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</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="./if-c.html" class="navigation-link">Prev</a> <a href="./eval-if-c.html" class="navigation-link">Next</a></span><span class="navigation-group-separator"> | </span><span class="navigation-group"><a href="./if-c.html" class="navigation-link">Back</a> <a href="./eval-if-c.html" class="navigation-link">Along</a></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.html" class="navigation-link">eval_if</a></td> 13</tr></table><div class="header-separator"></div> 14<div class="section" id="eval-if"> 15<h1><a class="toc-backref" href="./type-selection.html#id1522">eval_if</a></h1> 16<div class="section" id="id873"> 17<h3><a class="subsection-title" href="#synopsis" name="synopsis">Synopsis</a></h3> 18<pre class="literal-block"> 19template< 20 typename C 21 , typename F1 22 , typename F2 23 > 24struct <a href="./eval-if.html" class="identifier">eval_if</a> 25{ 26 typedef <em>unspecified</em> type; 27}; 28</pre> 29</div> 30<div class="section" id="id874"> 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 <tt class="literal"><span class="pre">C</span></tt>.</p> 34</div> 35<div class="section" id="id875"> 36<h3><a class="subsection-title" href="#header" name="header">Header</a></h3> 37<pre class="literal-block"> 38#include <<a href="../../../../boost/mpl/eval_if.hpp" class="header">boost/mpl/eval_if.hpp</a>> 39</pre> 40</div> 41<div class="section" id="id876"> 42<h3><a class="subsection-title" href="#parameters" name="parameters">Parameters</a></h3> 43<table border="1" class="docutils table"> 44<colgroup> 45<col width="15%" /> 46<col width="36%" /> 47<col width="48%" /> 48</colgroup> 49<thead valign="bottom"> 50<tr><th class="head">Parameter</th> 51<th class="head">Requirement</th> 52<th class="head">Description</th> 53</tr> 54</thead> 55<tbody valign="top"> 56<tr><td><tt class="literal"><span class="pre">C</span></tt></td> 57<td><a class="reference internal" href="./integral-constant.html">Integral Constant</a></td> 58<td>An evaluation condition.</td> 59</tr> 60<tr><td><tt class="literal"><span class="pre">F1</span></tt>, <tt class="literal"><span class="pre">F2</span></tt></td> 61<td>Nullary <a class="reference internal" href="./metafunction.html">Metafunction</a></td> 62<td>Metafunctions to select for evaluation from.</td> 63</tr> 64</tbody> 65</table> 66</div> 67<div class="section" id="id877"> 68<h3><a class="subsection-title" href="#expression-semantics" name="expression-semantics">Expression semantics</a></h3> 69<p>For any <a class="reference internal" href="./integral-constant.html">Integral Constant</a> <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> 70<pre class="literal-block"> 71typedef <a href="./eval-if.html" class="identifier">eval_if</a><c,f1,f2>::type t; 72</pre> 73<table class="docutils field-list" frame="void" rules="none"> 74<col class="field-name" /> 75<col class="field-body" /> 76<tbody valign="top"> 77<tr class="field"><th class="field-name">Return type:</th><td class="field-body">Any type.</td> 78</tr> 79<tr class="field"><th class="field-name">Semantics:</th><td class="field-body">If <tt class="literal"><span class="pre">c::value</span> <span class="pre">==</span> <span class="pre">true</span></tt>, <tt class="literal"><span class="pre">t</span></tt> is identical to <tt class="literal"><span class="pre">f1::type</span></tt>; otherwise <tt class="literal"><span class="pre">t</span></tt> is 80identical to <tt class="literal"><span class="pre">f2::type</span></tt>.</td> 81</tr> 82</tbody> 83</table> 84</div> 85<div class="section" id="id878"> 86<h3><a class="subsection-title" href="#example" name="example">Example</a></h3> 87<pre class="literal-block"> 88typedef <a href="./eval-if.html" class="identifier">eval_if</a>< <a href="./bool.html" class="identifier">true_</a>, <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.html" class="identifier">eval_if</a>< <a href="./bool.html" class="identifier">false_</a>, <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="id879"> 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-c.html">eval_if_c</a>, <a class="reference internal" href="./if.html">if_</a></p> 98<!-- Metafunctions/Type Selection//eval_if_c |40 --> 99</div> 100</div> 101 102<div class="footer-separator"></div> 103<table class="footer"><tr class="footer"><td class="header-group navigation-bar"><span class="navigation-group"><a href="./if-c.html" class="navigation-link">Prev</a> <a href="./eval-if-c.html" class="navigation-link">Next</a></span><span class="navigation-group-separator"> | </span><span class="navigation-group"><a href="./if-c.html" class="navigation-link">Back</a> <a href="./eval-if-c.html" class="navigation-link">Along</a></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> 104<td><div class="copyright-footer"><div class="copyright">Copyright © 2001-2009 Aleksey Gurtovoy and David Abrahams</div> 105Distributed under the Boost Software License, Version 1.0. (See accompanying 106file 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> 107</html> 108