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)