std::is_class

Defined in header `<type_traits>`
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

T

-

a type to check

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

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 <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() {}

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