C++面向对象程序设计双语教程（第3版）课件 ch07Polymorphism and Virtual Functions、ch08Templates

PolymorphismandVirtualFunctionsC++面向对象程序设计双语教程(第3版)Chapter701PolymorphismThekeyideabehindOOPispolymorphism.PolymorphismisderivedfromaGreekwordmeaning“manyforms”.Wespeakthetypesrelatedbyinheritanceaspolymorphictypes，becauseinmanycaseswecanusethe"manyforms³ofaderivedorbasetypeinterchangeably.IntroductiontoPolymorphismPolymorphismInprogramminglanguages，polymorphismmeans

that

some

codesoroperationsor

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

typestobehandledbyusingauniforminterface.多态性是一个面向对象程序设计的关键特性，它允许使用统一的接口处理不同数据类型。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.绑定是一种用于将标识符（如变量和函数名）转换成机器语言地址的过程。换句话说，绑定是一种关联，例如在标识符（变量或函数名）和操作（类型或特定函数体）之间的关联。根据绑定发生的时间，有两种类型的绑定，即静态绑定和动态绑定。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.纯虚函数是一个在基类中没有定义的函数。在类声明时使用pure说明符（=0）将一个虚成员函数声明为纯虚函数。AbstractBaseClassesAclasswhichcontainsoneormorepurevirtualfunctioniscalledanabstractbaseclass.Abstractbase

classcsactasexpressionsofgeneralconceptsfromwhichmorespecificclassescanbederived.包含一个或多个纯虚函数的类称为抽象基类。抽象基类作为通用概念的表示，从中可以导出更多具体的类。AbstractBaseClassesThestatementinLine6tellsthecompilertoreserveaslotforafunetionintheVTABLE，butnottoputanaddressinthatparticularslot.Evenifonlyonefunctioninaclassisdeclaredaspurevirtual，theVTABLEisincomplete.AbstractBaseClassesIftheVTABLEforaclassisincomplete，whatisthecompilersupposedtodowhen

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

andtellboththeuserandthecompilerhowitwasintendedtobeused.AbstractBaseClasses04CaseStudy:AMini-SystemWedesignamini-systemthatineludesdifferenttypesofpeopleintheuniversity.Therearefiveclasses，thatis，Person，Student，Employee，FacultyandStaff，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谢谢观看C++面向对象程序设计双语教程(第3版)TemplatesC++面向对象程序设计双语教程(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.类模板称为参数化类型，因为它们需要一个或多个类型参数来指定如何自定义“通用类”模板以形成类模板专用化。ClassTemplatesClassTemplateInstantiationItiseasytouseaclasstemplate.Createtherequiredclassesbypluggingintheactualtypeforthetypeparameters.Thisprocessiscommonlyknownas

"Instantiatingaelass".HereisasampledriverclassthatusesclasstemplateStack.ClassTemplatesTheprocessofgeneratingaclassdeclarationfromatemplateclassandatemplateargumentisoftencalledclasstemplateinstantiation.从模板类和模板参数生成类声明的过程通常称为类模板实例化。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