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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"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></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 (June 2015) compilers more recent | |
50 | than Visual C++ 8, clang, GCC-4.3, Greenhills 6.0, Intel-11.0, and Codegear | |
51 | 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_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"><</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">></span></code> | |
60 | or <code class="computeroutput"> <span class="preprocessor">#include</span> <span class="special"><</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">></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"><</span><span class="identifier">my_union</span><span class="special">></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"><</span><span class="keyword">const</span> <span class="identifier">my_union</span><span class="special">>::</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"><</span><span class="identifier">my_union</span><span class="special">>::</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"><</span><span class="identifier">my_union</span><span class="special">*>::</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"><</span><span class="identifier">T</span><span class="special">>::</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> | |
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91 | <td align="right"><div class="copyright-footer">Copyright © 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>) | |
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