| 123456789101112131415161718192021222324252627282930313233343536373839404142434445464748 |
- [/
- Copyright 2007 John Maddock.
- Distributed under the Boost Software License, Version 1.0.
- (See accompanying file LICENSE_1_0.txt or copy at
- http://www.boost.org/LICENSE_1_0.txt).
- ]
- [section:is_union is_union]
- template <class T>
- struct is_union : public __tof {};
-
- __inherit If T is a (possibly cv-qualified) union type then inherits from __true_type,
- otherwise inherits from __false_type. Currently requires some kind of compiler
- support, otherwise unions are identified as classes.
- __std_ref 3.9.2 and 9.5.
- __compat Without (some as yet unspecified) help from the
- compiler, we cannot distinguish between union and class types using only standard C++,
- as a result this type will never inherit from __true_type, unless the user explicitly
- specializes the template for their user-defined union types, or unless the compiler
- supplies some unspecified intrinsic that implements this functionality.
- Currently (June 2015) compilers more recent than Visual C++ 8, clang, GCC-4.3, Greenhills 6.0,
- Intel-11.0, and Codegear have the necessary compiler __intrinsics to ensure that this
- trait "just works". You may also test to see if the necessary __intrinsics are available
- by checking to see if the macro `BOOST_IS_UNION` is defined.
- __header ` #include <boost/type_traits/is_union.hpp>` or ` #include <boost/type_traits.hpp>`
- __examples
- Given `union my_union {};` then:
- [:`is_union<my_union>` inherits from `__true_type`.]
- [:`is_union<const my_union>::type` is the type `__true_type`.]
- [:`is_union<my_union>::value` is an integral constant
- expression that evaluates to /true/.]
- [:`is_union<my_union*>::value` is an integral constant
- expression that evaluates to /false/.]
- [:`is_union<T>::value_type` is the type `bool`.]
- [endsect]
|
