9.6 AbstractFactory and ConcreteFactory Quick Facts

• AbstractFactory synopsis:

  
  
  template
  
  
  <
  
  
     class TList,
  
  
     template <class> class Unit = AbstractFactoryUnit
  
  
  >
  
  
  class AbstractFactory;
  
  
  

  where TList is the typelist of abstract products that the factory creates, and Unit is the template that defines the interface for each type in TList. For example,

  
  
  typedef AbstractFactory<TYPELIST_3(Soldier, Monster, SuperMonster>
  
  
     AbstractEnemyFactory;
  
  
  

  defines an abstract factory capable of creating Soldiers, Monsters, and SuperMonsters.

• AbstractFactoryUnit<T> defines an interface consisting of a pure virtual function with the signature T* DoCreate(Type2Type<T>). Usually you don't call DoCreate directly; instead, you use AbstractFactory::Create.

• AbstractFactory exposes a Create template function. You can instantiate Create with any of the types of the abstract products. For example:

  
  
  AbstractEnemyFactory *pFactory = ...;
  
  
  Soldier *pSoldier = pFactory->Create<Soldier>();
  
  
  

• For implementing the interface that AbstractFactory defines, Loki provides the ConcreteFactory template. ConcreteFactory's synopsis is

  
  
  template
  
  
  <
  
  
     class AbstractFact,
  
  
     template <class, class> class FactoryUnit = OpNewFactoryUnit,
  
  
     class TList = AbstractFact::ProductList
  
  
  >
  
  
  class ConcreteFactory;
  
  
  

  where AbstractFact is the instantiation of AbstractFactory that is to be implemented, FactoryUnit is the implementation of the FactoryUnit creation policy, and TList is the typelist of concrete products.

• The FactoryUnit policy implementation has access to both the abstract product and the concrete product that it must create. Loki defines two Creator policies: OpNewFactoryUnit (Section 9.3) and PrototypeFactoryUnit (Section 9.4). They can also serve as examples for implementing custom implementations of the FactoryUnit policy.

• OpNewFactoryUnit uses the new operator for creating objects. If you use OpNewFactoryUnit, you must provide a concrete product typelist as the third parameter to ConcreteFactory. For example:

  
  
  typedef ConcreteFactory
  
  
  <
  
  
     AbstractEnemyFactory,
  
  
     OpNewFactoryUnit,
  
  
     TYPELIST_3(SillySoldier, SillyMonster, SillySuperMonster)
  
  
  >
  
  
  EasyLevelEnemyFactory;
  
  
  

• PrototypeFactoryUnit stores pointers to abstract product types and creates new objects by calling the Clone member function of their respective prototypes. This implies that PrototypeFactoryUnit requires each abstract product T to define a virtual member function Clone that returns a T* and whose semantics is to duplicate the object.

• When using PrototypeFactoryUnit with ConcreteFactory, you don't provide the third template argument to ConcreteFactory. For example:

  
  
  typedef ConcreteFactory
  
  
  <
  
  
     AbstractEnemyFactory,
  
  
     PrototypeFactoryUnit,
  
  
  >
  
  
  EnemyFactory;