Inheritance.

Inheritance enables classes to use the properties of other classes and add to those properties.

The thing to remember about object programming is that it attempts to create a software model as close to the real world as possible. Inheritance forms an important role to meet this need. Take the following example.

A fabric can have a colour and size and it is also used to make clothes as well as tents. Now looking at this from the other angle tents are made from fabric and poles but clothes are made from fabric and buttons. So, Tents and clothes have something in common, fabric! If we described tents and clothes in C++, we can seperate fabric into a seperate class that the Tent and Clothes classes can inherit. 


       -----------         
      |  Fabric   |        
       -----------         
          A    A           
          |    |           
       ---      ---        
      |            |       
 ---------     ----------- 
|  Tent   |   |  Clothes  |
 ---------     -----------

Now for some buzz words:

  1. Fabric is a base class
  2. Tent and Clothes are derived classes
This is what the code looks like: 


// ... The base class 'Fabric'
// ... is no different to normal.


  class Fabric
  {
  public:

       Fabric() {};
      ~Fabric() {};

      SetSize(int x, int y)
      {
          Length = x;
          Width  = y;
      };

      SetColour(char *C)
      {
         strcpy(Colour, C); 
      }

  private:
      int  Length;
      int  Width;
      char Colour[20];
  };




// ... The derived class 'Tent'
// ... names 'Fabric' as a base class.


// ... The derived class 'Clothes' also
// ... names 'Fabric' as a base class.


  class Tent : public Fabric
  {
  public:

       Tent() {};
      ~Tent() {};

      SetNumOfPoles(int P)
      {
          Poles = P;
      };

  private:
      int  Poles;
  };
        


  class Clothes : public Fabric
  {
  public:

      Clothes() {};
      ~Clothes() {};

      SetNumOfButtons(int B)
      {
          Buttons = B;
      };

  private:
      int  Buttons;
  };

What we now have are three classes with the following public methods.

Using the classes

So we have three classes, they are used in the same way as before but with one extra feature.

Consider this piece of code. 

  void Init(Fabric &Material);

  main()
  {
      Tent       Frame;
      Clothes    Jacket;

      Init(Frame);
      Init(Jacket);
  }

  void Init(Fabric &Material)
  {
      Material.SetColour("Red");
      Material.SetSize  (10, 20);
  }

The function 'Init' expects an object of type 'Fabric' but we are passing objects of type 'Tents' and 'Clothes', this is OK as 'Tents' and 'Clothes' are derived from Fabric.

Examples:

o The final program.

See Also:

Top Master Index Keywords Functions

Martin Leslie 29-Dec-97