12.1. Introduction

From Simula, C++ borrowed the concept of a class as a user-defined type and the concept of class hierarchies. In addition, it borrowed the idea for system design that classes should be used to model concepts in the programmer’s and the application’s world. C++ provides language constructs that directly support these design notions. Conversely, using the language features in support of design concepts distinguishes effective use of C++. Using language constructs only as notational props for more traditional types of programming is to miss key strengths of C++.

A concept does not exist in isolation. It coexists with related concepts and derives much of its power from relationships with related concepts. For example, try to explain what a car is. Soon you’ll have introduced the notions of wheels, engines, drivers, pedestrians, trucks, ambulances, roads, oil, speeding tickets, motels, etc. Since we use classes to represent concepts, the issue becomes how to represent relationships between concepts. However, we can’t express arbitrary relationships directly in a programming language. Even if we could, we wouldn’t want to. Our classes should be more narrowly defined than our everyday concepts – and more precise. The notion of a derived class and its associated language mechanisms are provided to express hierarchical relationships, that is, to express commonality between classes. For example, the concepts of a circle and a triangle are related in that they are both shapes; that is, they have the concept of a shape in common. Thus, we must explicitly define class Circle and class Triangle to have class Shape in common. Representing a circle and a triangle in a program without involving the notion of a shape would be to lose something essential. This chapter is an exploration of the implications of this simple idea, which is the basis for what is commonly called object-oriented programming.