Re: virtual functions & container interaction

Hi,

I've come upon a situation where I don't understand what's happening.
Basically, I'm implementing a kind of 'chain of responsibility' using
some covariant types:

#include <iostream>
#include <string>
#include <vector>
using namespace std;

class Shape {
public:
  int sides;
};

class Square : public Shape {
public:
  int sides;
  Square() : sides(4) {};
};

class Triangle : public Shape {
public:
  int sides;
  Triangle() : sides(3) {};
};

class ShapeMaker {
public:
  virtual Shape* CreateObject(void) const = 0;
};

class SquareMaker : public ShapeMaker {
public:
  Square* CreateObject(void) const {
    cout << "CreateObject: for SQUARE" << endl;
    return (new Square);
  }
};

class TriangleMaker : public ShapeMaker {
public:
  Triangle* CreateObject(void) const {
    cout << "CreateObject: for TRIANGLE" << endl;
    return (new Triangle);
  }
};

class ShapeContainer {
private:
  vector<Square*> squares;
  vector<Triangle*> triangles;
  vector<Shape*> shapes;
public:
  void enter(Square* s) { squares.push_back(s); }
  void enter(Triangle* t) { triangles.push_back(t); }
  void enter(Shape* s) { shapes.push_back(s); }

  friend ostream& operator<<(ostream& os, ShapeContainer &c) {
    return os << "Container has \t" << c.squares.size() << " squares
\n"
              << " \t" << c.triangles.size() << "
triangles\n"
          << " \t" << c.shapes.size() << " generic shapes
\n";
  }

};

int main ()
{

  SquareMaker* sfactory = new SquareMaker;
  TriangleMaker* tfactory = new TriangleMaker;

  cout << "\nFirst Set of Tests:" << endl;
  ShapeContainer container1;
  container1.enter(sfactory->CreateObject());
  container1.enter(tfactory->CreateObject());
  cout << container1;

  cout << "\n\nSecond Set of Tests:" << endl;
  vector<ShapeMaker*> master_factory;
  master_factory.push_back(sfactory);
  master_factory.push_back(tfactory);
  ShapeContainer container2;

  vector<ShapeMaker*>::iterator it = master_factory.begin();
  while(it != master_factory.end()) {
    // this calls the correct derived type function
    // but returns an object of the base class. huh?
    container2.enter( (*it)->CreateObject() );
    ++it;
  }

  cout << container2;
}

This compiles under gcc 4.0.1 (apple, x86). When I run it I get this:

First Set of Tests:
CreateObject: for SQUARE
CreateObject: for TRIANGLE
Container has 1 squares
                1 triangles
                0 generic shapes

Second Set of Tests:
CreateObject: for SQUARE
CreateObject: for TRIANGLE
Container has 0 squares
                0 triangles
                2 generic shapes

My question is why does (*it)->CreateObject() call the correct derived
class type, but result in a base class return type? Please be gentle;
I've been writing Perl code for years now and I'm lazy and used to
dynamic bindings.