双语版C++程序设计（第3版） 课件 9 Pointers and Dynamic Memory

Chapter9PointersandDynamicMemory

9.1VariableaddressesEveryvariableandobjectusedinaC++programisstoredinaspecificplaceinmemory.Eachlocationinmemoryhasauniqueaddressuses&togettheaddressofavariable9.1VariableaddressesHowthevariablesvar1andvar2arestoredinmemory?Differentcomputersmaygivedifferentaddressesfromtheonesabove.Variouscomputersandoperatingsystemswillstorevariablesatdifferentmemorylocations.Theaddressesareinhexadecimal(base16).9.2Pointervariables（指针变量）Apointervariableisavariablethatholdstheaddressofanothervariable.data_typeisanydatatype(suchaschar,int,float,astruct,aclassandsoon)variable_nameisanyvalidvariablename.Whitespaceinapointerdefinitionisnotrelevant.9.2PointervariablesPointerdefinitionsarereadbackwardsfromthevariablename,replacing*withthewords“isapointer”.int*int_ptrmeansthatint_ptrisapointertoanintfloat*float_ptrmeansthatfloat_ptrisapointertoafloat.9.2PointervariablesThetwovariablesptr1andptr2areusedtostoretheaddressesoftheothertwovariables,var1andvar2.9.3Thedereferenceoperator*（解引用运算符*）Thedereferenceoperator*isusedtoaccessthevalueofavariable,whoseaddressisstoredinapointer*ptrmeansthevalueofthevariableattheaddressstoredinthepointervariableptrInline9,the*isusedtodefineptrasapointertoanint.Inline13,the*isusedtoaccessthevalueofthememorylocation,theaddressofwhichisinptr.Line13displaysthevalueattheaddressheldinptrbyusingthedereferenceoperator*.Thisiscalleddereferencingthepointer

ptr.Thevalueof*ptristhesameasthevalueofvar.9.4UsingconstwithpointersWhendefiningapointer,thepointeritself,thevalueitpointstoorbothcanbemadeconstant.Thepositionofconstinthedefinitiondetermineswhichoftheseapply.Thedefinitionreadsas“pisapointertoanintegerconstant”.TheintegerisconstantandcannotbechangedusingpointerpThevalueoficanbechangedThepointermaybechanged9.4UsingconstwithpointersThisdefinitionofpreadsas“pisapointertoaconstantinteger”.thismeansthattheintegerisconstantandcannotbechangedusingpointerp.Thisdefinitionofpreads,“pisaconstantpointertoaninteger”.Thismeansthatthepointerisaconstantbutnotwhatitpointsto.9.4UsingconstwithpointersThisdefinitionofpreads,“pisaconstantpointertoanintegerconstant”.Thismeansthatboththepointerandtheintegeritpointstoareconstants.9.5Pointersandone-dimensionalarraysThenameofanarrayisapointertothefirstelementofthearray.Theelementsofthisarrayare:a[0],a[1],a[2],a[3],a[4]Thenameofthearrayaisequivalenttotheaddressofthefirstelement;aisthesameas&a[0]a+1istheaddressofthesecondelementa+2istheaddressofthethirdelement….9.5Pointersandone-dimensionalarraysAsthenameofanarrayisapointertothefirstelementofthearray,thedereferenceoperator*canbeusedtoaccesstheelementsofthearrayProgramExampleP9Etheexpression*(a+i)isnotequaltotheexpression*a+i9.5Pointersandone-dimensionalarraysUsepointerstoaccesstheelementsofanyarraynumbers[i]isequivalentto*(numbers+i).Althoughthenameofanarrayisapointertothefirstelementofthearray,youcannotchangeitsvalue;thisisbecauseitisaconstantpointer.a++ornumbers+=2areinvalid

9.5Pointersandone-dimensionalarraysAconstantmaybeaddedtoorsubtractedfromthevalueofapointer,allowingaccesstodifferentmemorylocations.Notallarithmeticoperationsarepermissibleonpointers.Themultiplicationoftwopointersisillegal,becausetheresultwouldnotbeavalidmemoryaddress.9.6Pointersandmulti-dimensionalarraysAccesstheelementsofamulti-dimensionalarrayusingpointers---increasinglycomplexAtwo-dimensionalarrayisstoredasan‘arrayofarrays’.Thismeansthataisaone-dimensionalarraywhoseelementsarethemselvesaone-dimensionalarraysofintegersThenameofatwo-dimensionalarrayisapointertothefirstelementofthearray.-------aisequivalentto&a[0]a[0]isitselfanarrayoftwointegers-------a[0]isequivalentto&a[0][0]9.6Pointersandmulti-dimensionalarraysa[0],a[1]anda[2]arepointers(datatypeisint*)andaisapointertoapointer(datatypeisint**).a[0]istheaddressofthefirstelementinthefirstrowofthearray.*a[0]isa[0][0],whichis4.a[1]istheaddressofthefirstelementinthesecondrow.*a[1]isa[1][0],whichis1.a[2]istheaddressofthefirstelementinthethirdrow.*a[2]isa[2][0],whichis9.9.6Pointersandmulti-dimensionalarraysa[0],a[1]anda[2]arepointers(datatypeisint*)andaisapointertoapointer(datatypeisint**).a[0]+1istheaddressofthesecondelementinthefirstrow.*(a[0]+1)isa[0][1],whichis6.*(a[1]+1)isa[1][1],whichis3.a[2]+1istheaddressofthesecondelementinthethirdrow.*(a[2]+1)isa[2][1],whichis7.a[1]+1istheaddressofthesecondelementinthesecondrow.9.6Pointersandmulti-dimensionalarrays*aisthesameasa[0]*(a+1)isthesameasa[1]*(a+2)isthesameasa[2]9.7Pointerstostructures（指向结构体的指针）DefineapointertoavariableofatypedefinedbystructorclassThegeneralformatfordefiningapointertoastructuretag_nameisthestructuretagvariable_nameisthenameofthepointervariableExample9.7PointerstostructuresDefineapointerptrtothestudent_recstructureAvaluecanbeassignedtoptrbyusingtheaddressoperator&Themembersofastructurevariablecanbereferencedbyusingthedereferenceoperator*.Notethatitistheaddressofthestructurevariablestudentandnottheaddressofthestructuretagstudent_recthatisassignedtoptrTheparenthesesarenecessary,becausetheselectionoperator.hasahigherprioritythanthedereferenceoperator*9.7PointerstostructuresForaccessingthemembersofastructure,thearrownotation->(-and>together)canbeusedinplaceofthedotnotationTheexpressionptr->numberreadsas“themembernumberofthestructurepointedtobyptr”.9.8PointerstoclassobjectsDefiningapointertoaclassobjectissimilartodefiningapointertoastructurevariable.class_nameisthenameoftheclass

variable_nameisthenameofthepointervariable.9.8PointerstoclassobjectsProgramP9FLine12definesac_ptrasapointertoabank_accountobjectLine14assignstheaddressofthebank_accountobjectactoac_ptr.9.8PointerstoclassobjectsThepublicmembersofaclassobjectmaybeaccessedbyusingthedereferenceoperator*.Thefirstpairofparenthesesarenecessary,becausetheselectionoperator.hasahigherprioritythanthedereferenceoperator*.Thearrownotation->canbeusedinplaceofthedotnotation->ismoreconvenientandcommon.9.9Pointersasfunctionarguments

（指针变量作为函数实参）ProgramExampleP9G:usespointersinplaceofreferencesLine19passestheaddressesofthetwofloating-pointvariablesnum1andnum2tothefunctionswap_vals().Theseaddressesarereceivedbytheparametersptr1andptr2,declaredaspointerstofloatsinthefunctionheaderonline24.Line26storesthevalueofnum1(=*ptr1)inthevariabletempLine28isequivalenttonum1=num2;Line29assignsthevalueoftemp(12.1)tonum2,because*ptr2isthesameasnum2.9.9PointersasfunctionargumentsItiseasiertousereferencesratherthanpointers&mustbeusedtopasstheaddressofthevariablestothefunctionWithinthefunctionthedereferenceoperator*mustbeusedtoaccessthevalueofeachofthenumbers.Thenumberoflibraryfunctionsusepointersasparametersctime(),convertingthetimeinsecondstoacharacterstringcontainingthedateandtime.9.9PointersasfunctionargumentsProgramExampleP9H9.10Dynamicmemoryallocation（动态内存分配）Problem:------Whendefininganarray,thenumberofelementsinthearraymustbespecified

inadvanceoftheprogramexecution.Sometimes,eitheralltheelementsspecifiedarenotusedormoreelementsthanwereoriginallyanticipatedarerequired.C++hastheabilitytoallocatememorywhileaprogramisexecutingUsingthememoryallocationoperatornew.9.10Dynamicmemoryallocation9.10.1AllocatingmemorydynamicallyforanarrayThenewmemoryallocationoperatorcanbeusedtoallocateacontiguous(连续的)blockofmemoryforanarrayofanydatatype,whetherthedatatypeisbuilt-inorisauser-definedstructureorclass.pointerisapointertotheallocatedmemorydata_typeisthedatatypeofthearray

sizeisthenumberofelementsinthearray9.10DynamicmemoryallocationWhenallocatingmemoryforanarrayofclassobjects,theremustbeadefaultconstructorfortheclasssothattheelementsofthearraygetinitialized.ProgramExampleP9I9.10DynamicmemoryallocationProgramExampleP9I…continuedLine15usesnewtoallocatetheexactnumberofelementsrequiredTheelementsofthenewlyallocatedarraycanbeaccessedusingeitherapointeroranindexTheallocatedmemoryisfreedusingthedeleteoperator.IMPORTANT:Includethesquarebrackets[]whenfreeingmemoryforapreviouslyallocatedarray.Withoutthesquarebrackets,onlythefirstelementofthearraywillbedeleted.9.10Dynamicmemoryallocation9.10.2InitializationwithnewWhenallocatingmemoryforanarrayofclassobjects,thedefaultconstructorfortheclassinitializestheelementsofthearray.DefaultconstructorforthebankaccountclassbeingcalledfivetimesNoinitializationisdonefordynamicallyallocatedarraysofbuilt-indatatypes9.10DynamicmemoryallocationSingleinstancesofanydatatype(built-in,auser-definedstructureoraclass)canbeinitializedusingasecondformofthenewoperator.pointerisapointertotheallocatedmemoryTheinitialvalueisoptionalAllocatememoryforaninteger,withaninitialvalueof100.Allocatememoryforaninteger,withnoinitialvalue.Allocatememoryforabankaccountobject.Thedefaultclassconstructoriscalledtodotheinitialization.Allocatememoryforabankaccountobject.Aconstructoriscalledtoassigninitialvaluestoaccountnumberandbalance.9.10DynamicmemoryallocationProgramExampleP9J9.10DynamicmemoryallocationProgramExampleP9J…continued9.10Dynamicmemoryallocation9.10.3Allocatingmemoryformulti-dimensionalarraysInC++,multi-dimensionalarraysareimplementedas‘arraysofarrays’.ProgramExampleP9K9.10DynamicmemoryallocationProgramExampleP9K…continuedLines44and47freethememoryallocatedinlines21and25separately.Foreachpointerreturnedfromnewinlines21and25thereisacorrespondingcalltodeletewiththatpointerinlines47and44.9.10Dynamicmemoryallocation9.10.4Outofmemoryerroritwasassumedthatthememoryrequestedwithnewwasallocated,regardlessofwhethermemorywasavailableornot.C++handlesinsufficientmemoryerrorsproducedbynewbycallingafunctionspecifiedinset_new_handler().ProgramExampleP9L9.10DynamicmemoryallocationLine27terminatestheprogramandexitstotheoperatingsystemwithastatuscodeof1.Anon-zerostatuscodeisusuallyusedtoindicateanabnormalexitfromaprogram.9.10DynamicmemoryallocationThefunctionout_of_memory()insertsanerrormessageintothestreamcerrratherthanintocout.Thestreamcerristypicallyusedforerrormessageswhilecoutisusedfordisplayingtheresultsofapro