std::is_base_of

 

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Type categories is_void (C++11) is_null_pointer (C++14) is_array (C++11) is_pointer (C++11) is_enum (C++11) is_union (C++11) is_class (C++11) is_function (C++11) is_object (C++11) is_scalar (C++11) is_compound (C++11) is_integral (C++11) is_floating_point (C++11) is_fundamental (C++11) is_arithmetic (C++11) is_reference (C++11) is_lvalue_reference (C++11) is_rvalue_reference (C++11) is_member_pointer (C++11) is_member_object_pointer (C++11) is_member_function_pointer (C++11) Type properties is_const (C++11) is_volatile (C++11) is_empty (C++11) is_polymorphic (C++11) is_final (C++14) is_abstract (C++11) is_aggregate (C++17) is_trivial (C++11) is_trivially_copyable (C++11) is_standard_layout (C++11) is_literal_type (C++11)(until C++20) is_pod (C++11)(deprecated in C++20) is_signed (C++11) is_unsigned (C++11) is_bounded_array (C++20) is_unbounded_array (C++20) is_scoped_enum (C++23) has_unique_object_representations (C++17) Type trait constants integral_constantbool_constanttrue_typefalse_type (C++11)(C++17)(C++11)(C++11) Metafunctions conjunction (C++17) disjunction (C++17) negation (C++17)	Supported operations is_constructibleis_trivially_constructibleis_nothrow_constructible (C++11)(C++11)(C++11) is_default_constructibleis_trivially_default_constructibleis_nothrow_default_constructible (C++11)(C++11)(C++11) is_copy_constructibleis_trivially_copy_constructibleis_nothrow_copy_constructible (C++11)(C++11)(C++11) is_move_constructibleis_trivially_move_constructibleis_nothrow_move_constructible (C++11)(C++11)(C++11) is_assignableis_trivially_assignableis_nothrow_assignable (C++11)(C++11)(C++11) is_copy_assignableis_trivially_copy_assignableis_nothrow_copy_assignable (C++11)(C++11)(C++11) is_move_assignableis_trivially_move_assignableis_nothrow_move_assignable (C++11)(C++11)(C++11) is_destructibleis_trivially_destructibleis_nothrow_destructible (C++11)(C++11)(C++11) has_virtual_destructor (C++11) is_swappable_withis_swappableis_nothrow_swappable_withis_nothrow_swappable (C++17)(C++17)(C++17)(C++17) Relationships and property queries is_same (C++11) is_base_of (C++11) is_convertibleis_nothrow_convertible (C++11)(C++20) is_layout_compatible (C++20) is_pointer_interconvertible_base_of (C++20) is_pointer_interconvertible_with_class (C++20) is_corresponding_member (C++20) reference_constructs_from_temporary (C++23) alignment_of (C++11) rank (C++11) extent (C++11) is_invocableis_invocable_ris_nothrow_invocableis_nothrow_invocable_r (C++17)(C++17)(C++17)(C++17) reference_converts_from_temporary (C++23) Type modifications remove_cvremove_constremove_volatile (C++11)(C++11)(C++11) add_cvadd_constadd_volatile (C++11)(C++11)(C++11) make_signed (C++11) make_unsigned (C++11) remove_reference (C++11) add_lvalue_referenceadd_rvalue_reference (C++11)(C++11) remove_pointer (C++11) add_pointer (C++11) remove_extent (C++11) remove_all_extents (C++11) Type transformations aligned_storage (C++11)(deprecated in C++23) aligned_union (C++11)(deprecated in C++23) decay (C++11) remove_cvref (C++20) enable_if (C++11) void_t (C++17) conditional (C++11) common_type (C++11) common_reference (C++20) underlying_type (C++11) result_ofinvoke_result (C++11)(until C++20)(C++17) type_identity (C++20)
Compile-time rational arithmetic
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integer_sequence (C++14)

 

Defined in header <type_traits>
template< class Base, class Derived > struct is_base_of;		(since C++11)

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, 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_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_convertibleis_nothrow_convertible (C++11)(C++20)	checks if a type can be converted to the other type (class template)
derived_from (C++20)	specifies that a type is derived from another type (concept)