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Implementing Inheritance 387

12

Mammal constructor...

Mammal constructor...

Dog constructor...

Mammal sound!

Woof!

Dog destructor...

Mammal destructor...

Mammal destructor...

Looking at the Mammalclass, you can see a method called Speak()defined on

line 15. The Dogclass declared on lines 23–37 inherits from Mammal(line 23),

and, therefore, has access to this Speak()method. The Dogclass, however, overrides this

method on line 33, causing Dogobjects to say Woof!when the Speak()method is called.

In the main()function, a Mammalobject,bigAnimal, is created on line 41, causing the

first line of output when the Mammalconstructor is called. On line 42, a Dogobject,Fido,

is created, causing the next two lines of output, where the Mammalconstructor and then

the Dogconstructor are called.

On line 43, the Mammalobject calls its Speak()method; then on line 44, the Dogobject

calls its Speak()method. The output reflects that the correct methods were called. The

bigAnimalmade a mammal sound and Fidowoofed. Finally, the two objects go out of

scope and the destructors are called.

OUTPUT

ANALYSIS

Overloading Versus Overriding

These terms are similar, and they do similar things. When you overload a method, you

create more than one method with the same name, but with a different signature. When

you override a method, you create a method in a derived class with the same name as a

method in the base class and the same signature.

Hiding the Base Class Method ......................................................................

In the previous listing, the Dogclass’s Speak()method hides the base class’s method.

This is what is wanted, but it can have unexpected results. If Mammalhas a method,

Move(), which is overloaded, and Dogoverrides that method, the Dogmethod hides all the

Mammalmethods with that name.

If Mammaloverloads Move()as three methods—one that takes no parameters, one that

takes an integer, and one that takes an integer and a direction—and Dogoverrides just the

Move()method that takes no parameters, it will not be easy to access the other two meth-

ods using a Dogobject. Listing 12.6 illustrates this problem.