1<?xml version="1.0" encoding="utf-8" ?> 2<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"> 3<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en"> 4<head> 5<meta http-equiv="Content-Type" content="text/html; charset=utf-8" /> 6<meta name="generator" content="Docutils 0.7: http://docutils.sourceforge.net/" /> 7<title>The MPL Reference Manual: eval_if_c</title> 8<link rel="stylesheet" href="../style.css" type="text/css" /> 9</head> 10<body class="docframe refmanual"> 11<table class="header"><tr class="header"><td class="header-group navigation-bar"><span class="navigation-group"><a href="./eval-if.html" class="navigation-link">Prev</a> <a href="./invocation.html" class="navigation-link">Next</a></span><span class="navigation-group-separator"> | </span><span class="navigation-group"><a href="./eval-if.html" class="navigation-link">Back</a> Along</span><span class="navigation-group-separator"> | </span><span class="navigation-group"><a href="./type-selection.html" class="navigation-link">Up</a> <a href="../refmanual.html" class="navigation-link">Home</a></span><span class="navigation-group-separator"> | </span><span class="navigation-group"><a href="./refmanual_toc.html" class="navigation-link">Full TOC</a></span></td> 12<td class="header-group page-location"><a href="../refmanual.html" class="navigation-link">Front Page</a> / <a href="./metafunctions.html" class="navigation-link">Metafunctions</a> / <a href="./type-selection.html" class="navigation-link">Type Selection</a> / <a href="./eval-if-c.html" class="navigation-link">eval_if_c</a></td> 13</tr></table><div class="header-separator"></div> 14<div class="section" id="eval-if-c"> 15<h1><a class="toc-backref" href="./type-selection.html#id1523">eval_if_c</a></h1> 16<div class="section" id="id880"> 17<h3><a class="subsection-title" href="#synopsis" name="synopsis">Synopsis</a></h3> 18<pre class="literal-block"> 19template< 20 bool c 21 , typename F1 22 , typename F2 23 > 24struct <a href="./eval-if-c.html" class="identifier">eval_if_c</a> 25{ 26 typedef <em>unspecified</em> type; 27}; 28</pre> 29</div> 30<div class="section" id="id881"> 31<h3><a class="subsection-title" href="#description" name="description">Description</a></h3> 32<p>Evaluates one of its two <a class="reference internal" href="./metafunction.html#nullary-metafunction">nullary-metafunction</a> arguments, <tt class="literal"><span class="pre">F1</span></tt> or <tt class="literal"><span class="pre">F2</span></tt>, depending 33on the value of integral constant <tt class="literal"><span class="pre">c</span></tt>. <tt class="literal"><span class="pre"><a href="./eval-if-c.html" class="identifier">eval_if_c</a><c,f1,f2>::type</span></tt> is a shorcut 34notation for <tt class="literal"><span class="pre"><a href="./eval-if.html" class="identifier">eval_if</a><</span> <span class="pre"><a href="./bool.html" class="identifier">bool_</a><c>,f1,f2</span> <span class="pre">>::type</span></tt>.</p> 35</div> 36<div class="section" id="id882"> 37<h3><a class="subsection-title" href="#header" name="header">Header</a></h3> 38<pre class="literal-block"> 39#include <<a href="../../../../boost/mpl/eval_if.hpp" class="header">boost/mpl/eval_if.hpp</a>> 40</pre> 41</div> 42<div class="section" id="id883"> 43<h3><a class="subsection-title" href="#parameters" name="parameters">Parameters</a></h3> 44<table border="1" class="docutils table"> 45<colgroup> 46<col width="15%" /> 47<col width="36%" /> 48<col width="48%" /> 49</colgroup> 50<thead valign="bottom"> 51<tr><th class="head">Parameter</th> 52<th class="head">Requirement</th> 53<th class="head">Description</th> 54</tr> 55</thead> 56<tbody valign="top"> 57<tr><td><tt class="literal"><span class="pre">c</span></tt></td> 58<td>An integral constant</td> 59<td>An evaluation condition.</td> 60</tr> 61<tr><td><tt class="literal"><span class="pre">F1</span></tt>, <tt class="literal"><span class="pre">F2</span></tt></td> 62<td>Nullary <a class="reference internal" href="./metafunction.html">Metafunction</a></td> 63<td>Metafunctions to select for evaluation from.</td> 64</tr> 65</tbody> 66</table> 67</div> 68<div class="section" id="id884"> 69<h3><a class="subsection-title" href="#expression-semantics" name="expression-semantics">Expression semantics</a></h3> 70<p>For any integral constant <tt class="literal"><span class="pre">c</span></tt> and nullary <a class="reference internal" href="./metafunction.html">Metafunction</a>s <tt class="literal"><span class="pre">f1</span></tt>, <tt class="literal"><span class="pre">f2</span></tt>:</p> 71<pre class="literal-block"> 72typedef <a href="./eval-if-c.html" class="identifier">eval_if_c</a><c,f1,f2>::type t; 73</pre> 74<table class="docutils field-list" frame="void" rules="none"> 75<col class="field-name" /> 76<col class="field-body" /> 77<tbody valign="top"> 78<tr class="field"><th class="field-name">Return type:</th><td class="field-body">Any type.</td> 79</tr> 80<tr class="field"><th class="field-name">Semantics:</th><td class="field-body">Equivalent to <tt class="literal"><span class="pre">typedef</span> <span class="pre"><a href="./eval-if.html" class="identifier">eval_if</a><</span> <span class="pre"><a href="./bool.html" class="identifier">bool_</a><c>,f1,f2</span> <span class="pre">>::type</span> <span class="pre">t;</span></tt></td> 81</tr> 82</tbody> 83</table> 84</div> 85<div class="section" id="id885"> 86<h3><a class="subsection-title" href="#example" name="example">Example</a></h3> 87<pre class="literal-block"> 88typedef <a href="./eval-if-c.html" class="identifier">eval_if_c</a>< true, <a href="./identity.html" class="identifier">identity</a><char>, <a href="./identity.html" class="identifier">identity</a><long> >::type t1; 89typedef <a href="./eval-if-c.html" class="identifier">eval_if_c</a>< false, <a href="./identity.html" class="identifier">identity</a><char>, <a href="./identity.html" class="identifier">identity</a><long> >::type t2; 90 91<a href="./assert.html" class="identifier">BOOST_MPL_ASSERT</a>(( is_same<t1,char> )); 92<a href="./assert.html" class="identifier">BOOST_MPL_ASSERT</a>(( is_same<t2,long> )); 93</pre> 94</div> 95<div class="section" id="id886"> 96<h3><a class="subsection-title" href="#see-also" name="see-also">See also</a></h3> 97<p><a class="reference internal" href="./metafunctions.html">Metafunctions</a>, <a class="reference internal" href="./integral-constant.html">Integral Constant</a>, <a class="reference internal" href="./eval-if.html">eval_if</a>, <a class="reference internal" href="./if.html">if_</a>, <a class="reference internal" href="./bool.html">bool_</a></p> 98</div> 99</div> 100 101<div class="footer-separator"></div> 102<table class="footer"><tr class="footer"><td class="header-group navigation-bar"><span class="navigation-group"><a href="./eval-if.html" class="navigation-link">Prev</a> <a href="./invocation.html" class="navigation-link">Next</a></span><span class="navigation-group-separator"> | </span><span class="navigation-group"><a href="./eval-if.html" class="navigation-link">Back</a> Along</span><span class="navigation-group-separator"> | </span><span class="navigation-group"><a href="./type-selection.html" class="navigation-link">Up</a> <a href="../refmanual.html" class="navigation-link">Home</a></span><span class="navigation-group-separator"> | </span><span class="navigation-group"><a href="./refmanual_toc.html" class="navigation-link">Full TOC</a></span></td> 103<td><div class="copyright-footer"><div class="copyright">Copyright © 2001-2009 Aleksey Gurtovoy and David Abrahams</div> 104Distributed under the Boost Software License, Version 1.0. (See accompanying 105file LICENSE_1_0.txt or copy at <a class="reference external" href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)</div></td></tr></table></body> 106</html> 107