ceph/src/boost/libs/type_traits/doc/html/boost_typetraits/reference/is_empty.html

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  26 <div class="titlepage"><div><div><h3 class="title">
  27 <a name="boost_typetraits.reference.is_empty"></a><a class="link" href="is_empty.html" title="is_empty">is_empty</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_empty</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 an empty class type (and
  34         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> 10p5.
  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_empty</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> In order to correctly
  47         detect empty classes this trait relies on either:
  48       </p>
  49 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
  50 <li class="listitem">
  51             the compiler implementing zero sized empty base classes, or
  52           </li>
  53 <li class="listitem">
  54             the compiler providing <a class="link" href="../intrinsics.html" title="Support for Compiler Intrinsics">intrinsics</a>
  55             to detect empty classes - this latter case can be tested for by checking
  56             to see if the macro BOOST_IS_EMPTY is defined.
  57           </li>
  58 </ul></div>
  59 <p>
  60         Can not be used with incomplete types.
  61       </p>
  62 <p>
  63         <span class="bold"><strong>Examples:</strong></span>
  64       </p>
  65 <div class="blockquote"><blockquote class="blockquote"><p>
  66           Given: <code class="computeroutput"><span class="keyword">struct</span> <span class="identifier">empty_class</span>
  67           <span class="special">{};</span></code>
  68         </p></blockquote></div>
  69 <div class="blockquote"><blockquote class="blockquote"><p>
  70           <code class="computeroutput"><span class="identifier">is_empty</span><span class="special">&lt;</span><span class="identifier">empty_class</span><span class="special">&gt;</span></code>
  71           inherits from <code class="computeroutput"><a class="link" href="integral_constant.html" title="integral_constant">true_type</a></code>.
  72         </p></blockquote></div>
  73 <div class="blockquote"><blockquote class="blockquote"><p>
  74           <code class="computeroutput"><span class="identifier">is_empty</span><span class="special">&lt;</span><span class="identifier">empty_class</span> <span class="keyword">const</span><span class="special">&gt;::</span><span class="identifier">type</span></code>
  75           is the type <code class="computeroutput"><a class="link" href="integral_constant.html" title="integral_constant">true_type</a></code>.
  76         </p></blockquote></div>
  77 <div class="blockquote"><blockquote class="blockquote"><p>
  78           <code class="computeroutput"><span class="identifier">is_empty</span><span class="special">&lt;</span><span class="identifier">empty_class</span><span class="special">&gt;::</span><span class="identifier">value</span></code> is an integral constant expression
  79           that evaluates to <span class="emphasis"><em>true</em></span>.
  80         </p></blockquote></div>
  81 <div class="blockquote"><blockquote class="blockquote"><p>
  82           <code class="computeroutput"><span class="identifier">is_empty</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>.
  83         </p></blockquote></div>
  84 </div>
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  88       Frederic Bron, Steve Cleary, Beman Dawes, Aleksey Gurtovoy, Howard Hinnant,
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