2. Abstract (easy to reuse)

Because you cannot master all the complex objects, choose to ignore their non-essential details and work with generalized and idealized object models.

class Point{
public:
	int x;
	int y;
};

class Line{
public:
	Point start;
    Point end;
	Line(const Point& start, const Point& end){
        this->start = start;
        this->end = end;
    }
};

class Rect{
public:
	Point leftUp;
    int width;
	int height;
	Rect(const Point& leftUp, int width, int height){
        this->leftUp = leftUp;
        this->width = width;
		this->height = height;
    }
};

//increase
class Circle{

};

 (2)MainForm1.cpp

class MainForm : public Form {       //Most used by public
private:
	Point p1;
	Point p2;
	vector<Line> lineVector;
	vector<Rect> rectVector;
	//change
	vector<Circle> circleVector;             //decompose

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){                          
		Line line(p1, p2);
		lineVector.push_back(line);
	}
	else if (rdoRect.Checked){
		int width = abs(p2.x - p1.x);
		int height = abs(p2.y - p1.y);
		Rect rect(p1, width, height);
		rectVector.push_back(rect);
	}
	//change
	else if (...){
		//...
		circleVector.push_back(circle);
	}

	//...
	this->Refresh();
	Form::OnMouseUp(e);
}

void MainForm::OnPaint(const PaintEventArgs& e){
	//For straight lines
	for (int i = 0; i < lineVector.size(); i++){
	    e.Graphics.DrawLine(Pens.Red,
			lineVector[i].start.x, 
			lineVector[i].start.y,
			lineVector[i].end.x,
			lineVector[i].end.y);
	}

	//For rectangles
	for (int i = 0; i < rectVector.size(); i++){
		e.Graphics.DrawRectangle(Pens.Red,
			rectVector[i].leftUp,
			rectVector[i].width,
			rectVector[i].height);
	}

	//change
	//For circles
	for (int i = 0; i < circleVector.size(); i++){
		e.Graphics.DrawCircle(Pens.Red,
			circleVector[i]);
	}

	//...
	Form::OnPaint(e);
}

class Shape{
public:
	virtual void Draw(const Graphics& g)=0;
	virtual ~Shape() { }     //General base classes have virtual destructors to prevent delete errors
};


class Point{
public:
	int x;
	int y;
};

class Line: public Shape{
public:
	Point start;
	Point end;
	Line(const Point& start, const Point& end){
		this->start = start;
		this->end = end;
	}

	//Implement your own Draw and be responsible for drawing yourself
	virtual void Draw(const Graphics& g){
		g.DrawLine(Pens.Red, 
			start.x, start.y,end.x, end.y);
	}

};

class Rect: public Shape{
public:
	Point leftUp;
	int width;
	int height;
	Rect(const Point& leftUp, int width, int height){
		this->leftUp = leftUp;
		this->width = width;
		this->height = height;
	}

	//Implement your own Draw and be responsible for drawing yourself
	virtual void Draw(const Graphics& g){
		g.DrawRectangle(Pens.Red,
			leftUp,width,height);
	}
};

//increase
class Circle : public Shape{
public:
	//Implement your own Draw and be responsible for drawing yourself
	virtual void Draw(const Graphics& g){
		g.DrawCircle(Pens.Red,
			         ...);
	}
};

(2) MainForm2.cpp

class MainForm : public Form {
private:
	Point p1;
	Point p2;
	//Abstract for all shapes
	vector<Shape*> shapeVector;    //Shape*implements its polymorphism

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));
	}
	//change
	else if (...){
		//...
		shapeVector.push_back(circle);
	}
	//...
	this->Refresh();
	Form::OnMouseUp(e);
}

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

	//For all shapes
	for (int i = 0; i < shapeVector.size(); i++){
		shapeVector[i]->Draw(e.Graphics); //Multiple calls, each responsible
	}
	//...
	Form::OnPaint(e);
}

Because of changes in demand, there is much less to change for an abstract solution than for a decomposition solution.

Eight object-oriented design principles: (more important than understanding the design pattern structure itself)

1. Dependency Inversion Principle (DIP)

2. Open and Closed Principle (OCP)