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| 6 | <meta name="generator" content="Docutils 0.7: http://docutils.sourceforge.net/" /> | |
| 7 | <title>The MPL Reference Manual: if_</title> | |
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| 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> |