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_</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="./type-selection.html" class="navigation-link">Prev</a> <a href="./if-c.html" class="navigation-link">Next</a></span><span class="navigation-group-separator"> | </span><span class="navigation-group">Back <a href="./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="./if.html" class="navigation-link">if_</a></td> 13</tr></table><div class="header-separator"></div> 14<div class="section" id="if"> 15<h1><a class="toc-backref" href="./type-selection.html#id1520">if_</a></h1> 16<div class="section" id="id859"> 17<h3><a class="subsection-title" href="#synopsis" name="synopsis">Synopsis</a></h3> 18<pre class="literal-block"> 19template< 20 typename C 21 , typename T1 22 , typename T2 23 > 24struct <a href="./if.html" class="identifier">if_</a> 25{ 26 typedef <em>unspecified</em> type; 27}; 28</pre> 29</div> 30<div class="section" id="id860"> 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 <tt class="literal"><span class="pre">C</span></tt>.</p> 33</div> 34<div class="section" id="id861"> 35<h3><a class="subsection-title" href="#header" name="header">Header</a></h3> 36<pre class="literal-block"> 37#include <<a href="../../../../boost/mpl/if.hpp" class="header">boost/mpl/if.hpp</a>> 38</pre> 39</div> 40<div class="section" id="id862"> 41<h3><a class="subsection-title" href="#parameters" name="parameters">Parameters</a></h3> 42<table border="1" class="docutils table"> 43<colgroup> 44<col width="15%" /> 45<col width="36%" /> 46<col width="48%" /> 47</colgroup> 48<thead valign="bottom"> 49<tr><th class="head">Parameter</th> 50<th class="head">Requirement</th> 51<th class="head">Description</th> 52</tr> 53</thead> 54<tbody valign="top"> 55<tr><td><tt class="literal"><span class="pre">C</span></tt></td> 56<td><a class="reference internal" href="./integral-constant.html">Integral Constant</a></td> 57<td>A selection condition.</td> 58</tr> 59<tr><td><tt class="literal"><span class="pre">T1</span></tt>, <tt class="literal"><span class="pre">T2</span></tt></td> 60<td>Any type</td> 61<td>Types to select from.</td> 62</tr> 63</tbody> 64</table> 65</div> 66<div class="section" id="id863"> 67<h3><a class="subsection-title" href="#expression-semantics" name="expression-semantics">Expression semantics</a></h3> 68<p>For any <a class="reference internal" href="./integral-constant.html">Integral Constant</a> <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> 69<pre class="literal-block"> 70typedef <a href="./if.html" class="identifier">if_</a><c,t1,t2>::type t; 71</pre> 72<table class="docutils field-list" frame="void" rules="none"> 73<col class="field-name" /> 74<col class="field-body" /> 75<tbody valign="top"> 76<tr class="field"><th class="field-name">Return type:</th><td class="field-body">Any type.</td> 77</tr> 78<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">t1</span></tt>; otherwise <tt class="literal"><span class="pre">t</span></tt> is 79identical to <tt class="literal"><span class="pre">t2</span></tt>.</td> 80</tr> 81</tbody> 82</table> 83</div> 84<div class="section" id="id864"> 85<h3><a class="subsection-title" href="#example" name="example">Example</a></h3> 86<pre class="literal-block"> 87typedef <a href="./if.html" class="identifier">if_</a><<a href="./bool.html" class="identifier">true_</a>,char,long>::type t1; 88typedef <a href="./if.html" class="identifier">if_</a><<a href="./bool.html" class="identifier">false_</a>,char,long>::type t2; 89 90<a href="./assert.html" class="identifier">BOOST_MPL_ASSERT</a>(( is_same<t1, char> )); 91<a href="./assert.html" class="identifier">BOOST_MPL_ASSERT</a>(( is_same<t2, long> )); 92</pre> 93<pre class="literal-block"> 94// allocates space for an object of class T on heap or "inplace" 95// depending on its <a href="./size.html" class="identifier">size</a> 96template< typename T > struct lightweight 97{ 98 // ... 99 typedef typename <a href="./if.html" class="identifier">if_</a>< 100 <a href="./less-equal.html" class="identifier">less_equal</a>< <a href="./sizeof.html" class="identifier">sizeof_</a><T>, <a href="./sizeof.html" class="identifier">sizeof_</a><T*> > 101 , inplace_storage<T> 102 , heap_storage<T> 103 >::type impl_t; 104 105 impl_t impl; 106}; 107</pre> 108</div> 109<div class="section" id="id865"> 110<h3><a class="subsection-title" href="#see-also" name="see-also">See also</a></h3> 111<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-c.html">if_c</a>, <a class="reference internal" href="./eval-if.html">eval_if</a></p> 112<!-- Metafunctions/Type Selection//if_c |20 --> 113</div> 114</div> 115 116<div class="footer-separator"></div> 117<table class="footer"><tr class="footer"><td class="header-group navigation-bar"><span class="navigation-group"><a href="./type-selection.html" class="navigation-link">Prev</a> <a href="./if-c.html" class="navigation-link">Next</a></span><span class="navigation-group-separator"> | </span><span class="navigation-group">Back <a href="./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> 118<td><div class="copyright-footer"><div class="copyright">Copyright © 2001-2009 Aleksey Gurtovoy and David Abrahams</div> 119Distributed under the Boost Software License, Version 1.0. (See accompanying 120file 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> 121</html> 122