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;
   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, (since C++17)
   Derived>::value; Inherited from std::integral_constant

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_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
// Run this code
 #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> );
 }

Output:
 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
   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)