Editing Abstract factory pattern

{{Short description|Software design pattern}}
[[Image:Abstract factory UML.svg|UML class diagram|thumb|right|317x317px]]
The '''abstract factory pattern''' in [[software engineering]] is a design pattern that provides a way to create families of related objects without imposing their concrete classes, by encapsulating a group of individual [[factory object|factories]] that have a common theme without specifying their concrete classes.<ref name="abstract factory">{{cite book | last1 = Freeman | first1 = Eric | last2 = Robson | first2 = Elisabeth | last3 = Sierra | first3 = Kathy | last4 = Bates | first4 = Bert | editor1-last = Hendrickson | editor1-first = Mike | editor2-last = Loukides | editor2-first = Mike | year = 2004 | title = Head First Design Patterns | volume = 1 | page = 156 | publisher = O'REILLY | format = paperback | isbn = 978-0-596-00712-6 | access-date = 2012-09-12 | url = http://shop.oreilly.com/product/9780596007126.do
}}</ref> According to this pattern, a client software component creates a concrete implementation of the abstract factory and then uses the generic [[Interface (object-oriented programming)|interface]] of the factory to create the concrete [[Object (computer science)|object]]s that are part of the family. The [[client (computing)|client]] does not know which concrete objects it receives from each of these internal factories, as it uses only the generic interfaces of their products.<ref name="abstract factory" /> This [[software design pattern|pattern]] separates the details of implementation of a set of objects from their general usage and relies on object composition, as object creation is implemented in methods exposed in the factory interface.<ref name="bullet points">{{cite book | last1 = Freeman | first1 = Eric | last2 = Robson | first2 = Elisabeth | last3 = Sierra | first3 = Kathy | last4 = Bates | first4 = Bert | editor1-last = Hendrickson | editor1-first = Mike | editor2-last = Loukides | editor2-first = Mike | year = 2004 | title = Head First Design Patterns | volume = 1 | page = 162 | publisher = O'REILLY | format = paperback | isbn = 978-0-596-00712-6 | access-date = 2012-09-12 | url = http://shop.oreilly.com/product/9780596007126.do
}}</ref>

Use of this pattern enables interchangeable concrete implementations without changing the code that uses them, even at [[Run time (program lifecycle phase)|runtime]]. However, employment of this pattern, as with similar [[Design pattern (computer science)|design pattern]]s, may result in unnecessary complexity and extra work in the initial writing of code. Additionally, higher levels of separation and abstraction can result in systems that are more difficult to debug and maintain.

==Overview==
The abstract factory design pattern is one of the 23 patterns described in the 1994 ''[[Design Patterns]]'' book. It may be used to solve problems such as:<ref>{{cite web|title=The Abstract Factory design pattern - Problem, Solution, and Applicability|url=http://w3sdesign.com/?gr=c01&ugr=proble|website=w3sDesign.com|access-date=2017-08-11}}</ref>
* How can an application be independent of how its objects are created?
* How can a class be independent of how the objects that it requires are created?
* How can families of related or dependent objects be created?

Creating objects directly within the class that requires the objects is inflexible. Doing so commits the class to particular objects and makes it impossible to change the instantiation later without changing the class. It prevents the class from being reusable if other objects are required, and it makes the class difficult to test because real objects cannot be replaced with mock objects.

A factory is the location of a concrete class in the code at which [[object creation|objects are constructed]]. Implementation of the pattern intends to insulate the creation of objects from their usage and to create families of related objects without depending on their concrete classes.<ref name="bullet points" /> This allows for new [[Subtyping|derived types]] to be introduced with no change to the code that uses the [[base class]].

The pattern describes how to solve such problems:
* [[Encapsulation (computer programming)|Encapsulate]] object creation in a separate (factory) object by defining and implementing an interface for creating objects.
* Delegate object creation to a factory object instead of creating objects directly.

This makes a class independent of how its objects are created. A class may be configured with a factory object, which it uses to create objects, and the factory object can be exchanged at runtime.
{{See also|#UML diagram}}

==Definition==
''Design Patterns'' describes the abstract factory pattern as "an interface for creating families of related or dependent objects without specifying their concrete classes."<ref name=":0">{{cite web | url = http://www.informit.com/ | title = Design Patterns: Abstract Factory | first = Erich | last = Gamma | author2 = Richard Helm | author3 = Ralph Johnson | author4 = John M. Vlissides | date = 2009-10-23 | publisher = informIT | archive-url = https://web.archive.org/web/20120516213805/http://www.informit.com/ | archive-date = 2012-05-16 | access-date = 2012-05-16 | quote = Object Creational: Abstract Factory: Intent: Provide an interface for creating families of related or dependent objects without specifying their concrete classes. | url-status = bot: unknown }}</ref>

==Usage==
The factory determines the concrete type of object to be created, and it is here that the object is actually created. However, the factory only returns a reference (in Java, for instance, by the '''new''' [[operator (programming)|operator]]) or a [[pointer (computer programming)|pointer]] of an abstract type to the created concrete object.

This insulates client code from [[object creation]] by having clients request that a [[factory object]] create an object of the desired [[abstract type]] and return an abstract pointer to the object.<ref>{{cite web | url = http://www.codeproject.com/ | title = Object Design for the Perplexed | first = David | last = Veeneman | date = 2009-10-23 | publisher = The Code Project | archive-url = https://web.archive.org/web/20110221224616/http://www.codeproject.com/ | archive-date = 2011-02-21 | access-date = 2012-05-16 | quote = The factory insulates the client from changes to the product or how it is created, and it can provide this insulation across objects derived from very different abstract interfaces. | url-status = bot: unknown }}</ref>

An example is an abstract factory class <code>DocumentCreator</code> that provides interfaces to create a number of products (e.g., <code>createLetter()</code> and <code>createResume()</code>). The system would have any number of derived concrete versions of the <code>DocumentCreator</code> class such as<code>FancyDocumentCreator</code> or <code>ModernDocumentCreator</code>, each with a different implementation of <code>createLetter()</code> and <code>createResume()</code> that would create corresponding objects such as<code>FancyLetter</code> or <code>ModernResume</code>. Each of these products is derived from a simple [[abstract class]] such as<code>Letter</code> or <code>Resume</code> of which the client is aware. The client code would acquire an appropriate [[Instance (computer science)|instance]] of the <code>DocumentCreator</code> and call its [[factory method]]s. Each of the resulting objects would be created from the same <code>DocumentCreator</code> implementation and would share a common theme. The client would only need to know how to handle the abstract <code>Letter</code> or <code>Resume</code> class, not the specific version that was created by the concrete factory.

As the factory only returns a reference or a pointer to an abstract type, the client code that requested the object from the factory is not aware of&mdash;and is not burdened by&mdash;the actual concrete type of the object that was created. However, the abstract factory knows the type of a concrete object (and hence a concrete factory). For instance, the factory may read the object's type from a configuration file. The client has no need to specify the type, as the type has already been specified in the configuration file. In particular, this means:

* The client code has no knowledge of the concrete [[Data type|type]], not needing to include any [[header file]]s or [[Class (computer science)|class]] [[Declaration (computer science)|declaration]]s related to it. The client code deals only with the abstract type. Objects of a concrete type are indeed created by the factory, but the client code accesses such objects only through their [[abstract interface|abstract interfaces]].<ref name="implementation">{{cite web | url = http://www.oodesign.com/abstract-factory-pattern.html | title = Abstract Factory: Implementation | publisher = OODesign.com | access-date = 2012-05-16
}}</ref>
* Adding new concrete types is performed by modifying the client code to use a different factory, a modification that is typically one line in one file. The different factory then creates objects of a different concrete type but still returns a pointer of the ''same'' abstract type as before, thus insulating the client code from change. This is significantly easier than modifying the client code to instantiate a new type. Doing so would require changing every location in the code where a new object is created as well as ensuring that all such code locations have knowledge of the new concrete type, for example, by including a concrete class header file. If all factory objects are stored globally in a [[singleton pattern|singleton]] object, and all client code passes through the singleton to access the proper factory for object creation, then changing factories is as easy as changing the singleton object.<ref name="implementation" />

== Structure ==

=== UML diagram ===
{{Plain image with caption|File:w3sDesign Abstract Factory Design Pattern UML.jpg|A sample UML class and sequence diagram for the abstract factory design pattern.
<ref>{{cite web|title=The Abstract Factory design pattern - Structure and Collaboration|url=http://w3sdesign.com/?gr=c01&ugr=struct|website=w3sDesign.com|access-date=2017-08-12}}</ref>|700px|align=left}}{{-}}
In the above [[Unified Modeling Language|UML]] [[class diagram]], 
the <code>Client</code> class that requires <code>ProductA</code> and <code>ProductB</code> objects does not instantiate the <code>ProductA1</code> and <code>ProductB1</code> classes directly. Instead, the <code>Client</code> refers to the <code>AbstractFactory</code> interface for creating objects, which makes the <code>Client</code> independent of how the objects are created (which concrete classes are instantiated). The <code>Factory1</code> class implements the <code>AbstractFactory</code> interface by instantiating the <code>ProductA1</code> and <code>ProductB1</code> classes.

The [[Unified Modeling Language|UML]] [[sequence diagram]] shows the runtime interactions. The <code>Client</code> object calls <code>createProductA()</code> on the <code>Factory1</code> object, which creates and returns a <code>ProductA1</code> object. Thereafter, the <code>Client</code> calls <code>createProductB()</code> on <code>Factory1</code>, which creates and returns a <code>ProductB1</code> object.

=== Variants ===
The original structure of the abstract factory pattern, as defined in 1994 in ''[[Design Patterns]]'', is based on abstract classes for the abstract factory and the abstract products to be created. The concrete factories and products are classes that specialize the abstract classes using inheritance.<ref name=":0" />  

A more recent structure of the pattern is based on interfaces that define the abstract factory and the abstract products to be created. This design  uses native support for interfaces or protocols in mainstream programming languages to avoid inheritance. In this case, the concrete factories and products are classes that realize the interface by implementing it.<ref name="abstract factory" /> 

== Example ==
This [[C++23]] implementation is based on the pre-C++98 implementation in the book.

<syntaxhighlight lang="c++">
import std;

enum class Direction {North, South, East, West};

class MapSite {
public:
  virtual void enter() = 0;
  virtual ~MapSite() = default;
};

class Room: public MapSite {
public:
  Room(): roomNumber(0) {}
  Room(int n): roomNumber(n) {}
  void setSide(Direction d, MapSite* ms) {
    std::println("Room::setSide {} ms", d);
  }
  virtual void enter() {}
  Room(const Room&) = delete; // rule of three
  Room& operator=(const Room&) = delete;
private:
  int roomNumber;
};

class Wall: public MapSite {
public:
  Wall() {}
  virtual void enter() {}
};

class Door: public MapSite {
public:
  Door(Room* r1 = nullptr, Room* r2 = nullptr): room1(r1), room2(r2) {}
  virtual void enter() {}
  Door(const Door&) = delete; // rule of three
  Door& operator=(const Door&) = delete;
private:
  Room* room1;
  Room* room2;
};

class Maze {
public:
  void addRoom(Room* r) {
    std::println("Maze::addRoom {}", r);
  }
  Room* roomNo(int) const {
    return nullptr;
  }
};

class MazeFactory {
public:
  MazeFactory() = default;
  virtual ~MazeFactory() = default;

  virtual Maze* makeMaze() const {
    return new Maze;
  }
  virtual Wall* makeWall() const {
    return new Wall;
  }
  virtual Room* makeRoom(int n) const {
    return new Room(n);
  }
  virtual Door* makeDoor(Room* r1, Room* r2) const {
    return new Door(r1, r2);
  }
};

// If createMaze is passed an object as a parameter to use to create rooms, walls, and doors, then you can change the classes of rooms, walls, and doors by passing a different parameter. This is an example of the Abstract Factory (99) pattern.

class MazeGame {
public:
  Maze* createMaze(MazeFactory& factory) {
    Maze* aMaze = factory.makeMaze();
    Room* r1 = factory.makeRoom(1);
    Room* r2 = factory.makeRoom(2);
    Door* aDoor = factory.makeDoor(r1, r2);
    aMaze->addRoom(r1);
    aMaze->addRoom(r2);
    r1->setSide(Direction::North, factory.makeWall());
    r1->setSide(Direction::East, aDoor);
    r1->setSide(Direction::South, factory.makeWall());
    r1->setSide(Direction::West, factory.makeWall());
    r2->setSide(Direction::North, factory.makeWall());
    r2->setSide(Direction::East, factory.makeWall());
    r2->setSide(Direction::South, factory.makeWall());
    r2->setSide(Direction::West, aDoor);
    return aMaze;
  }
};

int main() {
  MazeGame game;
  MazeFactory factory;
  game.createMaze(factory);
}
</syntaxhighlight>

The program output is:

<syntaxhighlight lang="output">
Maze::addRoom 0x1317ed0
Maze::addRoom 0x1317ef0
Room::setSide 0 0x1318340
Room::setSide 2 0x1317f10
Room::setSide 1 0x1318360
Room::setSide 3 0x1318380
Room::setSide 0 0x13183a0
Room::setSide 2 0x13183c0
Room::setSide 1 0x13183e0
Room::setSide 3 0x1317f10
</syntaxhighlight>

== See also ==
* [[Concrete class]]
* [[Factory method pattern]]
* [[Object creation]]
* [[Software design pattern]]

==References==
{{Reflist}}

==External links==
{{Wikibooks|Computer Science Design Patterns|Abstract Factory|Abstract Factory in action}}
* {{Commons-inline}}
* [https://web.archive.org/web/20151101110755/http://www.patterns.pl/abstractfactory.html Abstract Factory] Abstract Factory implementation example

{{Design patterns}}

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