Sams Teach Yourself C++ in 21 Days

Figure 11.16 models composition. This model says that the body is not only an aggrega-

tion of a head, two arms, and two legs, but that these objects (head, arms, legs) are cre-

ated when the body is created and disappear when the body disappears. That is, they

have no independent existence; the body is composed of these things and their lifetimes

are intertwined.

360 Day 11

FIGURE11.16

Composition. Body

Head Arms Legs

1

2

2

Discriminators and Powertypes How might you design the classes required to

reflect the various model lines of a typical car manufacturer? Suppose that you’ve been

hired to design a system for Acme Motors, which currently manufactures five cars: the

Pluto (a slow, compact car with a small engine), the Venus (a four-door sedan with a

middle-sized engine), the Mars (a sport coupe with the company’s biggest engine, engi-

neered for maximum performance), the Jupiter (a minivan with the same engine as the

sports coupe but designed to shift at a lower RPM and to use its power to move its

greater weight), and the Earth (a station wagon with a small engine but high RPM).

You might start by creating subtypes of car that reflect the various models, and then cre-

ate instances of each model as it rolls off the assembly line, as shown in Figure 11.17.

FIGURE11.17

Modeling subtypes.

Car

Earth Jupiter Mars Venus Pluto