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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> |
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| <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> |
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