计算机专业毕业设计中英对照外文翻译对象的创建和存在时间Word文档下载推荐.docx

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外文资料

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）.Smalltalkhasaverycompletesetofcontainers.Javaalsohascontainersinitsstandardlibrary.Insomelibraries,agenericcontainerisconsideredgoodenoughforallneeds,andinothers（Java,forexample）