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LISTING 18.1 VIRTUAL METHODS |
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1: // Listing 18.1 - virtual methods
2:
3: #include <iostream.h>
4: class Mammal
5: {
6: public:
7: Mammal():itsAge(1) { cout << Mammal constructor\n; }
8: virtual ~Mammal() { cout << Mammal destructor\n; }
9: virtual void Speak() const { cout << Mammal speak!\n; }
10: protected:
11: int itsAge;
12: };
13:
14: class Cat: public Mammal
15: {
16: public:
17: Cat() { cout << Cat constructor\n; }
18: ~Cat() { cout << Cat destructor\n; }
19: void Speak()const { cout << Meow!\n; }
20: };
21:
22: int main()
23: {
24: Mammal *pCat = new Cat;
25: pCat->Speak();
26: return 0;
27: } |
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Mammal constructor
Cat Constructor
Meow! |
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Analysis: On line 9, Speak() is declared to be a virtual method; it is overridden on lines 19 and invoked on line 25. Note, again, that pCat is a declared to be a pointer to Mammal, but the address of a Cat is assigned to it. This is, as discussed in Hour 17, Polymorphism and Derived Classes, the essence of polymorphism. |
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What happens, however, if you want to add a method to Cat that is inappropriate for Mammal? For example, suppose you want to add a method called Purr(). Cats purr, but no other mammals do. You would declare your class like this: |
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class Cat: public Mammal
{
public:
Cat() { cout << Cat constructor\n; }
~Cat() { cout << Cat destructor\n; }
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