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Implementing Inheritance 393

12

On line 29, a pointer to Mammalis created (pDog), but it is assigned the address of a new

Dogobject. Because a dog is a mammal, this is a legal assignment. The pointer is then

used on line 30 to call the Move()function. Because the compiler knows pDogonly to be

a Mammal, it looks to the Mammalobject to find the Move()method. On line 10, you can

see that this is a standard, nonvirtual method, so the Mammal’s version is called.

On line 31, the pointer then calls the Speak()method. Because Speak()is virtual (see

line 11), the Speak()method overridden in Dogis invoked.

This is almost magical. As far as the calling function knew, it had a Mammalpointer, but

here a method on Dogwas called. In fact, if you had an array of pointers to Mammal, each

of which pointed to a different subclass of Mammal, you could call each in turn, and the

correct function would be called. Listing 12.9 illustrates this idea.

LISTING12.9 Multiple Virtual Functions Called in Turn

1: //Listing 12.9 Multiple virtual functions called in turn

2: #include <iostream>

3: using namespace std;

4:

5: class Mammal

6: {

7: public:

8: Mammal():itsAge(1) { }

9: virtual ~Mammal() { }

10: virtual void Speak() const { cout << “Mammal speak!\n”; }

11:

12: protected:

13: int itsAge;

14: };

15:

16: class Dog : public Mammal

17: {

18: public:

19: void Speak()const { cout << “Woof!\n”; }

20: };

21:

22: class Cat : public Mammal

23: {

24: public:

25: void Speak()const { cout << “Meow!\n”; }

26: };

27:

28:

29: class Horse : public Mammal

30: {

31: public:

32: void Speak()const { cout << “Winnie!\n”; }