std::is_base_of - cppreference.com (original) (raw)

| | | | | ----------------------------------------------------------- | | ------------- | | 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.

Contents

• 1 Helper variable template
• 2 Inherited from std::integral_constant
  • 2.1 Member constants
  • 2.2 Member functions
  • 2.3 Member types
  • 2.4 Notes
  • 2.5 Possible Implementation
  • 2.6 Example
  • 2.7 Defect reports
  • 2.8 See also

[edit] 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

| | 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

| | converts the object to bool, returns value (public member function) | | ---------------------------------------------------------------------- | | | returns value (public member function) |

Member types

[edit] 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.

[edit] Possible Implementation

namespace details { template std::true_type test_ptr_conv(const volatile B*); template std::false_type test_ptr_conv(const volatile void*);   template<typename B, typename D> auto test_is_base_of(int) -> decltype(test_ptr_conv(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::value && std::is_class::value && decltype(details::test_is_base_of<Base, Derived>(0))::value > {};

[edit] Example

#include   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() {}

[edit] Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

[edit] See also