Re: How can I remove dynamic_cast and if statements from this code snippet?

On Nov 16, 2:12 pm, Ian Collins <ian-n...@hotmail.com> wrote:


This is basically the question I am asking this group.

Below is a pseudocode answer to the question that
 - leaves Shape, Circle, and Square untouched (good)
 - places no drawing dependency on the shapes (good)
 - leaves drawing code entirely in the Drawer classes (good)
 - does not have switch or if statements (good)
 * does use a dynamic cast (BAD)

Can you (or anyone else) come up with some c++ pseudocode that
 - leaves Shape, Circle, and Square untouched (good)
 - places no drawing dependency on the shapes (good)
 - leaves drawing code entirely in the Drawer classes (good)
 - does not have switch or if statements (good)
 - does not use a dynamic cast (good)

Chris

=====

//----------
struct Drawer
{
  virtual void Draw(Shape* shape) = 0;
};

//----------
struct CircleDrawer : public Drawer
{
  void Draw(Shape* shape)
  {
    // dynamic_cast<Circle*>(shape)
    // draw the circle
  }
};

//----------
struct SquareDrawer : public Drawer
{
  void Draw(Shape* shape)
  {
    // dynamic_cast<Square*>(shape)
    // draw the square
  }
};

//----------
struct DrawerCreator
{
  virtual Drawer* GetNewDrawer() = 0;
  virtual bool CanDraw(Shape* shape) = 0;
};

//----------
struct CircleDrawerCreator : public DrawerCreator
{
  Drawer* GetNewDrawer() { return new CircleDrawer; }
  bool CanDraw(Shape* shape) { return dynamic_cast<Circle*>(shape); }
};

//----------
struct SqareDrawerCreator : public DrawerCreator
{
  Drawer* GetNewDrawer() { return new SquareDrawer; }
  bool CanDraw(Shape* shape) { return dynamic_cast<Square*>(shape); }
};

//----------
struct DrawerManager
{
  void RegisterDrawerCreator(DrawerCreator* dc)
  {
    drawers.push_back(dc);
  }

  void Draw(Shape* shape)
  {
    if (Drawer* drawer = GetDrawer(shape))
    {
      drawer->Draw(shape);
    }
  }

  Drawer* GetDrawer(Shape* shape)
  {
    for (int i = 0; i < drawers.size(); ++i)
    {
      if (drawers[i]->CanDraw(shape))
      {
        return drawers[i]->GetNewDrawer();
      }
    }

    return 0;
  }

  vector<Drawer*> drawers;
};

//----------
int main()
{
  DrawerManager drawer_manager;

  drawer_manager.RegisterDrawerCreator(new CircleDrawerCreator);
  drawer_manager.RegisterDrawerCreator(new SquareDrawerCreator);

  Shape* shape1 = new Circle;
  Shape* shape2 = new Square;

  drawer_manager.Draw(shape1);
  drawer_manager.Draw(shape2);
}