std::is_class - cppreference.com

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`template< class T > struct is_class;`		(since C++11)

std::is_class is a 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.

If the program adds specializations for std::is_class or std::is_class_v, the behavior is undefined.

Template parameters

Helper variable template

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

Inherited from std::integral_constant

Member constants

true if T is a non-union class 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

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

#include <type_traits>

struct A {};
static_assert(std::is_class<A>::value);

class B {};
static_assert(std::is_class_v<B>);
static_assert(not std::is_class_v<B*>);
static_assert(not std::is_class_v<B&>);
static_assert(std::is_class_v<const B>);

enum class E {};
static_assert(not std::is_class<E>::value);

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

static_assert(not std::is_class_v<int>);

static_assert(std::is_class_v<struct S>, "incomplete class");
static_assert(std::is_class_v<class C>, "incomplete class");

int main() {}