Chapter11Inheritance

11.1Whatisinheritance?Inheritance

isoneofthefundamentalconceptsofobject-orientedprogramming.Inheritanceallowsanewclasstobeconstructedusinganexistingclassasabasis.Thenewclassincorporatesorinheritsthedatamembersandmemberfunctionsoftheexistingclass.Additionaldatamembersandmemberfunctionscanbeaddedtothenewclass,therebyextendingtheexistingclass.Theexistingclassisknownasthebase

class.Thenewclassisknownasthederived

orinherited

class.11.1Whatisinheritance?HowtoimplementthisinheritanceHierarchy?Doweneedtodefineemployeenumber,surname,forename,employedforeachclass?11.1Whatisinheritance?Employeesinthecompanyareeitherpart-timeorfull-time.Theemployeenumberandnamearecommontobothtypesofemployee.Whetherthepersoniscurrentlyemployedorisapastemployeeisalsocommontobothtypesofemployees.Part-timeemployeesarepaidanhourlyrate.full-timeemployeesareonasalaryandareallowedanannualpaidleave.Managersarealsofull-timeemployees,whoinadditiontotheirannualsalaryandpaidleavearealsopaidanannualbonus.11.1Whatisinheritance?Employeesclassisknownasthebase.thebaseclassisnotmodifiedinanywayandissimplyusedasabasisforwritingthederivedclass.Thisiscalledreusability(thebaseclassisbeingreused)andisamajorgoalofobject-orientedprogramming.Thederivedclasscan,inturn,beusedasabaseclassfromwhichotherclassesmaybederived,thuscreatingaclasshierarchy.11.2InheritancesyntaxInitssimplestform,inheritanceisusedbyfirstdefiningthebaseclass:Thisisthesameasdefininganyclass.Next

thederivedclassesaredefined:ProgramExampleP11B11.2Inheritancesyntax11.2Inheritancesyntax11.2Inheritancesyntaxclasspart_time:publicemployeeSyntax:

class

DerivedClassName

:

access-level

BaseClassName

access-levelspecifiesthetypeofderivationprivatebydefault,publicprotectedderivedclass.

Inheritancecreatesahierarchyofrelatedclasses(types)whichsharecodeandinterface.codereusability11.2InheritancesyntaxAderivedclasscanoverridemethodsdefinedinitsparentclass.Withoverriding,themethodinthesubclasshastheidenticalsignaturetothemethodinthebaseclass.asubclassimplementsitsownversionofabaseclassmethod.11.2Inheritancesyntax11.2Inheritancesyntax11.2Inheritancesyntaxthebaseclassdefaultconstructorisautomaticallycalledfromthederivedclass.Whichdisplay_data()iscalled??11.2InheritancesyntaxHowtoinitializethedatamembersofobjects?part_time

pt(123,"Smith","John",5.12);full_time

ft(124,"Jones","Mary",21500,21);managerman(125,"Other","A.N.",32000,30,9500);11.3Passingargumentstoabaseclass

constructorProgramP11Busedconstructorstoinitialisethedatamembersfromkeyboardinput.Toallowforinitialisationofanobjectwhenitiscreated,aconstructorwithaparameterlistisrequired.Whenaderivedobjectiscreated,itshouldtakecareoftheconstructionofthebaseobjectbycallingtheconstructorforthebaseclass.thetechnique.11.3Passingargumentstoabaseclass

constructorAsseeninthepreviousprogram,thebaseclassdefaultconstructorisautomaticallycalledfromthederivedclass.Whenanobjecthasinitialvalues,thenitialisationofthebaseclassobjectisdonebytheuseofaninitialisationlistinthederivedclassconstructor.Thenextprogramdemonstratesthetechnique.ProgramExampleP11C11.3Passingargumentstoabaseclass

constructorProgramExampleP11C11.3Passingargumentstoabaseclass

constructorProgramExampleP11C11.3Passingargumentstoabaseclass

constructorProgramExampleP11C11.3Passingargumentstoabaseclass

constructorProgramExampleP11C11.3Passingargumentstoabaseclass

constructorLine157nowinitialisestheobjectptwiththevaluesintheparentheses.Theconstructoronlines75to79iscalled.Theexpressionemployee(number,sname,fname)intheinitialisationlistonline78isanexplicitcalltothebaseclassconstructoronlines32to39.11.4ProtectedclassmembersThekeywordsprivateandpublicareusedtocontrolaccesstoboththedatamembersandthememberfunctionsofaclass.Aprivatedatamemberormemberfunctionofabaseclasscannotbeaccessedfromamemberfunctionofaderivedclass,thusensuringthattheprincipleofdatahidingisupheld.Inpracticeitcanbeconvenienttosharedatabetweenabaseclassandaderivedclass.11.4ProtectedclassmembersTherearetwowaysofdoingthis.Onepossibilityistochangetheaccessleveloftherelevantprivatememberstopublic,thusallowingthederivedclasstherequiredaccesstothesemembers.thisviolatestheprincipleofdatahidingbymakingthedatamembersavailableforuncontrolledmodificationfromanypartoftheprogram.Togetoverthisproblem,C++hasathirdlevelofaccessknownasprotectedaccess.11.4ProtectedclassmembersThislevelofaccessallowsderivedclasses(andonlyderivedclasses)tohaveaccesstospecifiedbaseclassmembers.Thenextprogramdemonstratestheuseofprotectedclassdatamembers.ProgramExampleP11D11.4ProtectedclassmembersProgramExampleP11D11.4ProtectedclassmembersLine61definesarectangleobjectrwithsidesofwidth1andheight2.Line62definesasquareobjectswithsidesofsize3.Line64displaysthedimensionsoftherectangleandline65displaysthedimensionofthesquare.Inordertodisplaythedimensionofthesquare,line51requiresaccesstothebaseclassdatamemberwidth.Thisisachievedbymakingwidthaprotecteddatamemberbyplacingitintheprotectedsectionoftheclass.11.4Protectedclassmembersifabaseclassmemberisdeclaredasprivate,thenthismemberisnotaccessibleoutsidethebaseclass,notevenfromaderivedclass.Protectedmembersareaccessiblefromwithinabaseclassandfromwithinanyofitsderivedclasses.Publicmembers(usuallymemberfunctions)areaccessiblefromanypartoftheprogram.11.5Typesofinheritance:public,protected

andprivateTheaccessrightsofabaseclassmemberwithinaderivedclasscanalsobespecifiedwhenthederivedclassisdeclared.Publicinheritancespecifiesthattheaccessrightsoftheinheritedmemberswillbethesameastheywereinthebaseclass.Replacingthekeywordpublicwiththekeywordprotectedmakesallpublicmembersinthebaseclassbecomeprotectedmembersinthederivedclass;.11.5Typesofinheritance:public,protected

andprivateThethreetypesofinheritancearesummarisedinthefollowingtable:11.5Typesofinheritance:public,protected

andprivateprotectedmembersinthebaseclassremainprotectedmembersinthederivedclass.Privateinheritanceisspecifiedbyusingthekeywordprivate.Withprivateinheritance,allthepublicandprotectedmembersofthebaseclassbecomeprivateintheinheritedclass.Inallcases,whetheritispublic,protectedorprivateinheritance,privatemembersinabaseclassareinaccessibleinaderivedclass.11.6CompositionInheritancedepicts“a-kind-of”relationshipbetweenclasses.Theothertypeofrelationshipthatmayexistbetweenclassesisthe“has-a”relationship,alsoknownascomposition.InC++thismeansthatobjectsofoneclassarecomposedofobjectsofanotherclass.ProgramExampleP11E11.6Composition11.6CompositionProgramExampleP11E11.6CompositionProgramExampleP11EProgramExampleP11E11.6CompositionAlineiscomposedoftwopointobjects.Whenalineobjectiscreated,theconstructorforthepointobjectembersiscalledbeforetheconstructorforthelineobjectiscalled.Whereasinheritancedefinesarelationshipbetweenclasses,compositiondefinesarelationshipbetweenobjects.11.7MultipleinheritanceC++allowsinheritancefrommorethanjustonebaseclass.AsshowninFigure11.5,thisallowstheconstructionofahierarchytoexpressarelationshipsuchas,classcis“a-kind-of”classaandisalso“a-kind-of”classb.11.7MultipleinheritanceThenextprogramusesmultipldate_timefromexistingdateandtimeclasses.Thedateclassiscalledcalendaranditisdeclaredinthefilecalendar.h:Thetimeclassisthetime24classfromprogramP10H.Theclassheaderisinthefiletime24.handcontainslines7to23ofprogramP10H.Theprivatedatamembershavebeenchangedtoprotectedsothattheycanbeusedbyinheritedclasses.Thetime24classmemberfunctionsareinthefiletime24.cpp.11.7Multipleinheritance11.7Multipleinheritance11.8Virtualbaseclasses（Classbisthebaseclassofbothd1andd2,whicharethebaseclassesofd3.Theprotectedandpublicmembersofbwillbeinheritedbyd1andd2fromtheirimmediatebaseclassb.Sinced3isderivedfrombothd1andd2,itinheritstheprotectedandpublicmembersofbfrombothd1andd2.Twocopiesoftheprotectedandpublicmembersofbnowexistind3.Programmingpitfalls1.Bydefault,theinheritancetypeisprivate,whichisnotthetypethatisusuallyrequired.