9.5 Summary

The Abstract Factory design pattern fosters an interface for creating a family of related or dependent polymorphic objects. Using Abstract Factory, you can divide implementation classes into disjoint families.

It is possible to implement a generic abstract factory interface by using typelists and policy templates. The typelists provide the product list (both concrete and abstract) and the policy templates.

The AbstractFactory class template provides a skeleton for defining abstract factories and works in conjunction with the AFUnit class template. AbstractFactory requires a user-provided abstract product typelist. Internally, AbstractFactory uses GenScatterHierarchy (see Chapter 3) to generate a granular interface that inherits AFUnit<T> for each product T in the abstract product typelist. This structure gives you the opportunity to reduce coupling by passing only individual factory units to various parts of an application.

The ConcreteFactory template helps with implementing the AbstractFactory interface. ConcreteFactory uses a FactoryUnit policy for creating objects. Internally, Concrete Factory uses GenLinearHierarchy (see Chapter 3). Loki provides two predefined implementations of the FactoryUnit policy: OpNewFactoryUnit, which creates objects by calling the new operator, and PrototypeFactoryUnit, which creates objects by cloning a prototype.