<?xml version="1.0" encoding="utf-8" ?> <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"> <html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en"> <head> <meta http-equiv="Content-Type" content="text/html; charset=utf-8" /> <meta name="generator" content="Docutils 0.7: http://docutils.sourceforge.net/" /> <title>The MPL Reference Manual: eval_if</title> <link rel="stylesheet" href="../style.css" type="text/css" /> </head> <body class="docframe refmanual"> <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> <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> </tr></table><div class="header-separator"></div> <div class="section" id="eval-if"> <h1><a class="toc-backref" href="./type-selection.html#id1522">eval_if</a></h1> <div class="section" id="id873"> <h3><a class="subsection-title" href="#synopsis" name="synopsis">Synopsis</a></h3> <pre class="literal-block"> template< typename C , typename F1 , typename F2 > struct <a href="./eval-if.html" class="identifier">eval_if</a> { typedef <em>unspecified</em> type; }; </pre> </div> <div class="section" id="id874"> <h3><a class="subsection-title" href="#description" name="description">Description</a></h3> <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 on the value <tt class="literal"><span class="pre">C</span></tt>.</p> </div> <div class="section" id="id875"> <h3><a class="subsection-title" href="#header" name="header">Header</a></h3> <pre class="literal-block"> #include <<a href="../../../../boost/mpl/eval_if.hpp" class="header">boost/mpl/eval_if.hpp</a>> </pre> </div> <div class="section" id="id876"> <h3><a class="subsection-title" href="#parameters" name="parameters">Parameters</a></h3> <table border="1" class="docutils table"> <colgroup> <col width="15%" /> <col width="36%" /> <col width="48%" /> </colgroup> <thead valign="bottom"> <tr><th class="head">Parameter</th> <th class="head">Requirement</th> <th class="head">Description</th> </tr> </thead> <tbody valign="top"> <tr><td><tt class="literal"><span class="pre">C</span></tt></td> <td><a class="reference internal" href="./integral-constant.html">Integral Constant</a></td> <td>An evaluation condition.</td> </tr> <tr><td><tt class="literal"><span class="pre">F1</span></tt>, <tt class="literal"><span class="pre">F2</span></tt></td> <td>Nullary <a class="reference internal" href="./metafunction.html">Metafunction</a></td> <td>Metafunctions to select for evaluation from.</td> </tr> </tbody> </table> </div> <div class="section" id="id877"> <h3><a class="subsection-title" href="#expression-semantics" name="expression-semantics">Expression semantics</a></h3> <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> <pre class="literal-block"> typedef <a href="./eval-if.html" class="identifier">eval_if</a><c,f1,f2>::type t; </pre> <table class="docutils field-list" frame="void" rules="none"> <col class="field-name" /> <col class="field-body" /> <tbody valign="top"> <tr class="field"><th class="field-name">Return type:</th><td class="field-body">Any type.</td> </tr> <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 identical to <tt class="literal"><span class="pre">f2::type</span></tt>.</td> </tr> </tbody> </table> </div> <div class="section" id="id878"> <h3><a class="subsection-title" href="#example" name="example">Example</a></h3> <pre class="literal-block"> typedef <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; typedef <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; <a href="./assert.html" class="identifier">BOOST_MPL_ASSERT</a>(( is_same<t1,char> )); <a href="./assert.html" class="identifier">BOOST_MPL_ASSERT</a>(( is_same<t2,long> )); </pre> </div> <div class="section" id="id879"> <h3><a class="subsection-title" href="#see-also" name="see-also">See also</a></h3> <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> <!-- Metafunctions/Type Selection//eval_if_c |40 --> </div> </div> <div class="footer-separator"></div> <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> <td><div class="copyright-footer"><div class="copyright">Copyright © 2001-2009 Aleksey Gurtovoy and David Abrahams</div> Distributed under the Boost Software License, Version 1.0. 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