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std::is_union

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

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.

The behavior of a program that adds specializations for is_union or is_union_v (since C++17) is undefined.

Template parameters

T - a type to check

Helper variable template

template< class T >
inline 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 <iostream>
#include <type_traits>
 
struct A {};
 
typedef union
{
    int a;
    float b;
} B;
 
struct C { B d; };
 
int main() 
{
    std::cout << std::boolalpha;
    std::cout << std::is_union<A>::value << '\n';
    std::cout << std::is_union<B>::value << '\n';
    std::cout << std::is_union<C>::value << '\n';
    std::cout << std::is_union<int>::value << '\n';
}

Output:

false
true
false
false

See also

(C++11)
checks if a type is a non-union class type
(class template)

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https://en.cppreference.com/w/cpp/types/is_union