Composition class is responsible for maintaining and updating the linebreaks of text displayed in a text viewer.

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1 Strategy 1 Intent Define a family f algrithms, encapsulate each ne, and make them interchangeable. Strategy lets the algrithm vary independently frm clients that use it. Als Knwn As Plicy Mtivatin The line breaking prblem Cmpsitin class is respnsible fr maintaining and updating the linebreaks f text displayed in a text viewer. SimpleCmpsitr implements a simple strategy that determines linebreaks ne at a time. TeXCmpsitr implements the TeX algrithm fr finding linebreaks. This strategy tries t ptimize linebreaks glbally, that is, ne paragraph at a time. ArrayCmpsitr implements a strategy that selects breaks s that each rw has a fixed number f items. Applicability many related classes differ nly in their behavir. Strategies prvide a way t cnfigure a class with ne f many behavirs. yu need different variants f an algrithm. an algrithm uses data that clients shuldn't knw abut. Use the Strategy pattern t avid expsing cmplex, algrithm-specific data structures. a class defines many behavirs, and these appear as multiple cnditinal statements in its peratins. Instead f many cnditinals, mve related cnditinal branches int their wn Strategy class.

2 Structure 2 Participants Strategy (Cmpsitr) declares an interface cmmn t all supprted algrithms. Cntext uses this interface t call the algrithm defined by a CncreteStrategy. CncreteStrategy (SimpleCmpsitr, TeXCmpsitr, ArrayCmpsitr) implements the algrithm using the Strategy interface. Cntext (Cmpsitin) is cnfigured with a CncreteStrategy bject. maintains a reference t a Strategy bject. may define an interface that lets Strategy access its data. Cllabratins Strategy and Cntext interact t implement the chsen algrithm. A cntext may pass all data required by the algrithm t the strategy when the algrithm is called. Alternatively, the cntext can pass itself as an argument t Strategy peratins. That lets the strategy call back n the cntext as required. A cntext frwards requests frm its clients t its strategy. Clients usually create and pass a CncreteStrategy bject t the cntext; thereafter, clients interact with the cntext exclusively. T here is ften a family f CncreteStrategy classes fr a client t chse frm. Cnsequences Families f related algrithms. An alternative t subclassing Cntext class Strategies eliminate cnditinal statements.

3 vid Cmpsitin::Repair () { switch (_breakingstrategy) { case SimpleStrategy: CmpseWithSimpleCmpsitr( break; case TeXStrategy: CmpseWithTeXCmpsitr( break; } // merge results with existing cmpsitin, if necessary } 3 vid Cmpsitin::Repair () { _cmpsitr->cmpse( // merge results with existing cmpsitin, if necessary } A chice f implementatins. Strategies can prvide different implementatins f the same behavir. 1. Clients must be aware f different Strategies. 2. Cmmunicatin verhead between Strategy and Cntext. 3. Increased number f bjects. Smetimes yu can reduce this verhead by implementing strategies as stateless bjects as flyweights that cntexts can share. Implementatin 1. Defining the Strategy and Cntext interfaces. The Strategy and Cntext interfaces must give a CncreteStrategy efficient access t any data it needs frm a cntext, and vice versa. One apprach is t have Cntext pass data in parameters t Strategy peratins in ther wrds, take the data t the strategy. Anther technique has a cntext pass itself as an argument, and the strategy requests data frm the cntext explicitly. Alternatively, the strategy can stre a reference t its cntext, eliminating the need t pass anything at all. Cntext must define a mre elabrate interface t its data, which cuples Strategy and Cntext mre clsely. The needs f the particular algrithm and its data requirements will determine the best technique.

4 Static Strategies as template parameters. 4 template <class AStrategy> class Cntext { vid Operatin() { thestrategy.dalgrithm( } private: AStrategy thestrategy; class MyStrategy { vid DAlgrithm( Cntext<MyStrategy> acntext; Making Strategy bjects ptinal (with default behavir) Sample Cde class Cmpsitin { Cmpsitin(Cmpsitr* vid Repair( private: Cmpsitr* _cmpsitr; Cmpnent* _cmpnents; int _cmpnentcunt; int _linewidth; int* _linebreaks; int _linecunt; // the list f cmpnents // the number f cmpnents // the Cmpsitin's line width // the psitin f linebreaks // in cmpnents // the number f lines class Cmpsitr { virtual int Cmpse( Crd natural[], Crd stretch[], Crd shrink[], int cmpnentcunt, int linewidth, int breaks[] ) = 0; prtected: Cmpsitr( vid Cmpsitin::Repair () { Crd* natural; Crd* stretchability; Crd* shrinkability; int cmpnentcunt; int* breaks; // prepare the arrays with the desired cmpnent sizes // determine where the breaks are: int breakcunt; breakcunt = _cmpsitr->cmpse( natural, stretchability, shrinkability, cmpnentcunt, _linewidth, breaks

5 } // lay ut cmpnents accrding t breaks 5 class SimpleCmpsitr : public Cmpsitr { SimpleCmpsitr( virtual int Cmpse( Crd natural[], Crd stretch[], Crd shrink[], int cmpnentcunt, int linewidth, int breaks[] class TeXCmpsitr : public Cmpsitr { TeXCmpsitr( virtual int Cmpse( Crd natural[], Crd stretch[], Crd shrink[], int cmpnentcunt, int linewidth, int breaks[] class ArrayCmpsitr : public Cmpsitr { ArrayCmpsitr(int interval virtual int Cmpse( Crd natural[], Crd stretch[], Crd shrink[], int cmpnentcunt, int linewidth, int breaks[] Cmpsitin* quick = new Cmpsitin(new SimpleCmpsitr Cmpsitin* slick = new Cmpsitin(new TeXCmpsitr Cmpsitin* icnic = new Cmpsitin(new ArrayCmpsitr(100) Knwn Uses RTL System fr cmpiler cde ptimizatin: Strategies define different register allcatin schemes (RegisterAllcatr) and instructin set scheduling plicies (RISCscheduler, CISCscheduler). ET++SwapsManager calculatin engine framewrk: A family f CncreteStrategy classes fr generating cash flws, valuing swaps, and calculating discunt factrs. Bch cllectin cmpnents: Memry allcatin strategies: managed (allcatin ut f a pl), cntrlled (allcatins/deallcatins are prtected by lcks), and unmanaged (the nrmal memry allcatr).. RApp is a system fr integrated circuit layut Ruter is a Strategy class.

6 Brland's ObjectWindws Strategies in dialgs bxes t ensure that the user enters valid data. ObjectWindws: uses Validatr bjects t encapsulate validatin strategies. 6 Related Patterns Flyweight: Strategy bjects ften make gd flyweights.

Provide a unified interface to a set of interfaces in a subsystem. Facade defines a higher-level interface that makes the subsystem easier to use.

Provide a unified interface to a set of interfaces in a subsystem. Facade defines a higher-level interface that makes the subsystem easier to use. Facade 1 Intent Prvide a unified interface t a set f interfaces in a subsystem. Facade defines a higher-level interface that makes the subsystem easier t use. Mtivatin Applicability Use the Facade pattern