网络即时通信系统的设计 外文文献翻译.doc
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本科毕业设计外文文献翻译
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XXX工业大学本科毕业设计外文文献翻译
Objectlandscapesandlifetimes
Technically,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.
1Collectionsanditerators
Ifyoudon’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).Smalltalkhasaverycompletesetofcon