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UsingPrototypicalObjectstoImplementSharedBehaviorinObjectOrientedSystems

HenryLieberman

ArtificialIntelligenceLaboratoryMassachusettsInstituteofTechnology

Cambridge,Mass.02139USA

Electronicmail:

Henry@AI.AI.MIT.Edu

Abstract

Atraditionalphilosophicalcontroversybetweenrepresentinggeneralconceptsasabstractsetsorclassesandrepresentingconceptsasconcreteprototypesisreflectedinacontroversybetweentwomechanismsforsharingbehaviorbetweenobjectsinobjectorientedprogramminglanguages.Inheritancesplitstheobjectworldintoclasses,whichencodebehaviorsharedamongagroupofinstances,whichrepresentindividualmembersofthesesets.Theclass/instancedistinctionisnotneededifthealternativeofusingprototypesisadopted.Aprototyperepresentsthedefaultbehaviorforaconcept,andnewobjectscanreusepartoftheknowledgestoredintheprototypebysayinghowthenewobjectdiffersfromtheprototype.Theprototypeapproachseemstoholdsomeadvantagesforrepresentingdefaultknowledge,andincrementallyanddynamicallymodifyingconcepts.Delegationisthemechanismforimplementingthisinobjectorientedlanguages.Aftercheckingitsidiosyncraticbehavior,anobjectcanforwardamessagetoprototypestoinvokemoregeneralknowledge.Becauseclassobjectsmustbecreatedbeforetheirinstancescanbeused,andbehaviorcanonlybeassociatedwithclasses,inheritancefixesthecommunicationpatternsbetweenobjectsatinstancecreationtime.Becauseanyobjectcanbeusedasaprototype,andanymessagescanbeforwardedatanytime,delegationisthemoreflexibleandgeneralofthetwotechniques.

1.Setsvs.prototypes:aphilosophicaldilemmawithpracticalconsequences

Howdopeoplerepresentknowledgeaboutgeneralizationstheymakefromexperiencewithconcretesituations?Philosophersconcernedwiththetheoryofknowledgehavedebatedthisquestion,butasweshallsee,theissueisnotwithoutpracticalconsequencesforthetaskofrepresentingknowledgeinobjectorientedsystems.Becausemuchofobjectorientedprogramminginvolvesconstructingrepresentationsofobjectsintherealworld,ourmechanismsforstoringandusingrealworldknowledgegetreflectedinmechanismsfordealingwithobjectsincomputerlanguages.We'llexaminehowthetraditionalcontroversybetweenrepresentingconceptsassetsversusrepresentingconceptsasprototypesgivesrisetotwomechanisms,inheritanceanddelegation,forsharingbehaviorbetweenrelatedobjectsinobject

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orientedlanguages.

Whenapersonhasexperienceinaparticularsituation,sayconcerningaparticularelephantnamedClyde,factsaboutClydecanoftenproveusefulwhenencounteringanotherelephant,sayoneFred.Ifwehavementalrepresentationsofaconceptfor(Clyde,andaconceptforFred,thequestionthen:HowdotherepresentationsofClydeandFredshareknowledge?Howcanweanswerquestions,suchasFred'scolor,numberoflegs,size,etc.byreferencetowhatwealreadyknowaboutClyde?Intheabsenceofanymechanismforsharingknowledgebetweenrelatedconcepts,we'dhavetorepeatalltheknowledgeaboutClydeinarepresentationofFred.

Therearetwopointsofviewwecanconsideradopting.Thefirstisbasedontheideaofabstractsets.FromlearningaboutClyde,wecanconstructaconceptoftheset[orclass]ofelephants,whichabstractsoutwhatwebelieveistrueaboutallindividualanimalssufficientlysimilartoClydetobecalledelephants.Thedescriptionofthesetcanenumerateallthe"essential"propertiesofelephants.WecanviewClydeasamemberorinstanceofthisclass.Inanobjectorientedsystem,thesetapproachinvolvescreatinganobjecttorepresentthesetofelephants,andestablishingalinkrepresentingthemembershiprelationbetweentheobjectrepresentingClydeandthesetobject.Sincethedescriptionofthesetrepresentswhatistrueaboutallitsmembers,wecananswerquestionsaboutClydebyreferringtothedescriptionoftheset.EstablishingthesamekindofmembershiplinkbetweenFredandthesetofelephantsenablesFredandClydetosharesomeofthesameknowledge.IfFredandClydesharesomeadditionalproperties,suchasthatofbeingIndianelephants,thatarenotsharedbysomeotherelephants,thesecanbeembodiedinasubclassobject,whichsharesallthepropertiesoftheelephantset,adjoiningtheadditionalpropertiesrelevanttoIndia.

Butthere'sanalternativepointofview.WecanconsiderClydetorepresenttheconceptofaprototypicalelephant.IfIaskyouto"thinkofanelephant",nodoubtthementalimageofsomeparticularelephantwillpoptomind,completewiththecharacteristicsofgraycolor,trunk,etc.IfClydewastheelephantmostfamiliartoyou,theprototypicalelephantmightbeanimageofClydehimself.IfIaskyouaquestionsuchas"Howmanylegsdoesanelephanthave?",awaytoanswerthequestionistoassumethattheansweristhesameashowmanylegsClydehas,unlessthere'sagoodreasontothinkotherwise.TheconceptofFredcanhaveaconnectionmarkingitsprototypeasClyde,asamechanismforsharinginformationbetweenthetwoweightypachyderms.ThedescriptionofFredcanstoreanyinformationthatisuniquetoFredhimself.IfIask"HowmanylegsdoesFredhave?",youassumetheansweristhesameforFredasforClyde,intheabsenceofanycontraryevidence.IfyouthenlearnthatFredisathreeleggedelephant,thatknowledgeisstoredwithFredandisalwayssearchedbeforereferencetotheprototypeismade.

2.Prototypeshaveadvantagesforincrementallearningofconcepts

Thoughtheconceptofasethasprovenfruitfulinmathematics,theprototypeapproachinsomewayscorrespondsmorecloselytothewaypeopleseemtoacquireknowledgefromconcretesituations.Thedifficultywithsetsstemsfromtheirabstractnesspeopleseemtobealotbetteratdealingwithspecificexamplesfirst,thengeneralizingfromthemthantheyareatabsorbinggeneralabstractprinciplesfirst,andlaterapplyingtheminparticularcases.Prototypesystemsallowcreatingindividualconceptsfirst,thengeneralizingthembysayingwhataspectsoftheconceptareallowedtovary.Setorientedsystemsrequirecreatingtheabstractdescriptionofthesetfirst,beforeindividualinstancescanbeinstalledasmembers.

Inmathematics,setsaredefinedeitherbyenumeratingtheirmembers,orbydescribingtheunifyingprinciplesthatidentifymembershipintheset.Wecanneitherenumeratealltheelephants,norarewegoodatmakingdefinitiveliststheessentialpropertiesofanelephant.Yetthemajorimpetusforcreating

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newconceptsalwaysseemstobeexperiencewithexamples.IfClydeisouronlyexperiencewithelephants,ourconceptofanelephantcanreallybenodifferentthantheconceptofClyde.Aftermeetingotherelephants,theanalogieswemakebetweenconceptslikeFredandClydeservetopickouttheimportantcharacteristicsofelephants.

Prototypesseemtobebetteratexpressingknowledgeaboutdefaults.Ifweassertgraynessasoneoftheidentifyingcharacteristicsofmembershipinthesetofelephants,wecan'tsaythatthereareexceptionalwhiteelephantswithoutriskingcontradictions.YetitiseasytosaythatFred,thewhiteelephant,isjustlikeClyde,exceptthatheiswhite.AsWittgensteinobserved,itisdifficulttosay,inadvance,exactlywhatcharacteristicsareessentialforaconcept.Itseemsthatasnewexamplesarise,peoplecanalwaysmakenewanalogiestopreviousconceptsthatpreservesomeofthe"defaults"forthatconceptandignoreothers.

3.Inheritanceimplementssets,delegationimplementsprototypes

Havingsetthestagewithourphilosophicaldiscussionoftheissuesofconceptrepresentation,weturnnowtohowtheseissuesaffectthemoremundanedetailsofimplementationofobjectorientedprogrammingsystems.

Implementingthesettheoreticapproachtosharingknowledgeinobjectorientedsystemsistraditionallydonebyamechanismcalledinheritance,firstpioneeredbythelanguageSimula,lateradoptedbySmallTalk,flavorsandLoops,amongothers.Anobjectcalledaclassencodescommonbehaviorforasetofobjects.Aclassalsohasadescriptionofwhatcharacteristicsareallowedtovaryamongmembersoftheset.Classeshavethepowertogenerateinstanceobjects,whichrepresentmembersofaset.Allinstancesofaclasssharethesamebehavior,butcanmaintainuniquevaluesforasetofstatevariablespredeclaredbytheclass.TorepresentClyde,youcreateadescriptionfortheclasselephant,withaninstancevariablefortheelephant'sname,valuesofwhichcanbeusedtodistinguishClydeandFred.Aclasscangiverisetosubclasses,whichaddadditionalvariablesandbehaviortotheclass.

Implementingtheprototypeapproachtosharingknowledgeinobjectorientedsystemsisanalternativemechanismcalleddelegation,appearingintheactorlanguages,andseveralLispbasedobjectorientedsystemssuchasDirector[Kahn79],T[Rees85],Orbit[Steels82],andothers.Delegationremovesthedistinctionbetweenclassesandinstances.Anyobjectcanserveasaprototype.Tocreateanobjectthatsharesknowledgewithaprototype,youconstructanextensionobject,whichhasalistcontainingitsprototypeswhichmaybesharedwithotherobjects,andpersonalbehavioridiosyncratictotheobjectitself.Whenanextensionobjectreceivesamessage,itfirstattemptstorespondtothemessageusingthebehaviorstoredinitspersonalpart.Iftheobject'spersonalcharacteristicsarenotrelevantforansweringthemessage,theobjectforwardsthemessageontotheprototypestoseeifonecanrespondtothemessage.Thisprocessofforwardingiscalleddelegatingthemessage.FredtheelephantwouldbeanextensionobjectthatstoredbehavioruniquetoFredinitspersonalpart,andreferencedtheprototypeClydeinitssharedpart.

4.Toolsforrepresentingbehaviorandinternalstatearethebuildingblocksofobjectorientedsystems

Eachobjectorientedsystemmustprovidesomelinguisticmechanismsfordefiningthebehaviorofobjects.Thephilosophyofobjectorientedprogrammingistousetheobjectrepresentationtoencodeboththeproceduresanddataofconventionallanguages.Ratherthandefinetheproceduralbehaviororthedatacontentofanobjectallatonce,itisconvenienttobreakbothaspectsofanobjectintoasetofpartsthatcanbeaccessedormodifiedindividuallybyname.

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Anobject'sinternalstateconsistsofvariablesoracquaintances,whichcanbeaccessedinmostobjectorientedsystemsbysendingtheobjectamessageconsistingofthevariable'sname.Anobject'sprocedureforrespondingtomessages[inactors,wesayitsscript]canbecomposedofasetofprocedurescalledmethods,eachofwhichisspecializedforhandlingonlyacertainsubsetofthemessagestheobjectreceives,identifiedbyname.Breakingupanobject'sstateintonamedvariablesmeansthatdifferentportionsofthestatecanbemodifiedincrementally,withoutaffectingtheothers.Breakingupanobject'sbehaviorintonamedmethodsmeansthatdifferentportionsofthebehaviorcanbemodifiedincrementally,withoutaffectingtheothers.Thelanguagemustthenprovidewaysofcombininggroupsofmethodsandvariablestoformobjectsandsomemeansofallowinganobjecttosharebehavior[implementedasmethodsandvariables]residinginpreviouslydefinedobjects.Wewillcallthesecompositeobjectsextensions.Thesebuildingblocksarerepresentedintheillustration"Toolsforsharingknowledge"with"icons"tobeusedinfurtherdiscussion.

Manyobjectorientedlanguagessupplyprimitivelinguisticmechanismsforcreatingobjectswithmethods,variablesandextensions.Analternativeapproach,whichisadvocatedintheactorformalism,istodefinemethods,variablesandextensionsasobjectsintheirownright,withtheirbehaviordeterminedbyamessagepassingprotocolamongthem.Obviously,anobjectrepresentingamethodcannotitselfhavemethods,otherwiseinfiniterecursionwouldresult.Usingsimpleobjectsprimitivetothesystem,avariableisdefinedtobeanobjectthatremembersanameandavalue,andrespondstoaccessandmodificationmessages.Amethodrespondsonlytothosemessagesforwhichitisdesigned,rejectingothers.Extensionobjectsusedelegationtoforwardmessagesfromonepartoftheobjecttoanothertolocatetheappropriateresponse.

Everyonewhoisalreadyconvincedoftheutilityofobjectorientedprogrammingshouldn'thavemuchtroublediscerningtheadvantagesofusingobjectorientedprogrammingintheimplementationoftheknowledgesharingmechanisms.Foremostamongthemistheabilitytodefineotherkindsofobjectswhichimplementalternativestothestandardversions.Insteadofanordinaryvariable,onemightliketohave"active"variablesthattakeactionwhenchanged,"readonly"variables,maybeeven"writeonly"variables,eachofwhichcouldbedefinedasadifferenttypeofvariableobject.Alternativekindsofmethodobjectscanusedifferingstrategiestocombinebehaviorfromcontributingcomponents,replacingthesocalled"methodcombination"featureoftheflavorssystemandmaking"multipleinheritance"easier.Differentkindsofextensionobjectscanmakedifferentefficiencytradeoffsontheissueofcopyingversussharing.

ToolsforSharingKnowledge

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Themechanismsforsharingknowledgeinobjectorientedlanguageshavenowgrownsocomplicatedthatitisimpossibletoreachuniversalconsensusonthebestmechanism.Usingobjectorientedprogrammingitselftoimplementthebasicbuildingblocksofstateandbehavioristhebestapproachforallowingexperimentationandcoexistenceamongcompetingformalisms.

5.ALogoexampleillustratesthedifferencesbetweendelegationandinheritance

AnexamplefromthedomainofLogoturtlegraphicswillillustratehowthechoicebetweendelegationandinheritanceaffectsthecontrolanddatastructuresinanobjectorientedsystem.Thedelegationapproachisillustratedinthefiguretitled"SharingKnowledgewithDelegation".Thefirstthingwewouldliketodoiscreateanobjectrepresentingapen,whichremembersalocationonthescreen,andcanbemovedtoadifferentlocation,drawinglinesbetweentheoldandnewlocations.

SharingKnowledgewithDelegation

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Westartoutbycreatingaprototypicalpenobject,whichhasaspecificlocationonthescreenx=200,y=50,andbehaviortorespondtothedrawmessage.Whenwewouldliketocreateanewpenobject,weneedonlydescribewhat'sdifferentaboutthenewpenfromthefirstone,inthiscasethexvariable.Sincetheyisthesameandbehaviorforthedrawmessageisthesame,theseneednotberepeated.

Thedrawmethodwillhavetousethevalueofthexvariable,andit'simportantthatthecorrectvalueofxisused.Whenthedrawmethodisdelegatedfromthenewpentotheoldpen,eventhoughthedrawmethodoftheoriginalpenisinvoked,itshouldbethexofthenewpenthatisused.

Toinsurethis,wheneveramessageisdelegated,itmustalsopassalongtheobjectthatoriginallyreceivedthemessage.ThisiscalledtheSELFvariableinSimula,SmallTalkandflavors,althoughIfindtheterm"self"alittlemisleading,sinceamethodoriginallydefinedforonekindofobjectoftenwindsupsendingtoa"self"ofadifferentkind.Inactorterminology,thisobjectiscalledtheclient,sincetheobjectbeingdelegatedtocanbethoughtofasperformingaservicefortheoriginalobject.Whenapendelegatesadrawmessagetoaprototypicalpen,itissaying"Idon'tknowhowtohandlethedrawmessage.I'dlikeyouansweritformeifyoucan,butifyouhaveanyfurtherquestions,likewhatisthevalueofmyxvariable,orneedanythingdone,youshouldcomebacktomeandask."Ifthemessageisdelegatedfurther,allquestionsaboutthevaluesofvariablesorrequeststoreplytomessagesareallinferredtotheobjectthatdelegatedthemessageinthefirstplace.

Supposenowwe'dliketocreateaturtleatthesamelocationastheoriginalpen,usingtheoriginalpenasaprototype.Howisaturtledifferentfromapen?Aturtlesharessomeofthebehaviorofapen,buthasadditionalstate,namelyit'sheading.Rememberingaheadingisessentialinimplementingtheadditionalbehaviorofbeingabletorespondtoforwardandbackmessagesbyrelyingonthebehavioroftheresponsetothedrawmessage.Wemaychooseeithertoprovideanewbehaviorfortheturtle'sdrawoperation,orrelyonthedrawoperationprovidedbytheoriginalpen.

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Let'slookatthesameexamplewiththeinheritanceapproachtosharingknowledgeasfoundinSimulaandSmallTalkinsteadofdelegation.Thisisillustratedinthefiguretitled"Sharingknowledgewithinheritance".Withinheritance,itisnecessarytocreateobjectsrepresentingclasses.Tomakeapen,itisfirstnecessarytomakeapenclassobject,whichspecifiesboththebehaviorandthenamesofvariables.

Individualpensarecreatedbysupplyingvaluesforalltheinstancevariablesofthepenclass,creatinganinstanceobject.Valuesforallthevariablesmustbespecified,eveniftheydonothaveuniquevaluesintheinstance.Nonewbehaviormaybeattachedtoanindividualpen.Extendingbehaviorisaccomplishedbyadifferentoperation,thatofcreatinganewsubclass.Thestepwhichgoesfromaninstancetobehaviorstoredinitsclassisperformedbya"hardwired"lookuploopinsystemslikeSimulaandSmallTalk,notbymessagepassing,asinthedelegationapproach.

Toextendpenswithnewbehavior,wemustfirstcreateanewclassobject.Hereaturtleclassaddsanewheadingalongwithnewbehaviorfortheforwardmessage.Noticethatthevariablesfromthepenclass,xandy,werecopieddownintotheturtleclass.Anindividualturtleinstancemustsupplyvaluesforallthevariablesofitsclass,superclass,andsoon.Thiscopyingleadstolargerinstanceobjectsforclassesfurtherandfurtherdowntheinheritancehierarchy.Thelookupofmethods,performedbyaprimitive,unchangeableroutineinsteadofmessagepassing,startsasearchformethodsintheclassofanobject,andproceedsupthesubclasstosuperclasschain.

Howdoesamethodinheritedfromthepenclasstotheturtleclassaccessamethodimplementedintheturtleclass?Sinceinheritancesystemsusuallydonotusemessagepassingtocommunicatefromsubclasstosuperclass,theycan'tpasstheturtleobjectalonginthemessage,aswewouldindelegation.

SharingKnowledgewithInheritance

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Insteadmostusevariablebindingtobindaspecialvariableselftotheobjectthatoriginallyreceivesamessage.Weshallseelateronthatthisleadstotrouble.

Inaddition,inheritancesystemsalsoallowthe"shortcut"ofbindingallthevariablesofaninstancesothattheycanbereferenceddirectlybycodenaminginmethodsasfreevariables.Whilethisissometimesmoreefficient,itshortcircuitsthemessagepassingmechanism,defeatingtheindependenceofinternalrepresentationwhichisthehallmarkofobjectorientedprogramming.Sincevariablereferencesusedifferentlinguisticsyntaxthanmessagesends,ifwewantedtochangethecoordinaterepresentationfromxandytopolarcoordinatesusingrhoandtheta,we'dhavetochangeallthereferencingmethods.Stickingtomessagepassingtoaccessxandymeansthatevenifthecoordinateswerechangedtopolar,wecouldstillprovidemethodsthatcomputetherectangularcoordinatesfromthepolar,andthechangewouldbetransparent.

Ihopethesediagramsleaveyouwiththeimpressionthatthedelegationapproachissimpler.Tocreatetwopensandaturtle,theinheritanceapproachrequirestheadditionalstepsofcreatingpenclassobjectsandturtleclassobjects.Also,wehavetohavetwodifferentkindsoflinksbetweenobjects,thesubclass

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linkandtheinstancelink,whereasthedelegationapproachonlyrequiresamessagepassingrelationshipbetweenthelinkedobjects.

6.Areinheritanceanddelegationequallypowerful?

Anobviousquestiontoaskabouttheprecedingdiscussionofinheritanceanddelegationiswhetherthetwotechniqueshavethesameexpressivepower.Theanswerisno.

Givendelegation,itiseasytoseehowwecouldimplementthefunctionalityofinheritance.Wecancreatespecialclassobjectsthatrespondtomessagestocreatenewinstances.Weneedonlyarrangethattheclassobjectsobservethecopyingofvariablesfromthesuperclasschainwhentheycreateinstances.Instanceobjectsaregivenbehaviorthatimplementsthelookupmethods,roughlyasfollows.

IfI'manINSTANCEobject

andIreceiveamessage

withaSELECTORandsomeARGUMENTS:

IftheSELECTORmatches

oneoftheVARIABLEnames

inmyCLASS[orSUPERCLASS,etc.],

Ireturnthecorrespondingvalue,

storedinmyself.

Otherwise,IlookforaMETHOD

whoseNAMEmatches

theSELECTORofthemessage

inthelistoflocalMETHODS

ofmyCLASS.

IfIfindone,

IbindthevariableSELFtomyself

[theINSTANCEobject].

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Ibindthenamesof

thevariablesofmyCLASS,

[andallthevariables

uptheSUPERCLASSchain]

totheirvaluesintheINSTANCE.

ThenIinvoketheMETHOD.

Ifthere'snomethod

inmyCLASS'sMETHODlist,

Itrytofindamethod

intheSUPERCLASS,

andsoonuptheSUPERCLASSchain.

Howabouttheotherway?Caninheritanceimplementdelegation?Unfortunatelynot.Thereasonisalittletrickytounderstand,butithastodowiththetreatmentoftheselfvariable,whichpreventsaproperimplementationofforwardingofmessages.

Often,amethodforhandlingamessagemayneedtoasktheobjectthatoriginallyreceivedthemessagetoperformsomeservice.Aturtleobjectwhichreceivesabackmessagewouldliketoturnitintoaforwardmessagesenttothesameobject,butnegatingthenumberofsteps,sothatback100islikeforward100.Indelegation,whenamethodisdelegatedamessage,itreceivesacomponentcalledtheclientinthedelegatemessage,whichhastheobjectthatoriginallyreceivedthemessage.

Ininheritancesystems,adistinguishedvariablenamedselfisautomaticallyboundtotherecipientofamessageduringtheexecutionofcodeforamethod.Whenthemethodsearchproceedsfromtheoriginalclasstoasuperclass,thevalueoftheselfvariabledoesn'tchange,sothatsuperclassmethodscanreplytothemessage"asif"theyweremethodsoftheoriginalobject.However,whenausersendsamessage,theselfvariableisalwaysrebound,sothatitisgenerallynotpossiblefortheusertodesignateanotherobjecttoreplyinplaceoftheobjectwhichoriginallyreceivedthemessage.Truedelegationcannotbeimplementedinthesesystems.

Anexample,illustratedinthefigure"TheSELFProblem,"willmakethisclear.Supposewewouldliketoextendaparticularturtleobjecttocreateaturtlewhichdrawsdashedinsteadofsolidlines.The

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obviouswaytodothisistohavethedashedturtleintercepttheforwardmessageandbreakuptheintervalintopieces,delegatingamessagetodrawaseriesofshorterlinestoasolidlineturtle.If,inaninheritancesystem,thedashedlineturtlesimplysendsaforwardmessagetothesolidlineturtle,thenselfwillbeboundtothesolidlineturtle.Ourearlierimplementationofbackintermsofforwardwillthenstopworking,sinceamessagetothedashedlineturtletogobackwilltrytosendaforwardmessagetoselfanddrawasolidlineinstead!

Becarefulaboutconfusingthisexamplewithanalternativeimplementationusinginheritancesystems,whichwouldcreateadashedturtleclassasasubclassofsolidturtleclass.Whilesuchanimplementationcouldhavethecorrectbehaviorwithrespecttothebackmessage,itstillwouldn'tcountasanimplementationofdelegation.Remember,whatweweretryingtodowastoseeifanobjectcouldforwardmessagestosomeotheralreadyexistingobject.Adashedturtleinstancewouldn'tbeforwardinganymessagestoaninstanceofsolidturtle,sinceitwouldjustinheritcopiesofthevariablesandmethodsfromsolidturtle.

7.Whataboutefficiency?

Theefficiencycomparisonbetweendelegationandinheritanceboilsdowntotime/spacetradeoffs.Somehavearguedthatinheritanceismoreefficientbecauseitrequiresfewermessages,butthiscomesatthecostofincreasingthesizeofobjects.Becausevariablesarecopieddownfromsuperclasstosubclass,instancesbecomelargerandlargerthefartherdownyougetintheinheritancehierarchy.Withdelegation,eachobjectneedonlyspecifywhat'sdifferentaboutitfromalreadyexistingprototypes,sothesizeofobjectsdoesnotnecessarilydependonthedepthinthehierarchyofsharedobjects.Alookatthediagramillustratingthedatastructuresforpenandturtleobjectswillconfirminheritance'sspeedadvantageanddelegation'sspaceadvantage.

Smallerobjectsmakeforfasterobjectcreationtimes,whichcanbeimportantinsystemsthatcreatelargenumbersofsmallobjectswithshortlifetimes,asopposedtosmallnumbersoflargeobjectswithlonglifetimes.Reducingthesizeofobjectsmayalsoimprovetheefficiencyofvirtualmemory,byimprovinglocalityofreference,allowingahigherdensityoffrequentlyreferencedobjectsintheprimarymemory.Withacopyinggarbagecollector,suchasthatdescribedin[LiebermanandHewitt83],smallerobjectscanimprovetheefficiencyofgarbagecollectionbyreducingthecopyingoverhead.

Implementorsshouldn'tgetscaredawaybythesearchrequiredtofindmethodsandvariablesinthedelegationapproach.There'sasimple,effectivetrickforreducingthesearchtime:cachingtheresultoflookups.Cachesareawayoftradingspaceforspeed,mitigatinganynegativeeffectsofthespeedforspacetradeoffmadebydelegation.Cachesmakeamoreeffectiveuseoftheextramemorythanindiscriminatelycopyinginstancevariables,becausethememorytheydouseissuretobeinconstantuse.Cachesdon'trestrictflexibilityininteractivelymodifyingtheprogrammingenvironmentthewaycopyingandcompilationoptimizationsdo.

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Onconventionalmachines,probablynoimplementationofdelegationisgoingtosurpassvariablelookupviaregistersandstackindexingforrawspeed.Butintheirzealtospeedupvariablelookup,implementorshaveforceddecisionssuchaslargeobjectsizeonobjectorientedlanguages,whichadverselyaffectefficiency.Parallelmachineswithlargeaddressspaceswillmaketheattractivenessofsuchregisterorientedoptimizationsfade.

SmallTalk[Krasner84]reportsa93%"hitrate"foramoderatelysizedcache,1000objects.Thismeansthatanysavingsbyinheritanceoverdelegationinlookupcouldatbestaffecttheremaining7%.Thebestthingtodoseemstobetokeepaglobalcache,andinvalidateitwheneveranychangesaremadetothesharinghierarchy.Achangewillthenslowthesystemdownforthenext1000messages,orwhatevertimethecachetakestofillupagain."Smarter"alternatives,suchasperobjectcachesareprobablynotworththeextratroubletheywouldcauseforincrementalsoftwaremodification,sincethehitrateonaglobalcacheissohigh.Sincebothinheritanceanddelegationcanbeimplementedalmostequallyefficiently,itseemsthatthere'slittlereasontosacrificetheextraflexibilityofdelegationonefficiencygrounds.

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8.RedirectingI/Ostreamsillustratesanimportantapplicationofdelegation

Manyobjectorientedsystemsmakegooduseofobjectorientedprogrammingtechniquestoimplementinputoutputstreams.Suchastreamisanobjectthatreceivesmessagestoinputoroutputacharacter,alineoftext,anexpression.Systemsusuallyhaveglobalvariablesdesignatingthe"current"sourcesofinputandoutput,whichisbydefaultboundtoanobjectrepresentingthestreamofcharactersbeingdisplayedonthewindowofascreenofaninteractivedisplay.

Thename"stream"suggeststhecontinualflowofcharactersorpixelsbetweentheuserandthesystem.Averyusefulkindofobjectisthatwhichimplementsa"dam"todivertthestreamtootherdestinations,or"plumbing"whichconnectsonestreamwithanother.Adribblefileisasequentialfilemaintainingarecordondiskofthehistoryofinputoutputinteractions,toprovideamorepermanentrecordingofinteractionsthantheephemeraltwinklingofpixels.Adribblefilecanbeimplementedbyreplacingthestreamwhichrepresentsinteractionsattheterminalwithonethatwritesthemtodiskalso.

Thedribblestreamneedstheabilitytomasqueradeastheterminalstream.Itshouldhavethesameresponsestoallthemessagesthattheordinaryterminalstreamhas,andalsoprovidetheadditionalbehaviorofwritingtothedisk.Thestreamsshouldbeconsideredindistinguishablefromthepointofviewofallprogramswhichperforminputoutput.

Toimplementthedribblestreamcleanly,we'dlikeittobethecasethattheimplementationofthedribblestreamshouldn'thavetoknowtheprecisedetailsoftheimplementationofthestreamwhichitisreplacing.Wemight,forexample,liketouseasingledribblestreamwithbothastreamtoadirectlyconnectedinteractiveterminalandastreaminteractingoveranetwork.

Canadribblestream"masquerade"asaterminalstream?

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Theimplementationusingdelegationisconvenientandstraightforward.Messageswhichdocharacteroutputareinterceptedandthediskoutputisinterposed.

ADRIBBLESTREAMisanobject

thatlogsinteractiononaSTREAM,

andrecordsitondiskusingaFILE

NAME

IfI'maDRIBBLESTREAMand

Igetamassagetoinputoroutput

aCHARACTER,

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IoutputtheCHARACTER

tothediskstreamtotheFILE

NAME.

ThenIdelegatethemessage

tooutputaCHARACTER

totheoriginalSTREAM.

IfI'maDRIBBLESTREAMand

Igetanyothermessage,

Isimplydelegatethemessage

totheSTREAM.

Itworkstotakecareofonlythesinglecharacterinputandoutputmessagesbecausepresumablyallhigherlevelmessages,likeprintofanobject,areultimatelyimplementedintermsofthesinglecharacterversions.Themethodwhichperformsahigherlevelprintoperationwouldultimatelysendacharacteroutputmessagetoitsclient[sendtoself].

Surprisingly,manyinheritancesystemsmakeitdifficulttoimplementthissimpleextensiontothebehaviorofstreams.OnevillainistheinsistenceofsystemslikeflavorsandSmallTalkondefiningseparateproceduresforhandlingeachtypeofmessage.Attemptingtotrytoimplementdribblestreamasasubclassofstreaminsystemsofthisilk,wewouldfindthatthere'snoeasywaytosay"...andsendalloftheirrelevantmessagesthroughtotheoriginalstream".Wewouldbeforcedtodefineonemethodtointerceptthecharacteroutputmessagetowritetothedisk,anothertointercepttheprintmessage,anothertointercepttheprintlinemessage,andsoonforeveryrelevantmessage.Everytimeanothermessagewasaddedtotheoriginalstreamanothermethodwouldhavetobeaddedtothedribblestream,withtediouslyrepetitivecode.Thisalsohastheunfortunateeffectofmakingtheimplementationofdribblestreamnowsensitivetothedetailsofexactlywhichmessagesitsembeddedstreamaccepts,inhibitingtheabilitytoreusetheimplementationwithdifferenttypesofstreams.

Addingtothesystemthedefinitionofadribblestreamclassorflavorwouldonlygivetheabilitytocreatenewinstancesofdribblestreamobjects.Itwouldnotbepossibletocreateadribblestreamwhichusedapreviouslyexistingstreamobject.We'dthenhavetomakenewterminalstreams,networkstreams,orotherkindofstreams,tobeabletotakeadvantageoftherecordingfunctionality.Weshouldn'thavetoreproduceeverykindofstreaminthesystemjusttohavethedribblecapability!

If,instead,weattempttomakeadribblestreamwhichholdstheinteractionstreamasoneofitsinstancevariables,wefacetheproblemthatthereisnowayforthedribblestreamtocorrectlyforwardamessagelikeprinttothevalueofthevariable.Becauseofthewaythesesystemshandletheselfvariable,the

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forwardingofmessagestotheoriginalstreamwon'twork,forthesamereasonasintheturtleexample.Sendingaprintmessagetotheinstancevariablewouldrebindtheselfvariable,soitwouldresultinsendinglowerlevelmessagesdirectlytotheinteractionstreamandnottothedribblestream.Soitseemsasthoughanystraightforwardattempttoimplementthedribblestreamasasimplebehavioralextensioninmanyinheritancesystemsisdoomed.

9.ParallelismcausesproblemsininheritancesystemsbecauseoftheSELFvariable

There'sanadditionalprobleminthecasethatthestreamcanacceptmessagesfrommorethanoneparallelprocess.Becausethestreamholdsmodifiablestate[suchasascreenbitmap],thestreammustbeprotectedagainsttimingerrorsresultingfromtwoprocessestryingtowritetothestreamatthesametime.Atechniquesuchasserializerobjects[Hewitt,Attardi,Lieberman79]ormonitorsmustbeused.Thismeansthatwhenthestreamreceivesawritemessage,it"locks",sothatsubsequentmessagestothestreammustwaitinaqueueforthestreamtofinishprocessingthefirstwritemessage.

Now,ifamessagetoadribblestreamtriestoprocessaprintmessagebysendingacharacteroutputmessagetotheselfvariable,itwillfindselfboundtoastreamwhichislockedwaitingforthatveryprintmessagetocomplete!Deadlock!

Sincedelegationusesmessagepassing,whenthedribblestreamdelegatestoaterminalsystemitcansupply[astheclientinthedelegatemessage]anunserializedversionofitself,whichcanprocessthemessagewithoutwaiting.

10.Delegationismoreflexiblethaninheritanceforcombiningbehaviorfrommultiplesources

Often,anobjectwillwanttoutilizebehaviorthatappearsinmorethanoneotheralreadyexistingobject.Thebehaviorthatasystemneedstoimplementaparticular"feature"canbepackagedupasasingleobject,andsometimesanobjectwillwanttocombineseveralofthesefeaturestoimplementitsbehavior.Forexample,windowobjectsmighthavetitles,borders,sizeadjustments,etc.Aparticularwindowobjectmaychoosesomeofthesefeaturesandnotothers.Featuresmaybeindependentofoneanother,ortheymayinteract.

Thesolutionininheritancesystemsistocreateaclassobjectthatmentionsalistofotherclasseswhosebehavioritwishestosham.Allthemethodsandvariablesmentionedinanyoftheclassesareinheritedbythecombinedobject.Systemslikeflavorsallowoptionally,onapermethodbasis,supplyinganoptionforhowtocombinebehaviorwhenmorethanonecomponentcontributesamethod.Typicaloptionsaretoinvokeallthecontributingmethods,imposeanorderonthem,orreturnalistoftheresults.

Theproblemwiththisstyleofcombiningbehaviorfrommultiplesourcesisthatitfixesthepatternofcommunicationbetweenobjectsbeforethetimeaninstanceobjectiscreated.Thislimitstheextenttowhichbehaviorfrompreviouslyexistingobjectscanbeuseddynamically.Bycontrast,withdelegation,thecommunicationpatternscanbedeterminedatthetimeamessageisreceivedbyanobject.

Withdelegation,amethodforanextensionobjectcansimplyaccesstheprototypicalobjectsfromwhichitderivesbehavioronthesharedlist.Awindowwhichwantstoinvokethedrawactionofapreviouslydefinedrectangleobjectactingasitsborderscansimplydelegatethedrawmessagetotherectangleobject.Thusdelegationdoesn'trequire"methodcombination"oraninventoryofesotericcombiningoperations.Thebehaviorissimplyprogrammedinthemethodforthecombinedextensionobject.Shouldaprogrammerwishtobuildalibraryofcommoncombinationtechniques,itiseasilydonebyconstructingvariantsonthestandardmethodobject,sodelegationcouldbemadeasconciseasmethodcombinationininheritancesystems.Withinheritance,ifawindowclassincludesa"bordersmixin",the

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windowdoesnotcontainanindependentobjectrepresentingitsborders,soitisnotpossibletosendamessagetothebordersofawindowindependentofthewindowobjectitself.Thewindowclassmerelycontainsamixtureofthemethodsandvariablesinheritedfromthebordersandothercontributingcomponents.

Inhighlyresponsiveinteractivesystems,itisoftennecessarytowaituntilamessageisreceivedtodeterminehowbehaviorfromcomponentobjectswillbeutilized.Here'sasimpleexampleinwhichdynamicutilizationofbehaviorfrommultiplesourcesisrequired,illustratedinthefigure"Delegationallowscommunicationpatternstobedecidedatruntime".

Aborderedbitmapcanbebuiltfromarectangle,whichcandisplayitsborders,andabitmapwhichcantransferanarrayofpixelstothescreen.Whatshouldthedrawresponsefortheborderedbitmapbe?Withinheritanceyoucreateaborderedbitmapclassthatinheritsbothfromrectangleandbitmap,sayingthatbothdrawmethodsaretobeused.Fine.

Butnowsupposewe'dliketogivetheusertheoptionofchangingdynamicallywhichbehaviorisused.Whenthebitmapisdraggedacrossthescreen,thetransferoftheentirearrayoneverymousemovementmightbetooslow,soitmightbepreferabletogivetheusertheoptionofjustdraggingtheoutlineofthebitmapinstead.Areasonablethingtodoistogivetheuseranonscreentoggleswitchtodecidethebehavior,andtheusercanpotentiallychangethebehavioratanytime.Sothebehavioroftheborderedbitmapcannotbedecidedbeforetheobjectiscreated.Withdelegation,whentheborderedbitmapgetsadrawmessage,itcandecidewhethertodelegatethemessagetotherectangleobjectthatitcontains,ortothebitmapobject,orboth.

Delegationallowscommunicationpatternstobedecidedatruntime

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InheritancesystemsarealsoplaguedbywhatIcalltheoneinstanceclassproblem.Whensystemsarecomposedoflargenumbersofobjectswithslightlyvaryingbehavior,youwinduphavingtocreatenewclassobjectsoftenjusttohaveoneorafewinstances.Itisnecessarytocreateadhocclassessuchas"windowwithawideborder,timesromanfontandnotitle"justtocombinefeaturesforasingleinstance.

11.Delegationisadvantageousforhighlyinteractive,incrementalsoftwaredevelopment

Animportantissuetoconsiderwhenevaluatingthetradeoffsbetweeninheritanceanddelegationistheconsequencesforincrementalsoftwaredevelopment.Aswehaveseenabove,inheritancetendstoencouragecopyingofvariablesandmethods,whiledelegationencouragessharing.Ifaprototypicalobjectchangesbehavior,thenallobjectswhichmentionthatprototypeontheirsharedlistwillautomatically"feel"thechange.Ifchangesaremadetoaninheritancehierarchy,suchasaddinganewinstancevariable,orchangingtheclassstructure,informationcopiedfromtheolddatastructuremayberenderedobsolete.Broadcastingtheresultofchangestocopiesputsaburdenontheoperationswhichmakeincrementalchangesinthesoftwareenvironment.Anextremeexampleofthisoccursintheflavorssystemwheresimplyaddingamethodtovanillaflavor,therootoftheinheritancehierarchy,resultsinrecompilationofeveryflavorintheentiresystem!Thiseffectivelyprohibitsanymodificationstoobjectsnearthetopoftheinheritancehierarchy.

Thoughdelegationhasbeentheminorityviewpointinobjectorientedlanguages,itisslowlybecomingrecognizedasimportantforitsaddedpowerandflexibility.Partofthereasonforneglectofthedelegationapproachhasbeenhistorical.Simula,oneofthefirstobjectorientedlanguages,adoptedtheinheritancetechnique.Itfixedcommunicationpatternsbetweenobjectsatcompiletime,aswasappropriateforacompiledlanguageoftheAlgolfamily.Thespecificmechanismsforthiswerethen"inherited"bySmallTalkandothers,withoutreconsideringwhethertheapproachwasstillappropriateforaninterpretivelanguageinamorehighlyinteractiveprogrammingenvironment.Ihopetheprecedingdiscussionhasconvincedyouthattheapproachofmodelingconceptsusingprototypesandimplementingbehaviorinobjectorientedlanguagesusingdelegationhasdistinctadvantagesoverthealternativepointofviewusingclassesandinheritance.

12.Acknowledgments

MajorsupportfortheworkdescribedinthispaperwasprovidedbytheSystemDevelopmentFoundation.OtherrelatedworkattheMITArtificialIntelligenceLaboratorywassupportedinpartbyDARPAunderONRcontractN0001480C0505.

CarlHewitt'sideasconcerningactors,andespeciallytheimpactofparallelismonobjectorientedprogrammingwereimportantinfluences.KennethKahnandLucSteelsimplementedobjectorientedlanguageswhichadopteddelegationmechanismsandalsoinfluencedtheseideas.AlanBorningreachedsimilarconclusionsinthecontextoftheThingLabsystemimplementedinSmallTalk.KoendeSmedtprovidedahelpfulcritiqueofatalkIgaveontheseissuesinNijmegen,theNetherlands.

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