Abstract Data Types, implemented as C++  classes,  define  our  approach
towards  designing  and building large systems.  Clearly, a large system
might involve a combination of many classes and complex classes  may  be
constructed  out  of simpler ones. This chapter explores these so called
"using relationships" among classes and introduces some features of  C++
that are necessary to support them.

The chapter also gives some further insights into how to analyse complex
problems and approach the design of modular programs.

The chapter is structured around the example of a  simplified  cashpoint
machine  which  allows users to carry out transactions on bank accounts.
In turn, the bank accounts make use of the Money class  defined  in  the
previous chapter.