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