Prototype
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
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]
C++Demos | Java Demos | Lab