Typically a program has to maintain several data structures each of which is manipulated in many different ways. It is best for the pieces of code that manipulate a particular data structure to be located in a small number of functions. And it would be desirable if the program could be written so that each data structure can only be accessed from its associated functions, i.e., it is not directly accessible to the rest of the program. In this way, we would then prevent a data structure from accidentally being misused. What we want is a way of building a wall around a data structure and the functions that manipulate it, and only allowing some of these functions to be accessible from outside the wall. Modern programming languages have a construct to do this: for example, Ada has packages, Fortran90 and Modula-2 both have modules, and C++ and Java both have classes.

In this section, we look at how to write our own class declarations.