libs/type_traits/doc/is_union.qbk
[/ 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]