C++面向?qū)ο蟪绦蛟O(shè)計雙語教程（第3版）課件 ch04Advance of Classes and Objects-Further Definition of Class Members and Objects

AdvanceofClassesandObjects—FurtherDefinitionofClassMembersandObjectsC++面向?qū)ο蟪绦蛟O(shè)計雙語教程(第3版)Chapter401ConstantMemberFunctionsandConstantObjectsTheDateclassdefinedsofarprovidesmemberfunctionsinordertogiveaDateobjectvaluesandchangeit.Unfortunately，wedidnotprovideawayofexaminingthevalueofaDate.Thisproblemcaneasilyberemediedbyaddingfunctionsforreadingtheday,month,andyear.ConstantMemberFunctionsandConstantObjectsNoteTheconstkeywordisplacedafterthe（empty）parameterlistinthefunctiondeclarations.It

indicatesthatthesefunctionseannotmodifythedatacontentofaDateclass.const關(guān)鍵字放置在函數(shù)聲明中的（空）參數(shù)列表之后，它表示這些函數(shù)不能修改Date類的數(shù)據(jù)內(nèi)容。ConstantMemberFunctionsandConstantObjectsSomeobjectsneedtobemodifiableandsomenot.Theconstkeywordcanbeusedtospecifythatanobjectisnotmodifiableandthatanyattempttomodifytheobjectshouldresultinacompilationerror.Aconstmemberfunctioncanbeinvokedforbothconstandnon-constobjects，whereasanon-constmemberfunctioncanbeinvokedonlyfornon-constobjects.ConstantMemberFunctionsandConstantObjects02thisPointersAnyobjecthasaccesstoitsownaddressthroughapointercalledthis.Butthethispointer

ofanobjectisnotpartoftheobjectsitself-i.e.thesizeofthememorytakenbythethispointerdoesnotaffecttheresultofasizeofoperationontheobjects.Forexample，boththesizesofthetodayandtomorrowobjeetsare12inthisexample.thisPointersWhenanon-staticmemberfunctioniscalledforanobjeet，thethispointerispassedasanimplicitargumenttoeachofthenon-staticmemberfunctionsofaclass，struct，oruniontype.However，staticmemberfunctionsdonothaveathispointer.thisPointersTheobject'saddressisavailablewithinthememberfunctionasthethispointer.Mostusagesofthisareimplicit.Itislegal，thoughunnecessary，toexplicitlyusethiswhenreferringtomembersoftheclass.thisPointersThethispointerisnotanordinaryvariable.Becausethethispointerisnon-modifiable，assignmentsto

this

are

not

allowed.this指針不是普通的變量。因為this指針是不可修改的，所以不允許給this指針賦值。thisPointers03StaticMembersInsideaclassdefinition，theclassmemberscanbedeclaredusingthestorageclassspecifierstatieintheclassmemberlist.Staticmembersofaclassarenotassociatedwiththeobjectsoftheclass：theyareindependentvariableswithstaticstorageduration.StaticMembersAstaticmemberisavariablethatispartofaclass，yetisnotpartofanobjectofthatclass.Onlyone

copyofthestaticmemberissharedbyallobjectsofaclassinaprogram.靜態(tài)成員是一個類的部分的變量，但不是該類對象的一部分。程序中一個類的所有對象只共享靜態(tài)成員的一個副本。StaticMembersAtypicaluseofstaticmembersisforrecordingdatacommontoallobjectsofaclass.Forexample，youcanuseastaticdatamemberasacountertostorethenumberofobjectsofaparticularclasstypethatarecreated.Eachtimeanewobjectiscreated，thisstaticdatamember

canbeincrementedtokeeptrackofthetotalnumberofobjects.StaticMembersWhenweinstantiateaclassobject，eachobjectgetsitsowncopyofallnormaldatamembers.Inthiscase，sincewehavedeclaredthreestudentclassobjects，weendupwiththreecopiesofcountoneinsidestl，thesecondinsidest2，andthethirdinsidest3.Theyshouldhavedifferentvaluesbecausethecountvariableisusedtokeeptrackofthetotalnumberof

objects.However，theygetthesamevalue0.StaticMembers04FreeStoreThefreestoreisapoolofmemoryavailableforyoutoalloeate（anddeallocate）storageforobjectsduringtheexecutionofyourprogram.Thenewanddeleteoperatorsareusedtoallocateanddeallocatefreestore，respectively.FreeStoreWhenyouaredonewithyourareaofmemory，youmustcallthedeletestatementonthepointer.Thedeletestatementreturnsthememorytothefreestore.Rememberthatthepointeritselfasopposedtothememorytowhichitpoints--isalocalvariable.Whenthefunctionin

whichitisdeclaredreturns，thatpointergoesoutofscopeandislost.FreeStoreThememoryallocated

byusingnewisnotfreedautomatically，however.Thatmemorybecomesunavailablea

situationcalledamemoryleak.Thenameisgiventodeseribethatthepartofmemorycannotberecovereduntiltheprogramends.Itisasthoughthememoryhasleakedoutofyourcomputer.FreeStoreJustasyoucancreateapointertoaninteger，youcancreateapointertoanyobject.IfyouhavedeclaredanobjectofclassDate，youcandeclareapointertothatclassandinstantiateaDateobjectonthefreestore，justasyoucanmakeoneonthestack.FreeStore05ObjectMembersDefinitionofObjectMembersSometimes，aclasshasadatamemberwhichisnotasimplebuilt-indatatypebutanaggregatebuilt-indata.Suchdatamemberisdefinedasanotherclassobject.Whenaclasshasobjectsofotherclassesasmembers，suchcapabilityiscalledcomposition.Compositionisaspecialcaseoftheaggregationrelationship.ObjectMembersForcreatingobjects，themembers

constructorsareinvokedbeforethebodyofthecontainingclass'ownconstructorisexecuted.Theconstructorsareinvokedintheorderinwhichthemembersaredeclaredintheclassratherthantheorderinwhichthemembersappear

intheinitializationlist.TheOrderofConstructorsandDestructorsforMemberObjectsObjectMembers為了確保成員對象的初始化要求，必須滿足下列條件之一：包含對象的類不需要構(gòu)造函數(shù)。包含對象的類有一個可訪問的默認構(gòu)造函數(shù)。包含類的構(gòu)造函數(shù)都顯式地初始化包含的對象。ObjectMembersClassMembersbyUsingInitializersMemberinitializersareessentialfortypesforwhichinitializationdiffersfromtheassignment，thatis，formemberobjectsofclasseswithoutadefaultconstructor，forconstmembers，andforreferencemembers.ObjectMembers06CopyMembersThecopyconstructortakesareferencetoaconstparameter.Itisaconsttoguaranteethatthecopyconstructordoesnotchangeit.Inaddition，itisareferencebecauseavalueparameterwouldrequiremakingacopy，whichwouldinvokethecopyconstructor，thereforemakinga

copyofitsparameter.CopyMembersDefinitionofCopyConstructorsTheaboveisanexampleofacopyconstructorfortheDateclass，whichdoesnotreallyneedonebecausethedefaultcopyconstructor'sactionofcopyingdatawouldworkfine.Still，itshowshowitworks.CopyMembersDefinitionofCopyConstructorsShallowCopyAshallowcopymeansthatC++copieseachmemberoftheclassindividuallyusingtheassignmentoperator（=）.Whenclassesaresimple

e.g.donotcontainanydynamically

allocatedmemory），thisworksverywellliketheexampleabove.CopyMembersDeepCopyTherootofthisproblemistheshallowcopydonebythecopyconstructordoingashallowcopyonpointervaluesinaeopyconstructororanoverloadedassignmentoperator（=）usuallymeanstrouble.Thesolutiontothisproblemistodoadeepcopyonanynon-null

pointersbeingcopied.CopyMembers07ArraysofObjectsInitializeanObjectArraybyUsingaDefaultConstructorSupposethatyoudeclareanarrayof4classobjects，forexample，4Dateobjects.youneedtodisplaytheirdays，monthsandyears.YouhavetodeclareonearrayarrayDateof4elementseach，whereineachelementisanobjectoftypeDate.ArraysofObjectsInitializeanObjectArraybyUsingaDefaultConstructorInthisease，itisimpracticaltospecifydifferentconstructorsforeachelement.Therefore，thedefaultconstructorisusedtoinitializeeach（array）classobject.Ifaclasshasconstructorsandyoudeclareanobjeetofthatclass，theclassshouldhavethedefaultconstructor.ArraysofObjects08FriendsAsdiscussedintheprevioussectionsonaccessspecifiers，toaccessdatamembersday，monthandyearfromtheoutsideoftheclassbythemainfunction，thedatamembersaredeclaredasprivateinsidetheDateclassinthepreviousexamples，becausethemainfunctionisnotaclassmemberthatwillnotbeabletoaceesstheprivatedata.FriendsToaccesstheprivatedatamembers，datamembersareputintothepublicpartoftheclass.However，thisbreakstheinformationhidingoftheclass.Infact，aprogrammermayhaveasituationwhereheorshewouldneedtoaccessprivatedatafromnon-memberfunctions.Forhandlingsuchcases，the

conceptoffriendfunetionsisausefultool.FriendsAfriendofaclassisafunetionorclassthatisnotamemberoftheclass，butisgrantedthesameaccess

totheclassasthemembersoftheclass.類的友元是一個函數(shù)或類，它不是類的成員，但是被賦予與類的成員相同的訪問權(quán)。FriendsFunct