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<div class="section" id="if">
<h1><a class="toc-backref" href="./type-selection.html#id1520">if_</a></h1>
<div class="section" id="id859">
<h3><a class="subsection-title" href="#synopsis" name="synopsis">Synopsis</a></h3>
<pre class="literal-block">
template&lt;
      typename C
    , typename T1
    , typename T2
    &gt;
struct <a href="./if.html" class="identifier">if_</a>
{
    typedef <em>unspecified</em> type;
};
</pre>
</div>
<div class="section" id="id860">
<h3><a class="subsection-title" href="#description" name="description">Description</a></h3>
<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>
</div>
<div class="section" id="id861">
<h3><a class="subsection-title" href="#header" name="header">Header</a></h3>
<pre class="literal-block">
#include &lt;<a href="../../../../boost/mpl/if.hpp" class="header">boost/mpl/if.hpp</a>&gt;
</pre>
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<div class="section" id="id862">
<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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<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>A selection condition.</td>
</tr>
<tr><td><tt class="literal"><span class="pre">T1</span></tt>, <tt class="literal"><span class="pre">T2</span></tt></td>
<td>Any type</td>
<td>Types to select from.</td>
</tr>
</tbody>
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<div class="section" id="id863">
<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 arbitrary types <tt class="literal"><span class="pre">t1</span></tt>, <tt class="literal"><span class="pre">t2</span></tt>:</p>
<pre class="literal-block">
typedef <a href="./if.html" class="identifier">if_</a>&lt;c,t1,t2&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">t1</span></tt>; otherwise <tt class="literal"><span class="pre">t</span></tt> is
identical to <tt class="literal"><span class="pre">t2</span></tt>.</td>
</tr>
</tbody>
</table>
</div>
<div class="section" id="id864">
<h3><a class="subsection-title" href="#example" name="example">Example</a></h3>
<pre class="literal-block">
typedef <a href="./if.html" class="identifier">if_</a>&lt;<a href="./bool.html" class="identifier">true_</a>,char,long&gt;::type t1;
typedef <a href="./if.html" class="identifier">if_</a>&lt;<a href="./bool.html" class="identifier">false_</a>,char,long&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>
<pre class="literal-block">
// allocates space for an object of class T on heap or &quot;inplace&quot;
// depending on its <a href="./size.html" class="identifier">size</a>
template&lt; typename T &gt; struct lightweight
{
    // ...
    typedef typename <a href="./if.html" class="identifier">if_</a>&lt;
          <a href="./less-equal.html" class="identifier">less_equal</a>&lt; <a href="./sizeof.html" class="identifier">sizeof_</a>&lt;T&gt;, <a href="./sizeof.html" class="identifier">sizeof_</a>&lt;T*&gt; &gt;
        , inplace_storage&lt;T&gt;
        , heap_storage&lt;T&gt;
        &gt;::type impl_t;

    impl_t impl;
};
</pre>
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<div class="section" id="id865">
<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="./if-c.html">if_c</a>, <a class="reference internal" href="./eval-if.html">eval_if</a></p>
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