CMPT 212 Object-Oriented Applications Design in C++ Fall 1997

Inheritance in C++

• We can create a derived C++ class from a base C++ class.

• We specify only the differences between the derived class and the base class.
  • There are three kinds of differences:
    1. The derived class may have additional properties (data members).
    2. The derived class may have a different implementation of a member function.
    3. The derived class may have addition member functions.

  • Note that we are not allowed to change the type of data members.

• Besides public and private, data members and member functions can also be protected. protected members are like private members except that they can also be accessed by member functions and friend functions of any class derived from the class.
  • Example:

       class myClass {
          public:
    	 int a;
          protected:
    	 int b;
          private:
    	 inc c;
          };
    	       

    a can be accessed publicly.
    c can only be accessed by member functions and friend functions of myClass.
    b can only be accessed by member functions and friend functions of myClass and by member functions and friend functions of any class derived from myClass.

• Example: Suppose we have this hierarchy of objects: point, circle, and disk.
  • A point has properties x_coord, y_coord, and colour, and functions move, setColour, and display.
    We can use this diagram to show a point object:

  • A circle is a point with an property radius, an additional function setRadius, and an overridden function display.
    An overridden function is a function in a derived class which replaces a function in a base class.

  • A disk is a circle which is coloured in. It contains an overridden function display.

  • We can draw a diagram to show how the classes are related.

  • Here is how we can implement this class hierarchy in C++.

       //  file graphobj.h
    
       #ifndef _graphobj_h_
       #define _graphobj_h_
    
       class Point {
          public:
    	 void move(float new_x, float new_y);
    	 void setColour(int new_colour);
    	 void display();
          protected:
    	 float x_coord;
    	 float y_coord;
    	 int   colour;
          }:
    
       class Circle : public Point {
          public:
    	 void display();
    	 void setRadius(float new_radius);
          protected:
    	 float radius;
          };
    
       class Disk : public Circle {
          public:
    	 void display();
          };
    
       #endif
    
    
       //  file graphobj.cpp
    
       #include "graphobj.h"
    
       void Point::move(float new_x, float new_y) {
          x_coord = new_x;
          y_coord = new_y;
       }
    
       void Point::setColour(int new_colour) {
          colour = new_colour;
       }
    
       void Point::display() {
          showpoint(x_coord, y_coord, colour);    //  Assume graphics function exists.
       }
    
       void Circle::display() {
          showcircle(x_coord, y_coord, colour, radius);    //  Assume graphics function exists.
       }
    
       void Circle::setRadius(float new_radius) {
          radius = new_radius;
       }
    
       void Disk::display() {
          Circle::display();
          fillarea(x_coord, y_coord, colour);    //  Assume graphics function exists.
       }
    	       

  • Here is an example which uses these C++ classes.

       #include "graphobj.h"
    
       int main() {
          Point  p;
          Circle c;
          Disk   d;
    
          p.move(1.0, 2.0);
          p.setColour(15).
          p.display();
    
          c.move(5.0, 3.0);
          c.setColour(12);
          c.setRadius(4.0);
          c.display();
    
          d.move(7.0, 4.0);
          d.setRadius(15.0);
          d.setColour(3);
          d.display();
    
          return 0;
       }
    	       

  • Note that we didn't need to write Circle::move or Disk::move or Disk::setRadius. These functions were inherited. Inheritance can save us a lot of time in writing and debugging code.

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