Sams Teach Yourself C++ in 21 Days

Implementing Inheritance 375

12

29:

30: class Dog : public Mammal

31: {

32: public:

33:

34: // Constructors

35: Dog();

36: ~Dog();

37:

38: // Accessors

39: BREED GetBreed() const;

40: void SetBreed(BREED);

41:

42: // Other methods

43: WagTail();

44: BegForFood();

45:

46: protected:

47: BREED itsBreed;

48: };

This program has no output because it is only a set of class declarations without their

implementations. Nonetheless, there is much to see here.

On lines 7–28, the Mammalclass is declared. Note that in this example,Mammal

does not derive from any other class. In the real world, mammals do derive—that

is, mammals are kinds of animals. In a C++ program, you can represent only a fraction

of the information you have about any given object. Reality is far too complex to capture

all of it, so every C++ hierarchy is a carefully limited representation of the data available.

The trick of good design is to represent the areas that you care about in a way that maps

back to reality in a reasonably faithful manner without adding unnecessary complication.

The hierarchy has to begin somewhere; this program begins with Mammal. Because of this

decision, some member variables that might properly belong in a higher base class are

now represented here. Certainly all animals have an age and weight, for example, so if

Mammalis derived from Animal, you would expect to inherit those attributes. As it is, the

attributes appear in the Mammalclass.

In the future, if another animal sharing some of these features were added (for instance,

Insect), the relevant attributes could be hoisted to a newly created Animalclass that

would become the base class of Mammaland Insect. This is how class hierarchies evolve

over time.

LISTING12.1 continued

ANALYSIS