std:: is_base_of

std::is_base_of is a BinaryTypeTrait .

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 should be a complete type ; otherwise the behavior is undefined.

If the program adds specializations for std::is_base_of or std::is_base_of_v (since C++17) , the behavior is undefined.

Helper variable template

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_ptr_conv(const volatile B*);
    template<typename>
    std::false_type test_ptr_conv(const volatile void*);
 
    template<typename B, typename D>
    auto test_is_base_of(int) -> decltype(test_ptr_conv<B>(static_cast<D*>(nullptr)));
    template<typename, typename>
    auto test_is_base_of(...) -> std::true_type; // private or ambiguous base
}
 
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_is_base_of<Base, Derived>(0))::value
    > {};

Example

#include <type_traits>
 
class A {};
class B : A {};
class C : B {};
class D {};
union E {};
using I = int;
 
static_assert
(
    std::is_base_of_v<A, A> == true &&
    std::is_base_of_v<A, B> == true &&
    std::is_base_of_v<A, C> == true &&
    std::is_base_of_v<A, D> != true &&
    std::is_base_of_v<B, A> != true &&
    std::is_base_of_v<E, E> != true &&
    std::is_base_of_v<I, I> != true
);
 
int main() {}

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

See also