Sams Teach Yourself C++ in 21 Days

Implementing Inheritance 391

12

Virtual Methods ..................................................................................................

This lesson has emphasized the fact that a Dogobject is a Mammalobject. So far that has

meant only that the Dogobject has inherited the attributes (data) and capabilities (meth-

ods) of its base class. In C++, the is-a relationship runs deeper than that, however.

C++ extends its polymorphism to allow pointers to base classes to be assigned to derived

class objects. Thus, you can write

Mammal* pMammal = new Dog;

This creates a new Dogobject on the heap and returns a pointer to that object, which it

assigns to a pointer to Mammal. This is fine because a dog is a mammal.

When calling overridden ancestor class functions using “::”, keep in mind

that if a new class is inserted in the inheritance hierarchy between the

descendant and its ancestor, the descendant will be now making a call that

skips past the intermediate class and, therefore, might miss invoking some

key capability implemented by the intermediate ancestor.

TIP

DOextend the functionality of existing,

tested classes by deriving.

DOchange the behavior of certain func-

tions in the derived class by overriding

the base class methods.

DON’Thide a base class function by

changing the function signature.

DON’Tforget that constis a part of the

signature.

DON’Tforget that the return type is not

part of the signature.

DO DON’T

This is the essence of polymorphism. For example, you could create many

types of windows, including dialog boxes, scrollable windows, and list boxes,

and give them each a virtual draw()method. By creating a pointer to a win-

dow and assigning dialog boxes and other derived types to that pointer, you

can call draw()without regard to the actual runtime type of the object

pointed to. The correct draw()function will be called.

NOTE

You can then use this pointer to invoke any method on Mammal. What you would like is

for those methods that are overridden in Dog()to call the correct function. Virtual