C++面向对象程序设计双语教程（第3版）课件 ch04Advance of Classes and Objects-Further Definition of Class Members and Objects

AdvanceofClassesandObjects—FurtherDefinitionofClassMembersandObjectsC++面向对象程序设计双语教程(第3版)Chapter401ConstantMemberFunctionsandConstantObjectsTheDateclassdefinedsofarprovidesmemberfunctionsinordertogiveaDateobjectvaluesandchangeit.Unfortunately，wedidnotprovideawayofexaminingthevalueofaDate.Thisproblemcaneasilyberemediedbyaddingfunctionsforreadingtheday,month,andyear.ConstantMemberFunctionsandConstantObjectsNoteTheconstkeywordisplacedafterthe（empty）parameterlistinthefunctiondeclarations.It

indicatesthatthesefunctionseannotmodifythedatacontentofaDateclass.const关键字放置在函数声明中的（空）参数列表之后，它表示这些函数不能修改Date类的数据内容。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指针不是普通的变量。因为this指针是不可修改的，所以不允许给this指针赋值。thisPointers03StaticMembersInsideaclassdefinition，theclassmemberscanbedeclaredusingthestorageclassspecifierstatieintheclassmemberlist.Staticmembersofaclassarenotassociatedwiththeobjectsoftheclass：theyareindependentvariableswithstaticstorageduration.StaticMembersAstaticmemberisavariablethatispartofaclass，yetisnotpartofanobjectofthatclass.Onlyone

copyofthestaticmemberissharedbyallobjectsofaclassinaprogram.静态成员是一个类的部分的变量，但不是该类对象的一部分。程序中一个类的所有对象只共享静态成员的一个副本。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为了确保成员对象的初始化要求，必须满足下列条件之一：包含对象的类不需要构造函数。包含对象的类有一个可访问的默认构造函数。包含类的构造函数都显式地初始化包含的对象。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.类的友元是一个函数或类，它不是类的成员，但是被赋予与类的成员相同的访问权。FriendsFunct