std:: is_abstract

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is_abstract
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Defined in header <type_traits>
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.

Template parameters

T - a type to check

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

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

Run this code 
#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

See also

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