Chapter8ObjectsandClasses

8.1Whatisanobject（對(duì)象）?Anobjectisacomponentofaprogramthatknowshowtoperformcertainactionsandknowshowtointeractwithotherpartsoftheprogram.Anobjectconsistsofoneormoredatavalues,whichdefinethestateorpropertiesoftheobjectfunctionsthatcanbeappliedtotheobjectThefunctionsassociatedwithanobjectrepresentwhatcanbedonetotheobjectandhowtheobjectbehaves.8.1Whatisanobject?Example：acomputergameAplayerwillhavedatavaluestorepresentcertainattributes,e.g.thestateoftheirhealthortheweaponstheypossess.Aplayermustbeabletoperformfunctionssuchaswalk,run,attackanenemy,andrescuethefairmaidentowin.8.2Whatisaclass?（什么是類(lèi)?）Aclassisageneralcategorythatdefines:Thecharacteristicsthatanobjectofthatcategorycontains.Thecharacteristicsarecalledpropertiesorclassdatamembers.Thefunctionsthatcanbeappliedtoobjectsofthatcategory.Thefunctionsarealsocalledclassmemberfunctionsormethods.encapsulation

Aclassdefinesboththetypeofdataandtheoperationsthatcanbeappliedtothatdata.Includingboththedataandfunctionsintooneunit,theclass,iscalledencapsulation.8.2Whatisaclass?Inourcomputeradventuregameaplayerclassmaybedefinedas:Note:Thisisonlyapseudo(notreal)-coderepresentationoftheclassusedforexplanatorypurposesonly.8.2Whatisaclass?AclassandinstanceoftheclassAclassisablueprintoratemplatethatcanbeusedtocreatemanyinstancesoftheclass.Aninstanceofaclassistheactualobject,createdfromthetemplate,thatcanbemanipulatedbythememberfunctionsoftheclass.Thedifferencebetweenaclassandaninstanceofaclass(anobject)islikethedifferencebetweenanounandapropernoun.personisanoun,JohnSmithisapropernounPersonistheclassandJohnSmithistheobject.8.2Whatisaclass?Example:theadventuregamethefollowingstatementcreatesaninstanceofadragon:Aclassmemberfunctioniscalledusingthememberselectionoperator(adot):Anynumberofdragonobjectscanbecreated.Heregeorgeisaninstanceoranobjectofthedragonclass.Thedataintheparenthesesrepresentinitialvaluesforsomeofthedatamembersofgeorge,e.g.sizeandnumber_of_claws.8.2Whatisaclass?Eachdragoncanperformdifferentfunctionsindependentofeachother.Tosummarise:AnobjectisaninstanceofaclassAnobjecthas:anidentity,i.e.itsname

astate,i.e.itsdatamembersabehaviour,i.e.itsmemberfunctions.8.3Furtherexamplesofclassesandobjects8.3.1AstudentclassSomeoftheproperties(classdatamembers)thatdefineastudentarethename,theaddress,thestudentnumber,thedateofbirth,thecourseandthecoursemark.Someofthefunctions(classmemberfunctionsormethods)thatcanbeperformedonastudentaretoupdatethestudent’scoursemark,displayaprogressreportandsoon.Astudentobjectisaspecificinstanceofthestudentclass.StudentsinacoursesuchasJohnSmithorLiuWeiareinstancesofthestudentClass.8.3Furtherexamplesofclassesandobjects8.3.2AbankaccountclassAbankaccounthasanaccountnumberandabalance.Operationsonabankaccountclassaretoopentheaccountwithanamountofmoneyandtodepositandwithdrawmoneyfromtheaccount.Theseoperationswillberepresentedbyclassmemberfunctionssuchasopen(amount),withdraw(amount)anddeposit(amount).8.4Abstraction（抽象）AbstractionEachsoftwareobjectisasimplificationofitsrealworldcounterpart.Inthecomputeradventuregame,eachsoftwareobjectbehavesonlyinsomerespectsasitsreal-worldcounterpart.theremaybenoneedforaplayertoeat,drinkorsleepinthegame.Thebankaccountclassdescribesonlythecharacteristicsandoperationsthatarerelevantforthepurposesoftheprogram.Abstractionisusedinoureverydaylives.8.4Abstraction（抽象）AbstractionAbstractionisusedinoureverydaylives.Alecturetimetablewillcontainasubject,alecturer’sname,aroomnumberandatime.Onthetimetable,theroomwherethelectureistobeheldisrepresentedsimplybyanumber.Forastudent,detailsoftheroomareunnecessaryandarethereforenotgiven.Foracollegeadministratorthesizeoftheroomisrelevantandsowouldbeincludedinanabstractionforthepurposesofcollegeadministration.8.5ConstructingaclassinC++ProgramExampleP8AOnline7,followingthekeywordclass,theclassisgiventhenamebank_accountLines10~13declarethememberfunctionsoftheclassLines15and16declarethedatamembersoftheclass.Themembersoftheclassaredividedintoprivateandpublicmembers.Thekeywordsprivateandpublicspecifytheaccesscontrollevelforthedataandfunctionmembersoftheclass.8.5ConstructingaclassinC++informationhidingTheprivatedatamembersareaccessibleonlytomemberfunctionsoftheclassandunavailabletoanyfunctionsthatarenotmembersoftheclass.Membersdeclaredwithpublicaccessareaccessibleinanypartofaprogram.Apublicmemberfunctioncanbecalledfromanypartofaprogram,whileaprivatememberfunctioncanonlybecalledfromwithinmemberfunctionsofthesameclass.Thepublicmemberfunctionsareknownasthepublicinterface

oftheclass.8.5ConstructingaclassinC++8.5ConstructingaclassinC++ThegeneralformatofaclassGenerally,datamembersofaclassareallprivateandthememberfunctionsoftheclassareallpublic.Thepublicsectionisusuallyplacedatthestartoftheclassbeforetheprivatesection.8.5ConstructingaclassinC++Theclassmemberfunctionsmustbedefined8.5ConstructingaclassinC++Aclassmemberfunctionhasthegeneralformat:Thescoperesolutionoperator::isusedheretospecifythatafunctionisamemberofaclass.8.6Usingaclass:definingandusingobjectsProgramExampleP8A...continued:Tousethebank_accountclass,placetheclassandmemberfunctiondefinitionsbeforemain().Theobjectsoftheclassaredefinedandusedinmain().Line44createsabank_accountobjectcalledmy_accountLine47initializesthedatamembersoftheclassbycallingthememberfunctionopen()withvaluesfortheaccount_noandbalance8.7Abstractdatatypes（抽象數(shù)據(jù)類(lèi)型）C++built-indatatypesEachdatatypehasauniquerangeofallowablevaluesandasetofallowableoperationsandfunctions.Theimplementationdetailsofthedatatypesarehiddenfromtheprogrammer.Thisiscalleddataabstraction.abstractdatatype(ADT)ThebankaccountclassdefinesanewdatatypethatisnotbuiltintotheC++language.Oncewritten,itisnotnecessarytoknowthedetailsofhoweachoftheclassmemberfunctionswork.knowhowtocallthepublicmemberfunctionsoftheclasssuchasopen(),deposit()andwithdraw().8.8Constructors（構(gòu)造函數(shù)）Constructoraclassmemberfunctionthathasthesamenameastheclassbeautomaticallycalledwhenanobjectoftheclassiscreatedfrequentlyusedtoprovidetheprivatedatamembersoftheobjectwithinitialvalues.Example:Theprivateaccount_numberandbalanceareassignedtheirinitialvaluesinopen().aconstructorisaddedtothebank_accountclassinplaceoftheopen()memberfunction.8.8ConstructorsProgramExampleP8BTheconstructorisneverexplicitlycalledandthereforecannotreturnavalue.That’swhyaclassconstructorhasnoreturntype,notevenvoid.8.8ConstructorsRewritingmain()tomakeuseoftheclassconstructorLine44ofthisprogramcreatesanobjectcalledmy_accountwiththesevalues:8.9DefaultclassconstructorAdefaultclassconstructorisaconstructorthathasnoparameters.ProgramExampleP8C8.9DefaultclassconstructorProgramExampleP8C…continued8.9DefaultclassconstructorProgramExampleP8C…continued8.10Overloadingclassconstructors

（重載類(lèi)構(gòu)造函數(shù)）ConstructorscanbeoverloadedTherecanbeseveralconstructorswithinaclass,eachwithadifferentnumberofparametersProgramExampleP8D8.10OverloadingclassconstructorsProgramExampleP8D…continuedOnlyoneargumentgivenwhentheobjectmy_accountisbeingcreatedtheclassconstructoronlines27to31iscalled,assigning123totheaccountnumberand0.0tothebalance.8.11ConstructorinitialisationlistsAdatamemberinitializationlistisfrequentlyusedinconstructorsinplaceofassignmentstatements.NostatementsleftinthisconstructorThechainbrackets{}arestillrequired8.12DefaultargumentvaluesinaconstructorConstructorsarefunctions,defaultargumentscanalsobeusedinconstructorsProgramExampleP8E8.12DefaultargumentvaluesinaconstructorProgramExampleP8E…continued8.12DefaultargumentvaluesinaconstructorProgramExampleP8E…continued8.13staticclassdatamembers（靜態(tài)類(lèi)數(shù)據(jù)成員）Astaticclassdatamemberisindependentofalltheobjectsthatarecreatedfromthatclass.Onlyonecopyofastaticdatamemberexistsandissharedbyallobjectsofaparticularclass.Thevalueofthestaticdatamemberisthereforethesameforalltheclassobjects.Ifevenoneoftheobjectsofaclassmodifiesthevalueofastaticdatamember,thenthevalueofthestaticdatamemberchangesforeveryobjectofthatclass.8.13staticclassdatamembersProgramExampleP8F8.13staticclassdatamembersProgramExampleP8F…continued8.13staticclassdatamembersProgramExampleP8F…continuedAstaticdatamemberisinitializedoutsidetheclassandoutsidemain()next_account_numberoccursonlyonceandissharedbyallobjectsoftheclassandbankaccountobjects.8.13staticclassdatamembersclassvariable:AssociatedwithaclassratherthananobjectinstancevariablesAssociatedwithinstancesoftheclassandarecalled.8.14UsingreturninamemberfunctionAclassmemberfunctioncanreturnavalueusingareturnstatementProgramExampleP8G8.14Usingreturninamemberfunction8.14Usingreturninamemberfunctioninspectororaccessorfunction(檢查或讀值函數(shù))Thememberfunctionget_balance(),Inspectorfunctionsallowprivatedatamembersofaclasstobeinspectedfromoutsidetheclass.mutatorfunction(設(shè)值函數(shù))Theclassmemberfunctionsdeposit()andwithdraw().Mutatorfunctionschangethevaluesoftheprivatedatamembersofaclass.8.15Inlineclassmemberfunctions（內(nèi)聯(lián)成員函數(shù)）Inlinefunctionsareusefulincaseslikethiswhereyouwanttouseafunctionbutdon’twantthefunctionoverheadsThememberfunctionsofaclasscanbemadeinlinebyprecedingthefunctionheaderwiththekeywordinline.Analternativewayofmakingamemberfunctioninlineistoincludethefunctiondefinitioninthebodyoftheclass.8.15InlineclassmemberfunctionsAdisadvantageofmakingfunctionsinlinebymovingtheirdefinitionsintotheclassisthattheclassbecomes‘crowded’withdetails.8.16Classinterfaceandclassimplementationclasspublicinterface:alistofthememberfunctionsoftheclassandhowtheycanbeused.Thepublicinterfacestartsatpublicandendsatthekeywordprivate.Aprogrammerusingaclassshouldonlyhavetoreadthepublicinterfaceinordertousetheclass;Tomakethepublicinterfaceeasiertoread,commentsshouldbeincludedexplainingthepurposeofthefunctions,theirparametersandtheirreturnvalues.classimplementation:detailsintheprivatesectionandinthememberfunctions8.16Classinterfaceandclassimplementation8.16Classinterfaceandclassimplementation8.16.1SeparationofclassinterfaceandclassimplementationItisnotpossibleinC++tocompletelyseparatethepublicinterfaceofaclassfromitsimplementation.InC++,theclassdeclarationisnormallyplacedinaheaderfile(e.g.bank_ac.h)andthememberfunctionsarenormallyplacedinaseparatefile(e.g.bank_ac.cpp).filebank_ac.h:Thelinesbeginningwith#arestandardpreprocessordirectivesusedtopreventmultipleinclusionsoftheheaderfileintoaprogram.8.16Classinterfaceandclassimplementation8.16ClassinterfaceandclassimplementationPlacinglines43to83ofthebankaccountclassintobank_ac.cpp:8.16ClassinterfaceandclassimplementationUsingthesetwofiles,ProgramExampleP8Gcannowbere-writtenas:Lines5and6arepreprocessordirectivesthatincorporatethebankaccountheaderandsourcefilesintotheprogram.Thequotesaroundthefilenamesonlines5and6tellthecompilerthatthefilestobeincludedareinthesamedirectoryastheprogram.8.16Classinterfaceandclassimplementation8.16.2UseofnamespacesinheaderfilesAprogrammaycontainmanyclassesandfunctions#includedfrommanydifferentheaderfiles.Anelementinoneoftheheaderfilesmayhaveinadvertentlybeengiventhesamenameasanelementinanotherheaderfile----resultinacompilererror.namespace:anamedblockofstatementsinaprogram.Withinanamespace,anidentifiernamemustbeuniquealthoughthenamemaybepreviouslyusedinothernamespaces.8.16Classinterfaceandclassi