Separate the construction of a complex object from its representation so that the same construction process can create different representations.

Size: px

Start display at page:

Download "Separate the construction of a complex object from its representation so that the same construction process can create different representations."

Matthew Berry
6 years ago
Views:

1 Builder 1 Intent Separate the cnstructin f a cmplex bject frm its representatin s that the same cnstructin prcess can create different representatins. Mtivatin. Applicability The algrithm fr creating a cmplex bject shuld be independent f the parts that make up the bject and hw they're assembled. The cnstructin prcess must allw different representatins fr the bject that's cnstructed. Structure

2 Participants 2 Builder (TextCnverter) specifies an abstract interface fr creating parts f a Prduct bject. CncreteBuilder (ASCIICnverter, TeXCnverter, TextWidgetCnverter) cnstructs and assembles parts f the prduct by implementing the Builder interface. defines and keeps track f the representatin it creates. prvides an interface fr retrieving the prduct (e.g., GetASCIIText, GetTextWidget). Directr (RTFReader) cnstructs an bject using the Builder interface. Prduct (ASCIIText, TeXText, TextWidget) represents the cmplex bject under cnstructin. CncreteBuilder builds the prduct's internal representatin and defines the prcess by which it's assembled. includes classes that define the cnstituent parts, including interfaces fr assembling the parts int the final result. Cllabratins

3 Cnsequences 3 1. It lets yu vary a prduct's internal representatin. 2. It islates cde fr cnstructin and representatin. It imprves mdularity by encapsulating the way a cmplex bject is cnstructed and represented. Clients needn't knw anything abut the classes that define the prduct's internal structure; such classes dn't appear in Builder's interface. Different Directrs can reuse CncreteBuilder t build Prduct variants frm the same set f parts. 3. It gives yu finer cntrl ver the cnstructin prcess. It cnstructs the prduct step by step. Builder interface reflects the prcess f cnstructing the prduct mre than ther creatinal patterns. Implementatin Standard implementatin: An abstract Builder class that defines an peratin fr each cmpnent that a directr may ask it t create. The peratins d nthing by default. A CncreteBuilder class verrides peratins fr cmpnents it's interested in creating. Other implementatin issues: 1. Assembly and cnstructin interface. The Builder class interface must be general enugh t allw the cnstructin f prducts fr all kinds f cncrete builders. A key design issue cncerns the mdel fr the cnstructin and assembly prcess. A mdel where the results f cnstructin requests are simply appended t the prduct is usually sufficient. But smetimes yu might need access t parts f the prduct cnstructed earlier. 2. Why n abstract class fr prducts? The prducts prduced by the cncrete builders differ s greatly in their representatin. The client is in a psitin t knw which cncrete subclass f Builder is in use and can handle its prducts accrdingly. 3. Empty methds as default in Builder.

4 Sample Cde 4 class MazeBuilder { public: virtual vid BuildMaze() { virtual vid BuildRm(int rm) { virtual vid BuildDr(int rmfrm, int rmt) { virtual Maze* GetMaze() { return 0; prtected: MazeBuilder(); ; Maze* MazeGame::CreateMaze (MazeBuilder& builder) { builder.buildmaze(); builder.buildrm(1); builder.buildrm(2); builder.builddr(1, 2); return builder.getmaze(); The builder hides the internal representatin f the Maze and hw these parts are assembled t cmplete the final maze. (N walls?) class StandardMazeBuilder : public MazeBuilder { public: StandardMazeBuilder(); virtual vid BuildMaze(); virtual vid BuildRm(int); virtual vid BuildDr(int, int); virtual Maze* GetMaze(); private: Directin CmmnWall(Rm*, Rm*); Maze* _currentmaze; ; StandardMazeBuilder::StandardMazeBuilder () { _currentmaze = 0; vid StandardMazeBuilder::BuildMaze () { _currentmaze = new Maze; Maze* StandardMazeBuilder::GetMaze () { return _currentmaze;

5 vid StandardMazeBuilder::BuildRm (int n) { if (!_currentmaze->rmn(n)) { Rm* rm = new Rm(n); _currentmaze->addrm(rm); 5 rm->setside(nrth, new Wall); rm->setside(suth, new Wall); rm->setside(east, new Wall); rm->setside(west, new Wall); vid StandardMazeBuilder::BuildDr (int n1, int n2) { Rm* r1 = _currentmaze->rmn(n1); Rm* r2 = _currentmaze->rmn(n2); Dr* d = new Dr(r1, r2); r1->setside(cmmnwall(r1,r2), d); r2->setside(cmmnwall(r2,r1), d); Maze* maze; MazeGame game; StandardMazeBuilder builder; game.createmaze(builder); maze = builder.getmaze(); class CuntingMazeBuilder : public MazeBuilder { public: CuntingMazeBuilder(); virtual vid BuildMaze(); virtual vid BuildRm(int); virtual vid BuildDr(int, int); virtual vid AddWall(int, Directin); vid GetCunts(int&, int&) cnst; private: int _drs; int _rms; ; CuntingMazeBuilder::CuntingMazeBuilder () { _rms = _drs = 0; vid CuntingMazeBuilder::BuildRm (int) { _rms++; vid CuntingMazeBuilder::BuildDr (int, int) { _drs++; vid CuntingMazeBuilder::GetCunts (int& rms, int& drs) cnst { rms = _rms; drs = _drs;

6 int rms, drs; MazeGame game; CuntingMazeBuilder builder; 6 game.createmaze(builder); builder.getcunts(rms, drs); cut << "The maze has " << rms << " rms and " << drs << " drs" << endl; Related Patterns Abstract Factry is similar t Builder fr cnstructing cmplex bjects. The primary difference is that the Builder pattern fcuses n cnstructing a cmplex bject step by step. Abstract Factry's emphasis is n families f prduct bjects (either simple r cmplex). Cmpsite is what the builder ften builds.

Software Quality Management

Software Quality Management 2004-2005 Marco Scotto (Marco.Scotto@unibz.it) Course Outline Creational Patterns Singleton Builder 2 Design pattern space Purpose Creational Structural Behavioral Scope Class Factory Method Adapter Interpreter