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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<
- typename C
- , typename T1
- , typename T2
- >
- 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 <<a href="../../../../boost/mpl/if.hpp" class="header">boost/mpl/if.hpp</a>>
- </pre>
- </div>
- <div class="section" id="id862">
- <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>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>
- </table>
- </div>
- <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><c,t1,t2>::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">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><<a href="./bool.html" class="identifier">true_</a>,char,long>::type t1;
- typedef <a href="./if.html" class="identifier">if_</a><<a href="./bool.html" class="identifier">false_</a>,char,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>
- <pre class="literal-block">
- // allocates space for an object of class T on heap or "inplace"
- // depending on its <a href="./size.html" class="identifier">size</a>
- template< typename T > struct lightweight
- {
- // ...
- typedef typename <a href="./if.html" class="identifier">if_</a><
- <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*> >
- , inplace_storage<T>
- , heap_storage<T>
- >::type impl_t;
- impl_t impl;
- };
- </pre>
- </div>
- <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>
- <!-- Metafunctions/Type Selection//if_c |20 -->
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