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外文资料ObjectlandscapesandlifetimesTechnically,OOPisjustaboutabstractdatatyping,inheritance,andpolymorphism,butotherissuescanbeatleastasimportant.Theremainderofthissectionwillcovertheseissues.Oneofthemostimportantfactorsisthewayobjectsarecreatedanddestroyed.Whereisthedataforanobjectandhowisthelifetimeoftheobjectcontrolled?Therearedifferentphilosophiesatworkhere.C++takestheapproachthatcontrolofefficiencyisthemostimportantissue,soitgivestheprogrammerachoice.Formaximumrun-timespeed,thestorageandlifetimecanbedeterminedwhiletheprogramisbeingwritten,byplacingtheobjectsonthestack(thesearesometimescalledautomaticorscopedvariables)orinthestaticstoragearea.Thisplacesapriorityonthespeedofstorageallocationandrelease,andcontrolofthesecanbeveryvaluableinsomesituations.However,yousacrificeflexibilitybecauseyoumustknowtheexactquantity,lifetime,andtypeofobjectswhileyou'rewritingtheprogram.Ifyouaretryingtosolveamoregeneralproblemsuchascomputer-aideddesign,warehousemanagement,orair-trafficcontrol,thisistoorestrictive.Thesecondapproachistocreateobjectsdynamicallyinapoolofmemorycalledtheheap.Inthisapproach,youdon'tknowuntilrun-timehowmanyobjectsyouneed,whattheirlifetimeis,orwhattheirexacttypeis.Thosearedeterminedatthespurofthemomentwhiletheprogramisrunning.Ifyouneedanewobject,yousimplymakeitontheheapatthepointthatyouneedit.Becausethestorageismanageddynamically,atrun-time,theamountoftimerequiredtoallocatestorageontheheapissignificantlylongerthanthetimetocreatestorageonthestack.(Creatingstorageonthestackisoftenasingleassemblyinstructiontomovethestackpointerdown,andanothertomoveitbackup.)Thedynamicapproachmakesthegenerallylogicalassumptionthatobjectstendtobecomplicated,sotheextraoverheadoffindingstorageandreleasingthatstoragewillnothaveanimportantimpactonthecreationofanobject.Inaddition,thegreaterflexibilityisessentialtosolvethegeneralprogrammingproblem.Javausesthesecondapproach,exclusively].Everytimeyouwanttocreateanobject,youusethenewkeywordtobuildadynamicinstanceofthatobject.There'sanotherissue,however,andthat'sthelifetimeofanobject.Withlanguagesthatallowobjectstobecreatedonthestack,thecompilerdetermineshowlongtheobjectlastsandcanautomaticallydestroyit.However,ifyoucreateitontheheapthecompilerhasnoknowledgeofitslifetime.InalanguagelikeC++,youmustdetermineprogrammaticallywhentodestroytheobject,whichcanleadtomemoryleaksifyoudon’tdoitcorrectly(andthisisacommonprobleminC++programs).Javaprovidesafeaturecalledagarbagecollectorthatautomaticallydiscoverswhenanobjectisnolongerinuseanddestroysit.Agarbagecollectorismuchmoreconvenientbecauseitreducesthenumberofissuesthatyoumusttrackandthecodeyoumustwrite.Moreimportant,thegarbagecollectorprovidesamuchhigherlevelofinsuranceagainsttheinsidiousproblemofmemoryleaks(whichhasbroughtmanyaC++projecttoitsknees).Therestofthissectionlooksatadditionalfactorsconcerningobjectlifetimesandlandscapes.1CollectionsanditeratorsIfyoudon’tknowhowmanyobjectsyou’regoingtoneedtosolveaparticularproblem,orhowlongtheywilllast,youalsodon’tknowhowtostorethoseobjects.Howcanyouknowhowmuchspacetocreateforthoseobjects?Youcan’t,sincethatinformationisn’tknownuntilrun-time.Thesolutiontomostproblemsinobject-orienteddesignseemsflippant:youcreateanothertypeofobject.Thenewtypeofobjectthatsolvesthisparticularproblemholdsreferencestootherobjects.Ofcourse,youcandothesamethingwithanarray,whichisavailableinmostlanguages.Butthere’smore.Thisnewobject,generallycalledacontainer(alsocalledacollection,buttheJavalibraryusesthatterminadifferentsensesothisbookwilluse“container”),willexpanditselfwhenevernecessarytoaccommodateeverythingyouplaceinsideit.Soyoudon’tneedtoknowhowmanyobjectsyou’regoingtoholdinacontainer.Justcreateacontainerobjectandletittakecareofthedetails.Fortunately,agoodOOPlanguagecomeswithasetofcontainersaspartofthepackage.InC++,it’spartoftheStandardC++LibraryandissometimescalledtheStandardTemplateLibrary(STL).ObjectPascalhascontainersinitsVisualComponentLibrary(VCL).Smalltalkhasaverycompletesetofcontainers.Javaalsohascontainersinitsstandardlibrary.Insomelibraries,agenericcontainerisconsideredgoodenoughforallneeds,andinothers(Java,forexample)thelibraryhasdifferenttypesofcontainersfordifferentneeds:avector(calledanArrayListinJava)forconsistentaccesstoallelements,andalinkedlistforconsistentinsertionatallelements,forexample,soyoucanchoosetheparticulartypethatfitsyourneeds.Containerlibrariesmayalsoincludesets,queues,hashtables,trees,stacks,etc.Allcontainershavesomewaytoputthingsinandgetthingsout;thereareusuallyfunctionstoaddelementstoacontainer,andotherstofetchthoseelementsbackout.Butfetchingelementscanbemoreproblematic,becauseasingle-selectionfunctionisrestrictive.Whatifyouwanttomanipulateorcompareasetofelementsinthecontainerinsteadofjustone?Thesolutionisaniterator,whichisanobjectwhosejobistoselecttheelementswithinacontainerandpresentthemtotheuseroftheiterator.Asaclass,italsoprovidesalevelofabstraction.Thisabstractioncanbeusedtoseparatethedetailsofthecontainerfromthecodethat’saccessingthatcontainer.Thecontainer,viatheiterator,isabstrac