1<html> 2<head> 3<meta http-equiv="Content-Type" content="text/html; charset=UTF-8"> 4<title>Actors in Detail</title> 5<link rel="stylesheet" href="../../../../../../doc/src/boostbook.css" type="text/css"> 6<meta name="generator" content="DocBook XSL Stylesheets V1.79.1"> 7<link rel="home" href="../../index.html" title="Chapter 1. Phoenix 3.2.0"> 8<link rel="up" href="../inside.html" title="Inside Phoenix"> 9<link rel="prev" href="../inside.html" title="Inside Phoenix"> 10<link rel="next" href="expression.html" title="Phoenix Expressions"> 11</head> 12<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"> 13<table cellpadding="2" width="100%"><tr> 14<td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../../../../boost.png"></td> 15<td align="center"><a href="../../../../../../index.html">Home</a></td> 16<td align="center"><a href="../../../../../../libs/libraries.htm">Libraries</a></td> 17<td align="center"><a href="http://www.boost.org/users/people.html">People</a></td> 18<td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td> 19<td align="center"><a href="../../../../../../more/index.htm">More</a></td> 20</tr></table> 21<hr> 22<div class="spirit-nav"> 23<a accesskey="p" href="../inside.html"><img src="../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../inside.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="expression.html"><img src="../../../../../../doc/src/images/next.png" alt="Next"></a> 24</div> 25<div class="section"> 26<div class="titlepage"><div><div><h3 class="title"> 27<a name="phoenix.inside.actor"></a><a class="link" href="actor.html" title="Actors in Detail">Actors in Detail</a> 28</h3></div></div></div> 29<h5> 30<a name="phoenix.inside.actor.h0"></a> 31 <span class="phrase"><a name="phoenix.inside.actor.actor"></a></span><a class="link" href="actor.html#phoenix.inside.actor.actor">Actor</a> 32 </h5> 33<p> 34 The main concept is the <code class="computeroutput"><span class="identifier">Actor</span></code>. 35 An <code class="computeroutput"><span class="identifier">Actor</span></code> is a model of the 36 <a href="http://www.boost.org/doc/libs/release/libs/fusion/doc/html/fusion/functional/concepts/poly.html" target="_top">Polymorphic 37 Function Object</a> concept (that can accept 0 to N arguments (where 38 N is a predefined maximum). 39 </p> 40<p> 41 An <code class="computeroutput"><span class="identifier">Actor</span></code> contains a valid 42 Phoenix Expression, a call to one of the function call operator overloads, 43 starts the evaluation process. 44 </p> 45<div class="note"><table border="0" summary="Note"> 46<tr> 47<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../../../../doc/src/images/note.png"></td> 48<th align="left">Note</th> 49</tr> 50<tr><td align="left" valign="top"><p> 51 You can set <code class="computeroutput"><span class="identifier">BOOST_PHOENIX_LIMIT</span></code>, 52 the predefined maximum arity an actor can take. By default, <code class="computeroutput"><span class="identifier">BOOST_PHOENIX_LIMIT</span></code> is set to 10. 53 </p></td></tr> 54</table></div> 55<p> 56 The <code class="computeroutput"><span class="identifier">actor</span></code> template class 57 models the <code class="computeroutput"><span class="identifier">Actor</span></code> concept: 58 </p> 59<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> 60<span class="keyword">struct</span> <span class="identifier">actor</span> 61<span class="special">{</span> 62 <span class="keyword">template</span> <span class="special"><</span><span class="keyword">typename</span> <span class="identifier">Sig</span><span class="special">></span> 63 <span class="keyword">struct</span> <span class="identifier">result</span><span class="special">;</span> 64 65 <span class="keyword">typename</span> <span class="identifier">result_of</span><span class="special">::</span><span class="identifier">actor</span><span class="special"><</span><span class="identifier">Expr</span><span class="special">>::</span><span class="identifier">type</span> 66 <span class="keyword">operator</span><span class="special">()()</span> <span class="keyword">const</span><span class="special">;</span> 67 68 <span class="keyword">template</span> <span class="special"><</span><span class="keyword">typename</span> <span class="identifier">T0</span><span class="special">></span> 69 <span class="keyword">typename</span> <span class="identifier">result_of</span><span class="special">::</span><span class="identifier">actor</span><span class="special"><</span><span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">T0</span> <span class="special">&>::</span><span class="identifier">type</span> 70 <span class="keyword">operator</span><span class="special">()(</span><span class="identifier">T0</span><span class="special">&</span> <span class="identifier">_0</span><span class="special">)</span> <span class="keyword">const</span><span class="special">;</span> 71 72 <span class="keyword">template</span> <span class="special"><</span><span class="keyword">typename</span> <span class="identifier">T0</span><span class="special">></span> 73 <span class="keyword">typename</span> <span class="identifier">result_of</span><span class="special">::</span><span class="identifier">actor</span><span class="special"><</span><span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">T0</span> <span class="keyword">const</span> <span class="special">&>::</span><span class="identifier">type</span> 74 <span class="keyword">operator</span><span class="special">()(</span><span class="identifier">T0</span> <span class="keyword">const</span> <span class="special">&</span> <span class="identifier">_0</span><span class="special">)</span> <span class="keyword">const</span><span class="special">;</span> 75 76 <span class="comment">//...</span> 77<span class="special">};</span> 78</pre> 79<div class="table"> 80<a name="phoenix.inside.actor.t0"></a><p class="title"><b>Table 1.9. Actor Concept Requirements</b></p> 81<div class="table-contents"><table class="table" summary="Actor Concept Requirements"> 82<colgroup> 83<col> 84<col> 85</colgroup> 86<thead><tr> 87<th> 88 <p> 89 Expression 90 </p> 91 </th> 92<th> 93 <p> 94 Semantics 95 </p> 96 </th> 97</tr></thead> 98<tbody> 99<tr> 100<td> 101 <p> 102 <code class="computeroutput"><span class="identifier">actor</span><span class="special">(</span><span class="identifier">arg0</span><span class="special">,</span> 103 <span class="identifier">arg1</span><span class="special">,</span> 104 <span class="special">...,</span> <span class="identifier">argN</span><span class="special">)</span></code> 105 </p> 106 </td> 107<td> 108 <p> 109 Function call operators to start the evaluation 110 </p> 111 </td> 112</tr> 113<tr> 114<td> 115 <p> 116 <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">result_of</span><span class="special"><</span><span class="identifier">Actor</span><span class="special"><</span><span class="identifier">Expr</span><span class="special">>(</span><span class="identifier">Arg0</span><span class="special">,</span> 117 <span class="identifier">Arg1</span><span class="special">,</span> 118 <span class="special">...,</span> <span class="identifier">ArgN</span><span class="special">)>::</span><span class="identifier">type</span></code> 119 </p> 120 </td> 121<td> 122 <p> 123 Result of the evaluation 124 </p> 125 </td> 126</tr> 127<tr> 128<td> 129 <p> 130 <code class="computeroutput"><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">actor</span><span class="special"><</span><span class="identifier">Expr</span><span class="special">,</span> 131 <span class="identifier">Arg0</span><span class="special">,</span> 132 <span class="identifier">Arg1</span><span class="special">,</span> 133 <span class="special">...,</span> <span class="identifier">ArgN</span><span class="special">>::</span><span class="identifier">type</span></code> 134 </p> 135 </td> 136<td> 137 <p> 138 Result of the evaluation 139 </p> 140 </td> 141</tr> 142</tbody> 143</table></div> 144</div> 145<br class="table-break"><h5> 146<a name="phoenix.inside.actor.h1"></a> 147 <span class="phrase"><a name="phoenix.inside.actor.function_call_operators"></a></span><a class="link" href="actor.html#phoenix.inside.actor.function_call_operators">Function 148 Call Operators</a> 149 </h5> 150<p> 151 There are 2*N function call operators for 0 to N arguments (N == <code class="computeroutput"><span class="identifier">BOOST_PHOENIX_LIMIT</span></code>). The actor class accepts 152 the arguments and forwards the arguments to the default evaluation action. 153 </p> 154<p> 155 Additionally, there exist function call operators accepting permutations 156 of const and non-const references. These operators are created for all N 157 <= <code class="computeroutput"><span class="identifier">BOOST_PHOENIX_PERFECT_FORWARD_LIMIT</span></code> 158 (which defaults to 3). 159 </p> 160<div class="note"><table border="0" summary="Note"> 161<tr> 162<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../../../../doc/src/images/note.png"></td> 163<th align="left">Note</th> 164</tr> 165<tr><td align="left" valign="top"> 166<p> 167 <span class="bold"><strong>Forwarding Function Problem</strong></span> 168 </p> 169<p> 170 There is a known issue with current C++ called the "<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2002/n1385.htm" target="_top">Forwarding 171 Function Problem</a>". The problem is that given an arbitrary 172 function <code class="computeroutput"><span class="identifier">F</span></code>, using current 173 C++ language rules, one cannot create a forwarding function <code class="computeroutput"><span class="identifier">FF</span></code> that transparently assumes the arguments 174 of <code class="computeroutput"><span class="identifier">F</span></code>. 175 </p> 176</td></tr> 177</table></div> 178<h5> 179<a name="phoenix.inside.actor.h2"></a> 180 <span class="phrase"><a name="phoenix.inside.actor.context"></a></span><a class="link" href="actor.html#phoenix.inside.actor.context">Context</a> 181 </h5> 182<p> 183 On an actor function call, before calling the evaluation function, the actor 184 created a <span class="bold"><strong>context</strong></span>. This context consists 185 of an <code class="computeroutput"><span class="identifier">Environment</span></code> and an 186 <code class="computeroutput"><span class="identifier">Action</span></code> part. These contain 187 all information necessary to evaluate the given expression. 188 </p> 189<div class="table"> 190<a name="phoenix.inside.actor.t1"></a><p class="title"><b>Table 1.10. Context Concept Requirements</b></p> 191<div class="table-contents"><table class="table" summary="Context Concept Requirements"> 192<colgroup> 193<col> 194<col> 195</colgroup> 196<thead><tr> 197<th> 198 <p> 199 Expression 200 </p> 201 </th> 202<th> 203 <p> 204 Semantics 205 </p> 206 </th> 207</tr></thead> 208<tbody> 209<tr> 210<td> 211 <p> 212 <code class="computeroutput"><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">context</span><span class="special"><</span><span class="identifier">Env</span><span class="special">,</span> 213 <span class="identifier">Actions</span><span class="special">>::</span><span class="identifier">type</span></code> 214 </p> 215 </td> 216<td> 217 <p> 218 Type of a Context 219 </p> 220 </td> 221</tr> 222<tr> 223<td> 224 <p> 225 <code class="computeroutput"><span class="identifier">context</span><span class="special">(</span><span class="identifier">e</span><span class="special">,</span> 226 <span class="identifier">a</span><span class="special">)</span></code> 227 </p> 228 </td> 229<td> 230 <p> 231 A Context containing environment <code class="computeroutput"><span class="identifier">e</span></code> 232 and actions <code class="computeroutput"><span class="identifier">a</span></code> 233 </p> 234 </td> 235</tr> 236<tr> 237<td> 238 <p> 239 <code class="computeroutput"><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">env</span><span class="special"><</span><span class="identifier">Context</span><span class="special">>::</span><span class="identifier">type</span></code> 240 </p> 241 </td> 242<td> 243 <p> 244 Type of the contained Environment 245 </p> 246 </td> 247</tr> 248<tr> 249<td> 250 <p> 251 <code class="computeroutput"><span class="identifier">env</span><span class="special">(</span><span class="identifier">ctx</span><span class="special">)</span></code> 252 </p> 253 </td> 254<td> 255 <p> 256 The environment 257 </p> 258 </td> 259</tr> 260<tr> 261<td> 262 <p> 263 <code class="computeroutput"><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">actions</span><span class="special"><</span><span class="identifier">Context</span><span class="special">>::</span><span class="identifier">type</span></code> 264 </p> 265 </td> 266<td> 267 <p> 268 Type of the contained Actions 269 </p> 270 </td> 271</tr> 272<tr> 273<td> 274 <p> 275 <code class="computeroutput"><span class="identifier">actions</span><span class="special">(</span><span class="identifier">ctx</span><span class="special">)</span></code> 276 </p> 277 </td> 278<td> 279 <p> 280 The actions 281 </p> 282 </td> 283</tr> 284</tbody> 285</table></div> 286</div> 287<br class="table-break"><h5> 288<a name="phoenix.inside.actor.h3"></a> 289 <span class="phrase"><a name="phoenix.inside.actor.environment"></a></span><a class="link" href="actor.html#phoenix.inside.actor.environment">Environment</a> 290 </h5> 291<p> 292 The Environment is a model of <a href="http://www.boost.org/doc/libs/release/libs/fusion/doc/html/fusion/sequence/concepts/random_access_sequence.html" target="_top">Random 293 Access Sequence</a>. 294 </p> 295<p> 296 The arguments passed to the actor's function call operator are collected 297 inside the Environment: 298 </p> 299<p> 300 <span class="inlinemediaobject"><img src="../../images/funnel_in.png"></span> 301 </p> 302<p> 303 Other parts of the library (e.g. the scope module) extends the <code class="computeroutput"><span class="identifier">Environment</span></code> concept to hold other information 304 such as local variables, etc. 305 </p> 306<h5> 307<a name="phoenix.inside.actor.h4"></a> 308 <span class="phrase"><a name="phoenix.inside.actor.actions"></a></span><a class="link" href="actor.html#phoenix.inside.actor.actions">Actions</a> 309 </h5> 310<p> 311 Actions is the part of Phoenix which are responsible for giving the actual 312 expressions a specific behaviour. During the traversal of the Phoenix Expression 313 Tree these actions are called whenever a specified rule in the grammar matches. 314 </p> 315<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">actions</span> 316<span class="special">{</span> 317 <span class="keyword">template</span> <span class="special"><</span><span class="keyword">typename</span> <span class="identifier">Rule</span><span class="special">></span> 318 <span class="keyword">struct</span> <span class="identifier">when</span><span class="special">;</span> 319<span class="special">};</span> 320</pre> 321<p> 322 The nested <code class="computeroutput"><span class="identifier">when</span></code> template 323 is required to be <a href="http://www.boost.org/doc/libs/release/doc/html/PrimitiveTransform.html" target="_top">Proto 324 Primitive Transform</a>. No worries, you don't have to learn <a href="http://www.boost.org/doc/libs/release/libs/proto/index.html" target="_top">Boost.Proto</a> 325 just yet! Phoenix provides some wrappers to let you define simple actions 326 without the need to dive deep into proto. 327 </p> 328<p> 329 Phoenix ships with a predefined <code class="computeroutput"><span class="identifier">default_actions</span></code> 330 class that evaluates the expressions with C++ semantics: 331 </p> 332<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">default_actions</span> 333<span class="special">{</span> 334 <span class="keyword">template</span> <span class="special"><</span><span class="keyword">typename</span> <span class="identifier">Rule</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Dummy</span> <span class="special">=</span> <span class="keyword">void</span><span class="special">></span> 335 <span class="keyword">struct</span> <span class="identifier">when</span> 336 <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">meta_grammar</span><span class="special">></span> 337 <span class="special">{};</span> 338<span class="special">};</span> 339</pre> 340<p> 341 For more information on how to use the default_actions class and how to attach 342 custom actions to the evaluation process, see <a class="link" href="actions.html" title="More on Actions">more 343 on actions</a>. 344 </p> 345<h5> 346<a name="phoenix.inside.actor.h5"></a> 347 <span class="phrase"><a name="phoenix.inside.actor.evaluation"></a></span><a class="link" href="actor.html#phoenix.inside.actor.evaluation">Evaluation</a> 348 </h5> 349<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">evaluator</span> 350<span class="special">{</span> 351 <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> 352 <span class="emphasis"><em>unspecified</em></span> <span class="keyword">operator</span><span class="special">()(</span><span class="identifier">Expr</span> <span class="special">&,</span> <span class="identifier">Context</span> <span class="special">&);</span> 353<span class="special">};</span> 354 355<span class="identifier">evaluator</span> <span class="keyword">const</span> <span class="identifier">eval</span> <span class="special">=</span> <span class="special">{};</span> 356</pre> 357<p> 358 The evaluation of a Phoenix expression is started by a call to the function 359 call operator of <code class="computeroutput"><span class="identifier">evaluator</span></code>. 360 </p> 361<p> 362 The evaluator is called by the <code class="computeroutput"><span class="identifier">actor</span></code> 363 function operator overloads after the context is built up. For reference, 364 here is a typical <code class="computeroutput"><span class="identifier">actor</span><span class="special">::</span><span class="keyword">operator</span><span class="special">()</span></code> 365 that accepts two arguments: 366 </p> 367<pre class="programlisting"><span class="keyword">template</span> <span class="special"><</span><span class="keyword">typename</span> <span class="identifier">T0</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">T1</span><span class="special">></span> 368<span class="keyword">typename</span> <span class="identifier">result_of</span><span class="special">::</span><span class="identifier">actor</span><span class="special"><</span><span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">T0</span> <span class="special">&,</span> <span class="identifier">T1</span> <span class="special">&>::</span><span class="identifier">type</span> 369<span class="keyword">operator</span><span class="special">()(</span><span class="identifier">T0</span> <span class="special">&</span><span class="identifier">t0</span><span class="special">,</span> <span class="identifier">T1</span> <span class="special">&</span><span class="identifier">t1</span><span class="special">)</span> <span class="keyword">const</span> 370<span class="special">{</span> 371 <span class="identifier">fusion</span><span class="special">::</span><span class="identifier">vector2</span><span class="special"><</span><span class="identifier">T0</span> <span class="special">&,</span> <span class="identifier">T1</span> <span class="special">&></span> <span class="identifier">env</span><span class="special">(</span><span class="identifier">t0</span><span class="special">,</span> <span class="identifier">t1</span><span class="special">);</span> 372 373 <span class="keyword">return</span> <span class="identifier">eval</span><span class="special">(*</span><span class="keyword">this</span><span class="special">,</span> <span class="identifier">context</span><span class="special">(</span><span class="identifier">env</span><span class="special">,</span> <span class="identifier">default_actions</span><span class="special">()));</span> 374<span class="special">}</span> 375</pre> 376<h5> 377<a name="phoenix.inside.actor.h6"></a> 378 <span class="phrase"><a name="phoenix.inside.actor.result_of__actor"></a></span><a class="link" href="actor.html#phoenix.inside.actor.result_of__actor">result_of::actor</a> 379 </h5> 380<p> 381 For reasons of symmetry to the family of <code class="computeroutput"><span class="identifier">actor</span><span class="special">::</span><span class="keyword">operator</span><span class="special">()</span></code> there is a special metafunction usable 382 for actor result type calculation named <code class="computeroutput"><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">actor</span></code>. 383 This metafunction allows us to directly specify the types of the parameters 384 to be passed to the <code class="computeroutput"><span class="identifier">actor</span><span class="special">::</span><span class="keyword">operator</span><span class="special">()</span></code> function. Here's a typical <code class="computeroutput"><span class="identifier">actor_result</span></code> that accepts two arguments: 385 </p> 386<pre class="programlisting"><span class="keyword">namespace</span> <span class="identifier">result_of</span> 387<span class="special">{</span> 388 <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">T0</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">T1</span><span class="special">></span> 389 <span class="keyword">struct</span> <span class="identifier">actor</span> 390 <span class="special">{</span> 391 <span class="keyword">typedef</span> <span class="identifier">fusion</span><span class="special">::</span><span class="identifier">vector2</span><span class="special"><</span><span class="identifier">T0</span><span class="special">,</span> <span class="identifier">T1</span><span class="special">></span> <span class="identifier">env_tpe</span><span class="special">;</span> 392 <span class="keyword">typedef</span> <span class="keyword">typename</span> <span class="identifier">result_of</span><span class="special">::</span><span class="identifier">context</span><span class="special"><</span><span class="identifier">env_type</span><span class="special">,</span> <span class="identifier">default_actions</span><span class="special">>::</span><span class="identifier">type</span> <span class="identifier">ctx_type</span> 393 <span class="keyword">typedef</span> <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">evaluator</span><span class="special">(</span><span class="identifier">Expr</span> <span class="keyword">const</span><span class="special">&,</span> <span class="identifier">ctx_type</span><span class="special">)>::</span><span class="identifier">type</span> <span class="identifier">type</span><span class="special">;</span> 394 <span class="special">};</span> 395<span class="special">}</span> 396</pre> 397</div> 398<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> 399<td align="left"></td> 400<td align="right"><div class="copyright-footer">Copyright © 2002-2005, 2010, 2014, 2015 Joel de Guzman, Dan Marsden, Thomas 401 Heller, John Fletcher<p> 402 Distributed under the Boost Software License, Version 1.0. (See accompanying 403 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>) 404 </p> 405</div></td> 406</tr></table> 407<hr> 408<div class="spirit-nav"> 409<a accesskey="p" href="../inside.html"><img src="../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../inside.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="expression.html"><img src="../../../../../../doc/src/images/next.png" alt="Next"></a> 410</div> 411</body> 412</html> 413
