...one of the most highly
regarded and expertly designed C++ library projects in the
world.
— Herb Sutter and Andrei
Alexandrescu, C++
Coding Standards
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 fromtrue_type
.
is_union<const void>::type
is the typetrue_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 typebool
.