Billy Bennett June 22, 2009. Intent Specify the kinds of objects to create using a prototypical instance, and create new objects by copying this prototype.

Billy BennettJune 22, 2009

Intent• Specify the kinds of objects to create using

a prototypical instance, and create new objects by copying this prototype

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

Structure

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

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

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

Implementation Tips• Using a prototype manager• Implementing the Clone operation• Initializing Clones

Related Patterns• Often used with Composite or Decorator• Abstract Factory

May compete with Prototype May store a set of Prototypes

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.

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?

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

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)

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)