C++面向对象程序设计双语教程（第3版）课件 ch06Inheritance

InheritanceC++面向对象程序设计双语教程(第3版)Chapter601ClassHierarchiesInthepreviouschapters，wediscusshowtobuildasingleclassthatrepresentsrealobjects.Theclassisrepresentedbyacollectionofabstractedconceptswithcommoncharacteristics.However，aconceptdoesnotexistinisolationalways.Itcoexistswithrelatedconcepts

and

derivesmuchofitspowerfromrelationshipswithrelatedconcepts.Itisafundamentalaspect

ofhumanintelligencetoseekout，recognize，andereaterelationshipsamongconcepts.ClassHierarchiesForexample，theconceptsofacirele，atriangleandarectanglearerelatedinthattheyare

allshapes；thatis，theyhavetheconceptofshapeincommon，thatis，theyhavetheoperationsofthemove，drawandcalculation.ClassHierarchiesHowever，wefindtheirdifferentpointsofthecircle，triangleandrectangle.Thus，wemustexplicitlydefineclassesCircle，TriangleandRectangletohaveclassShapeincommon.Representingacircle，atriangleandarectangleinaprogramwithoutinvolvingthenotionofshapewouldbetolosesomethingessential.ClassHierarchiesEachkindofshapeismorespecializedthanitsparentShape？WecanclassifyadifferentkindofshapeaccordingtothehierarchyshowninFigure6-1.ThesecouldberepresentedintheworldofclasseswithaShapeclassfromwhichwewouldderivethethreeotherones:Circle，TriangleandRectangle.ClassHierarchiesFigure6-1Aclasshierarchy.ClassHierarchies02DerivedClassesDeclarationofDerivedClassesThepublicaccessspecifiermaybereplacedbyanyoneoftheotheraccessspecifiersprotectedandprivate.Thisaccessspecifierlimitsthemostaccessiblelevelforthemembersinheritedfromthebaseclass.Thememberswithamore

accessiblelevelareinheritedwiththislevelinstead，whilethememberswithanequalormore

restrictiveaccesslevelkeeptheirrestrictivelevelinthederivedclass.DerivedClassesAsastudentisalsoaperson，thePersonclassdataisstoredasthepersonmemberofa

Studentobject.Thismaybeobvioustoahumanreader—especiallyacarefulreaderbutthere

isnothingthattellsthecompilerandothertoolsthatastudentisalsoaperson.AStudentobjectisnotaPersonobject，soonecannotsimplycreateoneobjectthroughtheobjectofanotherclass.StructureofDerivedClassesDerivedClassesIntherealworld，wedonotvieweverythingasunique；weoftenviewsomethingasbeinglikesomethingelsebutwithdifferencesoradditions.Studentsarelikeregularpersons；however，theremightbedifferences.DerivedClassesInheritanceisamechanismbywhiehoneclassacquirestheproperties（thedataandoperations）ofan

existingclass.Baseclass（superclassjtheclassbeinginheritedfrom.Derivedclass（subclassjtheclassthatinherits.DerivedClassesFigure6-3ClassesPersonandStudent.DerivedClasses03ConstructorsandDestructorsofDerivedClassesConstructorsofDerivedClassesAconstructorplaysavitalroleininitializinganobject.Somederivedclassesneedconstructors.Ifabaseclasshasconstructors，thentheymustbeinvoked.Althoughtheconstructorsanddestructorsofthebaseclassarenotinheritedthemselves，itsdefault

constructor（i.e.，itsconstructorwithoutparameters）anditsdestructorarealwayscalledwhen

anewobjectofaderivedclassiscreatedordestroyed.ConstructorsandDestructorsofDerivedClassesWhenaderivedclassobjectisdestroyed，theprogramcallsthatobject'sdestructor.Thisbeginsachain（orcascade）ofdestructorcallsinwhichthederivedclassdestructor，thedestructorsofthedirectandindirectbaseclasses，andtheclassesmembersexecuteinthereverseorderinwhichtheconstructorsexecuted.DestructorsofDerivedClassesConstructorsandDestructorsofDerivedClassesTheCallingOrderofDerivedClassObjectsThederivedclassobjectsareconstructedfromthebottomup：firstthebaseclass，thenthemembers，andthenthederivedclassitself.Theyaredestroyedintheoppositeorder：firstthederivedclassitself，thenthemembers，andthenthebaseclass.Membersandthebaseclassareconstructedinorderofdeclarationintheclassanddestroyedinthereverseorder.ConstructorsandDestructorsofDerivedClassesItturnsoutthatmuchofthesyntaxandbehavioraresimilarforbothcompositionandinheritance（whichmakessenseastheyarebothwaysofmakingnewclassesfromexistingclasses）.However，compositionandinheritancearedifferent.InheritanceandCompositionConstructorsandDestructorsofDerivedClasses04MemberFunctionsofDerivedClassesDefiningaMemberFunctionThescopeoperator（:）mustbeusedbeeausefunetionprinthasbeenredefinedinthederivedclass

Student.Suchreuseofnamesistypical.MemberFunctionsofDerivedClassesOverridingMemberFunctionsAStudentobjecthasaccesstoallmemberfunctionsinthePersonclass，aswellastoanymemberfunctions，suchasfull_name，whichisthefunctiondeclarationthattheStudentclassmightadd.MemberFunctionsofDerivedClassesOverridingMemberFunctionsSometimes，tokeepthesamefunctionnameasthatofitsbaseclass，theStudentclasscanalsooverrideabaseclassfunction.Overridingamemberfunctionmeanschangingtheimplementationofabaseclassfunctioninaderivedclass.Whenyoumakeanobjectofthederivedclass，thecorrectfunctioniscalled.MemberFunctionsofDerivedClassesWhenyouoverrideafunction，itmustagreeinreturning-valuetypeandinsignaturewiththefunetioninthebaseclass.Thesignatureisthefunctionprototyperatherthanthereturning-valuetype：thatis，thename，theparameterlist，andtheconstkeywordifused.OverloadingCombinatorialOperators05Access

ControlThisreflectstheviewthattherearethreekindsoffunctionsaccessingaclass：functionsimplementingtheclass（itsfriendsandmembers），functionsimplementingaderivedclass（itsfriendsandmembers），andtheotherfunctions.AccessControlinClassesAccessControlThiscanbepresentedgraphically，shownin

Figure6-5.AccessControlAccesstoBaseClassesIfamemberisprivateinthebaseclass，thenonlythebaseclassanditsfriendsmayaccessthatmember.Thederivedclasshasnoaccesstotheprivatemembersofitsbaseclass，norcanitmakethosemembersaccesstoitsownusers.AccessControlWecanseethisclearlyinTable6-1.AccessControl06TypeConversionofUser-DefinedTypesIntheprevioussections，eachclasshasinheritedfromasingleparent.Suchsingleinheritanceisenoughtodescribemostreal-worldrelationships.Someclasses，however，representtheblendingoftwoclassesintoone.MultipleInheritanceItcanalsobeamoregeneralgraphstructure，asshowninFigure6-7.MultipleInheritanceDeclarationofMultipleInheritanceInC++，itisperfectlypossiblethataclassinheritsmembersfrommorethanoneclass.Thisisdonebysimplyseparatingthedifferentbaseclasseswithcommasinthederivedclassdeclaration.MultipleInheritanceConstructorsofMultipleInheritanceFromtheobservationofExample6-10，whenaclasshasmultiplebaseclasses，baseclassesareinitializedinadepth-first，left-to-rightorderofappearaneeinthebase-specifier-listInExample6-10，whenthessobjeetiscreated，theorderofconstruetionisasfollows：Bed→Sofa→SleeperSofa.MultipleInheritance07VirtualInheritanceMultipleInheritanceAmbiguitiesAlthoughmultipleinheritanceisapowerfulfeature，itintroducesseveralpossibleproblems.Oneisapparentintheprecedingexample.NoticethatbothBedandSofacontainamemberweightinExample6-10.Thisislogicalbecausebothhaveameasurableweight.VirtualInheritanceAlthoughthissolutioncorrectstheproblem，specifyingthebaseclassintheapplicationfunctionisn'tdesirablebecauseitforcesclassinformationtoleakoutsidetheclassintoapplicationcode.Inthiscase，thefnfunctionmustknowthatSleeperSofainheritsfromSofa.Thesetypesofso-callednamecollisionsweren'tpossiblewithsingleinheritancebutarea

constantdangerwithmultipleinheritance.VirtualInheritanceInthepreviouscaseofSleeperSofa，thenamecollisiononweightwasmorethanamereaccident.ASleeperSofaclassdoesn'thaveabedweightseparatefromitssofaweight.Thecollisionoccurredbecausethisclasshierarchydoesnotcompletelydescribethereal-world.Specifically，theclasseshavenotbeencompletelyfactored.TryingtoSolveInheritanceAmbiguitiesVirtualInheritanceAlatticedclasshierarchyisdescribedinFigure6-9.VirtualInheritanceVirtualBaseClassesVirtualbaseclassesofferawaytosavespaeeandavoid