std::is_class

< 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

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

#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)