# std::is_abstract

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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

T | - | 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

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

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)

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