std::is_base_of

 

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

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

template<class Base, class Derived > constexprbool 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 or if both are the same non-union 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() (C++14)	returns value (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(constvolatile B*);template<typename>std::false_type test_ptr_conv(constvolatilevoid*);   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 Derived is an incomplete union type	the base characteristic is std::false_type in this case

See also

is_virtual_base_of (C++26)	checks if a type is a virtual base of the other type (class template)
is_convertibleis_nothrow_convertible (C++11)(C++20)	checks if a type can be converted to the other type (class template)
derived_from (C++20)	specifies that a type is derived from another type (concept)