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), 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 >
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
#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 ifDerived is an incomplete union type
|
the base characteristic is std::false_type in this case |