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Here are some useful techniques for implementing the Abstract Factory pattern.

  1. Factories as singletons. An application typically needs only one instance of a ConcreteFactory per product family. So it's usually best implemented as a Singleton .

  2. Creating the products. AbstractFactory only declares an interface for creating products. It's up to ConcreteProduct subclasses to actually create them. The most common way to do this is to define a factory method for each product. A concrete factory will specify its products by overriding the factory method for each. While this implementation is simple, it requires a new concrete factory subclass for each product family, even if the product families differ only slightly.

    If many product families are possible, the concrete factory can be implemented using the Prototype pattern. The concrete factory is initialized with a prototypical instance of each product in the family, and it creates a new product by cloning its prototype. The Prototype-based approach eliminates the need for a new concrete factory class for each new product family.

    Here's a way to implement a Prototype-based factory in Smalltalk. The concrete factory stores the prototypes to be cloned in a dictionary called partCatalog. The method make: retrieves the prototype and clones it:

    make: partName ^ (partCatalog at: partName) copy
    The concrete factory has a method for adding parts to the catalog.
    addPart: partTemplate named: partName partCatalog at: partName put: partTemplate
    Prototypes are added to the factory by identifying them with a symbol:
    aFactory addPart: aPrototype named: #ACMEWidget
    A variation on the Prototype-based approach is possible in languages that treat classes as first-class objects (Smalltalk and Objective C, for example). You can think of a class in these languages as a degenerate factory that creates only one kind of product. You can store classes inside a concrete factory that create the various concrete products in variables, much like prototypes. These classes create new instances on behalf of the concrete factory. You define a new factory by initializing an instance of a concrete factory with classes of products rather than by subclassing. This approach takes advantage of language characteristics, whereas the pure Prototype-based approach is language-independent. Like the Prototype-based factory in Smalltalk just discussed, the class-based version will have a single instance variable partCatalog, which is a dictionary whose key is the name of the part. Instead of storing prototypes to be cloned, partCatalog stores the classes of the products. The method make: now looks like this:
    make: partName ^ (partCatalog at: partName) new
  3. Defining extensible factories. AbstractFactory usually defines a different operation for each kind of product it can produce. The kinds of products are encoded in the operation signatures. Adding a new kind of product requires changing the AbstractFactory interface and all the classes that depend on it.

    A more flexible but less safe design is to add a parameter to operations that create objects. This parameter specifies the kind of object to be created. It could be a class identifier, an integer, a string, or anything else that identifies the kind of product. In fact with this approach, AbstractFactory only needs a single ``Make'' operation with a parameter indicating the kind of object to create. This is the technique used in the Prototype- and the class-based abstract factories discussed earlier.

    This variation is easier to use in a dynamically typed language like Smalltalk than in a statically typed language like C++. You can use it in C++ only when all objects have the same abstract base class or when the product objects can be safely coerced to the correct type by the client that requested them. The implementation section of Factory Method shows how to implement such parameterized operations in C++.

    But even when no coercion is needed, an inherent problem remains: All products are returned to the client with the same abstract interface as given by the return type. The client will not be able to differentiate or make safe assumptions about the class of a product. If clients need to perform subclass-specific operations, they won't be accessible through the abstract interface. Although the client could perform a downcast (e.g., with dynamic_cast in C++), that's not always feasible or safe, because the downcast can fail. This is the classic trade-off for a highly flexible and extensible interface.

Implementation of Abstract Factory, Adapter, Bridge, Builder, Chain of Responsibility, Command, Composite, Decorator, Facade, Factory Method, Flyweight, Interpreter, Iterator, Mediator, Memento, Observer, Prototype, Proxy, Singleton, State, Strategy, Template Method, Visitor
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