Virtual Functions, Pure Functions & Abstract Base Classes, Exercises of Advanced Computer Programming

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Virtual Functions, Pure Functions &

Abstract Base Classes

Casting

We saw the following:

Base *R = new Derived(); // okay

To use the member functions of Derived, one has to “remind” the compiler that we know that the object R is pointing to is a Derived object:

Derived Q = static_cast<Derived> (R);

Virtual and Nonvirtual

If a function is labeled virtual in the Base class, then the Derived version of a function is always used.

Otherwise, which version is used is determined by how the object is referenced. (We omit the details.)

Comment on Virtual Functions

Note that if a function is labeled virtual in the Base class, it is automatically virtual in the Derived class.

In Java all functions are implicitly virtual

Pure Functions

A virtual function might have no implementation at all in the Base class. This is indicated by writ- ing

virtual void scream( ) = 0;

in the class definition.

Abstract Base Classes

An abstract base class is one with at least one pure function. We cannot instantiate an object of an abstract base class.

(Java: An interface is a class without mem- ber variables all of whose member functions are pure.)