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"> 19 template< 20 typename C 21 , typename T1 22 , typename T2 23 > 24 struct <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"> 70 typedef <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 79 identical 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"> 87 typedef <a href="./if.html" class="identifier">if_</a><<a href="./bool.html" class="identifier">true_</a>,char,long>::type t1; 88 typedef <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> 96 template< 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> 119 Distributed under the Boost Software License, Version 1.0. (See accompanying 120 file 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