毕业论文5000字英文文献翻译c++.docx

毕业论文5000字英文文献翻译c++.docx

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毕业论文5000字英文文献翻译c++

英文翻译

英语原文：

.IntroducingClasses

Theonlyremainingfeatureweneedtounderstandbeforesolvingourbookstoreproblemishowtowriteadatastructuretorepresentourtransactiondata.InC++wedefineourowndatastructurebydefiningaclass.TheclassmechanismisoneofthemostimportantfeaturesinC++.Infact,aprimaryfocusofthedesignofC++istomakeitpossibletodefineclasstypesthatbehaveasnaturallyasthebuilt-intypesthemselves.Thelibrarytypesthatwe'veseenalready,suchasistreamandostream,arealldefinedasclassesthatis,theyarenotstrictlyspeakingpartofthelanguage.

Completeunderstandingoftheclassmechanismrequiresmasteringalotofinformation.Fortunately,itispossibletouseaclassthatsomeoneelsehaswrittenwithoutknowinghowtodefineaclassourselves.Inthissection,we'lldescribeasimpleclassthatwecanuseinsolvingourbookstoreproblem.We'llimplementthisclassinthesubsequentchaptersaswelearnmoreabouttypes,expressions,statements,andfunctionsallofwhichareusedindefiningclasses.

Touseaclassweneedtoknowthreethings:

Whatisitsname?

Whereisitdefined?

Whatoperationsdoesitsupport?

Forourbookstoreproblem,we'llassumethattheclassisnamedSales_itemandthatitisdefinedinaheadernamedSales_item.h.

TheSales_itemClass

ThepurposeoftheSales_itemclassistostoreanISBNandkeeptrackofthenumberofcopiessold,therevenue,andaveragesalespriceforthatbook.Howthesedataarestoredorcomputedisnotourconcern.Touseaclass,weneednotknowanythingabouthowitisimplemented.Instead,whatweneedtoknowiswhatoperationstheclassprovides.

Aswe'veseen,whenweuselibraryfacilitiessuchasIO,wemustincludetheassociatedheaders.Similarly,forourownclasses,wemustmakethedefinitionsassociatedwiththeclassavailabletothecompiler.Wedosoinmuchthesameway.Typically,weputtheclassdefinitionintoafile.Anyprogramthatwantstouseourclassmustincludethatfile.

Conventionally,classtypesarestoredinafilewithanamethat,likethenameofaprogramsourcefile,hastwoparts:

afilenameandafilesuffix.Usuallythefilenameisthesameastheclassdefinedintheheader.Thesuffixusuallyis.h,butsomeprogrammersuse.H,.hpp,or.hxx.Compilersusuallyaren'tpickyaboutheaderfilenames,butIDEssometimesare.We'llassumethatourclassisdefinedinafilenamedSales_item.h.

OperationsonSales_itemObjects

Everyclassdefinesatype.Thetypenameisthesameasthenameoftheclass.Hence,ourSales_itemclassdefinesatypenamed

Sales_item.Aswiththebuilt-intypes,wecandefineavariableofaclasstype.Whenwewrite"Sales_itemitem"wearesayingthatitemisanobjectoftypeSales_item.Weoftencontractthephrase"anobjectoftypeSales_item"to"aSales_itemobject"orevenmoresimplyto"aSales_item."

InadditiontobeingabletodefinevariablesoftypeSales_item,wecanperformthefollowingoperationsonSales_itemobjects:

Usetheadditionoperator,+,toaddtwoSales_items,

Usetheinputoperator,<

Usetheoutputoperator,>>towriteaSales_itemobject,

Usetheassignmentoperator,=,toassignoneSales_itemobjecttoanother,

Callthesame_isbnfunctiontodetermineiftwoSales_itemsrefertothesamebook.

ClassesarecentraltomostC++programs:

Classesletusdefineourowntypesthatarecustomizedfortheproblemsweneedtosolve,resultinginapplicationsthatareeasiertowriteandunderstand.Well-designedclasstypescanbeaseasytouseasthebuilt-intypes.

Aclassdefinesdataandfunctionmembers:

Thedatamembersstorethestateassociatedwithobjectsoftheclasstype,andthefunctionsperformoperationsthatgivemeaningtothedata.Classesletusseparateimplementationandinterface.Theinterfacespecifiestheoperationsthattheclasssupports.Onlytheimplementoroftheclassneedknoworcareaboutthedetailsoftheimplementation.Thisseparationreducesthebookkeepingaspectsthatmakeprogrammingtediousanderror-prone.

Classtypesoftenarereferredtoasabstractdatatypes.Anabstractdatatypetreatsthedataandoperationsonthatstateasasingleunit.Wecanthinkabstractlyaboutwhattheclassdoes,ratherthanalwayshavingtobeawareofhowtheclassoperates.Abstractdatatypesarefundamentaltobothobject-orientedandgenericprogramming.

Dataabstractionisaprogrammingtechniquethatreliesontheseparationofinterfaceandimplementation.Theclassdesignermustworryabouthowaclassisimplemented,butprogrammersthatusetheclassneednotknowaboutthesedetails.Instead,programmerswhouseatypeneedtoknowonlythetype'sinterface;theycanthinkabstractlyaboutwhatthetypedoesratherthanconcretelyabouthowthetypeworks.

Encapsulationisatermthatdescribesthetechniqueofcombininglower-levelelementstoformanew,higher-levelentity.Afunctionisoneformofencapsulation:

Thedetailedactionsperformedbythefunctionareencapsulatedinthelargerentitythatisthefunctionitself.Encapsulatedelementshidethedetailsoftheirimplementationwemaycallafunctionbuthavenoaccesstothestatementsthatitexecutes.Inthesameway,aclassisanencapsulatedentity:

Itrepresentsanaggregationofseveralmembers,andmostclasstypeshidethemembersthatimplementthetype.

Ifwethinkaboutthelibraryvectortype,itisanexampleofbothdataabstractionandencapsulation.Itisabstractinthattouseit,wethinkaboutitsinterfaceabouttheoperationsthatitcanperform.Itisencapsulatedbecausewehavenoaccesstothedetailsofhowthetypeisrepresentatednortoanyofitsimplementationartifacts.Anarray,ontheotherhand,issimilarinconcepttoavectorbutisneitherabstractnorencapsulated.Wemanipulateanarraydirectlybyaccessingthememoryinwhichthearrayisstored.

Notalltypesneedtobeabstract.Thelibrarypairclassisagoodexampleofauseful,well-designedclassthatisconcreteratherthanabstract.Aconcreteclassisaclassthatexposes,ratherthanhides,itsimplementation.

Someclasses,suchaspair,reallyhavenoabstractinterface.Thepairtypeexiststobundletwodatamembersintoasingleobject.Thereisnoneedoradvantagetohidingthedatamembers.Hidingthemembersinaclasslikepairwouldonlycomplicatetheuseofthetype.

Evenso,suchtypesoftenhavememberfunctions.Inparticular,itisagoodideaforanyclassthathasdatamembersofbuilt-inorcompoundtypetodefineconstructortoinitializethosemembers.Theuseroftheclasscouldinitializeorassigntothedatamembersbutitislesserror-pronefortheclasstodoso.

Programmerstendtothinkaboutthepeoplewhowillruntheirapplicationsas"users."Applicationsaredesignedforandevolveinresponsetofeedbackfromthosewhoultimately"use"theapplications.Classesarethoughtofinasimilarway:

Aclassdesignerdesignsandimplementsaclassfor"users"ofthatclass.Inthiscase,the"user"isaprogrammer,nottheultimateuseroftheapplication.

Authorsofsuccessfulapplicationsdoagoodjobofunderstandingandimplementingtheneedsoftheapplication'susers.Similarly,well-designed,usefulclassesaredesignedwithacloseattentiontotheneedsoftheusersoftheclass.

Inanotherway,thedivisionbetweenclassdesignerandclassuserreflectsthedivisionbetweenusersofanapplicationandthedesignersandimplementorsoftheapplication.Userscareonlyiftheapplicationmeetstheirneedsinacost-effectiveway.Similarly,usersofaclasscareonlyaboutitsinterface.Goodclassdesignersdefineaclassinterfacethatisintuitiveandeasytouse.Userscareabouttheimplementationonlyinsofarastheimplementationaffectstheiruseoftheclass.Iftheimplementationistoosloworputsburdensonusersoftheclass,thentheusersmustcare.Inwell-designedclasses,onlytheclassdesignerworriesabouttheimplementation.

Insimpleapplications,theuserofaclassandthedesigneroftheclassmightbeoneandthesameperson.Eveninsuchcases,itisusefultokeeptherolesdistinct.Whendesigningtheinterfacetoaclass,theclassdesignershouldthinkabouthoweasyitwillbetousetheclass.Whenusingtheclass,thedesignershouldn'tthinkabouthowtheclassworks.

Whenreferringtoa"user,"thecontextmakesitclearwhichkindofuserismeant.Ifwespeakof"usercode"orthe"user"oftheSales_itemclass,wemeanaprogrammerwhoisusingaclassinwritinganapplication.Ifwespeakofthe"user"ofthebookstoreapplication,wemeanthemanagerofthestorewhoisrunningtheapplication.

Dataabstractionandencapsulationprovidetwoimportantadvantages:

1.Classinternalsareprotectedfrominadvertentuser-levelerrors,whichmightcorruptthestateoftheobject.

2.Theclassimplementationmayevolveovertimeinresponsetochangingrequirementsorbugreportswithoutrequiringchangeinuser-levelcode.

Bydefiningdatamembersonlyintheprivatesectionoftheclass,theclassauthorisfreetomakechangesinthedata.Iftheimplementationchanges,onlytheclasscodeneedstobeexaminedtoseewhataffectthechangemayhave.Ifdataarepublic,thenanyfunctionthatdirectlyaccessesthedatamembersoftheoldrepresentationmightbebroken.Itwouldbenecessarytolocateandrewriteallthoseportionsofcodethatreliedontheoldpesentationbeforetheprogramcouldbeusedagain.

Similarly,iftheinternalstateoftheclassisprivate,thenchangestothememberdatacanhappeninonlyalimitednumbero