Design Pattern ->Factory Method

Layering & Contract Philosophy With additional indirection

Factory Method

The example code is as following:

 class CProduct //interface declaration
 class CConcreteProductOne: public CProduct;
 class CConcreteProductTwo: public CProduct;

 class CCreator
 {
      public: virtual CProduct* FactoryMethod()= ;
 }
 class CConcreteCreator: public CCreator
 {
      public: CProduct* FactoryMethod(){ return new CConcreteProductOne()  OR  new CConcreteProductTwo()};
 }
 class CClient
 {
         CCreator *pCreator = new CConreteCreator();
         CProduct *pProduct = pCreator->FacotryMethod();
 }

IoC = Inversion Of Control.

控制反转还有一个名字叫做依赖注入（Dependency Injection）。简称DI

DIP = Dependence-Inversion Principle ( low-level module is depended on high-level module by defining abstract interface in high-level).

Interface-Oriented Programming, not Implement-Oriented Programming

Applicability
Use the Factory Method pattern when

• A class can't anticipate the class of objects it must create.
• A class wants its subclasses to specify the objects it creates.
• Classes delegate responsibility to one of several helper subclasses, and you want to localize the knowledge of which helper subclass is the delegate.
• The new proper/appropriate instance of the concrete product can be got at run-time from the real situation.

Participants

• Product: defines the interface of objects the factory method creates.
• ConcreteProduct : implements the Product interface.
• Creator : declares the factory method, which returns an object of type Product.Creator may also define a default implementation of the factory method that returns a default ConcreteProduct object. may call the factory method to create a Product object.
• ConcreteCreator: overrides the factory method to return an instance of a ConcreteProduct.

Collaborations

• Creator relies on its subclasses to define the factory method so that it returns an instance of the appropriate ConcreteProduct.