Design Patterns - Decorator Pattern

Intent
Attach additional responsibilities to an object dynamically.
Decorators provide flexible alternatives to subclassing for extending functionality.

#include <iostream> 
#include <string>

using namespace std;
 
class Window 
{
public:
  virtual void draw() = 0;
  virtual string getDescription() = 0;
  virtual ~Window() {}
};
 
class SimpleWindow : public Window 
{
public:
  void draw() {
    // draw window 
  }
  string getDescription() {
	  return "simple window\n";
  }
};

class WindowDecorator : public Window 
{
protected:
	Window *m_decoratedWindow; 
 
public:
	WindowDecorator (Window *decoratedWindow): 
	  m_decoratedWindow(decoratedWindow) {}
};

class VerticalScrollBarDecorator : public WindowDecorator 
{
public:
	VerticalScrollBarDecorator (Window *decoratedWindow): 
		WindowDecorator(decoratedWindow) {}
 
	void draw() {
        drawVerticalScrollBar();
        m_decoratedWindow->draw();
    }
 
    string getDescription() {
        return m_decoratedWindow->getDescription() + "with vertical scrollbars\n";
    }

private:
	void drawVerticalScrollBar() {
        // draw the vertical scrollbar
    }
};

class HorizontalScrollBarDecorator : public WindowDecorator 
{
public:
	HorizontalScrollBarDecorator (Window *decoratedWindow): 
		WindowDecorator(decoratedWindow) {}
 
	void draw() {
        drawHorizontalScrollBar();
        m_decoratedWindow->draw();
    }
 
    string getDescription() {
        return m_decoratedWindow->getDescription() + "with horizontal scrollbars\n";
	}
private:
	void drawHorizontalScrollBar() {
        // draw the horizontal scrollbar
    }
};

int main()
{
  Window *simple = new SimpleWindow();
  cout << simple -> getDescription() << endl;

  Window *horiz = new HorizontalScrollBarDecorator ( new SimpleWindow());
  cout << horiz -> getDescription() << endl;

  Window *vert = new VerticalScrollBarDecorator ( new SimpleWindow());
  cout << vert -> getDescription() << endl;

  Window *decoratedWindow = new HorizontalScrollBarDecorator (
                new VerticalScrollBarDecorator(new SimpleWindow()));
  cout << decoratedWindow -> getDescription() << endl;

  return 0;
}

Output:

simple window

simple window
with horizontal scrollbars

simple window
with vertical scrollbars

simple window
with vertical scrollbars
with horizontal scrollbars

Let's take a look at code with the class diagram.

1. Each component (Window) can be used on its own, or wrapped by decorator.
2. The ConcreteComponent (SimpleWindow) is the object we're going to dynamically add new behavior (vertical/horizontal scrollbars) to. It inherits Component.

   class SimpleWindow : public Window
   

3. Each decorator has-a (wraps) a component, which means the decorator has an instance variable that holds a pointer to a component.

   class WindowDecorator : public Window 
   {
   protected:
   	Window *m_decoratedWindow; 
   ...
   

4. Decorators implement the same interface or abstract class as the component they are going to decorate.
5. The ConcreteDecorator has an instance variable for the thing it decorate (the Component the Decorator wraps).

   class VerticalScrollBarDecorator : public WindowDecorator 
   {
   public:
   	VerticalScrollBarDecorator (Window *decoratedWindow): 
   		WindowDecorator(decoratedWindow) {}
    
   	void draw() {
           drawVerticalScrollBar();
           m_decoratedWindow->draw();
       }
    
       string getDescription() {
           return m_decoratedWindow->getDescription() + "with vertical scrollbars\n";
       }
   ...
   };