Namespaces
Variants
Views
Actions

std::is_base_of

From cppreference.com
< cpp‎ | types
 
C++
 
Metaprogramming library
Type traits
Type categories
(C++11)
(C++14)  
(C++11)
(C++11)
(C++11)
(C++11)
(C++11)
(C++11)
(C++11)
(C++11)
(C++11)
(C++11)
(C++11)  
(C++11)
(C++11)
(C++11)
(C++11)
(C++11)
(C++11)
(C++11)
(C++11)
Type properties
(C++11)
(C++11)
(C++11)
(C++11)
(C++14)
(C++11)
(C++17)
(C++23)
(C++11)
(C++11)
(C++11)
(C++11)(until C++20*)
(C++11)(deprecated in C++20)
(C++11)
(C++11)
(C++20)
(C++20)
(C++23)
(C++17)
Type trait constants
(C++11)(C++17)(C++11)(C++11)
Metafunctions
(C++17)
(C++17)
(C++17)
Supported operations
Relationships and property queries
is_base_of
(C++11)
(C++26)
(C++11)
(C++11)
(C++11)
(C++17)(C++17)(C++17)(C++17)
Type modifications
(C++11)(C++11)(C++11)
(C++11)(C++11)(C++11)
(C++11)
(C++11)
Type transformations
(C++11)(deprecated in C++23)
(C++11)(deprecated in C++23)
(C++11)
(C++20)
(C++11)
(C++17)

(C++11)
(C++11)
(C++20)
(C++11)
(C++11)(until C++20*)(C++17)
(C++20)
Compile-time rational arithmetic
Compile-time integer sequences
(C++14)
 
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.

Contents

[edit] Helper variable template

template< class Base, class Derived >
constexpr bool is_base_of_v = is_base_of<Base, Derived>::value;
 (since C++17)
[edit]

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>

[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<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
    > {};

[edit] 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() {}

[edit] 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

[edit] See also

(C++26)
 checks if a type is a virtual base of the other type
(class template) [edit]
(C++11)(C++20)
 checks if a type can be converted to the other type
(class template) [edit]
(C++20)
 specifies that a type is derived from another type
(concept) [edit]
Retrieved from "https://en.cppreference.com/mwiki/index.php?title=cpp/types/is_base_of&oldid=176479"