NAME¶

std::is_class - std::is_class

Synopsis¶

Defined in header <type_traits>
template< class T > (since C++11)
struct 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¶

T - a type to check

Helper variable template

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

Inherited from std::integral_constant

Member constants¶

value true if T is a non-union class type, false otherwise
[static] (public static member constant)

Member functions¶

operator bool converts the object to bool, returns value
(public member function)
operator() returns value
(C++14) (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 checks if a type is a union type
(C++11) (class template)