An abstract base CLASS is a class definition that is always a base class for other classes to be derived from. No specific objects of the base class are declared by the program. A C++ abstract base class is one that has a pure virtual FUNCTION, a protected constructor, or a protected destructor.
At the design LEVEL, you create a pure virtual method by using = 0 in place of a method body. This notation specifies that the MEMBER function is a pure virtual function. This means that the base class is an abstract base class, and the class designer intends the class to be used only as a base class. The base class may or may not provide a function body for the pure virtual function. In either CASE, a program that uses this class may not directly declare any objects of the abstract base class. If the program declares an object of a class directly or indirectly derived from the abstract base class, the pure virtual function must be overridden explicitly.

An abstract base class is a class definition that is always a base class for other classes to be derived from. No specific objects of the base class are declared by the program. A C++ abstract base class is one that has a pure virtual function, a protected constructor, or a protected destructor.
At the design level, you create a pure virtual method by using = 0 in place of a method body. This notation specifies that the member function is a pure virtual function. This means that the base class is an abstract base class, and the class designer intends the class to be used only as a base class. The base class may or may not provide a function body for the pure virtual function. In either case, a program that uses this class may not directly declare any objects of the abstract base class. If the program declares an object of a class directly or indirectly derived from the abstract base class, the pure virtual function must be overridden explicitly.