The Factory Method pattern defines an interface for creating objects but lets subclasses decide which class to instantiate. It provides a way to delegate object instantiation to subclasses. For example, a hotel front desk acts as a factory by creating the appropriate type of room object based on a customer's needs. The pattern involves a Creator class with a factory method that returns a Product object, while ConcreteCreator subclasses override the method to instantiate a ConcreteProduct. This decouples client code from the instantiation process and makes a system extensible to new product types.

1. Factory Method Pattern CSE776-Design Patterns By Anjan Kumar Bollam Professor Jim Fawcett

3. It lets a class defer instantiation to subclasses at run time.

8. A class can’t anticipate the class of objects it must create.

9. A class wants its subclasses to specify the objects it creates.

12. Defines the interface of objects the factory method creates.

13. ConcreteProduct(ASP.SamplePage_aspx)

14. Implements the Product Interface

15. Creator(IHttpHandlerFactory)

16. Declares the factory method and may provide a default implementation for it.

17. It returns an object of type Product.

18. ConcreteCreator(PageHandlerFactory)

21. New concrete classes can be added without recompiling the existing code.

24. Abstract Creator class with no default implementation

25. Concrete Creator with default implementation.

26. Solutions:

27. Parameterized Methods

29. Templatized Factory Methods class Creator { public: Creator() { // You cannot call the factory method here (why?) // Use lazy initialization instead } virtual Product* CreateProduct() = 0; }; template <class T> class StandardCreator: public Creator { public: virtual Product* CreateProduct() { return new T; } } // In the Client StandardCreator<MyProduct> myCreator;

31. It is used in several places in the Java API. For example, URLConnection has a method getContent that returns the content as an appropriate object (html, gif etc.)

33. Template Methods

36. http://www.ondotnet.com/pub/a/dotnet/2003/08/11/factorypattern.html

37. http://en.wikibooks.org/wiki/More_C%2B%2B_Idioms/Virtual_Constructor