Chain of Responsibility
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.
UsageFactory 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: