Drived classes (10.22)

 (1)The notion of a drived class and its associated languanges machnisms are provided to express hierarchical

relationships ,that is,to express commonality between classes.

The aim is to help you attain sufficient understanding of the techniques

to be able to make intelligent and balanced choices among them for real problems.

(2)An obeject of a drived class can be treated as an object of a base class when

manipulated through pointers and reference. The opposite is not true.

A class must be defined in order to use as a base class.