英语文献翻译.docx

英语文献翻译.docx

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英语文献翻译

Objectlandscapesandlifetimes

Technically,OOPisjustaboutabstractdatatype,inheritanceandpolymorphism,butotherissuescanbeatleastasimportant.Theremainderofthissectionwillcovertheseissues.

Oneofthemostimportantfactorsisthewayobjectsarecreatedanddestroyed.Whereisthedataforanobjectandhowisthelifetimeoftheobjectcontrolled?

Therearedifferentphilosophiesatworkhere.C++takestheapproachthatcontrolofefficiencyisthemostimportantissue,soitgivestheprogrammerachoice.Formaximumrun-timespeed,thestorageandlifetimecanbedeterminedwhentheprogramisbeingwritten,byplacingtheobjectsonthestack（thesearesometimescalledautomaticorscopedvariables）orinthestaticstoragearea.Thisplacesapriorityonthespeedofstorageallocationandrelease,andcontrolofthesecanbeveryvaluableinsomesituations.However,yousacrificeflexibilitybecauseyoumustknowtheexactquantity,lifetime,andtypeofobjectswhileyou'rewritingtheprogram.Ifyouaretryingtosolveamoregeneralproblemsuchascomputer-aideddesign,warehousemanagement,orair-trafficcontrol,thisistoorestrictive.

Thesecondapproachistocreateobjectsdynamicallyinapoolofmemorycalledtheheap.Inthisapproach,youdon'tknowhowmanyobjectsyouneed,whattheirlifetimeis,orwhattheirexacttypeisuntilrun-time.Thosearedeterminedatthespurofthemomentwhiletheprogramisrunning.Ifyouneedanewobject,yousimplymakeitontheheapatthepointthatyouneedit.Becausethestorageismanageddynamically,atrun-time,theamountoftimerequiredtoallocatestorageontheheapissignificantlylongerthanthetimetocreatestorageonthestack.（Creatingstorageonthestackisoftenasingleassemblyinstructiontomovethestackpointerdown,andanothertomoveitbackup.）Thedynamicapproachmakesthegenerallylogicalassumptionthatobjectstendtobecomplicated,sotheextraoverheadoffindingstorageandreleasingthatstoragewillnothaveanimportantimpactonthecreationofanobject.Inaddition,thegreaterflexibilityisessentialtosolvethegeneralprogrammingproblem.

C++usesthesecondapproach,exclusively].Everytimeyouwanttocreateanobject,youusethenewkeywordtobuildadynamicinstanceofthatobject.

There'sanotherissue,however,andthat'sthelifetimeofanobject.Withlanguagesthatallowobjectstobecreatedonthestack,thecompilerdetermineshowlongtheobjectlastsandcanautomaticallydestroyit.However,ifyoucreateitontheheapthecompilerhasnoknowledgeofitslifetime.InalanguagelikeC++,youmustdetermineprogrammaticallywhentodestroytheobject,whichcanleadtomemoryleaksifyoudon’tdoitcorrectly（andthisisacommonprobleminC++programs）.Javaprovidesafeaturecalledagarbagecollectorthatautomaticallydiscoverswhenanobjectisnolongerinuseanddestroysit.Agarbagecollectorismuchmoreconvenientbecauseitreducesthenumberofissuesthatyoumusttrackandthecodeyoumustwrite.

Therestofthissectionlooksatadditionalfactorsconcerningobjectlandscapesandlifetimes.

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）thelibraryhasdifferenttypesofcontainersfordifferentneeds:

avector（calledanArrayListinJava）forconsistentaccesstoallelements,andalinkedlistforconsistentinsertionatallelements,forexample,soyoucanchoosethep