std::is_union

< cpp‎ | types

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

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