# std::is_class

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< cpp‎ | types

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

`std::is_class` is an UnaryTypeTrait.

Checks whether `T` is a non-union class type. Provides the member constant `value` which is equal to true, if `T` is a class type (but not union). Otherwise, `value` is equal to false.

The behavior of a program that adds specializations for `std::is_class` or `std::is_class_v` (since C++17) is undefined.

### Template parameters

T | - | a type to check

### Helper variable template

template< class T > inline constexpr bool is_class_v = is_class<T>::value; |  | (since C++17)

## Inherited from std::integral_constant

### Member constants

value [static] | true if `T` is a non-union class 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>

### Possible implementation

    namespace detail
    {
        template<class T>
        std::integral_constant<bool, !std::is_union<T>::value> test(int T::*);

        template<class>
        std::false_type test(...);
    }

    template<class T>
    struct is_class : decltype(detail::test<T>(nullptr)) {};

### Example

Run this code

    #include <iostream>
    #include <type_traits>

    struct A {};

    class B {};

    enum class E {};

    union U { class UC {}; };
    static_assert(not std::is_class_v<U>);
    static_assert(std::is_class_v<U::UC>);

    int main()
    {
        std::cout << std::boolalpha;
        std::cout << std::is_class<A>::value << ": A\n";
        std::cout << std::is_class_v<B> << ": B\n";
        std::cout << std::is_class_v<B*> << ": B*\n";
        std::cout << std::is_class_v<B&> << ": B&\n";
        std::cout << std::is_class_v<const B> << ": const B\n";
        std::cout << std::is_class<E>::value << ": E\n";
        std::cout << std::is_class_v<int> << ": int\n";
        std::cout << std::is_class_v<struct S> << ": struct S (incomplete)\n";
        std::cout << std::is_class_v<class C> << ": class C (incomplete)\n";
    }

Output:

    true: A
    true: B
    false: B*
    false: B&
    true: const B
    false: E
    false: int
    true: struct S (incomplete)
    true: class C (incomplete)

### See also

is_union (C++11) | checks if a type is a union type (class template)

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