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

   1 <html>
   2 <head>
   3 <meta http-equiv="Content-Type" content="text/html; charset=US-ASCII">
   4 <title>is_union</title>
   5 <link rel="stylesheet" href="../../../../../../doc/src/boostbook.css" type="text/css">
   6 <meta name="generator" content="DocBook XSL Stylesheets V1.78.1">
   7 <link rel="home" href="../../index.html" title="Chapter&#160;1.&#160;Boost.TypeTraits">
   8 <link rel="up" href="../reference.html" title="Alphabetical Reference">
   9 <link rel="prev" href="is_stateless.html" title="is_stateless">
  10 <link rel="next" href="is_unsigned.html" title="is_unsigned">
  11 </head>
  12 <body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF">
  13 <table cellpadding="2" width="100%"><tr>
  14 <td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../../../../boost.png"></td>
  15 <td align="center"><a href="../../../../../../index.html">Home</a></td>
  16 <td align="center"><a href="../../../../../../libs/libraries.htm">Libraries</a></td>
  17 <td align="center"><a href="http://www.boost.org/users/people.html">People</a></td>
  18 <td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td>
  19 <td align="center"><a href="../../../../../../more/index.htm">More</a></td>
  20 </tr></table>
  21 <hr>
  22 <div class="spirit-nav">
  23 <a accesskey="p" href="is_stateless.html"><img src="../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../reference.html"><img src="../../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../index.html"><img src="../../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="is_unsigned.html"><img src="../../../../../../doc/src/images/next.png" alt="Next"></a>
  24 </div>
  25 <div class="section">
  26 <div class="titlepage"><div><div><h3 class="title">
  27 <a name="boost_typetraits.reference.is_union"></a><a class="link" href="is_union.html" title="is_union">is_union</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_union</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         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         Currently requires some kind of compiler support, otherwise unions are identified
  37         as classes.
  38       </p>
  39 <p>
  40         <span class="bold"><strong>C++ Standard Reference:</strong></span> 3.9.2 and 9.5.
  41       </p>
  42 <p>
  43         <span class="bold"><strong>Compiler Compatibility:</strong></span> Without (some as
  44         yet unspecified) help from the compiler, we cannot distinguish between union
  45         and class types using only standard C++, as a result this type will never
  46         inherit from <a class="link" href="integral_constant.html" title="integral_constant">true_type</a>,
  47         unless the user explicitly specializes the template for their user-defined
  48         union types, or unless the compiler supplies some unspecified intrinsic that
  49         implements this functionality. Currently (May 2011) compilers more recent
  50         than Visual C++ 8, GCC-4.3, Greenhills 6.0, Intel-11.0, and Codegear have
  51         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_UNION</span></code>
  55         is defined.
  56       </p>
  57 <p>
  58         <span class="bold"><strong>Header:</strong></span> <code class="computeroutput"> <span class="preprocessor">#include</span>
  59         <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_union</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span></code>
  60         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>
  61       </p>
  62 <p>
  63         <span class="bold"><strong>Examples:</strong></span>
  64       </p>
  65 <p>
  66         Given <code class="computeroutput"><span class="keyword">union</span> <span class="identifier">my_union</span>
  67         <span class="special">{};</span></code> then:
  68       </p>
  69 <div class="blockquote"><blockquote class="blockquote"><p>
  70           <code class="computeroutput"><span class="identifier">is_union</span><span class="special">&lt;</span><span class="identifier">my_union</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_union</span><span class="special">&lt;</span><span class="keyword">const</span> <span class="identifier">my_union</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_union</span><span class="special">&lt;</span><span class="identifier">my_union</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_union</span><span class="special">&lt;</span><span class="identifier">my_union</span><span class="special">*&gt;::</span><span class="identifier">value</span></code> is an integral constant expression
  83           that evaluates to <span class="emphasis"><em>false</em></span>.
  84         </p></blockquote></div>
  85 <div class="blockquote"><blockquote class="blockquote"><p>
  86           <code class="computeroutput"><span class="identifier">is_union</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>.
  87         </p></blockquote></div>
  88 </div>
  89 <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
  90 <td align="left"></td>
  91 <td align="right"><div class="copyright-footer">Copyright &#169; 2000, 2011 Adobe Systems Inc, David Abrahams,
  92       Frederic Bron, Steve Cleary, Beman Dawes, Aleksey Gurtovoy, Howard Hinnant,
  93       Jesse Jones, Mat Marcus, Itay Maman, John Maddock, Alexander Nasonov, Thorsten
  94       Ottosen, Roman Perepelitsa, Robert Ramey, Jeremy Siek, Robert Stewart and Steven
  95       Watanabe<p>
  96         Distributed under the Boost Software License, Version 1.0. (See accompanying
  97         file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
  98       </p>
  99 </div></td>
 100 </tr></table>
 101 <hr>
 102 <div class="spirit-nav">
 103 <a accesskey="p" href="is_stateless.html"><img src="../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../reference.html"><img src="../../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../index.html"><img src="../../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="is_unsigned.html"><img src="../../../../../../doc/src/images/next.png" alt="Next"></a>
 104 </div>
 105 </body>
 106 </html>