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: 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="./if.html" class="navigation-link">Prev</a> <a href="./eval-if.html" class="navigation-link">Next</a></span><span class="navigation-group-separator"> | </span><span class="navigation-group"><a href="./if.html" class="navigation-link">Back</a> <a href="./eval-if.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="./if-c.html" class="navigation-link">if_c</a></td> 13</tr></table><div class="header-separator"></div> 14<div class="section" id="if-c"> 15<h1><a class="toc-backref" href="./type-selection.html#id1521">if_c</a></h1> 16<div class="section" id="id866"> 17<h3><a class="subsection-title" href="#synopsis" name="synopsis">Synopsis</a></h3> 18<pre class="literal-block"> 19template< 20 bool c 21 , typename T1 22 , typename T2 23 > 24struct <a href="./if-c.html" class="identifier">if_c</a> 25{ 26 typedef <em>unspecified</em> type; 27}; 28</pre> 29</div> 30<div class="section" id="id867"> 31<h3><a class="subsection-title" href="#description" name="description">Description</a></h3> 32<p>Returns one of its two arguments, <tt class="literal"><span class="pre">T1</span></tt> or <tt class="literal"><span class="pre">T2</span></tt>, depending on the value of 33integral constant <tt class="literal"><span class="pre">c</span></tt>. <tt class="literal"><span class="pre"><a href="./if-c.html" class="identifier">if_c</a><c,t1,t2>::type</span></tt> is a shorcut notation for 34<tt class="literal"><span class="pre"><a href="./if.html" class="identifier">if_</a><</span> <span class="pre"><a href="./bool.html" class="identifier">bool_</a><c>,t1,t2</span> <span class="pre">>::type</span></tt>.</p> 35</div> 36<div class="section" id="id868"> 37<h3><a class="subsection-title" href="#header" name="header">Header</a></h3> 38<pre class="literal-block"> 39#include <<a href="../../../../boost/mpl/if.hpp" class="header">boost/mpl/if.hpp</a>> 40</pre> 41</div> 42<div class="section" id="id869"> 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>A selection condition.</td> 60</tr> 61<tr><td><tt class="literal"><span class="pre">T1</span></tt>, <tt class="literal"><span class="pre">T2</span></tt></td> 62<td>Any type</td> 63<td>Types to select from.</td> 64</tr> 65</tbody> 66</table> 67</div> 68<div class="section" id="id870"> 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 arbitrary types <tt class="literal"><span class="pre">t1</span></tt>, <tt class="literal"><span class="pre">t2</span></tt>:</p> 71<pre class="literal-block"> 72typedef <a href="./if-c.html" class="identifier">if_c</a><c,t1,t2>::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="./if.html" class="identifier">if_</a><</span> <span class="pre"><a href="./bool.html" class="identifier">bool_</a><c>,t1,t2</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="id871"> 86<h3><a class="subsection-title" href="#example" name="example">Example</a></h3> 87<pre class="literal-block"> 88typedef <a href="./if-c.html" class="identifier">if_c</a><true,char,long>::type t1; 89typedef <a href="./if-c.html" class="identifier">if_c</a><false,char,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<pre class="literal-block"> 95// allocates space for an object of class T on heap or "inplace" 96// depending on its <a href="./size.html" class="identifier">size</a> 97template< typename T > struct lightweight 98{ 99 // ... 100 typedef typename <a href="./if-c.html" class="identifier">if_c</a>< 101 sizeof(T) <= sizeof(T*) 102 , inplace_storage<T> 103 , heap_storage<T> 104 >::type impl_t; 105 106 impl_t impl; 107}; 108</pre> 109</div> 110<div class="section" id="id872"> 111<h3><a class="subsection-title" href="#see-also" name="see-also">See also</a></h3> 112<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="./if.html">if_</a>, <a class="reference internal" href="./eval-if.html">eval_if</a>, <a class="reference internal" href="./bool.html">bool_</a></p> 113<!-- Metafunctions/Type Selection//eval_if |30 --> 114</div> 115</div> 116 117<div class="footer-separator"></div> 118<table class="footer"><tr class="footer"><td class="header-group navigation-bar"><span class="navigation-group"><a href="./if.html" class="navigation-link">Prev</a> <a href="./eval-if.html" class="navigation-link">Next</a></span><span class="navigation-group-separator"> | </span><span class="navigation-group"><a href="./if.html" class="navigation-link">Back</a> <a href="./eval-if.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> 119<td><div class="copyright-footer"><div class="copyright">Copyright © 2001-2009 Aleksey Gurtovoy and David Abrahams</div> 120Distributed under the Boost Software License, Version 1.0. (See accompanying 121file 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> 122</html> 123