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Decorator Design Pattern in C++

Decorator design pattern

  1. Create a "lowest common denominator" that makes classes interchangeable
  2. Create a second level base class for optional functionality
  3. "Core" class and "Decorator" class declare an "isa" relationship
  4. Decorator class "has a" instance of the "lowest common denominator"
  5. Decorator class delegates to the "has a" object
  6. Create a Decorator derived class for each optional embellishment
  7. Decorator derived classes delegate to base class AND add extra stuff
  8. Client has the responsibility to compose desired configurations
 #include <iostream> using namespace std; // 1. "lowest common denominator" 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(); }

Output

 TextField: 80, 24 ScrollDecorator BorderDecorator BorderDecorator

Code examples

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