C++ PROTECTED CLASS DATA ACCESS SPECIFIER

REVISED: Sunday, March 3, 2013

CONTENTS:
I.     C++ PROTECTED CLASS DATA ACCESS SPECIFIER INTRODUCTION
II.    C++ ACCESS SPECIFIERS
III.   C++ INHERITANCE
IV.  C++ BINARY SCOPE RESOLUTION OPERATOR ::
V.   C++ PRIVATE VERSUS PROTECTED MEMBERS
VI.  C++ PROTECTED CLASS DATA ACCESS SPECIFIER SUMMARY

YOU WILL LEARN:
1. C++ class data member access specifiers.
2. C++ class data member access rights.
3. C++ guidelines for defining base class data member access rights.
4. C++ private versus protected class data members.
5. To define a C++ base class as a base for other C++ classes.

• I. C++ PROTECTED CLASS DATA ACCESS SPECIFIER INTRODUCTION

Welcome to the “C++ Protected Class Data Access Specifier Tutorial.”

• II. C++ ACCESS SPECIFIERS

A class data access specifier is one of the following three keywords: private, protected, or public.

Class member functions are functions declared within a class definition. Class data access specifiers determine the data member access rights of base-class and derived-class member functions.

A class is a user defined type, a logical abstraction, and a key concept in C++. By default, functions and data members declared within a class are private to that class. Protected access is an intermediate level of data protection between private and public.

• III. C++ INHERITANCE

Inheritance involves an object acquiring the characteristics of another object.

When designing a program, one goal of a programmer is to create a base-class that has the core characteristics required by all of the planned derived-class objects. A derived-class contains characteristics specific to the derived-class and all of the characteristics of the base-class.

A class that is inherited is referred to as a base-class. The class that does the inheriting is called the derived-class. A derived class can be used as a base-class for another derived-class, and so forth, achieving multiple inheritances.

The syntax to establish inheritance is as follows:

class derivedClassName : classDataAccessSpecifier baseClassName

When a class inherits another class the function members and data members of the base class become members of the derived class. The only members of a base class that are not inherited by the derived class are the constructors, destructor, and any member functions overloading the assignment operator.

Base-classes should be designed as a base for other classes. Think of it as defining one class based on another class. The new class you are defining is the derived-class.

A.  The following should be kept in mind when designing base-classes:

1. Private data members of a base-class are accessible only from within other members of the same base-class; from within other members of their friends; and never accessible from within member functions of a derived-class.

It is a generally accepted programming practice to make all data members of a class private and to use non-private get and set functions to manipulate and validate the data members.

2. Public data members are accessed from within their own member functions; accessed from within their derived-class’ member functions, and accessed from within their friends member functions.

3. Protected data members are accessible from within members of their base-class; from within members of the friends of their base-class; and from within members and friends of any classes derived from that base-class.

Once an access specifier has been used, it remains in effect until either another access specifier is encountered or the end of the class declaration is reached. It is a generally accepted programming practice to have only one private, public, and protected section within each class. However, you can change access specifiers as often as you like.

• IV. C++ BINARY SCOPE RESOLUTION OPERATOR ::

The binary scope resolution operator is :: two colons. The binary scope resolution operator :: defines the class and scope of a member function in the same manner as if it had originally been included within the class definition.

The syntax for the binary scope resolution operator is as follows:

className::memberName

Base-class members are accessed in derived-classes by qualifying their names with the base-class name and the :: binary scope resolution operator.

• V. C++ PRIVATE VERSUS PROTECTED MEMBERS

A protected access specifier is normally only considered when a program contains inheritance. Inheritance is a major piece of object oriented programming (OOP) because it helps the programmer design hierarchical classifications.

A.  Two problems are created when protected data members are used.

1. Base-class protected data members can be assigned invalid values by derived-class objects because the derived-class object does not have to use a set function to change the value of the base-class’s protected data.

2. Derived-class member functions that depend on base-class implementation are fragile, or brittle, because a small change in the base class can break derived class implementation requiring the modification of all derived classes.

• VI. C++ PROTECTED CLASS DATA ACCESS SPECIFIER SUMMARY

Avoid using protected data members in a base class.

When possible make all data members of a class private and use non-private set and get functions to manipulate and validate the data members. 

YOU HAVE LEARNED:
1. C++ class data member access specifiers.
2. C++ class data member access rights.
3. C++ guidelines for defining base class data member access rights.
4. The difference between C++ private versus protected class data members.
5. How to use C++ to define a base class as a base for other classes.


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