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1、IntroductionFormanypeople,learningCOMandDCOMistough.YouknowthatlearningCOMistherightthingtodo-youhearconstanthypeandyouknowthatmanyofMicrosoft,sproductsandprogrammertoolsarebasedonCOM,soitisobviouslysomethingthatisimportant.ButyoualsoknowthatCOMisreallyhard.YoumayhavealreadytriedtolearnCOMonce,ormay
2、beevenseveraltimes.Youmayhaveslidthroughacoupleofbooks,playedwithsomewizards,etc.Butitjustdoesn,tmakeanysense.Everythingseemsextremelycomplicatedandmuchharderthanitneedstobe.There,salsothevocabulary:marshalling,apartmentthreads”,z,singletonobjects“andsoon.Whatisthis?Thepurposeofthissetoftutorialsist
3、ohelpyoutoquicklyunderstandwhatisgoingonintheworldofDCOMsothatyoucancreateCOMclientsandserverseasily.Wedothatstartingatthebeginningandlayingthingsoutforyousimplyandintherightorder.BythetimeyoufinishthesetutorialsyouwillunderstandallofthebasicconceptsdrivingDCOMandyouwillbeabletoproceedquicklytolearn
4、therest.YouwillbeamazedathoweasyDCOMcanbeonceyougetagoodstart. TheBaSiCSOfCOM-thebestplacetostartisatthebeginning. SimDIeCoMCIientS-COMclientsareeasy SimPIeCOMSerVerS-usingtheAT1.wizardtobuildaserverTheBasicsofCOMUnderstandinghowCOMworkscanbeintimidatingatfirst.Onereasonforthisintimidationisthefactt
5、hatCOMusesitsownvocabulary.AsecondreasonisthatCOMcontainsanumberofnewconcepts.OneoftheeasiestwaystomasterthevocabularyandconceptsistocompareCOMobjectstonormalC+objectstoidentifythesimilaritiesanddifferences.YoucanalsomapunfamiliarconceptsfromCOMintothestandardC+modelthatyoualreadyunderstand.Thiswill
6、giveyouacomfortablestartingpoint,fromwhichwelllookatCOMsfundamentalconcepts.Oncewehavedonethis,theexamplepresentedinthefollowingsectionswillbeextremelyeasytounderstand.ClassesandObjectsImaginethatyouhavecreatedasimpleclassinC+calledxxx.Ithasseveralmemberfunctions,namedMethodA,MethodB,andMethodC.Each
7、memberfunctionacceptsparametersandreturnsaresult.Theclassdeclarationisshownhere:classxxxpublic:intMethodA(inta);intMethodB(floatb);floatMethodC(floatc);；Theclassdeclarationitselfdescribestheclass.Whenyouneedtousetheclass,youmustcreateaninstanceoftheobject.Instantiationsaretheactualobjects;classesare
8、justthedefinitions.Eachobjectiscreatedeitherasavariable(localorglobal)oritiscreateddynamicallyusingthenewstatement.Thenewstatementdynamicallycreatesthevariableontheheapandreturnsapointertoit.Whenyoucallmemberfunctions,youdosobydereferencingthepointer.Forexample:xxx*px;/px=newxxx;/p-MethodA(l);/delet
9、epx;/pointertoxxxclasscreateobjectonheapcallmethodfreeobjectItisimportantforyoutounderstandandrecognizethatCOMfollowsthissameobjectedorientedmodel.COMhasclasses,memberfunctionsandinstantiationsjustlikeC+objectsdo.AlthoughyounevercallnewonaCOMobject,youmuststillcreateitinmemory.YouaccessCOMobjectswit
10、hpointers,andyoumustde-allocatethemwhenyouarefinished.WhenwewriteCOMcode,wewontbeusingnewanddelete.Althoughwe,regoingtouseC+asourlanguage,wellhaveawholenewsyntax.COMisimplementedbycallstotheCOMAPI,whichprovidesfunctionsthatcreateanddestroyCOMobjects.HeresanexampleCOMprogramwritteninpsedo-COMcode.ixx
11、*pi/CoCreateInstance(,&pi)/pi-MethodA();/pi-Release();/pointertoxxxCOMinterfacecreateinterfacecallmethodfreeinterfaceInthisexample,wellcallclassixxaninterface.Thevariablepiisapointertotheinterface.ThemethodCoCreateInstancecreatesaninstanceoftypeixx.Thisinterfacepointerisusedtomakemethodcalls.Release
12、deletestheinterface.vepurposelyomittedtheparameterstoCoCreateInstance.Ididthissoasnottoobscurethebasicsimplicityoftheprogram.CoCreateInstancetakesanumberofarguments,allofwhichneedsomemoredetailedcoverage.Fornow,letstakeastepbackandlookatthebiggerissueswithCOM.HowCOMIsDifferentCOMisnotC+,andforgoodre
13、ason.COMobjectsaresomewhatmorecomplicatedthentheirC+brethren.Mostofthiscomplicationisnecessarybecauseofnetworkconsiderations.TherearefourbasicfactorsdictatingthedesignofCOM:C+objectsalwaysruninthesameprocessspace.COMobjectscanrunacrossprocessesoracrosscomputers. COMmethodscanbecalledacrossanetwork.C
14、+methodnamesmustbeuniqueinagivenprocessspace.COMobjectnamesmustbeuniquethroughouttheworld. COMserversmaybewritteninavarietyofdifferentlanguagesandonentirelydifferentoperatingsystems,whileC+objectsarealwayswritteninC+.1.etslookatwhatthesedifferencesbetweenCOMandC+meantoyouasaprogrammer.COMcanrunacros
15、sprocessesInCOM,youastheprogrammerareallowedtocreateobjectsinotherprocesses,andonanymachineonthenetwork.ThatdoesnotmeanthatyouW川alwaysdoit(inmanycasesyouwont).However,thepossibilitymeansthatyoucantcreateaCOMobjectusingthenormalC+newstatement,andcallingitsmethodswithlocalprocedurecallswontsuffice.Toc
16、reateaCOMobject,someexecutingentity(anEXEoraService)willhavetoperformremotememoryallocationandobjectcreation.Thisisaverycomplextask.Byremote,wemeaninanotherprocessoronanotherprocess.ThisproblemissolvedbycreatingaconceptcalledaCOMserver.ThisotherentityW川havetomaintaintightcommunicationwiththeclient.C
17、OMmethodscanbecalledacrossanetworkIfyouhaveaccesstoamachineonthenetwork,andifaCOMserverfortheobjectyouwanttousehasbeeninstalledonthatmachine,thenyoucancreatetheCOMobjectonthatcomputer.Ofcourse,youmusttheproperprivileges,andeverythinghastobeset-upcorrectlyontheothercomputer.SinceyourCOMobjectwillnotn
18、ecessarilybeonthelocalmachine,youneedagoodwaytopointtoit,eventhoughitsmemoryissomewhereelse.Technically,thereisnowaytodothis.Inpractice,itcanbesimulatedbyintroducingawholenewlevelofobjects.OneofthewaysCOMdoesthisiswithaconceptcalledaproxystb.Welldiscussproxy/stubsinsomedetaillater.Anotherimportantis
19、sueispassingdatabetweentheCOMclientanditsCOMserver.Whendataispassedbetweenprocesses,threads,oroveranetwork,itiscalledMarshalling.Again,theproxystbtakescareofthemarshallingforyou.COMcanalsomarshaldataforcertaintypesofinterfaceusingType1.ibrariesandtheAutomationmarshaller.TheAutomationmarshallerdoesno
20、tneedtobespecificallybuiltforeachCOMserver.COMobjectsmustbeuniquethroughouttheworldThewholeworld?Comeon!Thismayseemlikeanexaggerationatfirst,butconsidertheInternettobeaworldwidenetwork.Evenifyoureworkingonasinglecomputer,COMmusthandlethepossibility.Uniquenessistheissue.InC+allclassesarehandledunequi
21、vocallybythecompiler.Thecompilercanseetheclassdefinitionforeveryclassusedinaprogramandmatchupallreferencestoittomakesuretheyconformtotheclassexactly.Thecompilercanalsoguaranteethatthereisonlyoneclassofagivenname.InCOMtheremustbeagoodwaytogetasimilarlyunequivocalmatch.COMmustguaranteethatthereW川OnIyb
22、eoneobjectofagivennameeventhoughthenumberofobjectsavailableonaworldwidenetworkishuge.ThisproblemissolvedbycreatingaconceptcalledaGUID.COMislanguageindpendentCOMserversmaybewrittenwithadifferentlanguageandanentirelydifferentoperatingsystem.COMobjectshavethecapabilityofbeingremotelyaccessible.Thatmean
23、stheymaybeinadifferentthread,process,orevenonadifferentcomputer.Theothercomputermayevenberunningunderadifferentoperatingsystem.Thereneedstobeagoodwaytotransmitparametersoverthenetworktoobjectsonothermachines.Thisproblemissolvedbycreatinganewwaytocarefullyspecifytheinterfacebetweentheclientandserver.
24、ThereisalsoanewcompilercalledMID1.(Microsoft?lnterfaceDefinition1.anguage).Thiscompilermakesitpossibletogenericallyspecifytheinterfacebetweentheserverandclient.MID1.definesCOMobjects,interfaces,methodsandparameters.COMVocabularyOneoftheproblemswe,regoingtohaveiskeepingtrackoftwosetsofterminology.You
25、reprobablyalreadyfamiliarwithC+andsomeObjectOrientedterminology.ThistableprovidesaroughequivalencybetweenCOMandconventionalterminology.ConceptConventional(C+OOP)COMClientAprogramthatrequestservicesfromaserver.AprogramthatcallsCOMmethods.ServerAprogramthatservesotherprograms.AprogramthatmakesCOMobjec
26、tsavailabletoaCOMclient.InterfaceNone.ApointertoagroupoffunctionsthatarecalledthroughCOM.ClassAdatatype.Definesagroupofmethodsanddatathatareusedtogether.ThedefinitionofanobjectthatimplementsoneormoreCOMinterfaces.Also,coclass.ObjectAninstanceofaclass.Theinstanceofacoclass.MarshallingNone.Movingdatab
27、etweenclientandserver.YoullnoticetheconceptsofInterfaceandMarshallingdonttranslatewellintotheC+model.TheclosestthingtoaninterfaceinC+istheexportdefinitionsofaD1.1.D1.1.sdomanyofthesamethingsasCOMwhendealingwithatightlycoupled(in-process)COMserver.MarshallinginC+isalmostentirelymanual.Ifyouretryingto
28、copydatabetweenprocessesandcomputers,youllhavetowritethecodeusingsomesortofinter-processcommunication.Youhaveseveralchoices,includingsockets,theclipboard,andmailslots.TheInterfaceThusfar,we,vebeenusingthewordinterfaceprettyloosely.Mydictionary(1947AmericanCollegeDictionary)definesaninterfaceasfollow
29、s:Interface,n.asurfaceregardedasthecommonboundaryoftwobodiesorsurfacesThatsactuallyausefulgeneraldescription.InCOMinterfacehasaveryspecificmeaning.COMinterfacesareacompletelynewconcept,notavailableinC+.Theconceptofaninterfaceisinitiallyhardtounderstandformanypeople.Aninterfaceisaghostlikeentitythatn
30、everhasaconcreteexistence.Itssortoflikeanabstractclass-butnotexactly.Atitssimplest,aninterfaceisnothingbutanamedcollectionoffunctions.InC+,aclass(usingthisterminology)isallowedonlyoneinterface.Thememberfunctionsofthatinterfaceareallthepublicmemberfunctionsoftheclass.Inotherwords,theinterfaceisthepub
31、liclyvisiblepartoftheclass.InC+thereisalmostnodistinctionbetweenaninterfaceandaclass.HeresanexampleC+class:classyyypublic:intDoThisO；private:voidHelperl();intcount;intx,y,z;)；Whensomeonetriestousethisclass,theyonlyhaveaccesstothepublicmembers.(Forthemomentwe,reignoringprotectedmembersandinheritance.
32、)TheycantcallHelperl,oruseanyoftheprivatevariables.Totheconsumerofthisclass,thedefinitionlookslikethis:classyyyintDoThisO;；Thispublicsubsetoftheclassistheinterfacetotheoutsideworld.Essentiallytheinterfacehidesthegutsoftheclassfromtheconsumer.ThisC+analogyonlygoessofar.ACOMinterfaceisnotaC+class.COMi
33、nterfacesandclasseshavetheirownspecialsetofrulesandconventions.COMallowsacoclass(COMclass)tohavemultipleinterfaces,eachinterfacehavingitsownnameanditsowncollectionoffunctions.Thereasonforthisfeatureistoallowformorecomplexandfunctionalobjects.ThisisanotherconceptthatisalientoC+.(Perhapsmultipleinterf
34、acescouldbeenvisionedasaunionoftwoclassdefinitions-somethingthatisntallowedinC+.)InterfacesisolatetheclientfromtheserverOneofthecardinalrulesofCOMisthatyoucanonlyaccessaCOMobjectthroughaninterface.Theclientprogramiscompletelyisolatedfromtheserversimplementationthroughinterfaces.Thisisanextremelyimpo
35、rtantpoint.TheclientprogramknowsnothingabouttheCOMobjectorC+classthatimplementstheCOMobject.Allitcanseeistheinterface.TheinterfaceislikewindowintotheCOMobject.Theinterfacedesignerallowstheclienttoseeonlythosepartsoftheobjectthatheorshewishestoexpose.Figure2-1illustrateshowallclientaccesstotheCOMobje
36、ctisfeledthroughtheinterface.FigUreThenotationusedhere,asmallcircleconnectedbyastick,istheconventionalwaytodrawaCOMinterface.Therearemanyimportantrulesassociatedwithinterfaces.WhilecriticalforunderstandingthedetailshowCOMworks,wecanleavethemuntillater.Fornow,wellconcentrateonthebroadconceptsofinterf
37、aces.ImaginingaCOMInterfaceHeresanotherwaytovisualizeaninterface.Inthissectionwe,llpresentaCOMinterfacewithoutanyoftheC+baggage.Welltrytolookataninterfaceinitsabstractform.Imagineacarobject.Allcarobjectsthatyouarefamiliarwithintherealworldhaveadrivinginterfacethatallowsyoutodirectthecarleftandrighta
38、ndalsotospeedthecarupandslowitdown.Thememberfunctionsforthedrivinginterfacemightbeleft,right,faster,slower,forwardandreverse.Manycarsalsohappentohavearadiointerfaceaswell,iftheyhavearadioinstalled.Thefunctionsfortheradiointerfacemightbeon,off,louder,softer,nextstationandpreviousstation.DrivingRadio1
39、.eft()On()Right()otf()Slower()1.ouder()Faster()Softer()ForwardQNextStationOReverse()PrevStationOTherearemanykindsofcars,butnotallofthemhaveradios.Therefore,theydonotimplementtheradiointerface,althoughtheydosupportthedrivinginterface.Inallcarsthatdohaveradiosthecapabilitiesoftheradioarethesame.Aperso
40、ndrivingacarwithoutaradiocanstilldrive,butcannothearmusic.Inacarthatdoeshavearadio,theradiointerfaceisavailable.COMsupportsthissamesortofmodelforCOMclasses.ACOMobjectcansupportacollectionofinterfaces,eachofwhichhasaname.ForCOMobjectsthatyoucreateyourself,youwilloftendefineandusejustasingleCOMinterfa
41、ce.ButmanyexistingCOMobjectssupportmultipleCOMinterfacesdependingonthefeaturestheysupport.Anotherimportantdistinctionisthatthedrivinginterfaceisnotthecar.Thedrivinginterfacedoesnttellyouanythingaboutthebrakes,orthewheels,ortheengineofthecar.Youdontdrivetheengineforexample,youusethefasterandslowermet
42、hods(acceleratorandbrakes)ofthedrivinginterface.Youdontreallycarehowtheslower(brake)methodisimplemented,aslongasthecarslowsdown.Whetherthecarhashydraulicorairbrakesisntimportant.ImagineacomponentWhenyourebuildingaCOMobject,youareveryconcernedabouthowtheinterfaceworks.Theuseroftheinterfacehowever,sho
43、uldntbeconcernedaboutitsimplementation.1.ikethebrakesonacar,theusercaresonlythattheinterfaceworks,notaboutthedetailsbehindtheinterface.ThisisolationofinterfaceandimplementationiscrucialforCOM.Byisolatingtheinterfacefromitsimplementation,wecanbuildcomponents.Componentscanbereplacedandre-used.Thisboth
44、simplifiesandmultipliestheusefulnessoftheobject.Whatsinaname?OneimportantfacttorecognizeisthatanamedCOMinterfaceisunique.Thatis,aprogrammerisallowedtomakeanassumptioninCOMthatifheaccessesaninterfaceofaspecificname,thememberfunctionsandparametersofthatinterfaceW川beexactlythesameinallCOMobjectsthatimp
45、lementtheinterface.So,followingourexample,theinterfacesnameddrivingandradiowillhaveexactlythesamememberfunctionsignatureinanyCOMobjectthatimplementsthem.Ifyouwanttochangethememberfunctionsofaninterfaceinanywaytyouhavetocreateanewinterfacewithanewname.Thesourceofallinterfaces-IUnknownTraditionalexpla
46、nationsofCOMstartoutwithathoroughdescriptionofthe(Unknowninterface.!UnknownisthefundamentalbasisforallCOMinterfaces.Despiteitsimportance,youdontneedtoknowabout!Unknowntounderstandtheinterfaceconcept.Theimplementationof!UnknownishiddenbythehigherlevelabstractionswellbeusingtobuildourCOMobjects.Actual
47、ly,payingtoomuchattentionto!Unknowncanbeconfusing.1.etsdealwithitatahighlevelheresoyouunderstandtheconcepts.(UnknownislikeanabstractbaseclassinC+.AllCOMinterfacesmustinheritfromIUnknown.!Unknownhandlesthecreationandmanagementoftheinterface.Themethodsof!Unknownareusedtocreate,referencecount,andreleas
48、eaCOMobject.AllCOMinterfacesimplementthese3methodsandtheyareusedinternallybyCOMtomanageinterfaces.Youwilllikelynevercallthese3methodsyourself.AtypicalCOMobjectNowletsputallofthesenewconceptstogetheranddescribeatypicalCOMobjectandaprogramthatwantstoaccessit.Inthenextsectionandthefollowingchapterswewillmakethisrealbyimplementingtheactualcodefortheobject.ImaginethatyouwanttocreatethesimplestpossibleCOMobject.Thisobjectwillsupportasingleinterface,andthatinterfacewillcontainasin
