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### is_union

    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 (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`.