外文翻译 - C++设计基本原理

0外文原文TheC+DesignRationaleC+isdesignedtobeanextensibleplatformforrapidlydevelopingprogramanalysistools.SeveralfeaturesofC+facilitatethis:Byconvention,C+writteninlowercaselettersrefersjusttotheprogramthatabstractsdatafromaC+program.CIA+writtenincapitallettersreferstothesystemmadeofthatabstractorandallofthetoolsthatusethedatabaseitgenerates.Entitiesincludeles,macros,types,functionsandvariables.SeparationofInformationExtractionandPresentation:Theprocessofextractinginformationfromaprogramandtheprocessofpresentingthatinformationshouldbeseparate.ThiseliminatestheneedforeachC+analysistooltoduplicatetheparsingprocessandallowsC+toolstosharetheinformationandpresentitindifferentways.AsimilardoctrinewasdevelopedintheInterlispproject.Unfortunately,manyCandC+toolstodaystillviolatethisbasicprinciple.AWell-ConceivedConceptualModel:Aconceptualmodelbasedontheentity-relationshipmodelwasdesignedtodescribetheentities,attributes,andrelationshipsinC+pro-grams.Themodelservesasanaccuratehighlevelspecificationforthecontentsoftheprogramdatabase.RelationalDatabase:Tosupportreuse,C+storesabstractedinformationintheformofarelationaldatabase.Thismakesitpossibleforthatinformationtobeaccessedbyawidevarietyofexistingdatabasequerysystems.IncrementalDatabaseConstruction:C+buildsandmaintainsaprogramdatabaseforlargesystemsefficientlybecauseitallowsincrementalupdatesofadatabase.WhenasetofC+sourcelesisupdated,onlythatportionhastobereabstractedtoupdatethedatabase.ThesebasicdesignprinciplesgivetoolsbasedonC+anadvantageovermanytraditionaldevelopmenttools,whichusuallymergetheinformationextractionand1presentationprocessesinasingletoolanddonotsharetheinformationwithothertools.TheCprogramdatabasegeneratedbyChashadmanynewCtoolsdevelopedontopofit.Thesetoolsweredevelopedinamatterofdaysbyusingthecommondatabase,butdifferentapproaches.Sometoolswerewrittenasawkorkshscriptsthatcalldatabasequerycommands.OthersareCprogramsthatcallaquerylibrary.OtherapplicationshavebeenwrittenthatuseINGRESorDataSharequeriestoextractinformationfromtheCprogramdatabase.WithminorchangestoadjusttothedifferentformatoftheC+programdatabase,thesetoolscanbeappliedtoC+programs.Manynewtoolscanbedevelopedinashortperiodoftimebysharingthewellstructuredinformationinthedatabase.ThismakesitpossibletoofferC+programmersahigherleveloftoolssupportthanhasbeenpossibleuptonow.OurconceptualmodelforaC+programdatabasecontainstheinformationtoanswerawidevarietyofqueries,includingwhatistheinheritancehierarchyofthisclass?whatareallthepublic,inheritedmembersofthisclass?whereareallthereferencestothisclassmember?whereareallthedeclarationsofthisoverloadedfunction?THELASERRANGEFINDERThisinventionrelatestoarangingmethodandapparatusformeasuringthedistancebetweentheapparatusandatargetbyemployingthelaserimpulsetransittime(timeofflight)principle.Itsimplifiesthedesignoftherangingapparatusandexecutesefficaciouslybyusingdigitallyprogrammabledelaygenerators(DPDG)initsprocessingunit.TheresolutionoftherangingapparatuscanbeeasilyupgradedbyserialarrangementofDPDGs.Alaserrangingapparatusemploysamodulatedlasersignalformeasuringthedistancebetweentherangingapparatusandatarget,wherebythetransittimeofthelasersignalfromtherangingapparatustothetargetandbackagainisdetermined.ThetimedifferenceT,i.e.time-of-flight,betweenthetransmittingofthelasersignalandthereceipt2ofthesignalreflectedbythetarget,isadirectmeasureofthedistanceDtobemeasured.However,thelasersignalistransmittedatthespeedoflight;therefore,apreciserangingrequires,achangeinthedistancetotargetbyfifteencentimetercorrespondstoachangeintimedifferenceofonlyonenanosecond.Inthepriorart，therearesomemethodstotransformthemeasuredvaluesinordertoobtaintherequiredtimeresolutionandcalculatethetime-of-flight.Thesemethods,eitherstretchortransformitintoanothermeasurablevaluesuchasavoltage,relyonagreatquantityofpreciseelectroniccomponents,increasethecomplexityofthecircuit.Theaccuracyofthesemethodsiseasilydegradedbytemperatureaffects.Methodsofcalculatingthephasedifferentbetweentheelectroniccircuitmorecomplicatedandtakealongertimetoperform.Generally,aspecificrangingmethodandapparatuslimitstherangeandresolutionobtainabletherewith.Thatmeans,itisalmostimpossibletochangethembyjustmodifyingapartofthecircuit.Object-OrientationandC+C+isjustoneofmanyprogramminglanguagesinusetoday.Whyaretheresomanylanguages?Whydonewonesappeareveryfewyears?Programminglanguageshaveevolvedtohelpprogrammerseasethetransitionfromdesigntoimplementation.Thefirstprogramminglanguageswereverydependentontheunderlyingmachinearchitecture.Writingprogramsatthislevelofdetailisverycumbersome.Justashardwareengineerslearnedhowtobuildcomputersystemsoutofothercomponents,languagedesignersalsorealizedthatprogramscouldbewrittenatamuchhigherlevel,therebyshieldingtheprogrammerfromthedetailsoftheunderlyingmachine.Whyaretheresuchalargenumberofhigh-levelprogramminglanguages?Therearelanguagesforaccessinglargeinventorydatabases,formattingfinancialreports,controllingrobotsonthefactoryfloor,processinglists,controllingsatellitesinrealtime,simulatinganuclearreactor,predictingchangingatmosphericconditions,playingchess,anddrawingcircuitboards.Eachoftheseproblemsrequiresdifferentsetsofdatastructuresand3algorithms.Programminglanguagesaretoolstohelpussolveproblems.However,thereisnotoneprogramminglanguagethatisbestforeverytypeofproblem.Newprogramminglanguagesareoftendevelopedtoprovidebettertoolsforsolvingaparticularclassofproblems.Otherlanguagesareintendedtobeusefulforavarietyofproblemdomainsandaremoregeneralpurpose.Eachprogramminglanguageimpartsaparticularprogrammingstyleordesignphilosophyonitsprogrammers.Withthemultitudeofprogramminglanguagesavailabletoday,anumberofsuchdesignphilosophieshaveemerged.Thesedesignphilosophies,calledprogrammingparadigms,helpustothinkaboutproblemsandformulatesolutions.SoftwareDesignthroughParadigmsWhendesigningsmallcomputerprogramsorlargesoftwaresystems,weoftenhaveamentalmodeloftheproblemwearetryingtosolve.Howdowedeviseamentalmodelofasoftwaresystem?Programmingparadigmsoffermanydifferentwaysofdesigningandthinkingaboutsoftwaresystems.Aparadigmcanbethoughtofasamentalmodelorasaframeworkfordesigninganddescribingasoftwaresystemsstructure.Themodelhelpsusthinkaboutandformulatesolutions.Wecanusethementalmodelofaparadigmindependentlyfromtheprogramminglanguagechosenforimplementation.However,whenthechosenlanguageprovidesconstructsandmechanismsthataresimilartothosethatarefoundintheparadigm,theimplementationwillbemorestraightforward.Usually,thereareseverallanguagesthatbelongtoaparadigm.Forthisreason,aprogrammingparadigmisalsoconsideredaclassoflanguages.Alanguagedoesnothavetofitintojustoneparadigm.Moreoften,languagesprovidefeaturesorcharacteristicsfromseveralparadigms.Hybridlanguages,suchasC+,combinecharacteristicsfromtwoormoreparadigms.C+includescharacteristicsfromtheimperativeandproceduralparadigms-justlikeitspredecessorlanguage,C-andtheobject-orientedparadigm.Theimperativeparadigmischaracterizedbyanabstractmodelofacomputerwitha4largememorystore.ThisistheclassicvonNeumannmodelofcomputerarchitecture.Computations,whichconsistofasequenceofcommands,arestoredasencodingwithinthestore.Commandsenablethemachinetofindsolutionsusingassignmenttomodifythestore,variablestoreadthestore,arithmeticandlogictoevaluateexpressions,andconditionalbranchingtocontroltheflowofexecution.Theproceduralparadigmincludestheimperativeparadigm,butextendsitwithanabstractionmechanismforgeneralizingcommandsandexpressionsintoprocedures.Parameters,whichareessentiallyaliasesforaportionofthestore,werealsointroducedbythisparadigm.Otherfeaturesincludeiteration,recursion,andselection.Mostmainstreamsprogrammingtodayisdoneinaprocedurallanguage.Theproceduralparadigmwasthefirstparadigmtointroducethenotionofabstractionintoprogramdesign.Thepurposeofabstractioninprogrammingistoseparatebehaviorfromimplementation.Proceduresareaformofabstraction.Theprocedureperformssometaskorfunction.Otherpartsoftheprogramcalltheprocedure,knowingthatitwillperformthetaskcorrectlyandefficiently,butwithoutknowingexactlyhowtheprocedureisimplemented.DATAABSTRACTIONisconcernedwithseparatingthebehaviorofadataobjectfromitsrepresentationorimplementation.Forexample,astackcontainstheoperationsPush,Pop,andIsEmpty.Astackobjectprovidesuserswiththeseoperations,butdoesnotrevealhowthestackisactuallyimplemented.Thestackcouldbeimplementedusinganarrayoralist.Usersofthestackobjectdonotcarehowthestackisimplemented,onlythatitperformstheaboveoperationscorrectlyandefficiently.Becausetheunderlyingimplementationofthedataobjectishiddenfromitsusers,theimplementationcaneasilybechangedwithoutaffectingtheprogramsthatuseit.Whenwedesignalgorithms,weoftenneedaparticulardatatypetouseinordertocarryoutthealgorithmsoperations.Thedesignofanalgorithmiseasierifwesimplyspecifythedatatypesofthevariables,withoutworryingabouthowtheactualdatatypeisimplemented.Wedescribethedatatypebyitspropertiesandoperationsandassumethat5whateverimplementationischosen,theoperationswillworkcorrectlyandefficiently.TypesdefinedinthiswayarecalledABSTRACTDATATYPES(ADTs).Theuseofabstractdatatypesmakesthedesignofthealgorithmmoregeneral,andallowsustoconcentrateonthealgorithmathandwithoutgettingboggeddowninimplementationdetails.Afterthealgorithmshavebeendesigned,theactualdatatypeswillneedtobeimplemented,alongwiththealgorithms.Recently,procedurallanguageshavebeenextendedtosupportthedefinitionofnewdatatypesandprovidefacilitiesfordataabstraction.Theobject-orientedparadigmretainsmuchofthecharacteristicsoftheproceduralparadigm,sinceproceduresarestilltheprimaryformforcomposingcomputations.However,ratherthanoperateonabstractvalues,programsintheobject-orientedparadigmoperateonobjects.Anobjectisverysimilartoanabstractdatatypeandcontainsdataaswellasprocedures.Therearethreeprimarycharacteristicsoftheobject-orientedparadigm.Wehavealreadydescribedthefirst,ENCAPSULATION,themechanismforenforcingdataabstraction.ThesecondcharacteristicisINHERITANCE.Inheritanceallowsnewobjectstobecreatedfromexisting,moregeneralones.Thenewobjectbecomesaspecializedversionofthegeneralobject.Newobjectsneedonlyprovidethemethodsordatathatdifferbecauseofthespecialization.Whenanobjectiscreated(orderived)fromanotherobject,itissaidtoinheritthemethodsanddataoftheparentobject,andincludesanynewrepresentationsandneworrevisedmethodsaddedtoit.Thethirdandfinalcharacteristicofobject-orientedprogrammingisPOLYMORPHISM.Polymorphismallowsmanydifferenttypesofobjectstoperformthesameoperationbyrespondingtothesamemessage.Forexample,wemayhaveacollectionofobjectswhichcanallperformasortoperation.However,wedonotknowwhattypesofobjectswillbecreateduntilrun-time.Object-orientedlanguagescontainmechanismsforensuringthateachsortmessageissenttotherightobject.Encapsulation,inheritance,andpolymorphismareconsideredthefundamental6characteristicsofobject-orientedprogrammingandallobject-orientedlanguagesmustprovidethesecharacteristicsinsomeway.Notsurprisingly,languagessupportthesecharacteristicsinverydifferentways.Smalltalk,C+,Objective-C,andLispwithCLOS(theCommonLispObjectSystem)areallexamplesofobject-orientedlanguages,andeachprovidessupportforencapsulation,inheritance,andpolymorphism.Constructinganobject-orientedprograminvolvesdeterminingtheobjectsthatareneededtosolvetheproblem.Theobjectsarethenusedtoconstructcomputationsthatdefinethebehaviorofthesoftwaresystem.Messagepassingisthefundamentalinteractionmechanismamongobjects.Messages(fromotherobjectsorprograms)aresenttoobjectstoinformthemtoperformoneoftheiroperations.Objectsareresponsibleformaintainingthestateoftheirdata.Onlytheobjectmaymodifyitsinternaldata.Objectsmaythemselvesbeimplementedviaothersub-objects.Implementinganobjectinvolvesarecursiveprocessofbreakingitintosub-objectsuntilatsomeleveltheobjectsandmethodsdefinedonthemareprimitives.Atthispoint,themethodsanddataconsistofelementsthatcanbeimplementedusingthebasicconstructsprovidedbytheprogramminglanguage.Oneofthemostimportantaspectsoftheobject-orientedparadigmishowitchangesourwayofthinkingaboutsoftwaresystems.Systemsarethoughtofasconsistingofindividualentitiesthatareresponsibleforcarryingouttheirownoperations.Eachobjectisconceivedandimplementedasself-contained.Suchamodelfacilitatessoftwaredesign(andlaterimplementation)becauseobjectsoftenmodelconceptualreal-worldentities.Designingsystemsusingtheobject-orientedparadigmresultsinsoftwaresystemsthatbehaveandappearmoreliketheirreal-lifecounterparts.TheObject-OrientedCharacteristicsofC+C+extendsCwithafacilityfordefiningnewdatatypes.AclassislikeaCstruct,butcontainsdataaswellasmethods.Inaddition,C+providesdifferentlevelsofaccesstothemembersofaclassinordertocontrolhowthemembersofaclasscanbemanipulatedfromoutsidetheclass.7Recallthattheimportanceofdataabstractionistohidetheimplementationdetailsofadataobjectfromtheuser.TheuseronlyaccessestheobjectthroughitsPUBLICINTERFACE.AC+classconsistsofapublicandprivatepart.Thepublicpartprovidestheinterfacetotheusersoftheclass,whiletheprivatepartcanonlybeusedbythefunctionsthatmakeuptheclass.C+provideskeywordstoindicatewhichmembersofaclassarehiddenandwhicharepartofitspublicinterface.Themembersofthehiddenimplementationaremarkedinsectionsbeginningwiththekeywordprivate.Thepublicinterfacepartoftheclassfollowsthekeywordpublic.Bydefault,thedeclarationswithinaclassareprivate,meaningthatonlythememberfunctions(andfriends)oftheclasshaveaccesstothem.Aclassdefinitiondoesnotallocateanymemory.Memoryisallocatedwhenanarrayobjectiscreatedthroughavariabledeclaration.Constructorsanddestructorsprovidetheinitializationandcleanupofanobject.Whenanobjectisdeclared,theconstructoriscalledtoinitializethememoryusedbytheobject.Thedestructorperformsanyclean-upfortheobjectwhentheobjectgoesoutofscopeandisdestroyed.Notethatwedidntreallyhidetheimplementationdetailsfromtheuser.C+doesnotprovideawaytocompletelyexcludeallofthedetailsoftheunderlyingimplementation,sincetheprivatepartoftheclassmustbeincludedwiththeclassdefinitionitisusefultorelaxtheaccesstovariableswithinaclass,particularlyunderinheritance.Oftenderivedclassesneedeasyaccesstotheprivatemembersoftheirparentclasses.C+definesthekeywordprotectedforthispurpose.Protectedmemberscanbeaccessedbythememberfunctionsofaclassaswellasbymemberfunctionsofderivedclasses.However,likeprivatemembers,protectedmemberscannotbeaccessedbyuserprograms.Onefinalnoteaboutobjects.Recallthatmessagepassingisthefundamentalmeansforcommunicationamongobjects.Whenwewriteia2.Size()weareeffectivelysendingamessagetothea2arrayobjecttodeterminethesizeofthearrayandreturnit.Inactuality,nomessageisreallysent.C+emulatesmessagepassingthroughtheuseof8functioncalls.Thecompilerensuresusthatthecorrectfunctionwillbecalledforthedesiredobject.So,inC+youcanthinkofmessagepassingasfunctioncalls.Object-orientationhasbecomeabuzzwordwithmanymeanings.Itisadesignmethodology,aparadigm(awayofthinkingaboutproblemsandfindingsolutions),andaformofprogramming.Asadesignmethodology,wecanuseobject-orientedtechniquestodesignsoftwaresystems.Butitismorethanadesignmethodology,itisawholenewwayofthinkingaboutproblems.Object-orienteddesignallowsustothinkabouttheactualreal-worldentitiesoftheproblemweareattemptingtoprovideasolutionfor.Beginningthedesignwithconceptsfromthereal-worldproblemdomainallowsthesameconceptstobecarriedovertoimplementation,makingthedesignandimplementationcyclemoreseamless.Onceadesignhasbeenconceived,aprogramminglanguagecanbechosenforimplementation.Byfactoringouttheinheritancerelationshipsfromtheobjecthierarchiesdiscoveredduringdesign,onecanevenimplementthesysteminatraditional,non-object-orientedlanguage.However,usinganobject-orientedlanguage,suchasC+,makesiteasiertorealizethedesignintoanimplementationbecausetheinherentrelationshipsamongobjectscanbedirectlysupportedinthelanguage.LanguagessuchasC+areconsideredhybridlanguagesbecausetheyaremulti-paradigmlanguages.C+isanobject-orientedextensionofCandcanbeusedasaprocedurallanguageorasanobject-orientedlanguage.Inthisissue,wecontinueourtouroftheobject-orientedfeaturesofC+.Oneofthemajorstrengthsofanyobject-orientedprogramminglanguageistheabilitytobuildotherclassesfromexistingclasses,therebyreusingcode.Inheritanceallowsexistingtypestobeextendedtoanassociatedcollectionofsub-types.Recallthatoneofthekeyactionsofobject-orienteddesignistoidentifyreal-worldentitiesandtherelationshipsamongthem.Whenasoftwaresystemisdesigned,avarietyofobjectsarise,whichmayberelatedinonewayoranother.Someclassesmaynotberelatedatall.Manytimesitmakessensetoorganizetheobjectclassesintoaninheritance9hierarchy.Organizingasetofclassesintoaclasshierarchyrequiresthatweunderstandtherelationshipsamongtheclassesindetail.Notallclassrelationshipsdictatethatinheritancebeused.C+providesthreeformsofinheritance:public,private,andprotected.Thesedifferentformsareusedfordifferentrelation-shipsbetweenobjects.Toillustratethesedifferenttypesofinheritance,letslookatseveraldifferentclassrelationships.ThefirstrelationshipistheIS-Arelationship.Thistypeofrelationshiprepresentsaspecializationbetweentypesorclasses.IS-Ainheritanceholdsfortwoclassesiftheobjectsdescribedbyoneclassbelongstothesetofobjectsdescribedbytheothermoregeneralclass.TheIS-Arelationshipisthetraditionalformofinheritancecalledsubtyping.Thesubtypeisaspecializationofsomemoregeneraltypeknownasthesupertype.InC+,thesupertypeiscalledthebaseclassandthesubtypethederivedclass.ToimplementtheIS-ArelationshipinC+weusepublicinheritance.Whenpublicinheritanceisusedthepublicpartsofthebaseclassbecomepublicinthederivedclassandtheprotectedpartsofthebaseclassbecomeprotectedinthederivedclass.ToimplementtheHAS-ArelationshipinC+weuseeithercompositionorprivateinheritance.Forexample,astackcanbeimplementedusinganarray.Wecaneitherusethestackasadatamember(composition)orderivethestackclassfromthearrayclassusingprivateinheritance.Itisalsopossibletouseinheritancetoachieveacontainershiprelationshipbetweentwoclasses.Privateinheritanceisusedwhentheinheritanceisnotpartoftheinterface;thebaseclassisanimplementationdetail.Underprivateinheritance,thepublicandprotectedpartsofthebaseclassbecomepartoftheprivatepartofthederivedclass.Usersofthederivedclasscannotaccessanyofth