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)