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<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&lt;
      typename C
    , typename F1
    , typename F2
    &gt;
struct <a href="./eval-if.html" class="identifier">eval_if</a>
{
    typedef <em>unspecified</em> type;
};
</pre>
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<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>
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<div class="section" id="id875">
<h3><a class="subsection-title" href="#header" name="header">Header</a></h3>
<pre class="literal-block">
#include &lt;<a href="../../../../boost/mpl/eval_if.hpp" class="header">boost/mpl/eval_if.hpp</a>&gt;
</pre>
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<h3><a class="subsection-title" href="#parameters" name="parameters">Parameters</a></h3>
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<tr><th class="head">Parameter</th>
<th class="head">Requirement</th>
<th class="head">Description</th>
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<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>
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<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>&lt;c,f1,f2&gt;::type t;
</pre>
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<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>
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<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>&lt; <a href="./bool.html" class="identifier">true_</a>, <a href="./identity.html" class="identifier">identity</a>&lt;char&gt;, <a href="./identity.html" class="identifier">identity</a>&lt;long&gt; &gt;::type t1;
typedef <a href="./eval-if.html" class="identifier">eval_if</a>&lt; <a href="./bool.html" class="identifier">false_</a>, <a href="./identity.html" class="identifier">identity</a>&lt;char&gt;, <a href="./identity.html" class="identifier">identity</a>&lt;long&gt; &gt;::type t2;

<a href="./assert.html" class="identifier">BOOST_MPL_ASSERT</a>(( is_same&lt;t1,char&gt; ));
<a href="./assert.html" class="identifier">BOOST_MPL_ASSERT</a>(( is_same&lt;t2,long&gt; ));
</pre>
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<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>
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