C++面向對象程序設計雙語教程（第3版）課件 ch07Polymorphism and Virtual Functions、ch08Templates

PolymorphismandVirtualFunctionsC++面向對象程序設計雙語教程(第3版)Chapter701PolymorphismThekeyideabehindOOPispolymorphism.PolymorphismisderivedfromaGreekwordmeaning“manyforms”.Wespeakthetypesrelatedbyinheritanceaspolymorphictypes，becauseinmanycaseswecanusethe"manyforms3ofaderivedorbasetypeinterchangeably.IntroductiontoPolymorphismPolymorphismInprogramminglanguages，polymorphismmeans

that

some

codesoroperationsor

objectsbehavedifferentlyindifferentcontexts.Forexample，the+operatorinC++implementtheadditionoperationwithdifferentdataordifferenttypes.IntroductiontoPolymorphismPolymorphismPolymorphismisakeyfeatureofobject-orientedprogrammingthatallowsthevaluesofdifferentdata

typestobehandledbyusingauniforminterface.多態(tài)性是一個面向對象程序設計的關鍵特性，它允許使用統(tǒng)一的接口處理不同數(shù)據(jù)類型。PolymorphismThepurposeofpolymorphismistoimplementastyleofprogrammingcalledmessage-passingintheliterature，inwhichtheobjectsofvarioustypesdefineacommoninterfaceofoperationsforusers.PolymorphismTypically，polymorphismoccurswhenthereisaclasshierarchyinwhichtheclassesarerelatedbyinheritance.PolymorphismBindingWhenaC++programisexecuted，itexecutessequentially，beginningatthetopof

functionmain.Whenafunctionealliseneountered，thepointofexecutionjumpstotheinitial

pointofthefunctionbeingcalled.HowdoestheCPUknowtodothis？PolymorphismWhenaprogramiscompiled，thecompilerconvertseachstatementinyourC++programintooneormorelinesofmachinelanguage.Eachlineofmachinelanguageisgivenitsownuniquesequentialaddress.Thisisnodifferentforfunctionswhenafunctionisencountered，itisconvertedintomachinelanguageandgiventhenextavailableaddress.Thus，eachfunction

endsupwithauniquemachinelanguageaddress.PolymorphismBindingisaprocessthatisusedtoconvertidentifiers

intomachinelanguageaddresses.Inotherwords，bindingisanassociation，suchasbetweenidentifiers

andoperations.Therearetwotypesofbindingaccordingtothetimeatwhichabindingtakesplace，i.e.staticbindingand

dynamicbinding.綁定是一種用于將標識符（如變量和函數(shù)名）轉換成機器語言地址的過程。換句話說，綁定是一種關聯(lián)，例如在標識符（變量或函數(shù)名）和操作（類型或特定函數(shù)體）之間的關聯(lián)。根據(jù)綁定發(fā)生的時間，有兩種類型的綁定，即靜態(tài)綁定和動態(tài)綁定。PolymorphismStaticBindingBydefault，Ct+matchesafunctioncallwiththecorrectfunctiondefinitionatcompiletime.Thisiscalledstatiebinding.Thiskindofbindingisalsoknownaseompile-timebinding.Forexample，theoperatoroverloadingandthefunctionoverloadingmentionedinthepreviouschaptersarethatthecompilerassociatestheoperatorfunctionsandoverloadedfunctionswiththeirdefinitionsatcompile-time.PolymorphismEachfunctionhasauniqueaddress.Thus，whenthecompilerencountersafunctioncall，itreplacesthefunctioncallwithamachinelanguageinstructionthattellstheCPUtojumptotheaddressofthefunction.PolymorphismWhenthecompilermatchesafunctioncallwiththecorreetfunctiondefinitionatrun-time，thisiscalleddynamiebinding.Thecodeassociatedwiththeproceduredoesnotknowuntiltheprogramisexecuted.Thiskindofbindingisalsoknownasrun-timebinding.Youcandeclare

afunctionwiththevirtualkeywordifyouwantthecompilertousedynamicbindingforthatspecificfunction.DynamicBindingPolymorphism02VirtualFunctionsAvirtualfunctionisamemberfunctionofthebaseclassandisredefinedbythederivedclass.Thecompilerwillguaranteethecorrectcorrespondencebetweenobjectsandthefunctionsappliedtothem.Avirtualfunctioniscreatedusingthekeywordvirtualwhichprecedesthenameofthefunction.VirtualFunctionsForexample，supposeagraphicsprogramincludesseveraldifferentshapes：acubeandacuboid，andsoon，asshowninFigure7-1.VirtualFunctionsToenablethiskindofbehavior，wedeclarefunctionvolumeinthebaseclassasavirtualfunction，andweoverridefunctionvolumeineachofthederivedclassestocalculatethevolumeofappropriateshape.VirtualFunctionsWhenaderivedclassinheritstheclasscontainingthevirtualfunction，ithastheabilityto

redefinethevirtualfunctions.Avirtualfunctionhasdifferentfunctionalityinthederivedclass.Thevirtualfunetionimplementsthephilosophyofoneinterfaceandmultiplemethods.VirtualFunctionsVirtualfunetionscanbeaccessedbyusingabaseclasspointer.Apointertothebaseclasscanbecreated.Abaseclasspointercancontaintheaddressofthederivedobjectasthederivedobjectcontainsthesubsetofabaseclassobject.VirtualFunctionsEveryderivedclassisalsoabaseclass.Whenabaseclasspointercontainstheaddressofthederivedclassobject，atruntime，whichversion

ofthevirtualfunctionbeingcalleddependsonthetypeofobjectcontainedbythepointer.

Hereisaprogramwhichillustratestheworkingofvirtualfunctions.VirtualFunctionsOnceafunctionisdeclaredasvirtual，itremainsvirtualallthewaydowntheinheritancehierarchy

fromthatpoint，evenifthatthefunctionisnotexplicitlydeclaredasvirtualwhenaclassoverrides

it.Ifthememberfunctioninthederivedclassisvirtual，thekeywordvirtualcanbeomittedinthe

derivedclass.Butthekeywordvirtualcannotbeomittedinthebaseclass.VirtualFunctionsExtensibilityIfthevolumefunetionisdefinedasvirtualinthebaseclass，youcanaddasmanynewclasstypesasyouwantwithoutchangingthefnfunction.Inawell-designedOOPprogram，mostorallyourfunetionswillfollowthemodeloffunctionfnandcommunicateonlywiththebase-classinterface.VirtualFunctionsExtensibilitySuchaprogramisextensiblebecauseyoucanaddnewfunctionalitybyinheritingnewdatatypesfromthecommonbaseclass.Thefunetionsthatmanipulatethebase

classinterfacewillnotneedtobechangedatalltoaccommodatethenewclasses.HereistheShapeexamplewithmorevirtualfunctionsandanumberofnewclasses，allofwhichworkcorrectlywiththeold，unchangedfnfunction.VirtualFunctionsTheextensibilityofclasshierarchymentionedinExample7-3isshowninFigure7-2.VirtualFunctionsHowcandynamiebindinghappenbyvirtualfunctions？Alltheworkgoesonbehindthescenesbythecompiler，whichinstallsthenecessarydynamicbindingmechanismwhenyouaskitto（youareaskedtocreatevirtualfunctions）.PrincipleofVirtualFunctionsVirtualFunctionsBecauseprogrammersoftenbenefitfromunderstandingthemechanismofvirtualfunctionsinC++，thissectionwillelaborateonthe

waythecompilerimplementsthismechanism，Thevirtualkeywordtellsthecompilerthatitshouldnotperformstaticbinding.PrincipleofVirtualFunctionsVirtualFunctionsIneachclasswithvirtualfunctions，itsecretlyplacesapointer，calledthevpointer（abbreviatedasVPTR），whichpoints

totheVTABLEforthatobject，asshowninFigure7-3.VirtualFunctionsVirtualDestructorsOnethingrecommendedfortheclasswithpointermembervariablesisthattheclassshouldhavethedestructor（seeSection4.6.2）.Thedestructorisautomaticallyexecutedwhentheclassobjectgoesoutofthescope.Then，iftheobjectcreatesdynamicobjects，thedestructorcanbedesignedtodeallocatethestorageforthem.VirtualFunctionsThebaseclassdestructorshouldalwaysbevirtual.Supposeyouusethedeleteoperatorwithabaseclasspointertoaderivedclassobjecttodestroythederivedclassobject.Ifthebaseclassdestructorisnotvirtual，butlikeanormalmemberfunction，thenthedeleteoperator

callsthedestruetorforthebaseclass，butnotthedestructorforthederivedclass.VirtualFunctionsIfnoneofthedestructorshasanythingimportanttodo（likedeallocatingmemoryobtainedwith

delete），thenvirtualdestructorsaren'tnecessary.But，ingeneral，toensurethatderivedclassobjects

aredestroyedproperly，youshouldmakevirtualthedestruetorsinallbaseclasses.VirtualFunctionsWehaveintroducedfunctionoverloadingandfunctionoverridingintheprevioussections.Thesetermsaresimilar，andtheydosimilarthings.Whenyouoverrideamemberfunetionoftheclass，youcreateamemberfunetioninaderivedclasswiththesamenameasafunetioninthebaseclassandthesamesignature.FunctionOverloadingandFunctionOverridingVirtualFunctionsIfwedefineafunctionasvirtualinthebaseclass，thefunctionwiththesame

name

andsamesignaturebutnokeywordvirtualinthederivedclassisalsothevirtualfunctionbecauseofoverriding.VirtualFunctions03AbstractBaseClassesOfteninadesign，youwantthebaseclasstopresentonlyaninterfaceforitsderivedclasses.Thatis，youdon'tallowanyonetocreateanobjectofthebaseclass，butonlywanttoupcasttoitsothatitsinterfacecanbeused.Thisisaccomplishedbymakingthatclassabstraet.AbstractBaseClassesAbstractbaseclassesactastheexpressionsofgeneralconceptsfromwhichmorespecific

classescanbederived.Ifanabstractbaseclasscannotbeinstantiated，itexistsextensivelyforinheritanceandit

mustbeinherited.AbstractBaseClassesTherearescenariosinwhichitisusefultodefineaclassthatisnotintendedtobeinstantiatedbecausesuchclassesnormallyareusedasbase-classesininheritance

hierarchies.AbstractBaseClassesThisclassmustbeinherited.Thisclassismostlyusedasabaseclass.Youcannotcreatean

objectofanabstractelasstype；however，youcanusepointersandreferencestoabstractclasstypes.AbstractBaseClassesAclassthatcontainsatleastonepurevirtualfunctionisconsideredasanabstractclass.Theclassesderivedfromtheabstractclassmustimplementthepurevirtualfunctionorthey，too，areabstractclasses.AbstractBaseClassesApurevirtualfunctionisafunctionwhichcontainsnodefinitioninthebaseclass.Youdeclareapurevirtualfunctionbyusingapurespecifier（=0）inthedeclarationofavirtualmemberfunctionintheclassdeclaration.純虛函數(shù)是一個在基類中沒有定義的函數(shù)。在類聲明時使用pure說明符（=0）將一個虛成員函數(shù)聲明為純虛函數(shù)。AbstractBaseClassesAclasswhichcontainsoneormorepurevirtualfunctioniscalledanabstractbaseclass.Abstractbase

classcsactasexpressionsofgeneralconceptsfromwhichmorespecificclassescanbederived.包含一個或多個純虛函數(shù)的類稱為抽象基類。抽象基類作為通用概念的表示，從中可以導出更多具體的類。AbstractBaseClassesThestatementinLine6tellsthecompilertoreserveaslotforafunetionintheVTABLE，butnottoputanaddressinthatparticularslot.Evenifonlyonefunctioninaclassisdeclaredaspurevirtual，theVTABLEisincomplete.AbstractBaseClassesIftheVTABLEforaclassisincomplete，whatisthecompilersupposedtodowhen

someonetriestomakeanobjectofthatclass？Itcannotsafelycreateanobjectofanabstractclass，soyougetanerrormessagefromthecompiler.Thus，thecompilerguaranteesthepurityoftheabstractclass.Bymakingaclassabstract，youcanensurethattheclientprogrammercannotmisuseit.AbstractBaseClassesPurevirtualfunctionsarehelpfulbecausetheymakeexplicittheabstractnessofaclass

andtellboththeuserandthecompilerhowitwasintendedtobeused.AbstractBaseClasses04CaseStudy:AMini-SystemWedesignamini-systemthatineludesdifferenttypesofpeopleintheuniversity.Therearefiveclasses，thatis，Person，Student，Employee，F(xiàn)acultyandStaff，tobedefined.AclassnamedPersonhastwoderivedclassesnamedStudentandEmployee.MakethederivedclassesFaculty

andStaffofEmployee.CaseStudy:AMini-SystemApersonhasaname，address，phonenumberande-mailaddress.A

studenthasaclassstatus（freshmen，sophomore，junior，orsenior）.Anemployeehasanoffice，salary，anddate-hired.DefineaclassnamedDatethatcontainsthefieldsyear，monthandday.CaseStudy:AMini-SystemAfacultymemberhasofficehoursandarank.Astaffmemberhasatitle.Defineaconstantvirtualto

StringfunctioninthePersonclassandoverrideitineachclasstodisplaytheclassnameand

theperson'sname.CaseStudy:AMini-SystemTheUMLdiagramofthesesixclassesisdescribedinFigure7-4.CaseStudy:AMini-System謝謝觀看C++面向對象程序設計雙語教程(第3版)TemplatesC++面向對象程序設計雙語教程(第3版)Chapter801Introduction

to

TemplatesManyC++programsusecommondatastructureslikestacks，queuesandlists.Aprogrammayrequireaqueueofcustomersandaqueueofmessages.Onecouldeasilyimplementaqueueofcustomers，thentaketheexistingcodeandimplementaqueueofmessages.IntroductiontoTemplatesTheprogramgrows，andnowthereisaneedforaqueueoforders.Sojusttakethequeueofmessagesandconvertthattoaqueueoforders（copy，paste，find，andreplace）.Needtomakesomechangestothequeueimplementation？IntroductiontoTemplatesNotaveryeasytask，sincethecodehasbeenduplicatedinmanyplaces.Reinventingsourcecodesisnotanintelligentapproachinanobject-orientedenvironmentwhichencouragesreusability.IntroductiontoTemplatesItseemstomakemoresensetoimplementaqueuethatcancontainanyarbitrarytyperatherthanduplicatingcode.Howdoesonedothat？Theansweristousetypeparameterization，ormorecommonlyreferredtoastemplates.IntroductiontoTemplatesToavoidrewritingcodethatwouldbeidenticalexceptfordifferenttypes.Sometimesyoudonotsimplyrelyonimplicittypeconversionorpromotionmatch，andyoucannotstuffeverythingintoaclasshierarchy.IntroductiontoTemplatesGenericprogrammingisastyleofprogramminginwhichalgorithmsarewrittenintermsoftypes

to-be-specified-later.Thenthesetypesworkingasparametersareinstantiatedwhenspecifictypesareneeded.Templatesprovidedirectsupportforgenericprogramming，thatis，programmingusingtypesasparameters.TheC++templatemechanismallowsatypetobeaparameterinthedefinitionofaclassorafunction.IntroductiontoTemplatesC++providestwokindsoftemplates：classtemplatesandfunctiontemplates.Usefunctiontemplatestowritegenericfunctionsthatcanbeusedwitharbitrarytypes.Forexample，onecanwritesearehingandsortingroutinesthatcanbeusedwithanyarbitrarytype.ThegenericalgorithmsintheStandardTemplateLibrary（i.e.STL）havebeenimplementedasfunctiontemplates，andthecontainershavebeenimplementedasclasstemplates.IntroductiontoTemplates02FunctionTemplatesLet'simaginethatwewanttowriteafunctiontocomparetwovaluesandindicatewhetherthefirstislessthan，equalto，orgreaterthanthesecondone.Inpractice，wewouldwanttodefineseveralsuchfunctions，eachofwhichcouldcomparevaluesofagiventype.DefinitionofFunctionTemplatesFunctionTemplatesThetwofunctionsarenearlyidentical.Theonlydifferencebetweenthemisthetypeoftheirparameters.Thefunetionbodyisthesameineachfunction.Havingtorepeatthebodyofthefunctionforeachtypethatwecompareistediousanderror-prone.FunctionTemplatesMoreimportantly，we

needtoknowinadvanceallthetypesthatwemighteverwanttocompare.Thisstrategy

cannotworkifwewanttobeabletousethefunctionontypesthatwedon'tknowabout.

Therefore，wethinkthatthesecomparefunctionsneedtobedefinedasagenericfunetion.FunctionTemplatesFunctionTemplateInstantiationWhenafunctiontemplateisfirstcalledforeachtype，thecompilercreatesaninstantiation.Eachinstantiationisaversionofthetemplatedfunctionspecializedforthetype.Thisinstantiationwillbeealledeverytimethefunctionisusedforthetype.FunctionTemplatesWhenthecompilerencountersthiscalltoatemplateinstantiation，itusesthetemplatetoautomaticallygenerateafunctionreplacingeachappearanceofTbythetypepassedastheactualtemplateparameter（intinthiscase）andthencallsit.Thisprocessisautomaticallyperformedbythecompilerandisinvisibletotheprogrammer.FunctionTemplatesFunctionTemplatewithDifferentParameterTypesSincethefunctiontemplateinExample8-1inchidesonlyonetemplateparameter(typenameT)andthefunctiontemplateitselfacceptstwoparameters，bothofthesameTtypes，wecannotcallourfunctiontemplatewithtwoobjectsofdifferenttypesasarguments.FunctionTemplatesFunctionTemplateOverloadingFunctiontemplatesandoverloadingareintimatelyrelated.Onecandeclareseveralfunctiontemplateswiththesamenameandevendeclareacombinationoffunctiontemplatesandordinaryfunctionswiththesamename.Whenanoverloadedfunctioniscalled，overloadingresolutionisnecessarytofindtherightfunctionortemplatefunction

toinvoke.FunctionTemplates03ClassTemplatesJustaswecandefinefunctiontemplates，wecanalsodefineclasstemplates.Forexample，a"stack3isadatastruetureintowhichweinsertitemsatthetopandretrievethoseitemsinlast-in，first-outorder.Itisindependentofthetypeoftheitemsbeingplacedinthestack.Normally，aStackclasscanbedefinedbyaspecifieddatatype.DefinitionofClassTemplatesClassTemplatesIfyouwanttocreatea

wonderfulopportunityforsoftwarereusability，youneedtodefineagenericStaekclassandtoinstantiatetheclassthatisthetype-specificversionsofthegenericStackclass.C++providesthiscapabilitythroughtheclasstemplate.ClassTemplatesAclasstemplateiscalledaparameterizedtype，becauseitrequiresoneormoretypeparameterstospecify

howtocustomizea"genericclass"templatetoformaclass-templatespecialization.類模板稱為參數(shù)化類型，因為它們需要一個或多個類型參數(shù)來指定如何自定義“通用類”模板以形成類模板專用化。ClassTemplatesClassTemplateInstantiationItiseasytouseaclasstemplate.Createtherequiredclassesbypluggingintheactualtypeforthetypeparameters.Thisprocessiscommonlyknownas

"Instantiatingaelass".HereisasampledriverclassthatusesclasstemplateStack.ClassTemplatesTheprocessofgeneratingaclassdeclarationfromatemplateclassandatemplateargumentisoftencalledclasstemplateinstantiation.從模板類和模板參數(shù)生成類聲明的過程通常稱為類模板實例化。ClassTemplatesNow，letusconsidertheuserprogramthatexercisesclasstemplateStack.Althoughtemplatesoffersoftware-reusabilitybenefits，rememberthatmultipleclass-templatespecializationsareinstantiatedinaprogram（atcompiletime），eventhoughthetemplateiswrittenonlyonce.ClassTemplatesTheprogrambeginsbyinstantiatinganobjeetdoubleStaekofsize5inLine5.ThisobjectisdeclaredtobeofclassStack<double>（pronounced

"Stackofdouble"）.ThecompilerassociatestypedoublewithtypeparameterTintheclasstemplatetoproducethesourcecode

foraStackclassoftypedouble.ClassTemplatesClassStackprovidestheisFullfunction，whichtheprogrammercanusetodeterminewhetherthestackis

fullbeforeattemptingapushoperation.Thiswouldavoidthepotentialerrorofpushingontoa

fullstack.ClassTemplates04Non-TypeParametersforTemplatesBesidesthetemplateargumentsthatareprecededbythetypenamekeywords，which

representtypes，templatescanalsohaveregulartypedparameters，likethosefoundinfunctions.Asanexample，takealookatthisclasstemplatethatisusedtocontainsequencesofelements.Non-TypeParametersforTemplatesInthisexample，integerNisanon-typetemplateparameter.Whentheobjectsofthe

templatearecreatedinLines24and25，aninteger5isdirectlypassedintoN.Itisalsopossibletosetdefaultvaluesortypesforclasstemplateparameters.Non-TypeParametersforTemplates05AbstractBaseClassesClasstemplatescaninheritorbeinheritedfrom.Formanypurposes，thereisnothing

significantlydifferentbetweenthetemplateandnon-templatescenarios.DerivationandClassTemplatesHowever，thereisoneimportantsubtletywhenderivingaclasstemplatefromabaseclassreferr