std::is_base_of

template< class Base, class Derived >
struct is_base_of;

If Derived is derived from Base or if both are the same non-union class (in both cases ignoring cv-qualification), provides the member constant value equal to true. Otherwise value is false.

If both Base and Derived are non-union class types, and they are not the same type (ignoring cv-qualification), Derived shall be a complete type; otherwise the behavior is undefined.

The behavior of a program that adds specializations for is_base_of or is_base_of_v (since C++17) is undefined.

Helper variable template

template< class Base, class Derived >
inline constexpr bool is_base_of_v = is_base_of<Base, Derived>::value;

Notes

std::is_base_of<A, B>::value is true even if A is a private, protected, or ambiguous base class of B. In many situations, std::is_convertible<B*, A*> is the more appropriate test.

Although no class is its own base, std::is_base_of<T, T>::value is true because the intent of the trait is to model the "is-a" relationship, and T is a T. Despite that, std::is_base_of<int, int>::value is false because only classes participate in the relationship that this trait models.

Possible Implementation

namespace details {
    template <typename B>
    std::true_type test_pre_ptr_convertible(const volatile B*);
    template <typename>
    std::false_type test_pre_ptr_convertible(const volatile void*);
 
    template <typename, typename>
    auto test_pre_is_base_of(...) -> std::true_type;
    template <typename B, typename D>
    auto test_pre_is_base_of(int) ->
        decltype(test_pre_ptr_convertible<B>(static_cast<D*>(nullptr)));
}
 
template <typename Base, typename Derived>
struct is_base_of :
    std::integral_constant<
        bool,
        std::is_class<Base>::value && std::is_class<Derived>::value &&
        decltype(details::test_pre_is_base_of<Base, Derived>(0))::value
    > { };

Example

#include <iostream>
#include <type_traits>
#define SHOW(...) \
    std::cout << #__VA_ARGS__ << " : " \
              << std:: __VA_ARGS__ << '\n'
int main()
{
    class A {};
    class B : A {};
    class C : B {};
    class D {};
 
    std::cout << std::boolalpha;
    SHOW( is_base_of_v<A, A> );
    SHOW( is_base_of_v<A, B> );
    SHOW( is_base_of_v<A, C> );
    SHOW( is_base_of_v<A, D> );
    SHOW( is_base_of_v<B, A> );
    SHOW( is_base_of_v<int, int> );
}

is_base_of_v<A, A> : true
is_base_of_v<A, B> : true
is_base_of_v<A, C> : true
is_base_of_v<A, D> : false
is_base_of_v<B, A> : false
is_base_of_v<int, int> : false

See also