7/31/2019 Daily Patterns #05

1/3

Factory methodDESIGN pattern

Intent

Define an interface for creating an object,but let subclasses decide which class toinstantiate. Factory Method lets a class

defer instantiation to subclasses.

Defining a virtual constructor.

The new operator considered harmful.

PROBLEM

A framework needs to standardize thearchitectural model for a range of

applications, but allow for individualapplications to define their own domain

objects and provide for their instantiation.

Factory Method is to creating objects asTemplate Method is to implementing an

algorithm. A superclass specifies allstandard and generic behavior (using pure

virtual placeholders for creation steps),and then delegates the creation details tosubclasses that are supplied by the client.

Factory Method makes a designmore customizable and only alittle more complicated. Other

design patterns require newclasses, whereas Factory

Method only requires a newoperation.

People often useFactory Method as the

standard way to createobjects; but it isnt

necessary if: the classthats instantiatednever changes, orinstantiation takes

place in an operationthat subclasses can

easily override (such asan initialization

operation).Factory Methods

are routinelyspecified by an

architecturalframework, and

then implementedby the user of the

framework.

Factory Method is similar toAbstract Factory but without

the emphasis on families.

seven nations -"the factory song"

http://www.youtube.com/watch?v=kH21Nr9gGEU

http://www.youtube.com/watch?v=kH21Nr9gGEUhttp://www.youtube.com/watch?v=kH21Nr9gGEU

7/31/2019 Daily Patterns #05

2/3

7/31/2019 Daily Patterns #05

3/3

Example

The Factory Method defines aninterface for creating objects, but

lets subclasses decide whichclasses to instantiate. Injection

molding presses demonstrate thispattern. Manufacturers of plastic

toys process plastic moldingpowder, and inject the plastic intomolds of the desired shapes. The

class of toy (car, action figure, etc.)is determined by the mold.

Factory Methods are usually calledwithin Template Methods.

Factory Method: creation throughinheritance. Prototype: creation through

delegation.

Prototype doesnt require subclassing,but it does require an Initialize

operation. Factory Method requiressubclassing, but doesnt require

Initialize.

Some GUYS recommend that as matter of

language design absolutely allconstructors should be private or

protected. Its no one elses businesswhether a class manufactures a new

object or recycles an old one.

Often, designs start out usingFactory Method (less complicated,more customizable, subclassesproliferate) and evolve towardAbstract Factory, Prototype, or

Builder (more flexible, morecomplex) as the designer discovers

where more flexibility is needed.

The advantage of a Factory Method isthat it can return the same instance

multiple times, or can return asubclass rather than an object of

that exact type.

Abstract Factory classes are oftenimplemented with Factory Methods,but they can be implemented using

Prototype.

The new operator consideredharmful. There is a difference

between requesting an object andcreating one. The new operator

always creates an object, and failsto encapsulate object creation. A

Factory Method enforces thatencapsulation, and allows an objectto be requested without

inextricable coupling to the act ofcreation.