面向对象程序设计英文.docx

面向对象程序设计英文.docx

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面向对象程序设计英文

Object-OrientedProgramming

Object-orientedprogrammingisbasedonthreefundamentalconcepts:

dataabstraction,inheritance,anddynamicbinding.InC++weuseclassesfordataabstractionandclassderivationtoinheritoneclassfromanother:

Aderivedclassinheritsthemembersofitsbaseclass（es）.Dynamicbindingletsthecompilerdetermineatruntimewhethertouseafunctiondefinedinthebaseorderivedclass.

Inheritanceanddynamicbindingstreamlineourprogramsintwoways:

Theymakeiteasiertodefinenewclassesthataresimilar,butnotidentical,tootherclasses,andtheymakeiteasierforustowriteprogramsthatcanignorethedetailsofhowthosesimilartypesdiffer.。

Manyapplicationsarecharacterizedbyconceptsthatarerelatedbutslightlydifferent.Forexample,ourbookstoremightofferdifferentpricingstrategiesfordifferentbooks.Somebooksmightbesoldonlyatagivenprice.Othersmightbesoldsubjecttosomekindofdiscountstrategy.Wemightgiveadiscounttopurchaserswhobuyaspecifiednumberofcopiesofthebook.Orwemightgiveadiscountforonlythefirstfewcopiespurchasedbutchargefullpriceforanyboughtbeyondagivenlimit.

Object-orientedprogramming（OOP）isagoodmatchtothiskindofapplication.Throughinheritancewecandefinetypesthatmodelthedifferentkindsofbooks.Throughdynamicbindingwecanwriteapplicationsthatusethesetypesbutthatcanignorethetype-dependentdifferences.

Theideasofinheritanceanddynamicbindingareconceptuallysimplebuthaveprofoundimplicationsforhowwebuildourapplicationsandforthefeaturesthatprogramminglanguagesmustsupport.BeforecoveringhowC++supportsOOP,we'lllookattheconceptsthatarefundamentaltothisstyleofprogramming.

AnOverview

ThekeyideabehindOOPispolymorphism.PolymorphismisderivedfromaGreekwordmeaning"manyforms."Wespeakoftypesrelatedbyinheritanceaspolymorphictypes,becauseinmanycaseswecanusethe"manyforms"ofaderivedorbasetypeinterchangeably.Aswe'llsee,inC++,polymorphismappliesonlytoreferencesorpointerstotypesrelatedbyinheritance.

Inheritance

Inheritanceletsusdefineclassesthatmodelrelationshipsamongtypes,sharingwhatiscommonandspecializingonlythatwhichisinherentlydifferent.Membersdefinedbythebaseclassareinheritedbyitsderivedclasses.Thederivedclasscanuse,withoutchange,thoseoperationsthatdonotdependonthespecificsofthederivedtype.Itcanredefinethosememberfunctionsthatdodependonitstype,specializingthefunctiontotakeintoaccountthepeculiaritiesofthederivedtype.Finally,aderivedclassmaydefineadditionalmembersbeyondthoseitinheritsfromitsbaseclass.

Classesrelatedbyinheritanceareoftendescribedasforminganinheritancehierarchy.Thereisoneclass,referredtoastheroot,fromwhichalltheotherclassesinherit,directlyorindirectly.Inourbookstoreexample,wewilldefineabaseclass,whichwe'llnameItem_base,torepresentundiscountedbooks.FromItem_basewewillinheritasecondclass,whichwe'llnameBulk_item,torepresentbookssoldwithaquantitydiscount.

Ataminimum,theseclasseswilldefinethefollowingoperations:

●anoperationnamedbookthatwillreturntheISBN

●anoperationnamednet_pricethatreturnsthepriceforpurchasingaspecifiednumberofcopiesofabook

ClassesderivedfromItem_basewillinheritthebookfunctionwithoutchange:

ThederivedclasseshavenoneedtoredefinewhatitmeanstofetchtheISBN.Ontheotherhand,eachderivedclasswillneedtodefineitsownversionofthenet_pricefunctiontoimplementanappropriatediscountpricingstrategy.

InC++,abaseclassmustindicatewhichofitsfunctionsitintendsforitsderivedclassestoredefine.Functionsdefinedasvirtualareonesthatthebaseexpectsitsderivedclassestoredefine.Functionsthatthebaseclassintendsitschildrentoinheritarenotdefinedasvirtual.

Giventhisdiscussion,wecanseethatourclasseswilldefinethree（const）memberfunctions:

∙Anonvirtualfunction,std:

:

stringbook（）,thatreturnstheISBN.ItwillbedefinedbyItem_baseandinheritedbyBulk_item.

∙Twoversionsofthevirtualfunction,doublenet_price（size_t）,toreturnthetotalpriceforagivennumberofcopiesofaspecificbook.BothItem_baseandBulk_itemwilldefinetheirownversionsofthisfunction.

DynamicBinding

Dynamicbindingletsuswriteprogramsthatuseobjectsofanytypeinaninheritancehierarchywithoutcaringabouttheobjects'specifictypes.Programsthatusetheseclassesneednotdistinguishbetweenfunctionsdefinedinthebaseorinaderivedclass.

Forexample,ourbookstoreapplicationwouldletacustomerselectseveralbooksinasinglesale.Whenthecustomerwasdoneshopping,theapplicationwouldcalculatethetotaldue.Onepartoffiguringthefinalbillwouldbetoprintforeachbookpurchasedalinereportingthetotalquantityandsalespriceforthatportionofthepurchase.

Wemightdefineafunctionnamedprint_totaltomanagethispartoftheapplication.Theprint_totalfunction,givenanitemandacount,shouldprinttheISBNandthetotalpriceforpurchasingthegivennumberofcopiesofthatparticularbook.Theoutputofthisfunctionshouldlooklike:

ISBN:

0-201-54848-8numbersold:

3totalprice:

98

ISBN:

0-201-82470-1numbersold:

5totalprice:

202.5

Ourprint_totalfunctionmightlooksomethinglikethefollowing:

//calculateandprintpriceforgivennumberofcopies,applyinganydiscounts

voidprint_total（ostream&os,

constItem_base&item,size_tn）

{

os<<"ISBN:

"<

:

book

<<"\tnumbersold:

"<

"

//virtualcall:

whichversionofnet_pricetocallisresolvedatruntime

<<_price（n）<

}

Thefunction'sworkistrivial:

Itprintstheresultsofcallingbookandnet_priceonitsitemparameter.Therearetwointerestingthingsaboutthisfunction.

First,eventhoughitssecondparameterisareferencetoItem_base,wecanpasseitheranItem_baseobjectoraBulk_itemobjecttothisfunction.

Second,becausetheparameterisareferenceandthenet_pricefunctionisvirtual,thecalltonet_pricewillberesolvedatruntime.Theversionofnet_pricethatiscalledwilldependonthetypeoftheargumentpassedtoprint_total.Whentheargumenttoprint_totalisaBulk_item,theversionofnet_pricethatisrunwillbetheonedefinedinBulk_itemthatappliesadiscount.IftheargumentisanItem_baseobject,thenthecallwillbetotheversiondefinedbyItem_base.

C++,dynamicbindinghappenswhenavirtualfunctioniscalledthroughareference（orapointer）toabaseclass.Thefactthatareference（orpointer）mightrefertoeitherabase-oraderived-classobjectisthekeytodynamicbinding.Callstovirtualfunctionsmadethroughareference（orpointer）areresolvedatruntime:

Thefunctionthatiscalledistheonedefinedbytheactualtypeoftheobjecttowhichthereference（orpointer）refers.

DefiningBaseandDerivedClasses

Inmanyways,baseandderivedclassesaredefinedlikeotherclasseswehavealreadyseen.However,therearesomeadditionalfeaturesthatarerequiredwhendefiningclassesinaninheritancehierarchy.Thissectionwillpresentthosefeatures.Subsequentsectionswillseehowuseofthesefeaturesimpactsclassesandtheprogramswewriteusinginheritedclasses.

DefiningaBaseClass

Likeanyotherclass,abaseclasshasdataandfunctionmembersthatdefineitsinterfaceandimplementation.Inthecaseofour（verysimplified）bookstorepricingapplication,ourItem_baseclassdefinesthebookandnet_pricefunctionsandneedstostoreanISBNandthestandardpriceforthebook:

//Itemsoldatanundiscountedprice

//derivedclasseswilldefinevariousdiscountstrategies

classItem_base{

public:

Item_base（conststd:

:

string&book="",

doublesales_price=0.0）:

isbn（book）,price（sales_price）{}

std:

:

stringbook（）const{returnisbn;}

//returnstotalsalespriceforaspecifiednumberofitems

//derivedclasseswilloverrideandapplydifferentdiscountalgorithms

virtualdoublenet_price（std:

:

size_tn）const

{returnn*price;}

virtual~Item_base（）{}

private:

std:

:

stringisbn;//identifierfortheitem

protected:

doubleprice;//normal,undiscountedprice

};

Forthemostpart,thisclasslookslikeotherswehaveseen.Itdefinesaconstructoralongwiththefunctionswehavealreadydescribed.Thatconstructorusesdefaultarguments（Section7.4.1,p.253）,whichallowsittobecalledwithzero,one,ortwoarguments.Itinitializesthedatamembersfromthesearguments.

Thenewpartsaretheprotectedaccesslabelandtheuseofthevirtualkeywordonthedestructorandthenet_pricefunction.We'llexplainvirtualdestructorsinSection15.4.4（p.587）,butfornowitisworthnotingthatclassesusedastherootclassofaninheritancehierarchygenerallydefineavirtualdestructor.

Base-ClassMemberFunctions

TheItem_baseclassdefinestwofunctions,oneofwhichisprecededbythekeywordvirtual.Thepurposeofthevirtualkeywordistoenabledynamicbinding.Bydefault,memberfunctionsarenonvirtual.Callstononvirtualfunctionsareresolvedatcompiletime.Tospecifythatafunctionisvirtual,weprecedeitsreturntypebythekeywordvirtual.Anynonstaticmemberfunction,otherthanaconstructor,maybevirtual.Thevirtualkeywordappearsonlyonthemember-functiondeclarationinsidetheclass.Thevirtualkeywordmaynotbeusedonafunctiondefinitionthatappearsoutsidetheclassbody.

AccessControlandInheritance

Inabaseclass,thepublicand