| [/ |
| 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 |
| (May 2005) only Visual C++ 8 has the necessary compiler __intrinsics to make this |
| trait "just work" without user intervention. |
| |
| __header ` #include <boost/type_traits/is_union.hpp>` or ` #include <boost/type_traits.hpp>` |
| |
| __examples |
| |
| [:`is_union<void>` inherits from `__true_type`.] |
| |
| [:`is_union<const void>::type` is the type `__true_type`.] |
| |
| [:`is_union<void>::value` is an integral constant |
| expression that evaluates to /true/.] |
| |
| [:`is_union<void*>::value` is an integral constant |
| expression that evaluates to /false/.] |
| |
| [:`is_union<T>::value_type` is the type `bool`.] |
| |
| [endsect] |
| |