virtual functions and abstract classes

Question

I read in my book:

An abstract class is a class that is designed to be specifically used as a base class. An abstract class contains at least one pure virtual function. You declare a pure virtual function by using a pure specifier (= 0) in the declaration of a virtual member function in the class declaration.

Is it mandatory for an abstract class to have a virtual function? Why?

What is the difference between pure virtual function and virtual function and what is the need of them?

Answer 1

A pure virtual function specifies an interface that must be overridden in a derived class to be able to create objects of the derived class.

A (non-pure) virtual function specifies an interface that can be overridden in a derived class, but the base class provides a default implementation of the interface.

For most practical purposes, yes, an abstract base class must contain at least one virtual function. The whole point of an abstract base class is to specify an interface that's implemented by derived classes. That interface is specified in terms of a number of virtual functions that can be called. Without virtual functions, you haven't specified an interface, which makes it pretty hard for the abstract base class to accomplish much.

Answer 2

If you use an abstract class, that means you don't want to instantiate this class incorrectly. And you must use pure virtual function in that class.But declaring or writing function in class is your choice. You can write the function in derived class too.