设计论文外文文献中英文翻译.docx

设计论文外文文献中英文翻译.docx

《设计论文外文文献中英文翻译.docx》由会员分享，可在线阅读，更多相关《设计论文外文文献中英文翻译.docx（5页珍藏版）》请在冰豆网上搜索。

设计论文外文文献中英文翻译

皮晨晖软件081班

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.

1.Thesinglyrootedhierarchy

OneoftheissuesinOOPthathasbecomeespeciallyprominentsincetheintroductionofC++iswhetherallclassesshouldultimatelybeinheritedfromasinglebaseclass.InJava（aswithvirtuallyallotherOOPlanguages）theansweris”yes”andthenameofthisultimatebaseclassissimplyObject.Itturnsoutthatthebenefitsofthesinglyrootedhierarchyaremany.

Allobjectsinasinglyrootedhierarchyhaveaninterfaceincommon,sotheyareallultimatelythesametype.Thealternative（providedbyC++）isthatyoudon’tknowthateverythingisthesamefundamentaltype.Fromabackward-compatibilitystandpointthisfitsthemodelofCbetterandcanbethoughtofaslessrestrictive,butwhenyouwanttodofull-onobject-orientedprogrammingyoumustthenbuildyourownhierarchytoprovidethesameconveniencethat’sbuiltintootherOOPlanguages.Andinanynewclasslibraryyouacquire,someotherincompatibleinterfacewillbeused.Itrequireseffort（andpossiblymultipleinheritance）toworkthenewinterfaceintoyourdesign.Istheextra”flexibility”ofC++worthit?

Ifyouneedit—ifyouhavealargeinvestmentinC—it’squitevaluable.Ifyou’restartingfromscratch,otheralternativessuchasJavacanoftenbemoreproductive.

Allobjectsinasinglyrootedhierarchy（suchasJavaprovides）canbeguaranteedtohavecertainfunctionality.Youknowyoucanperformcertainbasicoperationsoneveryobjectinyoursystem.Asinglyrootedhierarchy,alon