std:: is_class

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

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() {}

See also