CS 202, Fall 2007
Homework #8, Problem 1

The difference between a virtual function and a pure virtual function is that a virtual 
function has an implementation and gives the derived class the option of overriding the 
function. A pure virtual function does not provide an implementation and 
requires the derived class to override the function (for that derived class to be concrete; 
otherwise the derived class remains abstract) 

Declarations:

virtual void draw() const; // virtual function
virtual void draw() const = 0; // pure virtual function