third_party/boost/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 (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]