std::is_abstract

< cpp‎ | types

Defined in header `<type_traits>`
template< class T > struct is_abstract;		(since C++11)

std::is_abstract is an UnaryTypeTrait.

If T is an abstract class (that is, a non-union class that declares or inherits at least one pure virtual function), the base characteristic is std::true_type. For any other type, the base characteristic is std::false_type.

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

The behavior of a program that adds specializations for std::is_abstract or std::is_abstract_v (since C++17) is undefined.

Template parameters

a type to check

Helper variable template

template< class T > inline constexpr bool is_abstract_v = is_abstract<T>::value;		(since C++17)

Inherited from std::integral_constant

Member constants

value

[static]

true if T is an abstract class type, 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>

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);
 
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 was undefined if `T` is an incomplete union type	the base characteristic is std::false_type in this case

is_class (C++11)	checks if a type is a non-union class type (class template)
is_polymorphic (C++11)	checks if a type is a polymorphic class type (class template)