SavCpp8PowerPointsSavitch_ch_15

Chapter15,Inheritance,Overview,15.1InheritanceBasics15.2InheritanceDetails15.3Polymorphism,Slide15-3,15.1,Inheritance,InheritanceBasics,Inheritanceistheprocessbywhichanewclass,calledaderivedclass,iscreatedfromanotherclass,calledthebaseclassAderivedclassautomaticallyhasallthemembervariablesandfunctionsofthebaseclassAderivedclasscanhaveadditionalmembervariablesand/ormemberfunctionsThederivedclassisachildofthebaseorparentclass,Slide15-5,EmployeeClasses,Todesignarecord-keepingprogramwithrecordsforsalariedandhourlyemployeesSalariedandhourlyemployeesbelongtoaclassofpeoplewhosharethepropertyemployeeAsubsetofemployeesarethosewithafixedwageAnothersubsetofemployeesearnhourlywagesAllemployeeshaveanameandSSNFunctionstomanipulatenameandSSNarethesameforhourlyandsalariedemployees,Slide15-6,ABaseClass,WewilldefineaclasscalledEmployeeforallemployeesTheEmployeeclasswillbeusedtodefineclassesforhourlyandsalariedemployeesAdefinitionoftheemployeeclassisfoundin,Slide15-7,Display15.1,Display15.2,Functionprint_check,Functionprint_checkwillhavedifferentdefinitionstoprintdifferentchecksforeachtypeofemployeeAnEmployeeobjectlackssufficientinformationtoprintacheckEachderivedclasswillhavesufficientinformationtoprintacheck,Slide15-8,ClassHourlyEmployee,HourlyEmployeeisderivedfromClassEmployeeHourlyEmployeeinheritsallmemberfunctionsandmembervariablesofEmployeeTheclassdefinitionbeginsclassHourlyEmployee:publicEmployee:publicEmployeeshowsthatHourlyEmployeeisderivedfromclassEmployeeHourlyEmployeedeclaresadditionalmembervariableswage_rateandhours,Slide15-9,Display15.3,InheritedMembers,AderivedclassinheritsallthemembersoftheparentclassThederivedclassdoesnotre-declareorre-definemembersinheritedfromtheparent,exceptThederivedclassre-declaresandre-definesmemberfunctionsoftheparentclassthatwillhaveadifferentdefinitioninthederivedclassThederivedclasscanaddmembervariablesandfunctions,Slide15-10,ImplementingaDerivedClass,AnymemberfunctionsaddedinthederivedclassaredefinedintheimplementationfileforthederivedclassDefinitionsarenotgivenforinheritedfunctionsthatarenottobechangedTheHourlyEmployeeclassisdefinedin,Slide15-11,Display15.5,ClassSalariedEmployee,TheclassSalariedEmployeeisalsoderivedfromEmployeeFunctionprint_checkisredefinedtohaveameaningspecifictosalariedemployeesSalariedEmployeeaddsamembervariablesalaryTheinterfaceforSalariedEmployeeisfoundincontainstheimplementation,Slide15-12,Display15.4,Display15.6(1-2),ParentandChildClasses,RecallthatachildclassautomaticallyhasallthemembersoftheparentclassTheparentclassisanancestorofthechildclassThechildclassisadescendentoftheparentclassTheparentclass(Employee)containsallthecodecommontothechildclassesYoudonothavetore-writethecodeforeachchild,Slide15-13,DerivedClassTypes,AnhourlyemployeeisanemployeeInC+,anobjectoftypeHourlyEmployeecanbeusedwhereanobjectoftypeEmployeecanbeusedAnobjectofaclasstypecanbeusedwhereveranyofitsancestorscanbeusedAnancestorcannotbeusedwhereveroneofitsdescendentscanbeused,Slide15-14,DerivedClassConstructors,AbaseclassconstructorisnotinheritedinaderivedclassThebaseclassconstructorcanbeinvokedbytheconstructorofthederivedclassTheconstructorofaderivedclassbeginsbyinvokingtheconstructorofthebaseclassintheinitializationsection:HourlyEmployee:HourlyEmployee:Employee(),wage_rate(0),hours()/nocodeneeded,Slide15-15,AnyEmployeeconstructorcouldbeinvoked,DefaultInitialization,Ifaderivedclassconstructordoesnotinvokeabaseclassconstructorexplicity,thebaseclassdefaultconstructorwillbeusedIfclassBisderivedfromclassAandclassCisderivedfromclassBWhenaobjectofclassCiscreatedThebaseclassAsconstructoristhefirstinvokedClassBsconstructorisinvokednextCsconstructorcompletesexecution,Slide15-16,PrivateisPrivate,Amembervariable(orfunction)thatisprivateintheparentclassisnotaccessibletothechildclassTheparentclassmemberfunctionsmustbeusedtoaccesstheprivatemembersoftheparentThiscodewouldbeillegal:voidHourlyEmployee:print_check()net_pay=hours*wage_rage;net_payisaprivatememberofEmployee!,Slide15-17,TheprotectedQualifier,protectedmembersofaclassappeartobeprivateoutsidetheclass,butareaccessiblebyderivedclassesIfmembervariablesname,net_pay,andssnarelistedasprotected(notprivate)intheEmployeeclass,thiscode,illegalonthepreviousslide,becomeslegal:HourlyEmployee:print_check()net_pay=hours*wage_rage;,Slide15-18,ProgrammingStyle,Usingprotectedmembersofaclassisaconveniencetofacilitatewritingthecodeofderivedclasses.ProtectedmembersarenotnecessaryDerivedclassescanusethepublicmethodsoftheirancestorclassestoaccessprivatemembersManyprogrammingauthoritiesconsideritbadstyletouseprotectedmembervariables,Slide15-19,RedefinitionofMemberFunctions,Whendefiningaderivedclass,onlylistthetheinheritedfunctionsthatyouwishtochangeforthederivedclassThefunctionisdeclaredintheclassdefinitionHourlyEmployeeandSalariedEmployeeeachhavetheirowndefinitionsofprint_checkdemonstratestheuseofthederivedclassesdefinedinearlierdisplays.,Slide15-20,Display15.7(1-2),RedefiningorOverloading,AfunctionredefinedinaderivedclasshasthesamenumberandtypeofparametersThederivedclasshasonlyonefunctionwiththesamenameasthebaseclassAnoverloadedfunctionhasadifferentnumberand/ortypeofparametersthanthebaseclassThederivedclasshastwofunctionswiththesamenameasthebaseclassOneisdefinedinthebaseclass,oneinthederivedclass,Slide15-21,FunctionSignatures,Afunctionsignatureisthefunctionsnamewiththesequenceoftypesintheparameterlist,notincludinganyconstorsally_h.Employee:print_check();,Slide15-23,Section15.1Conclusion,CanyouExplainwhythedeclarationforget_nameisnotpartofthedefinitionofSalariedEmployee?GiveadefinitionforaclassTitledEmployeederivedfromclassSalariedEmployeewithoneadditionalstringcalledtitle?Addtwomemberfunctionsget_titleandset_title.Itshouldredefineset_name.,Slide15-24,15.2,InheritanceDetails,InheritanceDetails,Somespecialfunctionsare,forallpracticalpurposes,notinheritedbyaderivedclassSomeofthespecialfunctionsthatarenoteffectivelyinheritedbyaderivedclassincludeDestructorsCopyconstructorsTheassignmentoperator,Slide15-26,CopyConstructorsandDerivedClasses,Ifacopyconstructorisnotdefinedinaderivedclass,C+willgenerateadefaultcopyconstructorThiscopyconstructorcopiesonlythecontentsofmembervariablesandwillnotworkwithpointersanddynamicvariablesThebaseclasscopyconstructorwillnotbeused,Slide15-27,Operator=andDerivedClasses,Ifabaseclasshasadefinedassignmentoperator=andthederivedclassdoesnot:C+willuseadefaultoperatorthatwillhavenothingtodowiththebaseclassassignmentoperator,Slide15-28,DestructorsandDerivedClasses,AdestructorisnotinheritedbyaderivedclassThederivedclassshoulddefineitsowndestructor,Slide15-29,TheAssignmentOperator,Inimplementinganoverloadedassignmentoperatorinaderivedclass:ItisnormaltousetheassignmentoperatorfromthebaseclassinthedefinitionofthederivedclasssassignmentoperatorRecallthattheassignmentoperatoriswrittenasamemberfunctionofaclass,Slide15-30,TheOperator=Implementation,Thiscodesegmentshowshowtobegintheimplementationofthe=operatorforaderivedclass:Derivedstringname;andclassDog:publicPetpublic:virtualvoidprint();stringbreed;,Slide15-48,ASlicedDogisaPet,C+allowsthefollowingassignments:=Tiny;vdog.breed=GreatDane;vpet=vdog;However,vpetwillloosethebreedmemberofvdogsinceanobjectofclassPethasnobreedmemberThiscodewouldbeillegal:coutprint();islegalandproduces:name:Tinybreed:GreatDane,Slide15-52,voidDog:print()coutname:nameendl;coutbreed:breedbreed=GreatDane;ppet=pdog;,Display15.12(1-2),UseVirtualFunctions,Thepreviousexample:ppet-print();workedbecauseprintwasdeclaredasavirtualfunctionThiscodewouldstillproduceanerror:coutnamebreed;,Slide15-53,Why?,ppet-breedisstillillegalbecauseppetisapointertoaPetobjectthathasnobreedmemberFunctionprint()wasdeclaredvirtualbyclassPetWhenthecomputerseesppet-print(),itchecksthevirtualtableforclassesPetandDogandfindsthatppetpointstoanobjectoftypeDogBecauseppetpointstoaDogobject,codeforDog:print()isused,Slide15-54,RememberTwoRules,Tohelpmakesenseofobjectorientedprogrammingwithdynamicvariables,remembertheserulesIfthedomaintypeofthepointerp_ancestorisabaseclassfortheforthedomaintypeofpointerp_descendant,thefollowingassignmentofpointersisallowedp_ancestor=p_descendant;andnodatamemberswillbelostAlthoughallthefieldsofthep_descendantarethere,virtualfunctionsarerequiredtoaccessthem,Slide15-55,VirtualCompilation,Whenusingvirtualfunctions,youwillhavetodefineeachvirtualfunctionbeforecompilingDeclarationisnolongersufficientEvenifyoudonotcallthevirtualfunctionyoumayseeerrormessage:undefinedreferencetoClass_Namevirtualtable,Slide15-56,VirtualDestructors,DestructorsshouldbemadevirtualConsiderBase*pBase=newDerived;deletepBase;IfthedestructorinBaseisvirtual,thedestructorforDerivedisinvokedaspBasepointstoaDerivedobject,returningDerivedmemberstothefreestoreTheDeriveddestructorinturncalls