02 - Creational Design Patterns

Moshe Fresko

Bar-Ilan University

תשס"ח

2008

Design Patterns

Design Patterns help you learn from others’ successes, instead of your failures

Separate things that change, from the things that doesn’t change

Elegant and Cheap-to-Maintain Three classes of DPs

1. Creational

2. Behavioral

3. Structural

Creational Design Patterns

Creational DP: Abstracts the instantiation process. Help make a system independent of how objects are

created, composed, represented.

Two types of Creational Patterns 1. Class creational patterns

Use inheritance to vary the class that is instantiated

2. Object creational patternsDelegates instantiation to another object

Creational Design Patterns

Two recurring themes in these patterns1. They encapsulate knowledge about which concrete

classes the system use.2. They hide how instances of these classes are created

and put together. The system only knows the interfaces. Creational DP-s let you configure the system with

“product” objects Configuration can be static (compile-time) or dynamic

(run-time).

Example: To build a Maze

Maze Example – MapSite Abstract class

enum Direction

{ North, South, East, West } ;

class MapSite {

public:

void Enter() = 0 ;

} ;

Maze Example – Roomclass Room: public MapSite { public: Room(int roomNo) ; { roomNumber = number ; } MapSite* GetSide(Direction d) const { return sides[d] ; } void SetSide(Direction d, MapSite* m) { sides[d] = m ; } virtual void Enter() { /* … do something … */ } private: MapSite* sides[4] ; int roomNumber ; } ;

Maze Example – Wall and Door

class Wall: public MapSite {

public:

Wall() ;

virtual void Enter() ;

} ;

class Door: public MapSite {

public:

Door(Room*=0, Room*=0) ;

virtual void Enter() ;

Room* OtherSideFrom(Room*);

private:

Room* room1 ;

Room* room2 ;

bool isOpen;

} ;

Maze Example – Collection of Rooms

class Maze {

public:

Maze() ;

void addRoom(Room r) ;

Room* RoomNo(int) const;

private:

// …

};

Maze Example – Creation of Mazeclass MazeGame{

public: Maze* CreateMaze() {

Maze* maze = new Maze() ; Room* room1 = new Room(1) ; Room* room2 = new Room(2) ; Door* door = new Door(room1,room2) ;

maze->AddRoom(room1) ; maze->AddRoom(room2) ; room1->SetSide(North, new Wall()) ; room1->SetSide(East , door) ; room1->SetSide(South, new Wall()) ; room1->SetSide(West , new Wall()) ;

room2->SetSide(North, new Wall()) ; room2->SetSide(East , new Wall()) ; room2->SetSide(South, new Wall()) ; room2->SetSide(West , door) ;

return maze ;}

}

Room 1 Room 2

Creational Patterns Factory Method

Create-Maze calls virtual functions to create components Abstract Factory

Create-Maze is passed an object to use to create components

Prototype Create-Maze is parameterized by various prototypes

Builder Create-Maze is passed an object that can create entire

Maze Singleton

Can ensure that there is only one maze per game.

Factory Method

Moshe Fresko

Bar-Ilan University

2005-2006 - תשס"ו

Design Patterns Course

Factory Method

Intent: Define an interface for creating an object, but let subclasses decide which class to instantiate.

Motivation: Example: Framework of Abstract classes

Abstract classes: Document, Application Application has Open, New, etc. to create new documents Application cannot know which concrete document to instantiate

Concrete classes: DrawingDocument, DrawingApplication

Factory Method encapsulates the knowledge of which Document subclass to create and move this knowledge out of the framework.

Factory Method – Motivation

CreateDocument() is called Factory Method

Factory Method – Maze Exampleclass MazeGame{public:

virtual Maze* MakeMaze() const { return new Maze() ; }virtual Room* MakeRoom(int n) { return new Room(n) ; }virtual Wall* MakeWall() { return new Wall() ; }virtual Door* MakeDoor(Room* r1, Room* r2)

{ return new Door(r1,r2) ; }Maze* CreateMaze() {

Maze* maze = MakeMaze() ;Room* room1 = MakeRoom(1) ;Room* room2 = MakeRoom(2) ;Door* door = MakeDoor(room1,room2) ;………………return maze ;

}} ;

Factory Method – Maze, Customized Components

class BombedWall: public Wall {// …

} ;

class RoomWithABomb: public Room {public:

RoomWithABomb(int n) : Room(n) { }} ;

class BombedMazeGame: public MazeGame {public:

BombedMazeGame();virtual Wall* MakeWall()

{ return new BombedWall() ; }virtual Room* MakeRoom(int n)

{ return new RoomWithABomb(n) ; }} ;

Factory Method – Applicability

Use the Factory Method 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

Factory Method – Structure

Factory Method - Participants

Product (Document) The interface of objects the Factory Method creates

ConcreteProduct (MyDocument) Implements the product interface

Creator (Application) Declares the factory method which returns an object of

type Product

ConcreteCreator (MyApplication) Defines the Factory method to returnn an instance of

ConcreteProduct

Factory Method – Consequences

Eliminates the need to bind application-specific classes into the code.

Disadvantage: Clients might have to subclass the Creator class just to create a particular ConcreteProduct.

Provides hooks for subclasses to create extended version of an object.

Connects parallel class hierarchies.

Factory Method – Implementation Issues

1. Two Major Varieties Creator class has abstract factory methods Creator class defines default behavior for factory methods

2. Parameterized Factory Methods One factory method can create multiple kinds of products. All objects must have the same interface. Factory method can take a class ID.

3. Language-specific issues. Creator can keep the Class information for creating new

instances, dropping the need for the sub-classing.

4. Templates can be used to avoid subclassing.

Abstract Factory

Moshe Fresko

Bar-Ilan University

2005-2006 - תשס"ו

Design Patterns Course

Abstract Factory Intent: Provides an interface for creating families of

related or dependent objects without specifying their concrete classes.

Motivation: User interface Toolkit supporting multiple look-and- feel

standards. (Widgets like Scroll Bars, Windows, Buttons etc.)

Not to hard code these widgets for a particular look-and-feel otherwise hard to change it

We can define a WidgetFactory interface for creating each basic entity

Widget Factory enforces dependencies between the concrete Widget classes

Abstract Factory Example

Factory Method – Maze Exampleclass MazeFactory {public:

Maze* MakeMaze() { return new Maze() ; }Room* MakeRoom(int n) { return new Room(n) ; }Wall* MakeWall() { return new Wall() ; }Door* MakeDoor(Room r1, Room r2)

{ return new Door(r1,r2) ; }} ;class MazeGame {public:

Maze* CreateMaze(MazeFactory* factory) {Maze* maze = factory->newMaze() ;Room* room1 = factory->newRoom(1) ;Room* room2 = factory->newRoom(2) ;Door* door = factory->newDoor(room1,room2) ;………return maze ;

}} ;

Factory Method – Maze Exampleclass BombedWall: public Wall {

// …} ;

class RoomWithABomb: public Room {public:

RoomWithABomb(int n) : Room(n) { }} ;

class BombedMazeFactory: public MazeFactory {public:

BombedMazeGame();virtual Wall* MakeWall()

{ return new BombedWall() ; }virtual Room* MakeRoom(int n)

{ return new RoomWithABomb(n) ; }} ;

Abstract Factory – Applicability

Use Abstract Factory if A system must be independent of how its

products are created A system should be configured with one of

multiple families of products A family of related objects must be used together You want to reveal only interfaces of a family of

products and not their implementations

Abstract Factory – Structure

Abstract Factory – Participants AbstractFactory (WidgetFactory)

Declares an interface of methods to create abstract product objects

ConcreteFactory (MotifWidgetFactory,…) Implements the methods to create concrete product objects

AbstractProduct (Window, ScrollBar) Declares an interface for a product type

ConcreteProduct (MotifWindow, MotifScrollBar) Defines a product object Implements the AbstractProduct interface

Client Uses only interfaces declared by AbstractFactory and

AbstractProduct

Abstract Factory – Consequences

1. It isolates concrete classes

2. It makes exchanging product families easy

3. It promotes consistency among products

4. Supporting new kinds of products is difficult

Abstract Factory – Implementation

Factory better to be a Singleton If many product families are possible, the Concrete

Factory can be implemented using Prototype. Or alternatively the Class information of products can be kept (for languages supporting Class information).

Defining Extensible Factories: Adding a new Product type means to change AbstractFactory and all its subclasses. A more flexible but less safe design is to add a parameter to operations that create objects.

Singleton

Moshe Fresko

Bar-Ilan University

2005-2006 - תשס"ו

Design Patterns Course

Singleton

Intent: Ensure that a class has only one instance, and provide a global point of access to it.

Use Singleton There must be exactly one instance of a class, and it must

be accessible to clients from a well known access point. When this instance should be extensible by sub-classing

Singleton

Singleton Define an Instance operation to access its unique

instance. It must be a static method. Must create its own unique instance.

Singleton – Benefits

1. Controlled access to sole instance

2. Reduced namespace

3. May be sub-classed to refine operations

4. Can Permit a variable number of instances

5. More flexible than static methods

Singleton – Implementation

Ensure a unique instanceclass Singleton {private: static Singleton* inst = 0 ;protected: Singleton() { }public: static Singleton* getInstance() { if (inst==0) inst = new Singleton() ; return inst ;}

} ; Subclassing the singleton class

How to install the unique instance? To determine it in getInstance() method To rewrite getInstance() in the subclass To keep registry of Singletons

Singleton –Maze Factory

class MazeFactory {

protected: MazeFactory() { }

private: static MazeFactory* inst = null ;

public: static MazeFactory* getInst()

{ if (inst==null) inst = new MazeFactory() ;

return inst ; }

Maze* makeMaze()

{ return new Maze() ; }

Room* makeRoom(int n)

{ return new Room(n) ; }

Wall* makeWall()

{ return new Wall() ; }

Door* makeDoor(Room r1, Room r2)

{ return new Door(r1,r2) ; }

} ;

Singleton – Maze Exampleclass MazeGame{public:

Maze* createMaze() {Maze maze* = MazeFactory.getInst()->MakeMaze() ;Room room1* = MazeFactory.getInst()->MakeRoom(1) ;Room room2* = MazeFactory.getInst()->MakeRoom(2) ;Door door* = MazeFactory.getInst()->MakeDoor(room1,room2) ;

maze->AddRoom(room1) ;maze->AddRoom(room2) ;………return maze ;

}}

Singleton – Alternative Maze Factory

MazeFactory* MazeFactory::getInst() { if (inst==0) {

const char* style = getenv("MAZESTYLE") ; if (strcmp(style,"BOMBED“))

inst = new BombedMazeFactory() ;else if (strcmp(style,"BOMBED“))

inst = new EnchantedMazeFactory() ; else

inst = new MazeFactory() ; } return inst ; }

Template Singleton Class// in .h

template <class T>class Singleton : public T{public:

static Singleton* GetInstance() {

if (! ptrSingObject) ptrSingObject = new Singleton ;

return ptrSingObject ; }~Singleton() { delete ptrSingObject ; }

private:Singleton() { } ;static Singleton* ptrSingObject ;

};// In .cpp

template <class T>Singleton<T>* Singleton<T>::ptrSingObject = NULL ;

// usageclass CMyClass {

void myfunc() ;} ;

// In the program to useSingleton<CMyClass>::GetInstance()->myfunc() ;