Structural Design Patterns: Decorator Implementation

1
Structural Design Patterns Decorator
Implementation

• Ibrahim Jadalowen
• SENG 609.04, University of Calgary
• March 16, 2004

2
Introduction

• Decorators expand the functionality of an
  instance of a class without changing the class
  code
• More flexibility than static inheritance
• Adding items such as scroll bars to a window as
  needed provides more flexibility than requiring
  all windows to have scroll bars
• Decorators work behind the scenes, they are
  transparent to the interface

3
Decorator Structure
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A Working Example

• Normal cars can be made to look like sports cars
  by adding extra accessories to the body

• In this example we decorate the Car component
  with Wings, Side Skirts, Rear Aprons, and the
  Manufacturer Stamp.

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VehicleDecorator Structure
Vehicle DecorateVehicle()
Car DecorateVehicle()
CarDecorator DecorateVehicle()
_NextCarDecorator-gtDecorateVehicle()
CarWing DecorateVehicle()
CarApron DecorateVehicle()
CarDecoratorDecorateVehicle() AddApron()
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The Vehicle Component

• class Vehicle  public   virtual void
  Decorate_Vehicle()  protected   Vehicle()
  

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Concrete Component The Car

• The Car is the concrete component to which we
  will attach features
• class Car public Vehicle  public   Car()
  Vehicle()   void Decorate_Vehicle()

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The Decorator Class

• class CarDecorator public Vehicle
•                                                  
                  
• public
• CarDecorator(Vehicle a) Vehicle()
• _ACarDecorator a   void
  Decorate_Vehicle()
• _ACarDecorator-gtDecorate_Vehicle()
• private Vehicle _ACarDecorator

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Define the Car Wing

• class CarWing public CarDecorator
• public   CarWing(Vehicle a) CarDecorator(a)
    void Decorate_Vehicle()    cout ltlt "\t
  a car wing." ltlt endl        
•    CarDecoratorDecorate_Vehicle()
• //     and add extra stuff.

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Define the Rear Apron

• class CarApron public CarDecorator  public
    CarApron(Vehicle a) CarDecorator(a)  
  void Decorate_Vehicle()    cout ltlt "\t a car
  apron." ltlt endl    CarDecoratorDecorate_Vehicl
  e()

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Define the Side Skirt

• class CarSideSkirt public CarDecorator
   public  CarSideSkirt(Vehicle a)
  CarDecorator(a)  void Decorate_Vehicle()  
  cout ltlt "\t a car side skirt." ltlt endl  
  CarDecoratorDecorate_Vehicle()

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Manufacturer Stamp

• class CarStamp public CarDecorator  public
     CarStamp(Vehicle a, int cntry_num 1)
  CarDecorator(a)        _cntry_type
  cntry_num    void Decorate_Vehicle()
         Decorate()          CarDecoratorDecor
  ate_Vehicle()   protected     void
  Decorate() cout ltlt "\t made in" Type()    
  void Type()        if (_cntry_type gt 1)
  cout ltlt " Japan!..."ltlt endl        if
  (_cntry_type lt 1) cout ltlt " USA!..." ltlt endl
         if (_cntry_type 1) cout ltlt "
  Germany!..." ltlt endl  private
•   int _cntry_type

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Display Vehicle type

• void TellMeVehicleType(Vehicle a, char n)
   cout ltltn ltlt " has " ltlt endl  a-gtDecorate_Vehic
  le()  cout ltlt endl

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Client Main function

• void main(void) //Client has the
  responsibility to compose desired features
   Vehicle Honda   new CarWing(        new
  CarApron(         new CarSideSkirt(         
  new CarStamp(           new
  Car(),2))))  TellMeVehicleType(Honda, "Honda")
  

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Output

• Honda has
• a car wing
• a car apron
• a car side skirt
• made in Japan

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Conclusion

• The main function, or the client, doesnt have to
  be aware of the decorator presence. It is
  transparent.
• We add feature dynamically to the car objects as
  we wish.
• Decorator instances are not defined until we use
  them.

17
References

• Gamma 1995   Gamma, E., Helm, R., Johnson, R.
  Vlissides, J. (1995). Design Patterns Elements
  of Reusable Object-Oriented Software. Reading
  Mass., Addison Wesley. Rai 2000
•  Rai, Gurjit, Decorator Pattern, SENG609.04 Fall
  2000 Term Paper. University of Calgary.
  http//sern.ucalgary.ca/grai/seng/609.04/classli
  brarytemplate1.html