Chapter12Polymorphism

12.1WhatispolymorphismThewordpolymorphismisderivedfromaGreekwordmeaning“manyforms”.Polymorphismisoneofthemostimportantfeaturesinobject-orientedprogrammingandreferstotheabilityofdifferentobjectstoresponddifferentlytothesamecommand(or‘message’inobject-orientedprogrammingterminology).12.1WhatispolymorphismPolymorphismisusedwidelyinoureverydaylanguage.Forexample,thecommand‘open’meansdifferentthingswhenappliedtodifferentobjects.Openingabankaccountisverydifferentfromopeninga window,whichisdifferentfromopeningawindowonacomputerscreen.Thecommand(‘open’)isthesame,butdependingonwhatitisappliedto(theobject)theresultantactionsaredifferent.ProgramExampleP12A12.1WhatispolymorphismProgramExampleP12A12.1WhatispolymorphismItcanbeseenfromtheoutputthattheobjectsHALandPChaverespondeddifferentlytothesamehello()commandsenttothemonlines28and29.12.1WhatispolymorphismPolymorphismcanbedividedintotwobroadcategories:static(orcompile-time)anddynamic(orrun-time)polymorphism.Staticpolymorphismoccurswhentheprogramisbeingcompiled,dynamicpolymorphismwhentheprogramisactuallyrunning.C++hasthreestaticpolymorphismmechanisms:functionoverloading(chapter7)operatoroverloading(chapter10)andtemplates(chapter13).Functionswiththesamenamesanddifferentparameterlistsmeanfunctionoverloading.12.2VirtualfunctionsNormally,whichfunctionacallreferstoismadeatcompiletime.Thecompilerknowswhichfunctiontocallbasedontheobjectthatcallsit.Thisiscalledstaticorearlybinding.Indynamicorlatebinding,itisleftuntilrun-timetodeterminewhichfunctionshouldbecalled.Thisdecisionisbasedontheobjectmakingthecall.12.2VirtualfunctionsC++usesvirtualfunctionstoimplementdynamicbinding.Virtualfunctionsallowpolymorphismtoworkinallsituations.Thekeywordvirtualisoptionalinthederivedclasses,althoughitisprobablyagoodideatoincludeitforthesakeofclarity.Thisexampleclearlyshowsthatthevirtualfunctionarea()andtheobjectcallingitarenotboundtogetheruntilrun-timeProgramExampleP12D12.2VirtualfunctionsProgramExampleP12D12.2VirtualfunctionsProgramExampleP12D12.2VirtualfunctionsProgramExampleP12D12.2Virtualfunctions12.2VirtualfunctionsLine70definesptrtobeapointertothebaseclasscircle.Normally,apointertoonetypeofdatacannotbeusedtopointtodataofanothertype,e.g.apointertoanintdatatypecannotpointtoafloatdatatype.However,apointertoabaseclasscanalsobeusedasapointertoanobjectofaclassderivedfromthatbaseclass.Thisiswhyptrcanbeusedinlines82,88and92asapointertoanyofthenewlycreatedobjects.12.2VirtualfunctionsLine94usesptr(whichcanbepointingtoanyoftheobjects)tocallthememberfunctionmessage_area().12.2Virtualfunctions12.2.1WhentousevirtualfunctionsTherearememoryandexecutiontimeoverheadsassociatedwithvirtualfunctions,sobejudiciousinthechoiceofwhetherabaseclassfunctionisvirtualornot.Ingeneral,declareabaseclassmemberfunctionasvirtualifitmaybeoverriddeninaderivedclass.12.2.2OverridingandoverloadingOverloadingisacompilertechniquethatdistinguishesbetweenfunctionswiththesamenamebutwithdifferentparameterlists.Functionoverloadingiscoveredinchapter7.Overridingoccursininheritancewhenamemberfunctionofaderivedclasshasthesamenameandthesameparametersasamemberfunctionofitsbaseclass.12.3AbstractbaseclassesAbaseclassthatcontainsapurevirtualfunctioniscalledanabstractbaseclass.virtualfunction.Thebaseclassmemberfunctionhasnocodeandisassignedto0.virtualvoiddisplay_details()=0;Aclassmemberfunctiondefinedinthiswayiscalledapurevirtualfunction.Apurevirtualfunctionisrequiredbythecompilerbutdoesn’tactuallydoanything.Apurevirtualfunctionisoverriddeninallthederivedclassesandhencethereisnoneedtoimplementit.12.3AbstractbaseclassesAbstractbaseclasses(ABCs)arenotusedtocreateobjects,butexistsolelyasabaseforderivingotherclasses.Itisnotpossibletodefineanobjectofanabstractbaseclass,i.e.objectsofanabstractbaseclasscannotbeinstantiated.Althoughaclasshierarchydoesnothavetocontainanabstractbaseclass,typicallyoneisdefinedduringthefirststageofdevelopingaclasshierarchy.ShapeRectangleSquareTriangleCircle????inherits(isa)SumtheareaofdifferentShapes?Example:Shapeclasshierarchy12.3AbstractbaseclassesclassShape//Abstract

{

public:

//PurevirtualFunction

virtualvoidget_area()=0;};AclasswithoneormorepurevirtualfunctionsisanAbstractClassShapemakessenseonlyasabaseofsomeclassesderivedfromit.Servesasa“category”O(jiān)bjectsofabstractclasscan’tbecreatedShapes;//error:variableofanabstractclassAbstractClasses

&PureVirtualFunctions12.3AbstractbaseclassesCircle::Circle(doubler){radius=r;}doubleCircle::get_area(){constdoublepi=3.14;returnpi*radius*radius;}DerivedClassCircleclassCircle:publicShape{public: Circle(doubler); doubleget_area();private: doubleradius;};12.3AbstractbaseclassesRectangle::Rectangle(doublew,doublel){width=w;length=l;}doubleRectangle::get_area(){returnwidth*length;}classRectangle:publicShape{public: Rectangle(doublew,doublel); doubleget_area();private: doublewidth,length;};DerivedClassRectangle12.3AbstractbaseclassesHowtosumdifferentShape?intmain(){Circlecircle(3);Rectanglerect(4,5);doublesum=0;sum+=circle.get_area();sum+=rect.get_area();return0;}StaticBinding12.3AbstractbaseclassesHowtosumdifferentShape?intmain(){Shape*shapes[2]={newCircle(3),newRectangle(4,5)};doublesum=0;for(inti=0;i<2;i++){sum+=shapes[i]->get_area();}cout<<sum<<endl;return0;}DynamicBinding12.3AbstractbaseclassesProgrammingpitfalls1.Itiseasytogetconfusedbetweenvirtualmemberfunctionsandvirtualbaseclasses.Althoughbothareusedinthecontextofinheritance,theyarenotinfactrelated.Perhapsabetternameforvirtualfunctionswouldbe‘run-time-replaceable’functions?2.Anabstractbaseclassmusthaveatleastonederivedclass.Itisanerrortoattempttodefineanobjectofanabstractbaseclass.3.Avirtualmemberfunctioninabaseclasscanonlybeoverriddenin

aderivedclassifthememberfunctionintheder