Sams Teach Yourself C++ in 21 Days

To keep the program reasonably simple and manageable, only six methods have been put

in the Mammalclass—four accessor methods,Speak(), and Sleep().

The Dogclass inherits from Mammal, as indicated on line 30. You know Doginherits from

Mammalbecause of the colon following the class name (Dog), which is then followed by

the base class name (Mammal).

Every Dogobject will have three member variables:itsAge,itsWeight, and itsBreed.

Note that the class declaration for Dogdoes not include the member variables itsAgeand

itsWeight. Dog objects inherit these variables from the Mammalclass, along with all

Mammal’s methods except the copy operator and the constructors and destructor.

Private Versus Protected ......................................................................................

You might have noticed that a new access keyword,protected, has been introduced on

lines 25 and 46 of Listing 12.1. Previously, class data had been declared private.

However, private members are not available outside of the existing class. This privacy

even applies to prevent access from derived classes. You could make itsAgeand

itsWeightpublic, but that is not desirable. You don’t want other classes accessing these

data members directly.

376 Day 12

There is an argument to be made that you ought to make all member data

private and never protected. Stroustrup (the creator of C++) makes this

argument in The Design and Evolution of C++, ISBN 0-201-543330-3, Addison

Wesley, 1994. Protected methods, however, are not generally regarded as

problematic, and can be very useful.

NOTE

What you want is a designation that says, “Make these visible to this class and to classes

that derive from this class.” That designation is protected. Protected data members and

functions are fully visible to derived classes, but are otherwise private.

In total, three access specifiers exist: public, protected, and private. If a function has an

object of your class, it can access all the public member data and functions. The member

functions, in turn, can access all private data members and functions of their own class

and all protected data members and functions of any class from which they derive.

Thus, the function Dog::WagTail()can access the private data itsBreedand can access

the protected data of itsAgeand itsWeightin the Mammalclass.

Even if other classes are layered between Mammaland Dog(for example,

DomesticAnimals), the Dogclass will still be able to access the protected members of