Polymorphism question

Hi,

I have the following piece of code:

#include <iostream>
#include <vector>
#include <functional>

using namespace std;

class Shape
{
public:
     virtual void draw(){cout << "Drawing a shhape\n"; };
};

class Triangle : public Shape
{
public:
     void draw()
     {
         static int counter = 0;
         cout << "Drawing a trngle number " << counter++ << " from " <<
this << endl;
     }
};

class Square : public Shape
{
public:
     void draw()
     {
         static int counter = 0;
         cout << "Drawing a square number " << counter++ << " from " <<
this << endl;
     }
};

int main()
{
     vector<Shape*> shapes;
     Triangle t1,t2;
     Square s1, s2;
     shapes.push_back(&t1);
     shapes.push_back(&s1);
     shapes.push_back(&t2);
     shapes.push_back(&s2);

     cout << endl;
     cout << endl;
     for_each( shapes.begin(), shapes.end(), mem_fun(&Shape::draw));
     cout << endl;
     for_each( shapes.begin(), shapes.end(), mem_fun(&Shape::draw));
     cout << endl;
     for_each( shapes.begin(), shapes.end(), mem_fun(&Shape::draw));
     cout << endl;
     for_each( shapes.begin(), shapes.end(), mem_fun(&Shape::draw));
     cout << endl;
     for_each( shapes.begin(), shapes.end(), mem_fun(&Shape::draw));
     cout << endl;
     cout << endl;
}

The original example code was found here:
http://www-h.eng.cam.ac.uk/help/tpl/languages/C++/mem_fun.html

I've done some minor adaptations to understand the principle.

This is the output of the program:

[root@axis-00408c012351 /domotics]2595# ./main

Drawing a trngle number 0 from 0x9ff0be7f
Drawing a square number 0 from 0x9ff0be77
Drawing a trngle number 1 from 0x9ff0be7b
Drawing a square number 1 from 0x9ff0be73

Drawing a trngle number 2 from 0x9ff0be7f
Drawing a square number 2 from 0x9ff0be77
Drawing a trngle number 3 from 0x9ff0be7b
Drawing a square number 3 from 0x9ff0be73

Drawing a trngle number 4 from 0x9ff0be7f
Drawing a square number 4 from 0x9ff0be77
Drawing a trngle number 5 from 0x9ff0be7b
Drawing a square number 5 from 0x9ff0be73

Drawing a trngle number 6 from 0x9ff0be7f
Drawing a square number 6 from 0x9ff0be77
Drawing a trngle number 7 from 0x9ff0be7b
Drawing a square number 7 from 0x9ff0be73

Drawing a trngle number 8 from 0x9ff0be7f
Drawing a square number 8 from 0x9ff0be77
Drawing a trngle number 9 from 0x9ff0be7b
Drawing a square number 9 from 0x9ff0be73

One can clearly see the polymorphism mechanism is working fine.
However, I was expecting the following output:

[root@axis-00408c012351 /domotics]2595# ./main

Drawing a trngle number 0 from 0x9ff0be7f
Drawing a square number 0 from 0x9ff0be77
Drawing a trngle number 0 from 0x9ff0be7b
Drawing a square number 0 from 0x9ff0be73

Drawing a trngle number 1 from 0x9ff0be7f
Drawing a square number 1 from 0x9ff0be77
Drawing a trngle number 1 from 0x9ff0be7b
Drawing a square number 1 from 0x9ff0be73

Drawing a trngle number 2 from 0x9ff0be7f
Drawing a square number 2 from 0x9ff0be77
Drawing a trngle number 2 from 0x9ff0be7b
Drawing a square number 2 from 0x9ff0be73

Drawing a trngle number 3 from 0x9ff0be7f
Drawing a square number 3 from 0x9ff0be77
Drawing a trngle number 3 from 0x9ff0be7b
Drawing a square number 3 from 0x9ff0be73

Drawing a trngle number 4 from 0x9ff0be7f
Drawing a square number 4 from 0x9ff0be77
Drawing a trngle number 4 from 0x9ff0be7b
Drawing a square number 4 from 0x9ff0be73

Why? Since there's each time two instances of the same class created
(two of class triangle and two of class square), I was expecting that
each "for_each" loop was going to call the "draw()" function *of every
object*, not *of every class*.
One can clearly see the different objects that are called (shown by
the "this" value of the object), but I can only conclude that for
objects of the same class, the "draw" function called is always the
same.
Otherwise, I can't explain why the following happens:

Drawing a trngle number 0 from 0x9ff0be7f
Drawing a square number 0 from 0x9ff0be77
Drawing a trngle number 1 from 0x9ff0be7b
Drawing a square number 1 from 0x9ff0be73

There's a static variable in the "draw()" function and that variable
should only increase when calling "draw()" from the same class
instance ("this" value). Now, eg. when triangle object 0x9ff0be7f is
processed, "draw()" is called and the static variable "counter" (which
should belong to "draw()" of object 0x9ff0be7f) is increased.
But when the second triangle object is called (0x9ff0be7b), that same
static variable "counter" which was used with object 0x9ff0be7f, is
again increased.
I was expecting a "counter" variable for function "draw()" of object
0x9ff0be7f and another "counter" variable for function "draw()" of
object 0x9ff0be7b.

Can someone explain this to me, pls?

Best rgds,

--Geert

--
      [ See http://www.gotw.ca/resources/clcm.htm for info about ]
      [ comp.lang.c++.moderated. First time posters: Do this! ]