Sams Teach Yourself C++ in 21 Days

Powertypes enable you to create a variety of logicaltypes without using inheritance. You

can thus manage a large and complex set of types without the combinatorial explosion

you might encounter with inheritance.

Typically, you implementthe powertype in C++ with pointers. In this case, the Carclass

holds a pointer to an instance of PerformanceCharacteristicsclass (see Figure 11.20).

I’ll leave it as an exercise to the ambitious reader to convert the body and engine discrim-

inators into powertypes.

362 Day 11

FIGURE11.19

A discriminator as a

powertype.

Car

High Power Sedan Coupe Family Car

Low Power Sports Car

engine performance:PerformanceCharacteristics

body Performance Characteristics<<powertype>>

shift Pointmax RPM

accelerate

Family:PerformanceCharacteristics

Sport:PerformanceCharacteristics

Keep in mind that the practice of creating new types in this way at runtime

can reduce the benefits of C++ strong typing, in which the compiler can

enforce the correctness of interclass relationships. Therefore, use it carefully.

CAUTION

FIGURE11.20

The relationship

between a Carobject

and its powertype.

Car

High Power Sedan Coupe

Low Power

engine

body

Performance Characteristics

shift Point

max RPM

accelerate

Class Car : public Vehicle

{

public:

Car();