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Factory Design Pattern


In object-oriented computer programming, a factory is an object for creating other objects. It is an abstraction of a constructor, and can be used to implement various allocation schemes. For example, using this definition, singletons implemented by the singleton pattern are formal factories.
A factory object typically has a method for every kind of object it is capable of creating. These methods optionally accept parameters defining how the object is created, and then return the created object.
Factory objects are used in situations where getting hold of an object of a particular kind is a more complex process than simply creating a new object. The factory object might decide to create the object's class (if applicable) dynamically, return it from an object pool, do complex configuration on the object, or other things.
These kinds of objects have proven useful and several design patterns have been developed to implement them in any given language. For example, several "GoF patterns", like the "Factory method pattern", the "Builder" or even the "Singleton" are implementations of this concept. The "Abstract factory pattern" instead is a method to build collections of factories.

Usage

Factory objects are common in toolkits and frameworks where library code needs to create objects of types which may be subclassed by applications using the framework. They are also used in test-driven development to allow classes to be put under test.
Factories determine the actual concrete type of object to be created, and it is here that the object is actually created. As the factory only returns an abstract pointer, the client code does not know - and is not burdened by - the actual concrete type of the object which was just created. However, the type of a concrete object is known by the abstract factory.
In particular, this means:
  • The client code has no knowledge whatsoever of the concrete type, not needing to include any header files or class declarations relating to the concrete type. 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.
  • Adding new concrete types is done by modifying the client code to use a different factory, a modification which is typically one line in one file. This is significantly easier than modifying the client code to instantiate a new type, which would require changing every location in the code where a new object is created.


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