10: Design patterns

“… describes a problem which occurs over and over again in our environment, and then describes the core of the solution to that problem, in such a way that you can use this solution a million times over, without ever doing it the same way twice” – Christopher Alexander

This chapter introduces the important and yet non-traditional “patterns” approach to program design.

[[ Much of the prose in this chapter still needs work, but the examples all compile. Also, more patterns and examples are forthcoming ]]

Probably the most important step forward in object-oriented design is the “design patterns” movement, chronicled in Design Patterns, by Gamma, Helm, Johnson & Vlissides (Addison-

Wesley 1995).25 That book shows 23 different solutions to particular classes of problems. In this chapter, the basic concepts of design patterns will be introduced along with examples. This should whet your appetite to read Design Patterns (a source of what has now become an essential, almost mandatory, vocabulary for OOP programmers).

The latter part of this chapter contains an example of the design evolution process, starting with an initial solution and moving through the logic and process of evolving the solution to more appropriate designs. The program shown (a trash recycling simulation) has evolved over time, and you can look at that evolution as a prototype for the way your own design can start as an adequate solution to a particular problem and evolve into a flexible approach to a class of problems.

The pattern concept

Initially, you can think of a pattern as an especially clever and insightful way of solving a particular class of problems. That is, it looks like a lot of people have worked out all the angles of a problem and have come up with the most general, flexible solution for it. The problem could be one you have seen and solved before, but your solution probably didn’t have the kind of completeness you’ll see embodied in a pattern.

25 Conveniently, the examples are in C++.

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