Prototype pattern

See also: design pattern, Adapter pattern

Table of contents

1 Intent 2 Problem 3 Discussion 4 Structure 5 Example 6 Rules of thumb

Intent

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

Problem

Application "hard wires" the class of object to create in each "new" expression.

Discussion

Declare an abstract base class that specifies a pure virtual "clone" method, and, maintains a dictionary of all "cloneable" concrete derived classes. Any class that needs a "polymorphic constructor" capability: derives itself from the abstract base class, registers its prototypical instance, and implements the clone() operation.

The client then, instead of writing code that invokes the "new" operator on a hard-wired class name, calls a "clone" operation on the abstract base class, supplying a string or enumerated data type that designates the particular concrete derived class desired.

Structure

This diagram is from javacoder.net.

Example

The Prototype pattern specifies the kind of objects to create using a prototypical instance. Prototypes of new products are often built prior to full production, but in this example, the prototype is passive and does not participate in copying itself. The mitotic division of a cell - resulting in two identical cells - is an example of a prototype that plays an active role in copying itself and thus, demonstrates the Prototype pattern. When a cell splits, two cells of identical genotvpe result. In other words, the cell clones itself. [Michael Duell, "Non-software examples of software design patterns", Object Magazine, Jul 97, p54]

Non-software example

Rules of thumb

Sometimes creational patterns are competitors: there are cases when either Prototype or Abstract Factory could be used properly. At other times they are complementory: Abstract Factory might store a set of Prototypes from which to clone and return product objects [GOF, p126]. Abstract Factory, Builder, and Prototype can use Singleton in their implementations. [GOF, p81, 134]. Abstract Factory classes are often implemented with Factory Methods, but they can be implemented using Prototype. [GOF, p95]

Factory Method: creation through inheritance. Protoype: creation through delegation.

Often, designs start out using Factory Method (less complicated, more customizable, subclasses proliferate) and evolve toward Abstract Factory, Protoype, or Builder (more flexible, more complex) as the designer discovers where more flexibility is needed. [GOF, p136]

Prototype doesn't require subclassing, but it does require an "initialize" operation. Factory Method requires subclassing, but doesn't require Initialize. [GOF, p116]

Designs that make heavy use of the Composite and Decorator patterns often can benefit from Prototype as well. [GOF , p126]