Visitor. Intent. Motivation

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Junior Simmons
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1 Visitr 1 Intent Represent an peratin t be perfrmed n the elements f an bject structure. Visitr lets yu define a new peratin withut changing the classes f the elements n which it perates. Mtivatin

2 Applicability 2 an bject structure cntains many classes f bjects with differing interfaces, and yu want t perfrm peratins n these bjects that depend n their cncrete classes. many distinct and unrelated peratins need t be perfrmed n bjects in an bject structure, and yu want t avid "plluting" their classes with these peratins. Visitr lets yu keep related peratins tgether by defining them in ne class. When the bject structure is shared by many applicatins, use Visitr t put peratins in just thse applicatins that need them. the classes defining the bject structure rarely change, but yu ften want t define new peratins ver the structure. Changing the bject structure classes requires redefining the interface t all visitrs, which is ptentially cstly. Structure If the bject structure classes change ften, then it's prbably better t define the peratins in thse classes. Participants Visitr (NdeVisitr) declares a Visit peratin fr each class f CncreteElement in the bject structure. The peratin's name and signature identifies the class that sends the Visit request t the visitr.

3 3 CncreteVisitr (TypeCheckingVisitr) implements each peratin declared by Visitr. Each peratin implements a fragment f the algrithm defined fr the crrespnding class f bject in the structure. CncreteVisitr prvides the cntext fr the algrithm and stres its lcal state. This state ften accumulates results during the traversal f the structure. Element (Nde) defines an Accept peratin that takes a visitr as an argument. CncreteElement (AssignmentNde,VariableRefNde) implements an Accept peratin that takes a visitr as an argument. ObjectStructure (Prgram) can enumerate its elements. may prvide a high-level interface t allw the visitr t visit its elements. may either be a cmpsite r a cllectin such as a list r a set. Cllabratins A client that uses the Visitr pattern must create a CncreteVisitr bject and then traverse the bject structure, visiting each element with the visitr. When an element is visited, it calls the Visitr peratin that crrespnds t its class. The element supplies itself as an argument t this peratin t let the visitr access its state, if necessary. Cnsequences 1. Visitr makes adding new peratins easy. 2. A visitr gathers related peratins and separates unrelated nes. Related behavir isn't spread ver the classes defining the bject structure; it's lcalized in a visitr. Unrelated sets f behavir are partitined in their wn visitr subclasses. Any algrithm-specific data structures can be hidden in the visitr.

4 3. Adding new CncreteElement classes is hard. 4. Each new CncreteElement gives rise t a new abstract peratin n Visitr and a crrespnding implementatin in every CncreteVisitr class. The key cnsideratin in applying the Visitr pattern is whether yu are mstly likely t change the algrithm applied ver an bject structure r the classes f bjects that make up the structure. 4. Visiting acrss class hierarchies. An iteratr can't wrk acrss bject structures with different types f elements. Visitr des nt have this restrictin. 5. Accumulating state. 6. Breaking encapsulatin. The pattern ften frces yu t prvide public peratins that access an element's internal state, which may cmprmise its encapsulatin. Implementatin Each bject structure will have an assciated Visitr class. class Visitr { virtual vid VisitElementA(ElementA*); virtual vid VisitElementB(ElementB*); // and s n fr ther cncrete elements prtected: Visitr(); ; Each class f CncreteElement implements an Accept peratin that calls the matching Visit... peratin n the visitr fr that CncreteElement. class Element { virtual ~Element(); virtual vid Accept(Visitr&) = 0; prtected: Element(); ; class ElementA : public Element { ElementA(); virtual vid Accept(Visitr& v) { v.visitelementa(this); ;

5 5 class ElementB : public Element { ElementB(); virtual vid Accept(Visitr& v) { v.visitelementb(this); ; class CmpsiteElement : public Element { virtual vid Accept(Visitr&); List<Element*>* _children; ; vid CmpsiteElement::Accept (Visitr& v) { ListIteratr<Element*> i(_children); fr (i.first();!i.isdne(); i.next()) { i.currentitem()->accept(v); v.visitcmpsiteelement(this); Duble dispatch. single-dispatch: tw criteria determine which peratin will fulfill a request: the name f the request and the type f receiver. Duble-dispatch means the peratin that gets executed depends n the kind f request and the types f tw receivers. Accept is a duble-dispatch peratin. Its meaning depends n tw types: the Visitr's and the Element's. Duble-dispatching lets visitrs request different peratins n each class f element. Wh is respnsible fr traversing the bject structure? A visitr must visit each element f the bject structure. in the bject structure, in the visitr, r in a separate iteratr bject

6 Sample Cde 6 class Equipment { virtual ~Equipment(); cnst char* Name() { return _name; virtual Watt Pwer(); virtual Currency NetPrice(); virtual Currency DiscuntPrice(); virtual vid Accept(EquipmentVisitr&); prtected: Equipment(cnst char*); cnst char* _name; ; class EquipmentVisitr { virtual ~EquipmentVisitr(); virtual vid VisitFlppyDisk(FlppyDisk*); virtual vid VisitCard(Card*); virtual vid VisitChassis(Chassis*); virtual vid VisitBus(Bus*); // and s n fr ther cncrete subclasses f Equipment prtected: EquipmentVisitr(); ; vid FlppyDisk::Accept (EquipmentVisitr& visitr) { visitr.visitflppydisk(this); vid Chassis::Accept (EquipmentVisitr& visitr) { fr ( ListIteratr i(_parts);!i.isdne(); i.next() ) { i.currentitem()->accept(visitr); visitr.visitchassis(this); class PricingVisitr : public EquipmentVisitr { PricingVisitr(); Currency& GetTtalPrice(); virtual vid VisitFlppyDisk(FlppyDisk*); virtual vid VisitCard(Card*); virtual vid VisitChassis(Chassis*); virtual vid VisitBus(Bus*); //... Currency _ttal; ;

7 7 vid PricingVisitr::VisitFlppyDisk (FlppyDisk* e) { _ttal += e->netprice(); vid PricingVisitr::VisitChassis (Chassis* e) { _ttal += e->discuntprice(); class InventryVisitr : public EquipmentVisitr { InventryVisitr(); Inventry& GetInventry(); virtual vid VisitFlppyDisk(FlppyDisk*); virtual vid VisitCard(Card*); virtual vid VisitChassis(Chassis*); virtual vid VisitBus(Bus*); //... Inventry _inventry; ; vid InventryVisitr::VisitFlppyDisk (FlppyDisk* e) { _inventry.accumulate(e); vid InventryVisitr::VisitChassis (Chassis* e) { _inventry.accumulate(e); Equipment* cmpnent; InventryVisitr visitr; cmpnent->accept(visitr); cut << "Inventry " << cmpnent->name() << visitr.getinventry(); Related Patterns Cmpsite: Visitrs can be used t apply an peratin ver an bject structure defined by the Cmpsite pattern. Interpreter : Visitr may be applied t d the interpretatin.

Sometimes it's necessary to issue requests to objects without knowing anything about the operation being requested or the receiver of the request.

Sometimes it's necessary to issue requests to objects without knowing anything about the operation being requested or the receiver of the request. Cmmand 1 Intent Encapsulate a request as an bject, thereby letting yu parameterize clients with different requests, queue r lg requests, and supprt undable peratins. Als Knwn As Actin, Transactin Mtivatin