std::is_class

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

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 is_class or is_class_v (since C++17) is undefined.

Template parameters

T

-

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

Run this code

#include <iostream>
#include <type_traits>
 
struct A {};
 
class B {};
 
enum class C {};
 
union D { class E {}; };
static_assert(not std::is_class_v<D>);
 
int main() 
{
    std::cout << std::boolalpha;
    std::cout << std::is_class<A>::value << '\n';
    std::cout << std::is_class_v<B> << '\n';
    std::cout << std::is_class<C>::value << '\n';
    std::cout << std::is_class_v<int> << '\n';
}

Output:

true
true
false
false

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