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c++ polymorphism function

Posted on 2006-11-30
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Last Modified: 2010-04-01
Hello, I found the following code from one of the experts here, but I don't understand a lot of it.  Could someone please help me understand what the program is doing?  I understand the basics, but need help with understanding these lines.  I am not sure what teh ~ means.  Please help me understand.  

class Shape
{
public:
     Shape() {};
     virtual ~Shape() {};
     virtual void DrawShape() = 0;
};

class Circle : public Shape
{
public:
     Circle() {};
     virtual ~Circle() {};
     virtual void DrawShape() { cout << "This is a circle" << endl; }
};




-----------------------------------------------
Here's the code someone wrote:
-----------------------------------------------
#include <iostream>

using namespace std;

class Shape
{
public:
     Shape() {};
     virtual ~Shape() {};
     virtual void DrawShape() = 0;
};

class Circle : public Shape
{
public:
     Circle() {};
     virtual ~Circle() {};
     virtual void DrawShape() { cout << "This is a circle" << endl; }
};

class Triangle : public Shape
{
public:
     Triangle() {};
     virtual ~Triangle() {};
     virtual void DrawShape() { cout << "This is a triangle" << endl; }
};

class Rectangle : public Shape
{
public:
     Rectangle() {};
     virtual ~Rectangle() {};
     virtual void DrawShape() { cout << "This is a rectangle" << endl; }
};

int main()
{
     Shape* pShape = 0;
     int ask = 0;
     cout <<"=============================================="<<endl;
     cout << "Do you want to display (type 1,2,or 3):" <<endl;
     cout << "1. Circle"                                             <<endl;
     cout << "2. Triangle"                                   <<endl;
     cout << "3. Rectangle"                                             <<endl;
     cout <<"----------------------------------------------"<<endl;
     cin >> ask;
       cin.get();
     
     switch (ask)
     {    
     case 1:
          pShape = new Circle;
          break;
              cin.get();
     case 2:
          pShape = new Triangle;
          break;
              cin.get();
     case 3:
          pShape = new Rectangle;
          break;
              cin.get();
     default:
          cout << "Invalid choice.  Good-bye!" << endl;
              cin.get();
          return -1;
             
     }
     pShape->DrawShape();
     delete pShape;
       cin.get();

     return 0;
}
0
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Question by:meaar
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3 Comments
 
LVL 45

Accepted Solution

by:
sunnycoder earned 500 total points
ID: 18051859
Hi meaar,

~<functionName>
Where functionName is same as class name signifies destructor ...

Class Shape has 3 members ... a constructor which is empty and does nothing ...
A virtual destructor .... why we need a virtual destructor
http://blogs.msdn.com/oldnewthing/archive/2004/05/07/127826.aspx
http://www.codersource.net/cpp_virtual_destructors.html

And third member is a pure virtual function DrawShape() .. Note the =0. Pure virtual function is one whose implementation is mandatory for every class that derives from Shape class ...

Class Circle derives from Shape ... again an empty constructor and a virtual destructor ...
Since it derives from Shape, it provides an implementation for DrawShape - the pure virtual function in parent class ..

Now if you have
Shape * ptr;
Circle obj1;

ptr = &obj1;
ptr->DrawShape(); //this call will print "this is a circle"

Cheers!
sunnycoder
0
 

Author Comment

by:meaar
ID: 18051874
sunnycoder, your answer was nice you had to send it twice. :)
0
 
LVL 45

Expert Comment

by:sunnycoder
ID: 18051974
:) ...

It reported failure on first attempt which as I discovered later was successful.

Deleted the duplicate post.

sunnycoder
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