decorator.cpp

#include <iostream>
using namespace std;

// 1. "lowest common denom"
class Widget
{
  public:
    virtual void draw() = 0;
};

class TextField: public Widget
{
    // 3. "Core" class & "is a"
    int width, height;
  public:
    TextField(int w, int h)
    {
        width = w;
        height = h;
    }
 
    /*virtual*/
    void draw()
    {
        cout << "TextField: " << width << ", " << height << '\n';
    }
};

// 2. 2nd level base class
class Decorator: public Widget  // 4. "is a" relationship
{
    Widget *wid; // 4. "has a" relationship
  public:
    Decorator(Widget *w)
    {
        wid = w;
    }
 
    /*virtual*/
    void draw() 
    {
        wid->draw(); // 5. Delegation
    }
};

class BorderDecorator: public Decorator
{
  public:
    // 6. Optional embellishment
    BorderDecorator(Widget *w): Decorator(w){}
 
    /*virtual*/
    void draw()
    {
        // 7. Delegate to base class and add extra stuff
        Decorator::draw();
        cout << "   BorderDecorator" << '\n';
    }
};

class ScrollDecorator: public Decorator
{
  public:
    // 6. Optional embellishment
    ScrollDecorator(Widget *w): Decorator(w){}
 
    /*virtual*/
    void draw()
    {
        // 7. Delegate to base class and add extra stuff
        Decorator::draw();
        cout << "   ScrollDecorator" << '\n';
    }
};

int main()
{
  // 8. Client has the responsibility to compose desired configurations
  Widget *aWidget = new BorderDecorator(new BorderDecorator(new ScrollDecorator
    (new TextField(80, 24))));
  aWidget->draw();
}