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Showing content from https://en.cppreference.com/w/cpp/language/../error/error_code/../../types/is_abstract.html below:

std::is_abstract - cppreference.com

template< class T > struct is_abstract;

(since C++11)

std::is_abstract is a UnaryTypeTrait.

If T is an abstract class (that is, a non-union class that declares or inherits at least one pure virtual function), provides the member constant value equal to true. For any other type, value is false.

If T is an incomplete non-union class type, the behavior is undefined.

If the program adds specializations for std::is_abstract or std::is_abstract_v, the behavior is undefined.

[edit] Template parameters [edit] Helper variable template

template< class T > constexpr bool is_abstract_v = is_abstract<T>::value;

(since C++17) Inherited from std::integral_constant Member constants true if T is an abstract class type, 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] Example

#include <iostream>
#include <type_traits>
 
struct A { int m; };
static_assert(std::is_abstract_v<A> == false);
 
struct B { virtual void foo(); };
static_assert(std::is_abstract_v<B> == false);
 
struct C { virtual void foo() = 0; };
static_assert(std::is_abstract_v<C> == true);
 
struct D : C {};
static_assert(std::is_abstract_v<D> == true);
 
struct E : C { void foo() override {} };
static_assert(std::is_abstract_v<E> == false);
 
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 was undefined if
T is an incomplete union type the base characteristic is
std::false_type in this case [edit] See also checks if a type is a non-union class type
(class template) [edit] checks if a type is a polymorphic class type
(class template) [edit]

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