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Defining Base Class Acess (Inheritance)

In the previous article Introduction to Inheritance in C++ we saw how one class can inherit properties and functions from another, its general form is:


  class derived-class:access-specifier base-class
  {
    ...
    ...
    ...
  };

We discussed that the access-specifier here, can be anyone of the three private, public and protected. In this article we’ll discuss the meaning of each of these in detail.

We know that when a class is derived from another class (base class) then the members of the base class becomes the members of the derived class. The access-specifier used while deriving the class specifies the access status of the base class members inside the derived class.

If we don’t use any base class access-specifier then it’s taken to be private by default.

Please note that in no case are the private members of the base class be accessible to the members of its derived class.

So by using different access-specifier we only change the way public and protected members of a class are inherited inside derived class, since private members of the base class always remain inaccessible to the members of its derived class.

private:

If we use private access-specifier while deriving a class then both the public and protected members of the base class are inherited as private members of the derived class and hence are only accessible to its members.

Following example will illustrate this:


  // Introduction to Inheritance
  // ----------------------------
  // -- THIS PROGRAM WON'T RUN --
  // ----------------------------
  // THIS PROGRAM IS DESIGNED TO
  // HAVE ERRORS
  #include<iostream.h>

  // base class
  class base
  {
    int a;

  public:
    int b;
    void func()
    {
      cout<<b;
    }
  };

  // 'derived' class is 
  // inheriting 'base' 
  // privately
  class derived:private base
  {
    int c;

  public:
    int d;
    void func2()
    {
      // accessing base's members
      cout<<a;// ERROR
      // cannot access private
      // members of the base class

      cout<<b;// CORRECT
      func();// CORRECT

      // accessing its own
      // members
      cout<<d;
    }
  };

  void main(void)
  {
    base b;
    derived d;

    b.func();// CORRECT

    d.func();// ERROR
    // cannot access 'func()'
    // which is inherited
    // as private member of 
    // the derived class
  }

Here all the public members of the base class are inherited in the derived class as private members. Thus we can’t access the public members of the base class (b and func()) from the objects of the derived class although they are accessible to the members of the derived class.

public:

By deriving a class as public, the public members of the base class remains public members of the derived class and protected members remain protected members.

This is illustrated below:


  // ----------------------------
  // -- THIS PROGRAM WON'T RUN --
  // ----------------------------
  // THIS PROGRAM IS DESIGNED TO
  // HAVE ERRORS
  #include<iostream.h>

  // base class
  class base
  {
    int a;

  public:
    int b;
    void func()
    {
      cout<<b;
    }
  };

  // 'derived' class is 
  // inheriting 'base' 
  // publicly
  class derived:public base
  {
    int c;

  public:
    int d;
    void func2()
    {
      // accessing base's memebrs
      cout<<a;// ERROR
      // cannot access private
      // members of the base class

      cout<<b;// CORRECT
      func();// CORRECT

      // accessing its own
      // members
      cout<<d;
    }
  };

  void main(void)
  {
    base b;
    derived d;

    b.func();// CORRECT

    d.func();// CORRECT
  }

protected:

It makes the derived class to inherit the protected and public members of the base class as protected members of the derived class.

Protected members of the class and its use will be discussed in the coming articles.

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