> 10547<span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">></span> 10548<span class="keyword">struct</span> <span class="identifier">lambda</span> 10549<span class="special">{</span> 10550 <span class="identifier">BOOST_PROTO_BASIC_EXTENDS</span><span class="special">(</span><span class="identifier">T</span><span class="special">,</span> <span class="identifier">lambda</span><span class="special"><</span><span class="identifier">T</span><span class="special">>,</span> <span class="identifier">lambda_domain</span><span class="special">)</span> 10551 <span class="identifier">BOOST_PROTO_EXTENDS_ASSIGN</span><span class="special">()</span> 10552 <span class="identifier">BOOST_PROTO_EXTENDS_SUBSCRIPT</span><span class="special">()</span> 10553 10554 <span class="comment">// Calculate the arity of this lambda expression</span> 10555 <span class="keyword">static</span> <span class="keyword">int</span> <span class="keyword">const</span> <span class="identifier">arity</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">result_of</span><span class="special"><</span><span class="identifier">lambda_arity</span><span class="special">(</span><span class="identifier">T</span><span class="special">)>::</span><span class="identifier">type</span><span class="special">::</span><span class="identifier">value</span><span class="special">;</span> 10556 10557 <span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">Sig</span><span class="special">></span> 10558 <span class="keyword">struct</span> <span class="identifier">result</span><span class="special">;</span> 10559 10560 <span class="comment">// Define nested result<> specializations to calculate the return</span> 10561 <span class="comment">// type of this lambda expression. But be careful not to evaluate</span> 10562 <span class="comment">// the return type of the nullary function unless we have a nullary</span> 10563 <span class="comment">// lambda!</span> 10564 <span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">This</span><span class="special">></span> 10565 <span class="keyword">struct</span> <span class="identifier">result</span><span class="special"><</span><span class="identifier">This</span><span class="special">()></span> 10566 <span class="special">:</span> <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">eval_if_c</span><span class="special"><</span> 10567 <span class="number">0</span> <span class="special">==</span> <span class="identifier">arity</span> 10568 <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">eval</span><span class="special"><</span><span class="identifier">T</span> <span class="keyword">const</span><span class="special">,</span> <span class="identifier">lambda_context</span><span class="special"><</span><span class="identifier">fusion</span><span class="special">::</span><span class="identifier">tuple</span><span class="special"><></span> <span class="special">></span> <span class="special">></span> 10569 <span class="special">,</span> <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">identity</span><span class="special"><</span><span class="keyword">void</span><span class="special">></span> 10570 <span class="special">></span> 10571 <span class="special">{};</span> 10572 10573 <span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">This</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">A0</span><span class="special">></span> 10574 <span class="keyword">struct</span> <span class="identifier">result</span><span class="special"><</span><span class="identifier">This</span><span class="special">(</span><span class="identifier">A0</span><span class="special">)></span> 10575 <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">eval</span><span class="special"><</span><span class="identifier">T</span> <span class="keyword">const</span><span class="special">,</span> <span class="identifier">lambda_context</span><span class="special"><</span><span class="identifier">fusion</span><span class="special">::</span><span class="identifier">tuple</span><span class="special"><</span><span class="identifier">A0</span><span class="special">></span> <span class="special">></span> <span class="special">></span> 10576 <span class="special">{};</span> 10577 10578 <span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">This</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">A0</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">A1</span><span class="special">></span> 10579 <span class="keyword">struct</span> <span class="identifier">result</span><span class="special"><</span><span class="identifier">This</span><span class="special">(</span><span class="identifier">A0</span><span class="special">,</span> <span class="identifier">A1</span><span class="special">)></span> 10580 <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">eval</span><span class="special"><</span><span class="identifier">T</span> <span class="keyword">const</span><span class="special">,</span> <span class="identifier">lambda_context</span><span class="special"><</span><span class="identifier">fusion</span><span class="special">::</span><span class="identifier">tuple</span><span class="special"><</span><span class="identifier">A0</span><span class="special">,</span> <span class="identifier">A1</span><span class="special">></span> <span class="special">></span> <span class="special">></span> 10581 <span class="special">{};</span> 10582 10583 <span class="comment">// Define our operator () that evaluates the lambda expression.</span> 10584 <span class="keyword">typename</span> <span class="identifier">result</span><span class="special"><</span><span class="identifier">lambda</span><span class="special">()>::</span><span class="identifier">type</span> 10585 <span class="keyword">operator</span> <span class="special">()()</span> <span class="keyword">const</span> 10586 <span class="special">{</span> 10587 <span class="identifier">fusion</span><span class="special">::</span><span class="identifier">tuple</span><span class="special"><></span> <span class="identifier">args</span><span class="special">;</span> 10588 <span class="identifier">lambda_context</span><span class="special"><</span><span class="identifier">fusion</span><span class="special">::</span><span class="identifier">tuple</span><span class="special"><></span> <span class="special">></span> <span class="identifier">ctx</span><span class="special">(</span><span class="identifier">args</span><span class="special">);</span> 10589 <span class="keyword">return</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">eval</span><span class="special">(*</span><span class="keyword">this</span><span class="special">,</span> <span class="identifier">ctx</span><span class="special">);</span> 10590 <span class="special">}</span> 10591 10592 <span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">A0</span><span class="special">></span> 10593 <span class="keyword">typename</span> <span class="identifier">result</span><span class="special"><</span><span class="identifier">lambda</span><span class="special">(</span><span class="identifier">A0</span> <span class="keyword">const</span> <span class="special">&)>::</span><span class="identifier">type</span> 10594 <span class="keyword">operator</span> <span class="special">()(</span><span class="identifier">A0</span> <span class="keyword">const</span> <span class="special">&</span><span class="identifier">a0</span><span class="special">)</span> <span class="keyword">const</span> 10595 <span class="special">{</span> 10596 <span class="identifier">fusion</span><span class="special">::</span><span class="identifier">tuple</span><span class="special"><</span><span class="identifier">A0</span> <span class="keyword">const</span> <span class="special">&></span> <span class="identifier">args</span><span class="special">(</span><span class="identifier">a0</span><span class="special">);</span> 10597 <span class="identifier">lambda_context</span><span class="special"><</span><span class="identifier">fusion</span><span class="special">::</span><span class="identifier">tuple</span><span class="special"><</span><span class="identifier">A0</span> <span class="keyword">const</span> <span class="special">&></span> <span class="special">></span> <span class="identifier">ctx</span><span class="special">(</span><span class="identifier">args</span><span class="special">);</span> 10598 <span class="keyword">return</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">eval</span><span class="special">(*</span><span class="keyword">this</span><span class="special">,</span> <span class="identifier">ctx</span><span class="special">);</span> 10599 <span class="special">}</span> 10600 10601 <span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">A0</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">A1</span><span class="special">></span> 10602 <span class="keyword">typename</span> <span class="identifier">result</span><span class="special"><</span><span class="identifier">lambda</span><span class="special">(</span><span class="identifier">A0</span> <span class="keyword">const</span> <span class="special">&,</span> <span class="identifier">A1</span> <span class="keyword">const</span> <span class="special">&)>::</span><span class="identifier">type</span> 10603 <span class="keyword">operator</span> <span class="special">()(</span><span class="identifier">A0</span> <span class="keyword">const</span> <span class="special">&</span><span class="identifier">a0</span><span class="special">,</span> <span class="identifier">A1</span> <span class="keyword">const</span> <span class="special">&</span><span class="identifier">a1</span><span class="special">)</span> <span class="keyword">const</span> 10604 <span class="special">{</span> 10605 <span class="identifier">fusion</span><span class="special">::</span><span class="identifier">tuple</span><span class="special"><</span><span class="identifier">A0</span> <span class="keyword">const</span> <span class="special">&,</span> <span class="identifier">A1</span> <span class="keyword">const</span> <span class="special">&></span> <span class="identifier">args</span><span class="special">(</span><span class="identifier">a0</span><span class="special">,</span> <span class="identifier">a1</span><span class="special">);</span> 10606 <span class="identifier">lambda_context</span><span class="special"><</span><span class="identifier">fusion</span><span class="special">::</span><span class="identifier">tuple</span><span class="special"><</span><span class="identifier">A0</span> <span class="keyword">const</span> <span class="special">&,</span> <span class="identifier">A1</span> <span class="keyword">const</span> <span class="special">&></span> <span class="special">></span> <span class="identifier">ctx</span><span class="special">(</span><span class="identifier">args</span><span class="special">);</span> 10607 <span class="keyword">return</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">eval</span><span class="special">(*</span><span class="keyword">this</span><span class="special">,</span> <span class="identifier">ctx</span><span class="special">);</span> 10608 <span class="special">}</span> 10609<span class="special">};</span> 10610 10611<span class="comment">// Define some lambda placeholders</span> 10612<span class="identifier">lambda</span><span class="special"><</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special"><</span><span class="identifier">placeholder</span><span class="special"><</span><span class="identifier">mpl</span><span class="special">::</span><span class="identifier">int_</span><span class="special"><</span><span class="number">0</span><span class="special">></span> <span class="special">></span> <span class="special">>::</span><span class="identifier">type</span><span class="special">></span> <span class="keyword">const</span> <span class="identifier">_1</span> <span class="special">=</span> <span class="special">{{}};</span> 10613<span class="identifier">lambda</span><span class="special"><</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special"><</span><span class="identifier">placeholder</span><span class="special"><</span><span class="identifier">mpl</span><span class="special">::</span><span class="identifier">int_</span><span class="special"><</span><span class="number">1</span><span class="special">></span> <span class="special">></span> <span class="special">>::</span><span class="identifier">type</span><span class="special">></span> <span class="keyword">const</span> <span class="identifier">_2</span> <span class="special">=</span> <span class="special">{{}};</span> 10614 10615<span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">></span> 10616<span class="identifier">lambda</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="identifier">type</span><span class="special">></span> <span class="keyword">const</span> <span class="identifier">val</span><span class="special">(</span><span class="identifier">T</span> <span class="keyword">const</span> <span class="special">&</span><span class="identifier">t</span><span class="special">)</span> 10617<span class="special">{</span> 10618 <span class="identifier">lambda</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="identifier">type</span><span class="special">></span> <span class="identifier">that</span> <span class="special">=</span> <span class="special">{{</span><span class="identifier">t</span><span class="special">}};</span> 10619 <span class="keyword">return</span> <span class="identifier">that</span><span class="special">;</span> 10620<span class="special">}</span> 10621 10622<span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">></span> 10623<span class="identifier">lambda</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special"><</span><span class="identifier">T</span> <span class="special">&>::</span><span class="identifier">type</span><span class="special">></span> <span class="keyword">const</span> <span class="identifier">var</span><span class="special">(</span><span class="identifier">T</span> <span class="special">&</span><span class="identifier">t</span><span class="special">)</span> 10624<span class="special">{</span> 10625 <span class="identifier">lambda</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special"><</span><span class="identifier">T</span> <span class="special">&>::</span><span class="identifier">type</span><span class="special">></span> <span class="identifier">that</span> <span class="special">=</span> <span class="special">{{</span><span class="identifier">t</span><span class="special">}};</span> 10626 <span class="keyword">return</span> <span class="identifier">that</span><span class="special">;</span> 10627<span class="special">}</span> 10628 10629<span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">></span> 10630<span class="keyword">struct</span> <span class="identifier">construct_helper</span> 10631<span class="special">{</span> 10632 <span class="keyword">typedef</span> <span class="identifier">T</span> <span class="identifier">result_type</span><span class="special">;</span> <span class="comment">// for TR1 result_of</span> 10633 10634 <span class="identifier">T</span> <span class="keyword">operator</span><span class="special">()()</span> <span class="keyword">const</span> 10635 <span class="special">{</span> <span class="keyword">return</span> <span class="identifier">T</span><span class="special">();</span> <span class="special">}</span> 10636 10637 <span class="comment">// Generate BOOST_PROTO_MAX_ARITY overloads of the</span> 10638 <span class="comment">// following function call operator.</span> 10639<span class="preprocessor">#define</span> <span class="identifier">BOOST_PROTO_LOCAL_MACRO</span><span class="special">(</span><span class="identifier">N</span><span class="special">,</span> <span class="identifier">typename_A</span><span class="special">,</span> <span class="identifier">A_const_ref</span><span class="special">,</span> <span class="identifier">A_const_ref_a</span><span class="special">,</span> <span class="identifier">a</span><span class="special">)\</span> 10640 <span class="keyword">template</span><span class="special"><</span><span class="identifier">typename_A</span><span class="special">(</span><span class="identifier">N</span><span class="special">)></span> <span class="special">\</span> 10641 <span class="identifier">T</span> <span class="keyword">operator</span><span class="special">()(</span><span class="identifier">A_const_ref_a</span><span class="special">(</span><span class="identifier">N</span><span class="special">))</span> <span class="keyword">const</span> <span class="special">\</span> 10642 <span class="special">{</span> <span class="keyword">return</span> <span class="identifier">T</span><span class="special">(</span><span class="identifier">a</span><span class="special">(</span><span class="identifier">N</span><span class="special">));</span> <span class="special">}</span> 10643<span class="preprocessor">#define</span> <span class="identifier">BOOST_PROTO_LOCAL_a</span> <span class="identifier">BOOST_PROTO_a</span> 10644<span class="preprocessor">#include</span> <span class="identifier">BOOST_PROTO_LOCAL_ITERATE</span><span class="special">()</span> 10645<span class="special">};</span> 10646 10647<span class="comment">// Generate BOOST_PROTO_MAX_ARITY-1 overloads of the</span> 10648<span class="comment">// following construct() function template.</span> 10649<span class="preprocessor">#define</span> <span class="identifier">M0</span><span class="special">(</span><span class="identifier">N</span><span class="special">,</span> <span class="identifier">typename_A</span><span class="special">,</span> <span class="identifier">A_const_ref</span><span class="special">,</span> <span class="identifier">A_const_ref_a</span><span class="special">,</span> <span class="identifier">ref_a</span><span class="special">)</span> <span class="special">\</span> 10650<span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">,</span> <span class="identifier">typename_A</span><span class="special">(</span><span class="identifier">N</span><span class="special">)></span> <span class="special">\</span> 10651<span class="keyword">typename</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">make_expr</span><span class="special"><</span> <span class="special">\</span> 10652 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">function</span> <span class="special">\</span> 10653 <span class="special">,</span> <span class="identifier">lambda_domain</span> <span class="special">\</span> 10654 <span class="special">,</span> <span class="identifier">construct_helper</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="special">\</span> 10655 <span class="special">,</span> <span class="identifier">A_const_ref</span><span class="special">(</span><span class="identifier">N</span><span class="special">)</span> <span class="special">\</span> 10656<span class="special">>::</span><span class="identifier">type</span> <span class="keyword">const</span> <span class="special">\</span> 10657<span class="identifier">construct</span><span class="special">(</span><span class="identifier">A_const_ref_a</span><span class="special">(</span><span class="identifier">N</span><span class="special">))</span> <span class="special">\</span> 10658<span class="special">{</span> <span class="special">\</span> 10659 <span class="keyword">return</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">make_expr</span><span class="special"><</span> <span class="special">\</span> 10660 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">function</span> <span class="special">\</span> 10661 <span class="special">,</span> <span class="identifier">lambda_domain</span> <span class="special">\</span> 10662 <span class="special">>(</span> <span class="special">\</span> 10663 <span class="identifier">construct_helper</span><span class="special"><</span><span class="identifier">T</span><span class="special">>()</span> <span class="special">\</span> 10664 <span class="special">,</span> <span class="identifier">ref_a</span><span class="special">(</span><span class="identifier">N</span><span class="special">)</span> <span class="special">\</span> 10665 <span class="special">);</span> <span class="special">\</span> 10666<span class="special">}</span> 10667<span class="identifier">BOOST_PROTO_REPEAT_FROM_TO</span><span class="special">(</span><span class="number">1</span><span class="special">,</span> <span class="identifier">BOOST_PROTO_MAX_ARITY</span><span class="special">,</span> <span class="identifier">M0</span><span class="special">)</span> 10668<span class="preprocessor">#undef</span> <span class="identifier">M0</span> 10669 10670<span class="keyword">struct</span> <span class="identifier">S</span> 10671<span class="special">{</span> 10672 <span class="identifier">S</span><span class="special">()</span> <span class="special">{}</span> 10673 <span class="identifier">S</span><span class="special">(</span><span class="keyword">int</span> <span class="identifier">i</span><span class="special">,</span> <span class="keyword">char</span> <span class="identifier">c</span><span class="special">)</span> 10674 <span class="special">{</span> 10675 <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="string">"S("</span> <span class="special"><<</span> <span class="identifier">i</span> <span class="special"><<</span> <span class="string">","</span> <span class="special"><<</span> <span class="identifier">c</span> <span class="special"><<</span> <span class="string">")\n"</span><span class="special">;</span> 10676 <span class="special">}</span> 10677<span class="special">};</span> 10678 10679<span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span> 10680<span class="special">{</span> 10681 <span class="comment">// Create some lambda objects and immediately</span> 10682 <span class="comment">// invoke them by applying their operator():</span> 10683 <span class="keyword">int</span> <span class="identifier">i</span> <span class="special">=</span> <span class="special">(</span> <span class="special">(</span><span class="identifier">_1</span> <span class="special">+</span> <span class="number">2</span><span class="special">)</span> <span class="special">/</span> <span class="number">4</span> <span class="special">)(</span><span class="number">42</span><span class="special">);</span> 10684 <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="identifier">i</span> <span class="special"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span> <span class="comment">// prints 11</span> 10685 10686 <span class="keyword">int</span> <span class="identifier">j</span> <span class="special">=</span> <span class="special">(</span> <span class="special">(-(</span><span class="identifier">_1</span> <span class="special">+</span> <span class="number">2</span><span class="special">))</span> <span class="special">/</span> <span class="number">4</span> <span class="special">)(</span><span class="number">42</span><span class="special">);</span> 10687 <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="identifier">j</span> <span class="special"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span> <span class="comment">// prints -11</span> 10688 10689 <span class="keyword">double</span> <span class="identifier">d</span> <span class="special">=</span> <span class="special">(</span> <span class="special">(</span><span class="number">4</span> <span class="special">-</span> <span class="identifier">_2</span><span class="special">)</span> <span class="special">*</span> <span class="number">3</span> <span class="special">)(</span><span class="number">42</span><span class="special">,</span> <span class="number">3.14</span><span class="special">);</span> 10690 <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="identifier">d</span> <span class="special"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span> <span class="comment">// prints 2.58</span> 10691 10692 <span class="comment">// check non-const ref terminals</span> 10693 <span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="identifier">_1</span> <span class="special"><<</span> <span class="string">" -- "</span> <span class="special"><<</span> <span class="identifier">_2</span> <span class="special"><<</span> <span class="char">'\n'</span><span class="special">)(</span><span class="number">42</span><span class="special">,</span> <span class="string">"Life, the Universe and Everything!"</span><span class="special">);</span> 10694 <span class="comment">// prints "42 -- Life, the Universe and Everything!"</span> 10695 10696 <span class="comment">// "Nullary" lambdas work too</span> 10697 <span class="keyword">int</span> <span class="identifier">k</span> <span class="special">=</span> <span class="special">(</span><span class="identifier">val</span><span class="special">(</span><span class="number">1</span><span class="special">)</span> <span class="special">+</span> <span class="identifier">val</span><span class="special">(</span><span class="number">2</span><span class="special">))();</span> 10698 <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="identifier">k</span> <span class="special"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span> <span class="comment">// prints 3</span> 10699 10700 <span class="comment">// check array indexing for kicks</span> 10701 <span class="keyword">int</span> <span class="identifier">integers</span><span class="special">[</span><span class="number">5</span><span class="special">]</span> <span class="special">=</span> <span class="special">{</span><span class="number">0</span><span class="special">};</span> 10702 <span class="special">(</span><span class="identifier">var</span><span class="special">(</span><span class="identifier">integers</span><span class="special">)[</span><span class="number">2</span><span class="special">]</span> <span class="special">=</span> <span class="number">2</span><span class="special">)();</span> 10703 <span class="special">(</span><span class="identifier">var</span><span class="special">(</span><span class="identifier">integers</span><span class="special">)[</span><span class="identifier">_1</span><span class="special">]</span> <span class="special">=</span> <span class="identifier">_1</span><span class="special">)(</span><span class="number">3</span><span class="special">);</span> 10704 <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="identifier">integers</span><span class="special">[</span><span class="number">2</span><span class="special">]</span> <span class="special"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span> <span class="comment">// prints 2</span> 10705 <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="identifier">integers</span><span class="special">[</span><span class="number">3</span><span class="special">]</span> <span class="special"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span> <span class="comment">// prints 3</span> 10706 10707 <span class="comment">// Now use a lambda with an STL algorithm!</span> 10708 <span class="keyword">int</span> <span class="identifier">rgi</span><span class="special">[</span><span class="number">4</span><span class="special">]</span> <span class="special">=</span> <span class="special">{</span><span class="number">1</span><span class="special">,</span><span class="number">2</span><span class="special">,</span><span class="number">3</span><span class="special">,</span><span class="number">4</span><span class="special">};</span> 10709 <span class="keyword">char</span> <span class="identifier">rgc</span><span class="special">[</span><span class="number">4</span><span class="special">]</span> <span class="special">=</span> <span class="special">{</span><span class="char">'a'</span><span class="special">,</span><span class="char">'b'</span><span class="special">,</span><span class="char">'c'</span><span class="special">,</span><span class="char">'d'</span><span class="special">};</span> 10710 <span class="identifier">S</span> <span class="identifier">rgs</span><span class="special">[</span><span class="number">4</span><span class="special">];</span> 10711 10712 <span class="identifier">std</span><span class="special">::</span><span class="identifier">transform</span><span class="special">(</span><span class="identifier">rgi</span><span class="special">,</span> <span class="identifier">rgi</span><span class="special">+</span><span class="number">4</span><span class="special">,</span> <span class="identifier">rgc</span><span class="special">,</span> <span class="identifier">rgs</span><span class="special">,</span> <span class="identifier">construct</span><span class="special"><</span><span class="identifier">S</span><span class="special">>(</span><span class="identifier">_1</span><span class="special">,</span> <span class="identifier">_2</span><span class="special">));</span> 10713 <span class="keyword">return</span> <span class="number">0</span><span class="special">;</span> 10714<span class="special">}</span> 10715</pre> 10716<p> 10717 </p> 10718</div> 10719<div class="section"> 10720<div class="titlepage"><div><div><h4 class="title"> 10721<a name="boost_proto.users_guide.examples.checked_calc"></a><a class="link" href="users_guide.html#boost_proto.users_guide.examples.checked_calc" title="Checked Calculator: A Simple Example of External Transforms">Checked 10722 Calculator: A Simple Example of External Transforms</a> 10723</h4></div></div></div> 10724<p> 10725 This is an advanced example that shows how to externally parameterize a 10726 grammar's transforms. It defines a calculator EDSL with a grammar that 10727 can perform either checked or unchecked arithmetic. 10728 </p> 10729<p> 10730</p> 10731<pre class="programlisting"><span class="comment">// Copyright 2011 Eric Niebler. Distributed under the Boost</span> 10732<span class="comment">// Software License, Version 1.0. (See accompanying file</span> 10733<span class="comment">// LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)</span> 10734<span class="comment">//</span> 10735<span class="comment">// This is an example of how to specify a transform externally so</span> 10736<span class="comment">// that a single grammar can be used to drive multiple differnt</span> 10737<span class="comment">// calculations. In particular, it defines a calculator grammar</span> 10738<span class="comment">// that computes the result of an expression with either checked</span> 10739<span class="comment">// or non-checked division.</span> 10740 10741<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">iostream</span><span class="special">></span> 10742<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">assert</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> 10743<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">mpl</span><span class="special">/</span><span class="keyword">int</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> 10744<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">mpl</span><span class="special">/</span><span class="identifier">next</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> 10745<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">mpl</span><span class="special">/</span><span class="identifier">min_max</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> 10746<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">fusion</span><span class="special">/</span><span class="identifier">container</span><span class="special">/</span><span class="identifier">vector</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> 10747<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">fusion</span><span class="special">/</span><span class="identifier">container</span><span class="special">/</span><span class="identifier">generation</span><span class="special">/</span><span class="identifier">make_vector</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> 10748<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">proto</span><span class="special">/</span><span class="identifier">proto</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> 10749<span class="keyword">namespace</span> <span class="identifier">mpl</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">mpl</span><span class="special">;</span> 10750<span class="keyword">namespace</span> <span class="identifier">proto</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">proto</span><span class="special">;</span> 10751<span class="keyword">namespace</span> <span class="identifier">fusion</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">fusion</span><span class="special">;</span> 10752 10753<span class="comment">// The argument placeholder type</span> 10754<span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">I</span><span class="special">></span> <span class="keyword">struct</span> <span class="identifier">placeholder</span> <span class="special">:</span> <span class="identifier">I</span> <span class="special">{};</span> 10755 10756<span class="comment">// Give each rule in the grammar a "name". This is so that we</span> 10757<span class="comment">// can easily dispatch on it later.</span> 10758<span class="keyword">struct</span> <span class="identifier">calc_grammar</span><span class="special">;</span> 10759<span class="keyword">struct</span> <span class="identifier">divides_rule</span> <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">divides</span><span class="special"><</span><span class="identifier">calc_grammar</span><span class="special">,</span> <span class="identifier">calc_grammar</span><span class="special">></span> <span class="special">{};</span> 10760 10761<span class="comment">// Use external transforms in calc_gramar</span> 10762<span class="keyword">struct</span> <span class="identifier">calc_grammar</span> 10763 <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">or_</span><span class="special"><</span> 10764 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special"><</span> 10765 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special"><</span><span class="identifier">placeholder</span><span class="special"><</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_</span><span class="special">></span> <span class="special">></span> 10766 <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">functional</span><span class="special">::</span><span class="identifier">at</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_state</span><span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_value</span><span class="special">)</span> 10767 <span class="special">></span> 10768 <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special"><</span> 10769 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special"><</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">convertible_to</span><span class="special"><</span><span class="keyword">double</span><span class="special">></span> <span class="special">></span> 10770 <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_value</span> 10771 <span class="special">></span> 10772 <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special"><</span> 10773 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">plus</span><span class="special"><</span><span class="identifier">calc_grammar</span><span class="special">,</span> <span class="identifier">calc_grammar</span><span class="special">></span> 10774 <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_default</span><span class="special"><</span><span class="identifier">calc_grammar</span><span class="special">></span> 10775 <span class="special">></span> 10776 <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special"><</span> 10777 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">minus</span><span class="special"><</span><span class="identifier">calc_grammar</span><span class="special">,</span> <span class="identifier">calc_grammar</span><span class="special">></span> 10778 <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_default</span><span class="special"><</span><span class="identifier">calc_grammar</span><span class="special">></span> 10779 <span class="special">></span> 10780 <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special"><</span> 10781 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">multiplies</span><span class="special"><</span><span class="identifier">calc_grammar</span><span class="special">,</span> <span class="identifier">calc_grammar</span><span class="special">></span> 10782 <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_default</span><span class="special"><</span><span class="identifier">calc_grammar</span><span class="special">></span> 10783 <span class="special">></span> 10784 <span class="comment">// Note that we don't specify how division nodes are</span> 10785 <span class="comment">// handled here. Proto::external_transform is a placeholder</span> 10786 <span class="comment">// for an actual transform.</span> 10787 <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special"><</span> 10788 <span class="identifier">divides_rule</span> 10789 <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">external_transform</span> 10790 <span class="special">></span> 10791 <span class="special">></span> 10792<span class="special">{};</span> 10793 10794<span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">E</span><span class="special">></span> <span class="keyword">struct</span> <span class="identifier">calc_expr</span><span class="special">;</span> 10795<span class="keyword">struct</span> <span class="identifier">calc_domain</span> <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">domain</span><span class="special"><</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">generator</span><span class="special"><</span><span class="identifier">calc_expr</span><span class="special">></span> <span class="special">></span> <span class="special">{};</span> 10796 10797<span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">E</span><span class="special">></span> 10798<span class="keyword">struct</span> <span class="identifier">calc_expr</span> 10799 <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">extends</span><span class="special"><</span><span class="identifier">E</span><span class="special">,</span> <span class="identifier">calc_expr</span><span class="special"><</span><span class="identifier">E</span><span class="special">>,</span> <span class="identifier">calc_domain</span><span class="special">></span> 10800<span class="special">{</span> 10801 <span class="identifier">calc_expr</span><span class="special">(</span><span class="identifier">E</span> <span class="keyword">const</span> <span class="special">&</span><span class="identifier">e</span> <span class="special">=</span> <span class="identifier">E</span><span class="special">())</span> <span class="special">:</span> <span class="identifier">calc_expr</span><span class="special">::</span><span class="identifier">proto_extends</span><span class="special">(</span><span class="identifier">e</span><span class="special">)</span> <span class="special">{}</span> 10802<span class="special">};</span> 10803 10804<span class="identifier">calc_expr</span><span class="special"><</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special"><</span><span class="identifier">placeholder</span><span class="special"><</span><span class="identifier">mpl</span><span class="special">::</span><span class="identifier">int_</span><span class="special"><</span><span class="number">0</span><span class="special">></span> <span class="special">></span> <span class="special">>::</span><span class="identifier">type</span><span class="special">></span> <span class="identifier">_1</span><span class="special">;</span> 10805<span class="identifier">calc_expr</span><span class="special"><</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special"><</span><span class="identifier">placeholder</span><span class="special"><</span><span class="identifier">mpl</span><span class="special">::</span><span class="identifier">int_</span><span class="special"><</span><span class="number">1</span><span class="special">></span> <span class="special">></span> <span class="special">>::</span><span class="identifier">type</span><span class="special">></span> <span class="identifier">_2</span><span class="special">;</span> 10806 10807<span class="comment">// Use proto::external_transforms to map from named grammar rules to</span> 10808<span class="comment">// transforms.</span> 10809<span class="keyword">struct</span> <span class="identifier">non_checked_division</span> 10810 <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">external_transforms</span><span class="special"><</span> 10811 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special"><</span> <span class="identifier">divides_rule</span><span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_default</span><span class="special"><</span><span class="identifier">calc_grammar</span><span class="special">></span> <span class="special">></span> 10812 <span class="special">></span> 10813<span class="special">{};</span> 10814 10815<span class="keyword">struct</span> <span class="identifier">division_by_zero</span> <span class="special">:</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">exception</span> <span class="special">{};</span> 10816 10817<span class="keyword">struct</span> <span class="identifier">do_checked_divide</span> 10818 <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">callable</span> 10819<span class="special">{</span> 10820 <span class="keyword">typedef</span> <span class="keyword">int</span> <span class="identifier">result_type</span><span class="special">;</span> 10821 <span class="keyword">int</span> <span class="keyword">operator</span><span class="special">()(</span><span class="keyword">int</span> <span class="identifier">left</span><span class="special">,</span> <span class="keyword">int</span> <span class="identifier">right</span><span class="special">)</span> <span class="keyword">const</span> 10822 <span class="special">{</span> 10823 <span class="keyword">if</span> <span class="special">(</span><span class="identifier">right</span> <span class="special">==</span> <span class="number">0</span><span class="special">)</span> <span class="keyword">throw</span> <span class="identifier">division_by_zero</span><span class="special">();</span> 10824 <span class="keyword">return</span> <span class="identifier">left</span> <span class="special">/</span> <span class="identifier">right</span><span class="special">;</span> 10825 <span class="special">}</span> 10826<span class="special">};</span> 10827 10828<span class="comment">// Use proto::external_transforms again, this time to map the divides_rule</span> 10829<span class="comment">// to a transforms that performs checked division.</span> 10830<span class="keyword">struct</span> <span class="identifier">checked_division</span> 10831 <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">external_transforms</span><span class="special"><</span> 10832 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special"><</span> 10833 <span class="identifier">divides_rule</span> 10834 <span class="special">,</span> <span class="identifier">do_checked_divide</span><span class="special">(</span><span class="identifier">calc_grammar</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_left</span><span class="special">),</span> <span class="identifier">calc_grammar</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_right</span><span class="special">))</span> 10835 <span class="special">></span> 10836 <span class="special">></span> 10837<span class="special">{};</span> 10838 10839<span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span> 10840<span class="special">{</span> 10841 <span class="identifier">non_checked_division</span> <span class="identifier">non_checked</span><span class="special">;</span> 10842 <span class="keyword">int</span> <span class="identifier">result2</span> <span class="special">=</span> <span class="identifier">calc_grammar</span><span class="special">()(</span><span class="identifier">_1</span> <span class="special">/</span> <span class="identifier">_2</span><span class="special">,</span> <span class="identifier">fusion</span><span class="special">::</span><span class="identifier">make_vector</span><span class="special">(</span><span class="number">6</span><span class="special">,</span> <span class="number">2</span><span class="special">),</span> <span class="identifier">non_checked</span><span class="special">);</span> 10843 <span class="identifier">BOOST_ASSERT</span><span class="special">(</span><span class="identifier">result2</span> <span class="special">==</span> <span class="number">3</span><span class="special">);</span> 10844 10845 <span class="keyword">try</span> 10846 <span class="special">{</span> 10847 <span class="identifier">checked_division</span> <span class="identifier">checked</span><span class="special">;</span> 10848 <span class="comment">// This should throw</span> 10849 <span class="keyword">int</span> <span class="identifier">result3</span> <span class="special">=</span> <span class="identifier">calc_grammar</span><span class="special">()(</span><span class="identifier">_1</span> <span class="special">/</span> <span class="identifier">_2</span><span class="special">,</span> <span class="identifier">fusion</span><span class="special">::</span><span class="identifier">make_vector</span><span class="special">(</span><span class="number">6</span><span class="special">,</span> <span class="number">0</span><span class="special">),</span> <span class="identifier">checked</span><span class="special">);</span> 10850 <span class="identifier">BOOST_ASSERT</span><span class="special">(</span><span class="keyword">false</span><span class="special">);</span> <span class="comment">// shouldn't get here!</span> 10851 <span class="special">}</span> 10852 <span class="keyword">catch</span><span class="special">(</span><span class="identifier">division_by_zero</span><span class="special">)</span> 10853 <span class="special">{</span> 10854 <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="string">"caught division by zero!\n"</span><span class="special">;</span> 10855 <span class="special">}</span> 10856<span class="special">}</span> 10857</pre> 10858<p> 10859 </p> 10860</div> 10861</div> 10862<div class="section"> 10863<div class="titlepage"><div><div><h3 class="title"> 10864<a name="boost_proto.users_guide.resources"></a><a class="link" href="users_guide.html#boost_proto.users_guide.resources" title="Background and Resources">Background and Resources</a> 10865</h3></div></div></div> 10866<p> 10867 Proto was initially developed as part of <a href="../../../libs/xpressive/index.html" target="_top">Boost.Xpressive</a> 10868 to simplify the job of transforming an expression template into an executable 10869 finite state machine capable of matching a regular expression. Since then, 10870 Proto has found application in the redesigned and improved Spirit-2 and the 10871 related Karma library. As a result of these efforts, Proto evolved into a 10872 generic and abstract grammar and tree transformation framework applicable 10873 in a wide variety of EDSL scenarios. 10874 </p> 10875<p> 10876 The grammar and tree transformation framework is modeled on Spirit's grammar 10877 and semantic action framework. The expression tree data structure is similar 10878 to Fusion data structures in many respects, and is interoperable with Fusion's 10879 iterators and algorithms. 10880 </p> 10881<p> 10882 The syntax for the grammar-matching features of <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">matches</span><span class="special"><></span></code> is inspired by MPL's lambda expressions. 10883 </p> 10884<p> 10885 The idea for using function types for Proto's composite transforms is inspired 10886 by Aleksey Gurtovoy's <a href="http://lists.boost.org/Archives/boost/2002/11/39718.php" target="_top">"round" 10887 lambda</a> notation. 10888 </p> 10889<h5> 10890<a name="boost_proto.users_guide.resources.h0"></a> 10891 <span class="phrase"><a name="boost_proto.users_guide.resources.references"></a></span><a class="link" href="users_guide.html#boost_proto.users_guide.resources.references">References</a> 10892 </h5> 10893<div class="blockquote"><blockquote class="blockquote"><p> 10894 <a name="boost_proto.users_guide.resources.SYB"></a>Ren, D. and Erwig, 10895 M. 2006. A generic recursion toolbox for Haskell or: scrap your boilerplate 10896 systematically. In <span class="emphasis"><em>Proceedings of the 2006 ACM SIGPLAN Workshop 10897 on Haskell</em></span> (Portland, Oregon, USA, September 17 - 17, 2006). 10898 Haskell '06. ACM, New York, NY, 13-24. DOI=<a href="http://doi.acm.org/10.1145/1159842.1159845" target="_top">http://doi.acm.org/10.1145/1159842.1159845</a> 10899 </p></blockquote></div> 10900<h5> 10901<a name="boost_proto.users_guide.resources.h1"></a> 10902 <span class="phrase"><a name="boost_proto.users_guide.resources.further_reading"></a></span><a class="link" href="users_guide.html#boost_proto.users_guide.resources.further_reading">Further 10903 Reading</a> 10904 </h5> 10905<p> 10906 A technical paper about an earlier version of Proto was accepted into the 10907 <a href="http://lcsd.cs.tamu.edu/2007/" target="_top">ACM SIGPLAN Symposium on Library-Centric 10908 Software Design LCSD'07</a>, and can be found at <a href="http://lcsd.cs.tamu.edu/2007/final/1/1_Paper.pdf" target="_top">http://lcsd.cs.tamu.edu/2007/final/1/1_Paper.pdf</a>. 10909 The tree transforms described in that paper differ from what exists today. 10910 </p> 10911</div> 10912<div class="section"> 10913<div class="titlepage"><div><div><h3 class="title"> 10914<a name="boost_proto.users_guide.glossary"></a><a class="link" href="users_guide.html#boost_proto.users_guide.glossary" title="Glossary">Glossary</a> 10915</h3></div></div></div> 10916<div class="variablelist"> 10917<p class="title"><b></b></p> 10918<dl class="variablelist"> 10919<dt><span class="term"> <a name="boost_proto.users_guide.glossary.callable_transform"></a> callable transform</span></dt> 10920<dd><p> 10921 A transform of the form <code class="computeroutput"><span class="identifier">R</span><span class="special">(</span><span class="identifier">A0</span><span class="special">,</span><span class="identifier">A1</span><span class="special">,...)</span></code> (i.e., a function type) where 10922 <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">is_callable</span><span class="special"><</span><span class="identifier">R</span><span class="special">>::</span><span class="identifier">value</span></code> is <code class="computeroutput"><span class="keyword">true</span></code>. 10923 <code class="computeroutput"><span class="identifier">R</span></code> is treated as a polymorphic 10924 function object and the arguments are treated as transforms that yield 10925 the arguments to the function object. 10926 </p></dd> 10927<dt><span class="term"> <a name="boost_proto.users_guide.glossary.context"></a> context</span></dt> 10928<dd><p> 10929 In Proto, the term <span class="emphasis"><em>context</em></span> refers to an object 10930 that can be passed, along with an expression to evaluate, to the <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">eval</span><span class="special">()</span></code> 10931 function. The context determines how the expression is evaluated. All 10932 context structs define a nested <code class="computeroutput"><span class="identifier">eval</span><span class="special"><></span></code> template that, when instantiated 10933 with a node tag type (e.g., <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">plus</span></code>), 10934 is a binary polymorphic function object that accepts an expression 10935 of that type and the context object. In this way, contexts associate 10936 behaviors with expression nodes. 10937 </p></dd> 10938<dt><span class="term"> <a name="boost_proto.users_guide.glossary.domain"></a> domain</span></dt> 10939<dd><p> 10940 In Proto, the term <span class="emphasis"><em>domain</em></span> refers to a type that 10941 associates expressions within that domain with a <span class="emphasis"><em>generator</em></span> 10942 for that domain and optionally a <span class="emphasis"><em>grammar</em></span> for the 10943 domain. Domains are used primarily to imbue expressions within that 10944 domain with additional members and to restrict Proto's operator overloads 10945 such that expressions not conforming to the domain's grammar are never 10946 created. Domains are empty structs that inherit from <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">domain</span><span class="special"><></span></code>. 10947 </p></dd> 10948<dt><span class="term"> <a name="boost_proto.users_guide.glossary.dsl"></a> domain-specific language</span></dt> 10949<dd><p> 10950 A programming language that targets a particular problem space by providing 10951 programming idioms, abstractions and constructs that match the constructs 10952 within that problem space. 10953 </p></dd> 10954<dt><span class="term"> <a name="boost_proto.users_guide.glossary.edsl"></a> embedded domain-specific language</span></dt> 10955<dd><p> 10956 A domain-specific language implemented as a library. The language in 10957 which the library is written is called the "host" language, 10958 and the language implemented by the library is called the "embedded" 10959 language. 10960 </p></dd> 10961<dt><span class="term"> <a name="boost_proto.users_guide.glossary.expression"></a> expression</span></dt> 10962<dd><p> 10963 In Proto, an <span class="emphasis"><em>expression</em></span> is a heterogeneous tree 10964 where each node is either an instantiation of <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">expr</span><span class="special"><></span></code>, <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">basic_expr</span><span class="special"><></span></code> or some type that is an extension 10965 (via <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">extends</span><span class="special"><></span></code> 10966 or <code class="computeroutput"><span class="identifier">BOOST_PROTO_EXTENDS</span><span class="special">()</span></code>) of such an instantiation. 10967 </p></dd> 10968<dt><span class="term"> <a name="boost_proto.users_guide.glossary.expression_template"></a> expression template</span></dt> 10969<dd><p> 10970 A C++ technique using templates and operator overloading to cause expressions 10971 to build trees that represent the expression for lazy evaluation later, 10972 rather than evaluating the expression eagerly. Some C++ libraries use 10973 expression templates to build embedded domain-specific languages. 10974 </p></dd> 10975<dt><span class="term"> <a name="boost_proto.users_guide.glossary.generator"></a> generator</span></dt> 10976<dd><p> 10977 In Proto, a <span class="emphasis"><em>generator</em></span> is a unary polymorphic function 10978 object that you specify when defining a <span class="emphasis"><em>domain</em></span>. 10979 After constructing a new expression, Proto passes the expression to 10980 your domain's generator for further processing. Often, the generator 10981 wraps the expression in an extension wrapper that adds additional members 10982 to it. 10983 </p></dd> 10984<dt><span class="term"> <a name="boost_proto.users_guide.glossary.grammar"></a> grammar</span></dt> 10985<dd><p> 10986 In Proto, a <span class="emphasis"><em>grammar</em></span> is a type that describes a 10987 subset of Proto expression types. Expressions in a domain must conform 10988 to that domain's grammar. The <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">matches</span><span class="special"><></span></code> metafunction evaluates whether 10989 an expression type matches a grammar. Grammars are either primitives 10990 such as <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_</span></code>, composites such as <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">plus</span><span class="special"><></span></code>, 10991 control structures such as <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">or_</span><span class="special"><></span></code>, or some type derived from 10992 a grammar. 10993 </p></dd> 10994<dt><span class="term"> <a name="boost_proto.users_guide.glossary.object_transform"></a> object transform</span></dt> 10995<dd><p> 10996 A transform of the form <code class="computeroutput"><span class="identifier">R</span><span class="special">(</span><span class="identifier">A0</span><span class="special">,</span><span class="identifier">A1</span><span class="special">,...)</span></code> (i.e., a function type) where 10997 <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">is_callable</span><span class="special"><</span><span class="identifier">R</span><span class="special">>::</span><span class="identifier">value</span></code> is <code class="computeroutput"><span class="keyword">false</span></code>. 10998 <code class="computeroutput"><span class="identifier">R</span></code> is treated as the 10999 type of an object to construct and the arguments are treated as transforms 11000 that yield the parameters to the constructor. 11001 </p></dd> 11002<dt><span class="term"> <a name="boost_proto.users_guide.glossary.polymorphic_function_object"></a> polymorphic function object</span></dt> 11003<dd><p> 11004 An instance of a class type with an overloaded function call operator 11005 and a nested <code class="computeroutput"><span class="identifier">result_type</span></code> 11006 typedef or <code class="computeroutput"><span class="identifier">result</span><span class="special"><></span></code> 11007 template for calculating the return type of the function call operator. 11008 </p></dd> 11009<dt><span class="term"> <a name="boost_proto.users_guide.glossary.primitive_transform"></a> primitive transform</span></dt> 11010<dd><p> 11011 A type that defines a kind of polymorphic function object that takes 11012 three arguments: expression, state, and data. Primitive transforms 11013 can be used to compose callable transforms and object transforms. 11014 </p></dd> 11015<dt><span class="term"> <a name="boost_proto.users_guide.glossary.subdomain"></a> sub-domain</span></dt> 11016<dd><p> 11017 A sub-domain is a domain that declares another domain as its super-domain. 11018 Expressions in sub-domains can be combined with expressions in the 11019 super-domain, and the resulting expression is in the super-domain. 11020 </p></dd> 11021<dt><span class="term"> <a name="boost_proto.users_guide.glossary.transform"></a> transform</span></dt> 11022<dd><p> 11023 Transforms are used to manipulate expression trees. They come in three 11024 flavors: primitive transforms, callable transforms, or object transforms. 11025 A transform <code class="computeroutput"><em class="replaceable"><code>T</code></em></code> can be made into 11026 a ternary polymorphic function object with <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special"><></span></code>, as in <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special"><</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_</span><span class="special">,</span> <em class="replaceable"><code>T</code></em><span class="special">></span></code>. 11027 Such a function object accepts <span class="emphasis"><em>expression</em></span>, <span class="emphasis"><em>state</em></span>, 11028 and <span class="emphasis"><em>data</em></span> parameters, and computes a result from 11029 them. 11030 </p></dd> 11031</dl> 11032</div> 11033</div> 11034<div class="footnotes"> 11035<br><hr style="width:100; text-align:left;margin-left: 0"> 11036<div id="ftn.boost_proto.users_guide.front_end.customizing_expressions_in_your_domain.per_domain_as_child.f0" class="footnote"><p><a href="#boost_proto.users_guide.front_end.customizing_expressions_in_your_domain.per_domain_as_child.f0" class="para"><sup class="para">[34] </sup></a> 11037 It's not always possible to hold something by value. By default, <code class="computeroutput"><a class="link" href="../boost/proto/as_expr.html" title="Function as_expr">proto::as_expr()</a></code> makes an exception 11038 for functions, abstract types, and iostreams (types derived from <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">ios_base</span></code>). These objects are held 11039 by reference. All others are held by value, even arrays. 11040 </p></div> 11041<div id="ftn.boost_proto.users_guide.back_end.expression_transformation.example__calculator_arity.f0" class="footnote"><p><a href="#boost_proto.users_guide.back_end.expression_transformation.example__calculator_arity.f0" class="para"><sup class="para">[35] </sup></a> 11042 This error message was generated with Microsoft Visual C++ 9.0. Different 11043 compilers will emit different messages with varying degrees of readability. 11044 </p></div> 11045</div> 11046</div> 11047<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> 11048<td align="left"></td> 11049<td align="right"><div class="copyright-footer">Copyright © 2008 Eric Niebler<p> 11050 Distributed under the Boost Software License, Version 1.0. (See accompanying 11051 file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>) 11052 </p> 11053</div></td> 11054</tr></table> 11055<hr> 11056<div class="spirit-nav"> 11057<a accesskey="p" href="../proto.html"><img src="../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../proto.html"><img src="../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../index.html"><img src="../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="reference.html"><img src="../../../doc/src/images/next.png" alt="Next"></a> 11058</div> 11059</body> 11060</html> 11061
