class MainForm : public Form {

private:

Point p1;

Point p2;

//针对所有形状

vector<Shape*> shapeVector;

public:

MainForm(){

//...

}

protected:

virtual void OnMouseDown(const MouseEventArgs& e);

virtual void OnMouseUp(const MouseEventArgs& e);

virtual void OnPaint(const PaintEventArgs& e);

};

void MainForm::OnMouseDown(const MouseEventArgs& e){

p1.x = e.X;

p1.y = e.Y;

//...

Form::OnMouseDown(e);

}

void MainForm::OnMouseUp(const MouseEventArgs& e){

p2.x = e.X;

p2.y = e.Y;

if (rdoLine.Checked){

shapeVector.push_back(new Line(p1,p2));

}

else if (rdoRect.Checked){

int width = abs(p2.x - p1.x);

int height = abs(p2.y - p1.y);

shapeVector.push_back(new Rect(p1, width, height));

}

//改变

else if (...){

//...

shapeVector.push_back(circle);

}

//...

this->Refresh();

Form::OnMouseUp(e);

}

void MainForm::OnPaint(const PaintEventArgs& e){

//针对所有形状

for (int i = 0; i < shapeVector.size(); i++){

shapeVector[i]->Draw(e.Graphics); //多态调用，各负其责

}

//...

Form::OnPaint(e);

}