Design Pattern ->Prototype

Layering & Contract Philosophy With additional indirection

Prototype

The example code is as following:

  class CObject
  {
     public:
               virtual CObject* clone() const = ;
               virtual void BasicOperation() = ;
  }
  //clone is a virtual method, its return type can be different from base class
  class CConcreteObject: public CObject
  {
     public:
       virtual CConcreteObject* clone() const { return new CConcreteObject (*this);}
 }

 class Client
 {
     public: CObject* pObject = NULL;
     public:
     void setObject(CObject *p)
     {
        if (pObject != NULL )
           delete pObject;
        pObject = NULL;
        pObject = p->clone();
     }
     void operation() { pObject->BasicOperation();}
 }

Applicability

• Use the Prototype pattern when:a system should be independent of how its products are created, composed, and represented;
• Use the Prototype pattern when: the classes to instantiate are specified at run-time, for example, by dynamic loading; or to avoid building a class hierarchy of factories that parallels the class hierarchy of products; or when instances of a class can have one of only a few different combinations of state. It may be more convenient to install a corresponding number of prototypes and clone them rather than instantiating the class manually,each time with the appropriate state.

Participants

• Prototype (Graphic): declares an interface for cloning itself which must be implemented by derived class.
• ConcretePrototype (Staff, WholeNote, HalfNote): implements an operation for cloning itself.
• Client (GraphicTool): creates a new object by asking a prototype to clone itself.

Collaborations

• A client asks a prototype to clone itself and manipulates them as own.

From:Design Patterns:Elements of Reusable Object-Oriented Software, by GoF