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Showing content from https://cplusplus.com/reference/type_traits/is_abstract/ below:

class template

<type_traits>

std::is_abstract

template <class T> struct is_abstract;

Is abstract class

Trait class that identifies whether T is an abstract class.

An abstract class is a class that has at least one pure virtual function.

is_abstract inherits from integral_constant as being either true_type or false_type, depending on whether T is a polymorphic class type.

Template parameters

T

A complete type, or void (possible cv-qualified), or an array of unknown bound of a complete element type.

A type.
If a class type, it shall be a complete type or a union (for unions, this trait is always false).

Member types Inherited from integral_constant:

Member constants Inherited from integral_constant:
member constant definition value either true or false
Member functions Inherited from integral_constant:

operator bool: Returns value (public member function)

Example

// is_abstract example
#include <iostream>
#include <type_traits>

struct A { };
struct B {
  virtual void fn() = 0;  // pure virtual function
};
struct C : B { };
struct D : C { virtual void fn(){} };

int main() {
  std::cout << std::boolalpha;
  std::cout << "is_abstract:" << std::endl;
  std::cout << "A: " << std::is_abstract<A>::value << std::endl;
  std::cout << "B: " << std::is_abstract<B>::value << std::endl;
  std::cout << "C: " << std::is_abstract<C>::value << std::endl;
  std::cout << "D: " << std::is_abstract<D>::value << std::endl;
  return 0;
}

Output:

is_abstract:
A: false
B: true
C: true
D: false