std::is_base_of

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), the base characteristic is std::true_type. Otherwise the base characteristic is std::false_type.

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.

The behavior of a program that adds specializations for std::is_base_of or std::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, 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(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 `Derived` is an incomplete union type	the base characteristic is std::false_type in this case

Compiler support
Freestanding and hosted
Language
Standard library
Standard library headers
Named requirements
Feature test macros (C++20)
Language support library
Concepts library (C++20)
Metaprogramming library (C++11)
Diagnostics library
General utilities library
Strings library
Containers library
Iterators library
Ranges library (C++20)
Algorithms library
Numerics library
Localizations library
Input/output library
Filesystem library (C++17)
Regular expressions library (C++11)
Concurrency support library (C++11)
Technical specifications
Symbols index
External libraries

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)