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Showing content from https://en.cppreference.com/w/cpp/language/../types/is_union.html below:

std::is_union - cppreference.com

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.

[edit] Template parameters [edit] 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 true if T is a union type, false otherwise
(public static member constant) Member functions converts the object to bool, returns value
(public member function) returns value
(public member function) Member types [edit] 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() {}

[edit] See also checks if a type is a non-union class type
(class template) [edit]

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