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template <class T>
struct is_union : public true_type-or-false_type {};
Inherits: 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.
C++ Standard Reference: 3.9.2 and 9.5.
        Compiler Compatibility: 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 fromtrue_type.
is_union<const my_union>::typeis the typetrue_type.
is_union<my_union>::valueis an integral constant expression that evaluates to true.
is_union<my_union*>::valueis an integral constant expression that evaluates to false.
is_union<T>::value_typeis the typebool.