Chapter 15, Problem 1RQE

A class that cannot be instantiated is a(n) _____ .

Program Description Answer

The classes which cannot be instantiated are referred as “abstract base class”.

Explanation of Solution

Inheritance:

Inheritance is the concept of inheriting the members and properties of a base class from the derived class. The main advantages of inheritance are,

• Code reusability and fast implementation.
• Reduces the program code.

Abstract base class:

Abstract base class is a class which contains at least one pure virtual function.

• Compiler will not permit the user to instantiate an abstract class.
• Normally, in function the base class contains the implementation and the derived class overrides this implementation with its own implementation.
• But, the class which inherits an abstract base class must provide definition to the pure virtual function.
  • The pure virtual functions implementation is not provided, and the function can be made pure virtual by adding (= 0) at the end of the function.
  • The pure virtual function is mostly implemented in the derived class and not in a base class.

Example for abstract base class with virtual function:

/*Abstract base class as it uses virtual function*/

class Base

{

/*Access specifier*/

public:

    /*virtual function*/

    virtual void disp() = 0

};

/*Virtual function definition*/

void Base::disp()     

{

    //statement;

}

Explanation:

In the above example,

• Define the abstract base class with the name of “Base”.
  • Declare the pure virtual function with the name of “disp()” to make the class as the abstract class.
• The implementation of pure virtual function “disp()” is not provided in base class. But, it is given outside of the class definition.
• Hence, the abstract base class “Base” cannot be instantiated.