std::is_union

Defined in header `<type_traits>`
template< class T > struct is_union;		(since C++11)

std::is_union is a UnaryTypeTrait.

Checks whether T is a union type. Provides the member constant value, which is equal to true if T is a union type. Otherwise, value is equal to false.

If the program adds specializations for std::is_union or std::is_union_v, the behavior is undefined.

Template parameters

a type to check

Helper variable template

template< class T >
constexpr bool is_union_v = is_union<T>::value;

(since C++17)

Inherited from std::integral_constant

Member constants

value

[static]

true if T is a union type, false otherwise
(public static member constant)

Member functions

operator bool	converts the object to bool, returns `value` (public member function)
operator() (C++14)	returns `value` (public member function)

Member types

Type	Definition
`value_type`	bool
`type`	std::integral_constant<bool,value>

Example

#include <type_traits>

struct A {};
static_assert(!std::is_union_v<A>);

typedef union
{
    int a;
    float b;
} B;
static_assert(std::is_union_v<B>);

struct C { B d; };
static_assert(!std::is_union_v<C>);

static_assert(!std::is_union_v<int>);

int main() {}

is_class (C++11)	checks if a type is a non-union class type (class template)
is_union_type (C++26)	checks if reflected type is a union type (function)