CS 202, Fall 2007
Homework #8, Problem 2

A class becomes abstract if it contains one or more pure virtual functions.
Abstract classes cannot be used to instantiate objects as can be done with 
ordinary classes, the purpose of abstract class is to provide an appropriate 
base class from which other classes can be derived. Any pure virtual functions
must be overridden in the derived class, otherwise the derived class is also
an abstract class.