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>Chapter 47. Boost.YAP</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="libraries.html" title="Part I. The Boost C++ Libraries (BoostBook Subset)"> 10<link rel="prev" href="xpressive/appendices.html" title="Appendices"> 11<link rel="next" href="boost_yap/manual.html" title="Manual"> 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="xpressive/appendices.html"><img src="../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="libraries.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="boost_yap/manual.html"><img src="../../doc/src/images/next.png" alt="Next"></a> 25</div> 26<div class="chapter"> 27<div class="titlepage"><div> 28<div><h2 class="title"> 29<a name="yap"></a>Chapter 47. Boost.YAP</h2></div> 30<div><div class="author"><h3 class="author"> 31<span class="firstname">Zach</span> <span class="surname">Laine</span> 32</h3></div></div> 33<div><p class="copyright">Copyright © 2018 T. Zachary Laine</p></div> 34<div><div class="legalnotice"> 35<a name="yap.legal"></a><p> 36 Distributed under the Boost Software License, Version 1.0. (See accompanying 37 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>) 38 </p> 39</div></div> 40</div></div> 41<div class="toc"> 42<p><b>Table of Contents</b></p> 43<dl class="toc"> 44<dt><span class="section"><a href="yap.html#boost_yap.introduction">Introduction</a></span></dt> 45<dt><span class="section"><a href="boost_yap/manual.html">Manual</a></span></dt> 46<dd><dl> 47<dt><span class="section"><a href="boost_yap/manual.html#boost_yap.manual.an_expression_template_primer">An Expression 48 Template Primer</a></span></dt> 49<dt><span class="section"><a href="boost_yap/manual.html#boost_yap.manual.the_yap_way">The YAP Way</a></span></dt> 50<dt><span class="section"><a href="boost_yap/manual.html#boost_yap.manual.expressions">Expressions</a></span></dt> 51<dt><span class="section"><a href="boost_yap/manual.html#boost_yap.manual.mix_and_match_expression_templates">Mix-and-Match 52 Expression Templates</a></span></dt> 53<dt><span class="section"><a href="boost_yap/manual.html#boost_yap.manual.kinds_of_expressions">Kinds of Expressions</a></span></dt> 54<dt><span class="section"><a href="boost_yap/manual.html#boost_yap.manual.operators">Operators</a></span></dt> 55<dt><span class="section"><a href="boost_yap/manual.html#boost_yap.manual.transforming_expressions">Transforming 56 Expressions</a></span></dt> 57<dt><span class="section"><a href="boost_yap/manual.html#boost_yap.manual.evaluating_expressions">Evaluating Expressions</a></span></dt> 58<dt><span class="section"><a href="boost_yap/manual.html#boost_yap.manual.operator_macros">Operator Macros</a></span></dt> 59<dt><span class="section"><a href="boost_yap/manual.html#boost_yap.manual.how_expression_operands_are_treated">How 60 Expression Operands Are Treated</a></span></dt> 61<dt><span class="section"><a href="boost_yap/manual.html#boost_yap.manual.printing">Printing</a></span></dt> 62<dt><span class="section"><a href="boost_yap/manual.html#boost_yap.manual.examples">Examples</a></span></dt> 63<dt><span class="section"><a href="boost_yap/manual.html#boost_yap.manual.header_organization">Header Organization</a></span></dt> 64<dt><span class="section"><a href="boost_yap/manual.html#boost_yap.manual.configuration">Configuration</a></span></dt> 65<dt><span class="section"><a href="boost_yap/manual.html#boost_yap.manual.object_code">Object Code</a></span></dt> 66</dl></dd> 67<dt><span class="section"><a href="boost_yap/concepts.html">Concepts</a></span></dt> 68<dt><span class="section"><a href="boost_yap/compiler_support.html">Compiler Support</a></span></dt> 69<dt><span class="section"><a href="boost_yap/dependencies.html">Dependencies</a></span></dt> 70<dt><span class="section"><a href="yap/reference.html">Reference</a></span></dt> 71<dd><dl><dt><span class="section"><a href="yap/reference.html#headers">Headers</a></span></dt></dl></dd> 72<dt><span class="section"><a href="boost_yap/rationale.html">Rationale</a></span></dt> 73</dl> 74</div> 75<div class="section"> 76<div class="titlepage"><div><div><h2 class="title" style="clear: both"> 77<a name="boost_yap.introduction"></a><a class="link" href="yap.html#boost_yap.introduction" title="Introduction">Introduction</a> 78</h2></div></div></div> 79<p> 80 "I like to start documentation with a quote. A nice, pithy one." 81 </p> 82<p> 83 <span class="emphasis"><em><span class="bold"><strong>— Eric Niebler (paraphrased)</strong></span></em></span> 84 </p> 85<h4> 86<a name="boost_yap.introduction.h0"></a> 87 <span class="phrase"><a name="boost_yap.introduction.motivation"></a></span><a class="link" href="yap.html#boost_yap.introduction.motivation">Motivation</a> 88 </h4> 89<p> 90 Expression templates are rad. They are used in lots of libraries; here are 91 just three of the most impressive: 92 </p> 93<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "> 94<li class="listitem"> 95 <a href="http://boost-spirit.com" target="_top">Boost.Spirit</a> allows you to 96 write an EBNF-style grammar that gets transformed into a PEG parser. 97 </li> 98<li class="listitem"> 99 <a href="http://eigen.tuxfamily.org" target="_top">Eigen</a> allows you to do 100 linear algebra using a very natural and mathematical expression syntax 101 that <a href="http://eigen.tuxfamily.org" target="_top">Eigen</a> uses to heavily 102 optimize your expressions. 103 </li> 104<li class="listitem"> 105 <a href="https://www.lri.fr/~falcou/nt2" target="_top">NT2</a> takes slightly 106 modified MatLab code and allows it to be parsed and run as highly optimized 107 C++ code. 108 </li> 109</ul></div> 110<p> 111 However, this can come at a high cost. Expression templates are costly to implement 112 and maintain. Each of <a href="http://eigen.tuxfamily.org" target="_top">Eigen</a> 113 and Boost.Ublas has a large volume of complex expression template code that 114 cannot be reused elsewhere. 115 </p> 116<p> 117 With the language facilities available in the C++14 and C++17 standards, an 118 expression template library is now straightforward to write and use, and has 119 very reasonable compile times. 120 </p> 121<p> 122 As a quick example, let's say we are doing a bit of matrix math, and we write 123 this statement: 124 </p> 125<pre class="programlisting"><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> 126</pre> 127<p> 128 in which all the variables are matrices. It turns out that making a temporary 129 for <code class="computeroutput"><span class="identifier">A</span> <span class="special">*</span> 130 <span class="identifier">B</span></code> and then another temporary for 131 the resulting product plus <code class="computeroutput"><span class="identifier">C</span></code> 132 is very inefficient. Most matrix math libraries will have a single function 133 that does it in one go: 134 </p> 135<pre class="programlisting"><span class="identifier">mul_add_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> 136</pre> 137<p> 138 If you use a matrix library that offers both kinds of syntax, you have to notice 139 when some bit of operator-using code should be replaced with some more efficient 140 function; this is tedious and error-prone. If the library does not provide 141 the operator syntax at all, only providing the more-efficient function calls, 142 code using the library is a lot less writable and readable. 143 </p> 144<p> 145 Using Boost.YAP, you can write some library code that enables expressions like 146 <code class="computeroutput"><span class="identifier">D</span> <span class="special">=</span> 147 <span class="identifier">A</span> <span class="special">*</span> <span class="identifier">B</span> <span class="special">+</span> <span class="identifier">C</span></code> 148 to be automatically transformed into expressions like <code class="computeroutput"><span class="identifier">mul_add_assign</span><span class="special">(</span><span class="identifier">D</span><span class="special">,</span> 149 <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></code>. 150 </p> 151<p> 152 Consider another example. Many of us have used Unix command line tools to remove 153 duplicate lines in a file: 154 </p> 155<pre class="programlisting"><span class="identifier">sort</span> <span class="identifier">file_with_duplicates</span> <span class="special">|</span> <span class="identifier">uniq</span> <span class="special">></span> <span class="identifier">file_without_duplicates</span> 156</pre> 157<p> 158 We can do something very similar with the standard algorithms, of course: 159 </p> 160<p> 161</p> 162<pre class="programlisting"><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">v1</span> <span class="special">=</span> <span class="special">{</span><span class="number">0</span><span class="special">,</span> <span class="number">2</span><span class="special">,</span> <span class="number">2</span><span class="special">,</span> <span class="number">7</span><span class="special">,</span> <span class="number">1</span><span class="special">,</span> <span class="number">3</span><span class="special">,</span> <span class="number">8</span><span class="special">};</span> 163<span class="identifier">std</span><span class="special">::</span><span class="identifier">sort</span><span class="special">(</span><span class="identifier">v1</span><span class="special">.</span><span class="identifier">begin</span><span class="special">(),</span> <span class="identifier">v1</span><span class="special">.</span><span class="identifier">end</span><span class="special">());</span> 164<span class="keyword">auto</span> <span class="identifier">it</span> <span class="special">=</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">unique</span><span class="special">(</span><span class="identifier">v1</span><span class="special">.</span><span class="identifier">begin</span><span class="special">(),</span> <span class="identifier">v1</span><span class="special">.</span><span class="identifier">end</span><span class="special">());</span> 165<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="keyword">const</span> <span class="identifier">v2</span><span class="special">(</span><span class="identifier">v1</span><span class="special">.</span><span class="identifier">begin</span><span class="special">(),</span> <span class="identifier">it</span><span class="special">);</span> 166<span class="identifier">assert</span><span class="special">(</span><span class="identifier">v2</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="keyword">int</span><span class="special">>({</span><span class="number">0</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">7</span><span class="special">,</span> <span class="number">8</span><span class="special">}));</span> 167</pre> 168<p> 169 </p> 170<p> 171 However, it would be much better if our code did exactly that, but with a more 172 concise syntax: 173 </p> 174<p> 175</p> 176<pre class="programlisting"><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">v1</span> <span class="special">=</span> <span class="special">{</span><span class="number">0</span><span class="special">,</span> <span class="number">2</span><span class="special">,</span> <span class="number">2</span><span class="special">,</span> <span class="number">7</span><span class="special">,</span> <span class="number">1</span><span class="special">,</span> <span class="number">3</span><span class="special">,</span> <span class="number">8</span><span class="special">};</span> 177<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="keyword">const</span> <span class="identifier">v2</span> <span class="special">=</span> <span class="identifier">sort</span><span class="special">(</span><span class="identifier">v1</span><span class="special">)</span> <span class="special">|</span> <span class="identifier">unique</span><span class="special">;</span> 178<span class="identifier">assert</span><span class="special">(</span><span class="identifier">v2</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="keyword">int</span><span class="special">>({</span><span class="number">0</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">7</span><span class="special">,</span> <span class="number">8</span><span class="special">}));</span> 179</pre> 180<p> 181 </p> 182<p> 183 This looks much more similar to the Unix command line above. (Let's pretend 184 that <a href="https://github.com/ericniebler/range-v3" target="_top">Range-v3</a> 185 doesn't already do almost exactly this.) 186 </p> 187<p> 188 Boost.YAP can be used to do both of these things, in a pretty small amount 189 of code. In fact, you can jump right into the <a class="link" href="boost_yap/manual.html#boost_yap.manual.examples.pipable_algorithms" title="Pipable Algorithms">Pipable 190 Algorithms</a> example if you want to see how the second one can be implemented. 191 </p> 192<h4> 193<a name="boost_yap.introduction.h1"></a> 194 <span class="phrase"><a name="boost_yap.introduction.features"></a></span><a class="link" href="yap.html#boost_yap.introduction.features">Features</a> 195 </h4> 196<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "> 197<li class="listitem"> 198 Simple <a class="link" href="boost_yap/concepts.html#boost_yap.concepts.expressiontemplate">ExpressionTemplate</a> 199 and <a class="link" href="boost_yap/concepts.html#boost_yap.concepts.expression">Expression</a> concepts 200 easily modeled by user code. Member and non-member functions on <a class="link" href="boost_yap/concepts.html#boost_yap.concepts.expressiontemplate">ExpressionTemplates</a> 201 and <a class="link" href="boost_yap/concepts.html#boost_yap.concepts.expression">Expressions</a> can 202 be added with compact macros, and a reference template that models <a class="link" href="boost_yap/concepts.html#boost_yap.concepts.expressiontemplate">ExpressionTemplate</a> 203 exists for prototyping or experimentation. 204 </li> 205<li class="listitem"> 206 Evaluation of Boost.YAP expressions matches the semantics of builtin C++ 207 expressions as closely as possible. This leads to clearer understanding 208 of the semantics of expression evaluation, because the definitions are 209 local to the types involved. 210 </li> 211<li class="listitem"> 212 Expressions may be transformed explicitly in a user-defined way. This is 213 accomplished with overloaded call operators in a transform class, which 214 are matched against subexpressions in the overall expression. While these 215 member functions may transform a subexpression into anything, a common 216 pattern is to transform only some subexpressions into either new subexpressions 217 or appropriate values and to leave other subexpressions unchanged. This 218 <code class="computeroutput"><span class="identifier">evaluate</span><span class="special">(</span><span class="identifier">transform</span><span class="special">(</span><span class="identifier">expr</span><span class="special">))</span></code> 219 idiom is expected to be one of the most common ways of using Yap to manipulate 220 and evaluate expressions. 221 </li> 222<li class="listitem"> 223 Functions that operate on or create expressions. Functions are provided 224 (and used within Boost.YAP) that manipulate expressions or their subexpressions. 225 These simplify the process of writing user-defined transforms, for example. 226 </li> 227</ul></div> 228</div> 229</div> 230<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> 231<td align="left"><p><small>Last revised: August 11, 2020 at 15:03:11 GMT</small></p></td> 232<td align="right"><div class="copyright-footer"></div></td> 233</tr></table> 234<hr> 235<div class="spirit-nav"> 236<a accesskey="p" href="xpressive/appendices.html"><img src="../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="libraries.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="boost_yap/manual.html"><img src="../../doc/src/images/next.png" alt="Next"></a> 237</div> 238</body> 239</html> 240
