Boost C++ Libraries

...one of the most highly regarded and expertly designed C++ library projects in the world. Herb Sutter and Andrei Alexandrescu, C++ Coding Standards

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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.


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