CMPT 212 Object-Oriented Applications Design in C++ Spring 1998

Protected Members

• Problem: We want member functions of a derived class to be able to access members (data members or member functions) of a base class, but we don't want these members to be publicly available.
  • If we make the members private, then the derived class cannot access them.

  • If we make them public, then anyone can access them.

• Solution: Make them protected.

• protected members are like private members except that they can be accessed by derived classes.

• protected members are most often used for data members, so that the derived classes can access them directly without going through public access functions.

Public, Private, and Protected Inheritance

• So far we have only seen public inheritance.
  • Example:

       class baseClass {
          public:
             int a;
          private:
             int b;
          };
    
       class derivedClass {
          public:
             int c;
          private:
             int d;
          };
                   

    In an instance of derivedClass, the members a and c are publicly accessible, and b and d are not.

• Problem: Suppose we do not want the public members of the base class to be public in the derived class. Suppose we want them to be private or protected in the derived class.

• Solution: Use private or protected inheritance.

• Do not confuse public, private, and protected inheritance with public, private, and protected members.

• Access in base class	Base class inherited as	Access in derived class
  public protected private	public	public protected no access
  public protected private	protected	protected protected no access
  public protected private	private	private private no access

• Both private and protected inheritance affect how users of a derived class can access members inherited from a base class.

• private and protected inheritance differ in how they affect classes derived from the derived class.

• Example:

     class A {
        public:
           int x;
        protected:
           int y;
        private:
           int z;
        };
  
     class B1 : public A { ... };
  
     class B2 : protected A { ... };
    
     class B3 : private A { ... };
  
     class C1 : public B1 { ... };
    
     class C2 : public B2 { ... };
  
     class C3 : public B3 { ... };
           

  This table shows where the members x, y and z can be accessed:

  	x	y	z
  A member functions	yes	yes	yes
  B1 member functions	yes	yes	no
  B2 member functions	yes	yes	no
  B3 member functions	yes	yes	no
  C1 member functions	yes	yes	no
  C2 member functions	yes	yes	no
  C3 member functions	no	no	no
  A users	yes	no	no
  B1 users	yes	no	no
  B2 users	no	no	no
  B3 users	no	no	no
  C1 users	yes	no	no
  C2 users	no	no	no
  C3 users	no	no	no

Inheriting Friendship

• Friendship is not inherited.
  • Friends of a base class are not automatically friends of a derived class.

Graphical User Interface (GUI) Fundamentals

• In a DOS or other command-line-based application, the application controls when the user can enter input and what input can be entered.
  • When the application wants to do input or output, it call some subroutine in the operating system.

• In a GUI-based application, the application has less control over when the user can enter input and what input can be entered.
  • A user can normally open a menu, resize the window, or close the application at any time.

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