FoundationofClassesandObjects—DataAbstractionandDefinitionofClassesC++面向?qū)ο蟪绦蛟O(shè)計(jì)雙語教程(第3版)Chapter301IntroductiontoStructuresAstructuretypeisauser-definedtype.Structuresareaggregatedatatypebuiltbyusingelementsofothertypes，thatis，structuresareawayofstoringmanydifferentvaluesinvariablesofpotentiallydifferenttypesunderthesamename.Thismakesitamoremodularprogram，whichiseasiertomodifybecauseitsdesignmakesthingsmorecompact.Forexample，astructurecanbeusedtostoreinformationaboutstudentsinadatabase.IntroductiontoStructuresWhenyoudefineastructure，youmustrememberthatthemaindifferencebetweenstructuresandarraysisthatthestruetureisacollectionofdifferentdatatypesandthearrayisacollectionofthesamedatatype.IntroductiontoStructuresDefiningaStructureinC++Theprecedingstructuredefinitiondoesnotreserveanyspaceinmemory，rather，thedefinitioncreatesanewdatatypethatisusedtodeclareavariable.Structure'svariableis

declaredlikevariablesofothertypes.IntroductiontoStructuresTheprogramdefinesasingleDatestructurecalledtodayandusesthedotoperatortoinitializethestructurememberswiththevalues18forday，11formonth，and2010foryear.Theprogramthenaddsanumbertomemberyearbycallingfunctionaddyear，andprintsthedateintheEnglishstandardformat.AccessingMembersofStructuresIntroductiontoStructuresStructureswithMemberFunctionsTheresultofthisprogramisthesameastheoneofExample3-1.However，thefunctionsdefinedinLines9，10and11areplacedinsidetheDatestruetureasmembers.Thesefunctionsareaccessedusingamemberaccessoperator()，asmentionedinLines18and21.IntroductiontoStructures02DataAbstractionandClassesDataAbstractionAsthedevelopedsoftware

becomes

more

complex，wedesignmethodsanddata

structuresinparallel.Weprogressfromthelogicalorabstractdatastructureenvisionedatthe

toplevelthroughtherefinementprocessuntilwereachtheconcretecoding.DataAbstractionandClassesChapter1introducedtheprogrammingmethodologycalledobject-orientedprogramming

（OOP）.InOOP，thefirststepistoidentifythecomponents（calledobjects）byusingtheprincipleofdataabstraction.Anobjectisthevariableofaclasswhichcombinesdataandtheoperationsonthatdatainasingleunit.TheaimoftheC++classconceptistoprovidetheprogrammerwithatoolforcreatingnewtypesthatcanbeusedasconvenientlyasthebuilt-intypes.DefiningClassesDataAbstractionandClassesAnobjeetdoesnotexistuntilaninstanceoftheclasshasbeencreated；theclassisjustadefinition.Whentheobjectisphysicallycreated，thespaceforthatobjectisallocatedinthememory.Itispossibletohavemultipleobjeetscreatedfromoneclass.DefiningObjectsDataAbstractionandClassesFigure3-1showsavisualimageoftheclassobjectstodayandtomorrow.DataAbstractionandClassesIn-ClassMemberFunctionDefinitionAmemberfunctiondefinedwithintheclassdefinitionratherthansimplydeclaredthereistakentobeaninlinememberfunction.Thatis，in-classdefinitionofmemberfunctionsisforsmall，frequently-usedfunctions.DataAbstractionandClassesFileStructureofanAbstractDataTypeAnabstractdatatype（ADT）consistsoftwoparts：aspecificationandanimplementation.Thespecificationdescribesthebehaviorofthedatatypewithoutrefcrencetoitsimplementation.Theimplementationcreatesanabstractionbarrierbyhidingtheconcretedatarepresentationaswellasthecodefor

theoperations.DataAbstractionandClasses03InformationHidingTheprevioussectiondefinedtheDateclasstoimplementthedataoperationsinaprogram.Wethenwroteaprogram（Date.cpp）thatusedtheDateclass.Infaet，wecombinedtheDateclasswiththefunctiondefinitionstoimplementtheoperationsandthemainfunetionsoastocompletetheprogram.Thatis，thespecificationandimplementationdetailsoftheDateclass

weredirectlyincorporatedintotheprogram.InformationHidingIfseveralprogrammersusethesameobjectinaprojectwithdirectaccesstotheinternalpartsoftheobject，thereisnoguaranteethateveryprogrammerwillusethesameobjectinexactlythesameway.Thus，wemusthidetheimplementationdetails.Theusershouldknow

onlywhattheobjectdoes，nothowitdoes.InformationHidingHidingtheimplementationdetailsfreestheuserfromhavingtofitthisextrapieceofcodeintheprogram.Also，byhidingthedetails，wecanensurethatanobjectwillbeusedinexactlythesamewaythroughouttheproject.Furthermore，onceanobjecthasbeenwritten，debugged，andtestedproperly，itbecomes（andremains）error-free.InformationHidingToimplementDateinaprogram，theusermustdeclareobjectsofclassDate，andknow

whatoperationsareallowedandwhattheoperationsdo.Todoso，theusermusthaveaccesstothespecificationdetails.Astheuserisnotconcernedwiththeimplementationdetails，wemustputthosedetailsinaseparatefile.InformationHiding04AccessControlAkeyfeatureofobject-orientedprogrammingisinformation（data）hiding.Thistermdoesnotrefertotheactivitiesofparticularlyparanoidprogrammers；ratheritmeansthatdataisconcealedwithinaclasssothatitcannotbeaccessedmistakenlybyfunctionsoutsidetheclass.AccessControlTheprimarymechanismforhidingdataistoputitinaclassandmakeitprivate.Privatedataorfunctionscanonlybeaccessedfromwithintheclass.Publiedataorfunctions，ontheotherhand，areaccessiblefromoutsidetheclass.AccessControlThedeclarationoftheDateclassintheprevioussectionprovidesasetoffunctionsformanipulatingadate.However，itdoesnotspecifythatthosefunctionsshouldbetheonlyonestodependdirectlyonDate'srepresentationandtheonlyonestoaccessdirectlytotheDateobjects.AccessControlThepublicspecifierseparatestheclassbodyintotwoparts.Thenamesinthefirstpartwiththeprivatekeyword，includingdatamembersday，monthandyear，canbeusedonlybymemberfunctions.Thesecondpartwiththepublickeyword，includingmemberfunctionsinit_date，add_yearandprint，eonstitutesthepublicinterfacetoobjectsoftheclass.AccessControl05ConstructorsTheuseofthefunction，suchasinit_date，toprovideinitializationforclassobjectsis

inelegant.Becauseitisnowherestatedthatanobjectmustbeinitialized，aprogrammermay

forgettodosoordosotwice（oftenwithequallydisastrousresults）.ConstructorsDefinitionofConstructorsAconstructorisrecognizedbyhavingthesame

name

astheclassitself.Itservesthe

purposeofinitializingobjects.ConstructorsToguaranteethatthemembersofaclassareinitialized，youuseconstructors.Therearetwotypesofconstructors:withparametersandwithoutparameters.Theconstructorwithoutparametersiscalledthedefaultconstructor.OverloadingConstructorsConstructorsConstructorswithDefaultParametersAconstructorcanalsohavedefaultparameters.Insuchclass，therulesfordeclaringformalparametersarethesameasthosefordeclaringdefaultparametersinafunction.Moreover，actualparameterstoaconstructorwithdefaultparametersarepassedaccordingtotherulesforfunctionswithdefaultparameters.Constructors06DestructorsDefinitionofDestructorsAconstructorinitializesanobjeet.Inotherwords，itcreatestheenvironmentinwhichthememberfunctionsoperate.Sometimes，creatingthatenvironmentinvolvesacquiringaresource，suchasafile，alock，orsomememory，thatmustbereleasedafteruse.Thus，someclassesneedafunctionthatisguaranteedtobeinvokedwhenanobjectisdestroyedina

mannersimilartothewayaconstruetorisguaranteedtobeinvokedwhenanobjectiscreated.DestructorsUMLDiagramforClassesTheUnifiedModellingLanguage（UML）isnowthemostwidelyusedgraphicalrepresentationschemeformodelingobject-orientedsystem.Ithasindeedunifiedthevariouspopularschemes.Thosewhodesignsystemsusethelanguage（intheformofthediagram）tomodeltheirsystem，aswedothroughoutthisbook.DestructorsTheOrderofConstructorandDestructorCallsConstructorsanddestructorarecalledautomaticallybythecompiler.Theorderinwhichthesefunctioncallsaremadedependsontheorderinwhichtheirexecutionentersandleavesthescopesinwhichobjectsareinstantiated.Generally，thedestructorcallsaremadeinthereverseorderofthecorrespondingconstructorcalls.Destructors07EncapsulationEncapsulationisoneofthefundamentalprinciplesofobject-orientedprogramming.Itisthegroupingofrelatedideasintooneunit，whichcanthereafterbereferredtobyasinglename.Theprocessiscombiningdataandfunctionsintoasingleunitcalledclass.EncapsulationEncapsulationmeansthatalloftheobject'sdataiscontainedandhiddenintheobjectandsuchdataaccessisrestrictedtothemembersofthatclass，thatis，theprogrammercannotdirectlyaccessthedata.Dataisonlyaccessiblethroughthefunctionsexistinginsidetheclass.EncapsulationTheCounterclassinerementsanumberbyonesuccessivelyandreturnsthesum.ThepublicmemberfunctionsaddandgetSumaretheinterfacestotheoutsideworldandauserneedstoknowthemtousetheclass.Theprivatemembersumissomethingthatishiddenfrom

theoutsideworldbutisneededforproperclassoperation.EncapsulationInaword，Dataencap