1. Billy Bennett June 22, 2009

3.  Intent Specify the kinds of objects to create using a prototypical instance, and create new objects by copying this prototype

4.  Applicability System must be independent of how products are created, composed, and represented (usually true of Creational patterns) AND Classes to instantiate are specified at run-time To avoid building a class hierarchy of factories for a class hierarchy of products Instances of classes have only a few different combinations of state

5.  Structure

6.  Participants Prototype – declares an interface for cloning itself ConcretePrototype – implements an operation for cloning itself Client – creates a new object by asking a prototype for a clone of itself

7.  Consequences – GOOD Can add/remove products at run time Can specify new objects by varying values Can specify new objects by varying structure Less subclassing – why is everyone so down on inheritance? Configuring an application with classes dynamically

8.  Consequences – BAD Every subclass of Prototype must implement the Clone operation  Can be difficult when classes to be constructed already exist  Can be difficult when internal objects within a Prototype can’t be copied or have circular references

9.  Implementation Tips Using a prototype manager Implementing the Clone operation Initializing Clones

10.  Related Patterns Often used with Composite or Decorator Abstract Factory  May compete with Prototype  May store a set of Prototypes

12.  To Start: We have an abstract base class “Event” Event has few operations  virtual long timestamp() = 0; (time of event)  virtual const char* rep() = 0; (packet of data) The problem: no universal interface to define events  A vending machine would need CoinType() and CoinReturn(), etc.

13.  The Questions: 1. How does the framework create instances of domain-specific subclasses? 2. How does the application code access subclass-specific operations, when all it gets is an Event object?

14.  The Questions: 1. How does the framework create instances of domain-specific subclasses?  How about Prototype?  Add an operation to the base Event class: virtual Event* copy()  Can be an instance of any Event subclass

15.  The Questions: 2. How does the application code access subclass-specific operations, when all it gets is an Event object?  We could just dynamic_cast over and over…Yuck  We could use Visitor…  add void accept(EventVisitor&) to base Event class  But then we have to define a bunch of domain-specific events within the Visitor class…Yuck (just like dynamic casting)

16.  Another solution – Memento Implement a cursor (a la Iterator) as a Memento itself “Type Laundering” – define a cursor base class that indicates which aspects of the cursor should be public (e.g. the destructor)