libs/type_traits/doc/html/boost_typetraits/reference/is_class.html

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  26 <div class="titlepage"><div><div><h3 class="title">
  27 <a name="boost_typetraits.reference.is_class"></a><a class="link" href="is_class.html" title="is_class">is_class</a>
  28 </h3></div></div></div>
  29 <pre class="programlisting"><span class="keyword">template</span> <span class="special">&lt;</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">&gt;</span>
  30 <span class="keyword">struct</span> <span class="identifier">is_class</span> <span class="special">:</span> <span class="keyword">public</span> <em class="replaceable"><code><a class="link" href="integral_constant.html" title="integral_constant">true_type</a>-or-<a class="link" href="integral_constant.html" title="integral_constant">false_type</a></code></em> <span class="special">{};</span>
  31 </pre>
  32 <p>
  33         <span class="bold"><strong>Inherits:</strong></span> If T is a (possibly cv-qualified)
  34         class type (and not a union type) then inherits from <a class="link" href="integral_constant.html" title="integral_constant">true_type</a>,
  35         otherwise inherits from <a class="link" href="integral_constant.html" title="integral_constant">false_type</a>.
  36       </p>
  37 <p>
  38         <span class="bold"><strong>C++ Standard Reference:</strong></span> 3.9.2 and 9.2.
  39       </p>
  40 <p>
  41         <span class="bold"><strong>Header:</strong></span> <code class="computeroutput"> <span class="preprocessor">#include</span>
  42         <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">type_traits</span><span class="special">/</span><span class="identifier">is_class</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span></code>
  43         or <code class="computeroutput"> <span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">type_traits</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span></code>
  44       </p>
  45 <p>
  46         <span class="bold"><strong>Compiler Compatibility:</strong></span> Without (some as
  47         yet unspecified) help from the compiler, we cannot distinguish between union
  48         and class types, as a result this type will erroneously inherit from <a class="link" href="integral_constant.html" title="integral_constant">true_type</a> for
  49         union types. See also <a class="link" href="is_union.html" title="is_union">is_union</a>.
  50         Currently (May 2011) compilers more recent than Visual C++ 8, GCC-4.3, Greenhills
  51         6.0, Intel-11.0, and Codegear have the necessary compiler <a class="link" href="../intrinsics.html" title="Support for Compiler Intrinsics">intrinsics</a>
  52         to ensure that this trait "just works". You may also test to see
  53         if the necessary <a class="link" href="../intrinsics.html" title="Support for Compiler Intrinsics">intrinsics</a>
  54         are available by checking to see if the macro <code class="computeroutput"><span class="identifier">BOOST_IS_CLASS</span></code>
  55         is defined.
  56       </p>
  57 <p>
  58         <span class="bold"><strong>Examples:</strong></span>
  59       </p>
  60 <div class="blockquote"><blockquote class="blockquote"><p>
  61           Given: <code class="computeroutput"><span class="keyword">class</span> <span class="identifier">MyClass</span><span class="special">;</span></code> then:
  62         </p></blockquote></div>
  63 <div class="blockquote"><blockquote class="blockquote"><p>
  64           <code class="computeroutput"><span class="identifier">is_class</span><span class="special">&lt;</span><span class="identifier">MyClass</span><span class="special">&gt;</span></code>
  65           inherits from <code class="computeroutput"><a class="link" href="integral_constant.html" title="integral_constant">true_type</a></code>.
  66         </p></blockquote></div>
  67 <div class="blockquote"><blockquote class="blockquote"><p>
  68           <code class="computeroutput"><span class="identifier">is_class</span><span class="special">&lt;</span><span class="identifier">MyClass</span> <span class="keyword">const</span><span class="special">&gt;::</span><span class="identifier">type</span></code>
  69           is the type <code class="computeroutput"><a class="link" href="integral_constant.html" title="integral_constant">true_type</a></code>.
  70         </p></blockquote></div>
  71 <div class="blockquote"><blockquote class="blockquote"><p>
  72           <code class="computeroutput"><span class="identifier">is_class</span><span class="special">&lt;</span><span class="identifier">MyClass</span><span class="special">&gt;::</span><span class="identifier">value</span></code> is an integral constant expression
  73           that evaluates to <span class="emphasis"><em>true</em></span>.
  74         </p></blockquote></div>
  75 <div class="blockquote"><blockquote class="blockquote"><p>
  76           <code class="computeroutput"><span class="identifier">is_class</span><span class="special">&lt;</span><span class="identifier">MyClass</span><span class="special">&amp;&gt;::</span><span class="identifier">value</span></code> is an integral constant expression
  77           that evaluates to <span class="emphasis"><em>false</em></span>.
  78         </p></blockquote></div>
  79 <div class="blockquote"><blockquote class="blockquote"><p>
  80           <code class="computeroutput"><span class="identifier">is_class</span><span class="special">&lt;</span><span class="identifier">MyClass</span><span class="special">*&gt;::</span><span class="identifier">value</span></code> is an integral constant expression
  81           that evaluates to <span class="emphasis"><em>false</em></span>.
  82         </p></blockquote></div>
  83 <div class="blockquote"><blockquote class="blockquote"><p>
  84           <code class="computeroutput"><span class="identifier">is_class</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;::</span><span class="identifier">value_type</span></code> is the type <code class="computeroutput"><span class="keyword">bool</span></code>.
  85         </p></blockquote></div>
  86 </div>
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