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
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
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.