# std::is_union

From cppreference.com
< cpp‎ | types

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

`std::is_union` is an 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.

The behavior of a program that adds specializations for `std::is_union` or `std::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

Run this code

    #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

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

Retrieved from "https://en.cppreference.com/mwiki/index.php?title=cpp/types/is_union&oldid=152462"