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Advanced Inheritance 571

16

Adding Friend Classes ........................................................................................

Sometimes, you will create classes together, as a set. For example,PartNodeand

PartsListwere tightly coupled, and it would have been convenient if PartsListcould

have read PartNode’s Partpointer,itsPart, directly.

You wouldn’t want to make itsPartpublic, or even protected, because this is an imple-

mentation detail of PartNodeand you want to keep it private. You do want to expose it to

PartsList,however.

If you want to expose your private member data or functions to another class, you must

declare that class to be a friend. This extends the interface of your class to include the

friend class.

After a class declares another to be its friend, all of the declaring classes’ member data

and functions are public to the friend class. For example, if PartsNodedeclares

PartsListto be a friend, all PartsNode’s member data and functions are public as far as

PartsListis concerned.

It is important to note that friendship cannot be transferred. Although you are my friend

and Joe is your friend, that doesn’t mean Joe is my friend. Friendship is not inherited,

either. Again, although you are my friend and I’m willing to share my secrets with you,

that doesn’t mean I’m willing to share my secrets with your children.

Finally, friendship is not commutative. Assigning Class One to be a friend of Class Two

does not make Class Two a friend of Class One. You might be willing to tell me your

secrets, but that doesn’t mean I am willing to tell you mine.

DOinherit publicly when the derived

object is a kind of the base class.

DOuse aggregation when you want to

delegate functionality to another class,

but you don’t need access to its pro-

tected members.

DOuse private inheritance when you

need to implement one class in terms of

another, and you need access to the base

class’s protected members.

DON’Tuse private inheritance when you

need to use more than one instance of

the base class. You must use aggrega-

tion. For example, if PartsCatalog

needed two PartsLists, you could not

have used private inheritance.

DON’Tuse public inheritance when

members of the base class should not be

available to clients of the derived class.

DO DON’T