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1 | [/ |
2 | Copyright 2007 John Maddock. | |
3 | Distributed under the Boost Software License, Version 1.0. | |
4 | (See accompanying file LICENSE_1_0.txt or copy at | |
5 | http://www.boost.org/LICENSE_1_0.txt). | |
6 | ] | |
7 | ||
8 | [section:is_union is_union] | |
9 | template <class T> | |
10 | struct is_union : public __tof {}; | |
11 | ||
12 | __inherit If T is a (possibly cv-qualified) union type then inherits from __true_type, | |
13 | otherwise inherits from __false_type. Currently requires some kind of compiler | |
14 | support, otherwise unions are identified as classes. | |
15 | ||
16 | __std_ref 3.9.2 and 9.5. | |
17 | ||
18 | __compat Without (some as yet unspecified) help from the | |
19 | compiler, we cannot distinguish between union and class types using only standard C++, | |
20 | as a result this type will never inherit from __true_type, unless the user explicitly | |
21 | specializes the template for their user-defined union types, or unless the compiler | |
22 | supplies some unspecified intrinsic that implements this functionality. | |
23 | Currently (June 2015) compilers more recent than Visual C++ 8, clang, GCC-4.3, Greenhills 6.0, | |
24 | Intel-11.0, and Codegear have the necessary compiler __intrinsics to ensure that this | |
25 | trait "just works". You may also test to see if the necessary __intrinsics are available | |
26 | by checking to see if the macro `BOOST_IS_UNION` is defined. | |
27 | ||
28 | ||
29 | __header ` #include <boost/type_traits/is_union.hpp>` or ` #include <boost/type_traits.hpp>` | |
30 | ||
31 | __examples | |
32 | ||
33 | Given `union my_union {};` then: | |
34 | ||
35 | [:`is_union<my_union>` inherits from `__true_type`.] | |
36 | ||
37 | [:`is_union<const my_union>::type` is the type `__true_type`.] | |
38 | ||
39 | [:`is_union<my_union>::value` is an integral constant | |
40 | expression that evaluates to /true/.] | |
41 | ||
42 | [:`is_union<my_union*>::value` is an integral constant | |
43 | expression that evaluates to /false/.] | |
44 | ||
45 | [:`is_union<T>::value_type` is the type `bool`.] | |
46 | ||
47 | [endsect] | |
48 |