c++英语文献.docx

c++英语文献.docx

《c++英语文献.docx》由会员分享，可在线阅读，更多相关《c++英语文献.docx（11页珍藏版）》请在冰豆网上搜索。

c++英语文献

Object-OrientationandC++

C++isjustoneofmanyprogramminglanguagesinusetoday.Whyaretheresomanylanguages?

Whydonewonesappeareveryfewyears?

Programminglanguageshaveevolvedtohelpprogrammerseasethetransitionfromdesigntoimplementation.

Thefirstprogramminglanguageswereverydependentontheunderlyingmachinearchitecture.Writingprogramsatthislevelofdetailisverycumbersome.Justashardwareengineerslearnedhowtobuildcomputersystemsoutofothercomponents,languagedesignersalsorealizedthatprogramscouldbewrittenatamuchhigherlevel,therebyshieldingtheprogrammerfromthedetailsoftheunderlyingmachine.

Whyaretheresuchalargenumberofhigh-levelprogramminglanguages?

Therearelanguagesforaccessinglargeinventorydatabases,formattingfinancialreports,controllingrobotsonthefactoryfloor,processinglists,controllingsatellitesinrealtime,simulatinganuclearreactor,predictingchangingatmosphericconditions,playingchess,anddrawingcircuitboards.Eachoftheseproblemsrequiresdifferentsetsofdatastructuresandalgorithms.Programminglanguagesaretoolstohelpussolveproblems.However,thereisnotoneprogramminglanguagethatisbestforeverytypeofproblem.Newprogramminglanguagesareoftendevelopedtoprovidebettertoolsforsolvingaparticularclassofproblems.Otherlanguagesareintendedtobeusefulforavarietyofproblemdomainsandaremoregeneralpurpose.

Eachprogramminglanguageimpartsaparticularprogrammingstyleordesignphilosophyonitsprogrammers.Withthemultitudeofprogramminglanguagesavailabletoday,anumberofsuchdesignphilosophieshaveemerged.Thesedesignphilosophies,calledprogrammingparadigms,helpustothinkaboutproblemsandformulatesolutions.

SoftwareDesignthroughParadigms

Whendesigningsmallcomputerprogramsorlargesoftwaresystems,weoftenhaveamentalmodeloftheproblemwearetryingtosolve.Howdowedeviseamentalmodelofasoftwaresystem?

Programmingparadigmsoffermanydifferentwaysofdesigningandthinkingaboutsoftwaresystems.Aparadigmcanbethoughtofasamentalmodelorasaframeworkfordesigninganddescribingasoftwaresystem'sstructure.Themodelhelpsusthinkaboutandformulatesolutions.

Wecanusethementalmodelofaparadigmindependentlyfromtheprogramminglanguagechosenforimplementation.However,whenthechosenlanguageprovidesconstructsandmechanismsthataresimilartothosethatarefoundintheparadigm,theimplementationwillbemorestraightforward.Usually,thereareseverallanguagesthatbelongtoaparadigm.Forthisreason,aprogrammingparadigmisalsoconsideredaclassoflanguages.

Alanguagedoesnothavetofitintojustoneparadigm.Moreoften,languagesprovidefeaturesorcharacteristicsfromseveralparadigms.Hybridlanguages,suchasC++,combinecharacteristicsfromtwoormoreparadigms.C++includescharacteristicsfromtheimperativeandproceduralparadigms--justlikeitspredecessorlanguage,C--andtheobject-orientedparadigm.

Theimperativeparadigmischaracterizedbyanabstractmodelofacomputerwithalargememorystore.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,weoftenneedaparticulardatatypetouseinordertocarryoutthealgorithm'soperations.Thedesignofanalgorithmiseasierifwesimplyspecifythedatatypesofthevariables,withoutworryingabouthowtheactualdatatypeisimplemented.Wedescribethedatatypebyitspropertiesandoperationsandassumethatwhateverimplementationischosen,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,andpolymorphismareconsideredthefundamentalcharacteristicsofobject-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.

Recallthattheimportanceofdataabstractionistohidetheimplementationdetails