std::is_base_of

Defined in header `<type_traits>`
template< class Base, class Derived > struct is_base_of;		(since C++11)

If Derived is derived from Base or if both are the same non-union class, 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.

Helper variable template

template< class Base, class Derived > inline constexpr bool is_base_of_v = is_base_of<Base, Derived>::value;		(since C++17)

Inherited from std::integral_constant

Member constants

value

[static]

true if Derived is derived from Base , 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>

Notes

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 Base> std::true_type is_base_of_test_func(Base*);
    template <typename Base> std::false_type is_base_of_test_func(void*);
    template <typename Base, typename Derived>
    using pre_is_base_of = decltype(is_base_of_test_func<Base>(std::declval<Derived*>()));
 
    // with <experimental/type_traits>:
    // template <typename Base, typename Derived>
    // using pre_is_base_of2 = std::experimental::detected_or_t<std::true_type, pre_is_base_of, Base, Derived>;
    template <typename Base, typename Derived, typename = void>
    struct pre_is_base_of2 : public std::true_type { };
    // note std::void_t is a C++17 feature
    template <typename Base, typename Derived>
    struct pre_is_base_of2<Base, Derived, std::void_t<pre_is_base_of<Base, Derived>>> :
        public pre_is_base_of<Base, Derived> { };
}
 
template <typename Base, typename Derived>
struct is_base_of :
    public std::conditional_t<
        std::is_class<Base>::value && std::is_class<Derived>::value,
        details::pre_is_base_of2<Base, Derived>,
        std::false_type
    > { };

Example

Run this code

#include <iostream>
#include <type_traits>
 
class A {};
 
class B : A {};
 
class C {};
 
int main() 
{
    std::cout << std::boolalpha;
    std::cout << "a2b: " << std::is_base_of<A, B>::value << '\n';
    std::cout << "b2a: " << std::is_base_of<B, A>::value << '\n';
    std::cout << "c2b: " << std::is_base_of<C, B>::value << '\n';
    std::cout << "same type: " << std::is_base_of<C, C>::value << '\n';
}

Output:

a2b: true
b2a: false
c2b: false
same type: true

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