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<HTML><HEAD><TITLE>Creational Patterns</TITLE><SCRIPT>function setFocus() { if ((navigator.appName != "Netscape") && (parseFloat(navigator.appVersion) == 2)) { return; } else { self.focus(); }}</SCRIPT></HEAD><BODY BGCOLOR = #FFFFFF TEXT = #000000onLoad="setFocus()";><A NAME="top"></A><A NAME="creation_top1"></A><P>Creational design patterns abstract the instantiation process.They help make a system independent of how its objects are created,composed, and represented. A class creational pattern uses inheritanceto vary the class that's instantiated, whereas an object creationalpattern will delegate instantiation to another object.<A NAME="creation_top2"></A><P>Creational patterns become important as systems evolve to depend moreon object composition than class inheritance. As that happens,emphasis shifts away from hard-coding a fixed set of behaviors towarddefining a smaller set of fundamental behaviors that can be composedinto any number of more complex ones. Thus creating objects withparticular behaviors requires more than simply instantiating a class.<A NAME="creation_top2"></A><P>There are two recurring themes in these patterns. First, they allencapsulate knowledge about which concrete classes the system uses.Second, they hide how instances of these classes are created and puttogether. All the system at large knows about the objects is theirinterfaces as defined by abstract classes. Consequently, thecreational patterns give you a lot of flexibility in <EM>what</EM> getscreated, <EM>who</EM> creates it, <EM>how</EM> it gets created, and <EM>when</EM>. They let you configure a system with "product" objects thatvary widely in structure and functionality. Configuration can bestatic (that is, specified at compile-time) or dynamic (atrun-time).<A NAME="creation_top3"></A><P>Sometimes creational patterns are competitors. For example,there are cases when either <A HREF="pat3dfs.htm" TARGET="_mainDisplayFrame">Prototype (117)</A>or <A HREF="pat3afs.htm" TARGET="_mainDisplayFrame">Abstract Factory (87)</A> couldbe used profitably. At other times they are complementary:<A HREF="pat3bfs.htm" TARGET="_mainDisplayFrame">Builder (97)</A> can use one of the otherpatterns to implement which components get built.<A HREF="pat3dfs.htm" TARGET="_mainDisplayFrame">Prototype (117)</A> can use<A HREF="pat3efs.htm" TARGET="_mainDisplayFrame">Singleton (127)</A> in its implementation.<A NAME="creation_top4"></A><P>Because the creational patterns are closely related, we'll study allfive of them together to highlight their similarities and differences.We'll also use a common example—building a maze for a computergame—to illustrate their implementations. The maze and the game willvary slightly from pattern to pattern. Sometimes the game will besimply to find your way out of a maze; in that case the player willprobably only have a local view of the maze. Sometimes mazes containproblems to solve and dangers to overcome, and these games may providea map of the part of the maze that has been explored.<A NAME="creation_top5"></A><P>We'll ignore many details of what can be in a maze and whether a mazegame has a single or multiple players. Instead, we'll just focus onhow mazes get created. We define a maze as a set of rooms. A roomknows its neighbors; possible neighbors are another room, a wall, or adoor to another room.<A NAME="creation_top6"></A><P>The classes <CODE>Room</CODE>, <CODE>Door</CODE>, and <CODE>Wall</CODE>define the components of the maze used in all our examples. We defineonly the parts of these classes that are important for creating amaze. We'll ignore players, operations for displaying and wanderingaround in a maze, and other important functionality that isn'trelevant to building the maze.<P>The following diagram shows the relationships between these classes:<A NAME="room-82c"></A><P ALIGN=CENTER><IMG SRC="Pictures/maze.gif"></P><P>Each room has four sides. We use an enumeration <CODE>Direction</CODE> inC++ implementations to specify the north, south, east, and west sides ofa room:</P><PRE> enum Direction {North, South, East, West};</PRE><P>The Smalltalk implementations use corresponding symbols to representthese directions.</P><A NAME="mapsite"></A><P>The class <CODE>MapSite</CODE> is the common abstract class for all thecomponents of a maze. To simplify the example, <CODE>MapSite</CODE> definesonly one operation, <CODE>Enter</CODE>. Its meaning depends on what you'reentering. If you enter a room, then your location changes. If you try toenter a door, then one of two things happen: If the door is open, you gointo the next room. If the door is closed, then you hurt your nose.</P><PRE> class MapSite { public: virtual void Enter() = 0; };</PRE><A NAME="mapsite2"></A><P><CODE>Enter</CODE> provides a simple basis for more sophisticated gameoperations. For example, if you are in a room and say "Go East," thegame can simply determine which <CODE>MapSite</CODE> is immediately to theeast and then call <CODE>Enter</CODE> on it. The subclass-specific<CODE>Enter</CODE> operation will figure out whether your location changedor your nose got hurt. In a real game, <CODE>Enter</CODE> could take theplayer object that's moving about as an argument.</P><A NAME="def-room"></A><P><CODE>Room</CODE> is the concrete subclass of <CODE>MapSite</CODE> thatdefines the key relationships between components in the maze. Itmaintains references to other <CODE>MapSite</CODE> objects and stores aroom number. The number will identify rooms in the maze.</P><PRE> class Room : public MapSite { public: Room(int roomNo); MapSite* GetSide(Direction) const; void SetSide(Direction, MapSite*); virtual void Enter(); private: MapSite* _sides[4]; int _roomNumber; };</PRE><A NAME="wall-code"></A><A NAME="door-code"></A><P>The following classes represent the wall or door that occurs on eachside of a room.</P><PRE> 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; };</PRE><A NAME="Door-def"></A><A NAME="maze-def"></A><P>We need to know about more than just the parts of a maze. We'll alsodefine a <CODE>Maze</CODE> class to represent a collection of rooms.<CODE>Maze</CODE> can also find a particular room given a room numberusing its <CODE>RoomNo</CODE> operation.</P><PRE> class Maze { public: Maze(); void AddRoom(Room*); Room* RoomNo(int) const; private: // ... };</PRE><P><CODE>RoomNo</CODE> could do a look-up using a linear search, a hash table,or even a simple array. But we won't worry about such details here.Instead, we'll focus on how to specify the components of a maze object.</P><A NAME="mazegame"></A><P>Another class we define is <CODE>MazeGame</CODE>, which creates the maze.One straightforward way to create a maze is with a series of operationsthat add components to a maze and then interconnect them. Forexample, the following member function will create a maze consistingof two rooms with a door between them:</P><A NAME="CreateMaze-def"></A><PRE> Maze* MazeGame::CreateMaze () { Maze* aMaze = new Maze; Room* r1 = new Room(1); Room* r2 = new Room(2); Door* theDoor = new Door(r1, r2); aMaze->AddRoom(r1); aMaze->AddRoom(r2); r1->SetSide(North, new Wall); r1->SetSide(East, theDoor); r1->SetSide(South, new Wall); r1->SetSide(West, new Wall); r2->SetSide(North, new Wall); r2->SetSide(East, new Wall); r2->SetSide(South, new Wall); r2->SetSide(West, theDoor); return aMaze; }</PRE><A NAME="CreateMaze-text1"></A><P>This function is pretty complicated, considering that all it does is createa maze with two rooms. There are obvious ways to make it simpler. Forexample, the <CODE>Room</CODE> constructor could initialize the sideswith walls ahead of time. But that just moves the code somewhere else.The real problem with this member function isn't its size but its <EM>inflexibility</EM>. It hard-codes the maze layout. Changing the layoutmeans changing this member function, either by overriding it—whichmeans reimplementing the whole thing—or by changing parts ofit—which is error-prone and doesn't promote reuse.</P><A NAME="CreateMaze-text2"></A><P>The creational patterns show how to make this design more <EM>flexible</EM>, not necessarily smaller. In particular, they will make iteasy to change the classes that define the components of a maze.</P><A NAME="CreateMaze-text2"></A><P>Suppose you wanted to reuse an existing maze layout for a new gamecontaining (of all things) enchanted mazes. The enchanted maze game hasnew kinds of components, like <CODE>DoorNeedingSpell</CODE>, a door thatcan be locked and opened subsequently only with a spell; and<CODE>EnchantedRoom</CODE>, a room that can have unconventional items init, like magic keys or spells. How can you change <CODE>CreateMaze</CODE>easily so that it creates mazes with these new classes of objects?</P><A NAME="CreateMaze-text3"></A><P>In this case, the biggest barrier to change lies in hard-coding theclasses that get instantiated. The creational patterns providedifferent ways to remove explicit references to concrete classesfrom code that needs to instantiate them:</P><UL><A NAME="CreateMaze-text4"></A><LI>If <CODE>CreateMaze</CODE> calls virtual functions instead of constructorcalls to create the rooms, walls, and doors it requires, then you canchange the classes that get instantiated by making a subclass of<CODE>MazeGame</CODE> and redefining those virtual functions. This approachis an example of the<A HREF="pat3cfs.htm" TARGET="_mainDisplayFrame">Factory Method (107)</A> pattern.</LI><P></P><A NAME="CreateMaze-text5"></A><LI>If <CODE>CreateMaze</CODE> is passed an object as a parameter to use tocreate rooms, walls, and doors, then you can change the classes ofrooms, walls, and doors by passing a different parameter. This is anexample of the <A HREF="pat3afs.htm" TARGET="_mainDisplayFrame">Abstract Factory (87)</A> pattern.</LI><P></P><A NAME="CreateMaze-text6"></A><LI>If <CODE>CreateMaze</CODE> is passed an object that can create a new mazein its entirety using operations for adding rooms, doors, and walls tothe maze it builds, then you can use inheritance to change parts ofthe maze or the way the maze is built. This is an example of the<A HREF="pat3bfs.htm" TARGET="_mainDisplayFrame">Builder (97)</A> pattern.</LI><P></P><A NAME="CreateMaze-text7"></A><LI>If <CODE>CreateMaze</CODE> is parameterized by various prototypical room,door, and wall objects, which it then copies and adds to the maze,then you can change the maze's composition by replacing theseprototypical objects with different ones. This is an example of the<A HREF="pat3dfs.htm" TARGET="_mainDisplayFrame">Prototype (117)</A> pattern.</LI></UL><A NAME="CreateMaze-text8"></A><P>The remaining creational pattern, <A HREF="pat3efs.htm"TARGET="_mainDisplayFrame">Singleton (127)</A>, canensure there's only one maze per game and that all game objects haveready access to it—without resorting to global variables orfunctions. Singleton also makes it easy to extend or replace the mazewithout touching existing code.</P><A NAME="last"></A><P><A HREF="#top"><IMG SRC="gifsb/up3.gif" BORDER=0></A><BR><A HREF="pat3afs.htm" TARGET="_mainDisplayFrame"><IMG SRC="gifsb/rightar3.gif" ALIGN=TOP BORDER=0></A> <A HREF="pat3afs.htm" TARGET="_mainDisplayFrame">Abstract Factory</A><BR><A HREF="patcafs.htm" TARGET="_mainDisplayFrame"><IMG SRC="gifsb/leftarr3.gif" ALIGN=TOP BORDER=0></A> <A HREF="patcafs.htm" TARGET="_mainDisplayFrame">Pattern Catalog</A></P></BODY></HTML>⌨️ 快捷键说明
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