std::is_base_of(3)

NAME
std::is_base_of - std::is_base_of

Synopsis
   Defined in header <type_traits>
   template< class Base, class Derived > (since C++11)
   struct 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.

Member constants
   value true if Derived is derived from Base or if both are the same non-union
   [static] class (in both cases ignoring cv-qualification), false otherwise
            (public static member constant)

Member functions
   operator bool converts the object to bool, returns value
                 (public member function)
   operator() returns value
   (C++14) (public member function)

Member types
   Type Definition
   value_type bool
   type std::integral_constant<bool, 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_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
// Run this code
 #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.
      DR Applied to Behavior as published Correct behavior
   LWG 2015 C++11 the behavior might be undefined if the base characteristic is
                       Derived is an incomplete union type std::false_type in this case

See also
   is_convertible
   is_nothrow_convertible checks if a type can be converted to the other type
   (C++11) (class template)
   (C++20)
   derived_from specifies that a type is derived from another type
   (C++20) (concept)