C++面向对象程序设计双语教程（第3版）课件 ch05Operator Overloading

OperatorOverloadingC++面向对象程序设计双语教程(第3版)Chapter501IntroductiontoOperatorOverloadingThepreviouschapterdefinedandimplementedtheDateclass.ItalsoshowedhowtheDateclasscanbeusedtorepresentadateintheuser'sprogram.Let'sreviewthedefinitionoftheDateclass.IntroductiontoOperatorOverloadingThefirststatementprintsthevalueofthemyDayobject.Whethercanweusetheinsertionoperator<<tooutputthevalueofmyDay？ThesecondstatementincrementsthevalueofmyDaybyoneyear，whethercanweusetheincrementoperator++toincrementthevalueofmyDay？IntroductiontoOperatorOverloadingLikewise，whethercanweusearelationaloperatorforcomparison？Ifyes，wecanenhancetheflexibilityformanipulatingobjectmyDay.Thus，wecanusethefollowingstatementsinsteadofthepreviousstatements.IntroductiontoOperatorOverloadingAsweknow，C++supportsasetofoperatorsforbuilt-intypes.However，theseoperatorscannotbeapplieddirectlytouser-definedtypes.IntroductiontoOperatorOverloadingAlthoughwewanttousetheoperators<<，++andtorepresentthenotionsofthemyDayobjeet，thiswillcausecompileerrorssincewedonotdefinethebehavioroftheDateclasswith<<，++and==.Therefore，wemustextendthedefinitionoftheseoperatorsinourclassDate.Thisisealledoperatoroverloading，inC++terminology.

Operatoroverloadingisoneofthemostexcitingfeaturesofobject-orientedprogramming.IntroductiontoOperatorOverloading02OperatorFunctionsC++allowstheusertooverloadmostoftheoperatorssothattheoperationscanwork

effectivelyinaspecificapplication.Hereisalistofalltheoperatorsthatcanbeoverloaded：OperatorFunctionsThefollowingoperators

cannotbedefinedbyauser：scoperesolution;memberselection;memberselectionthroughpointertomember;ternaryoperator.OperatorFunctionsOperatorFunctionsInordertooverloadoperators，youmustwritefunetions.Thefunctionthatoverloadsan

operatorisnamedbythereservedwordoperatorfollowedbytheoperatortobeoverloaded.OperatorFunctionsThefirstfunctionoverloadsanadditionoperator（+）asamemberfunction.Thesecondfunctionoverloadsaminusoperator（）asanon-memberfunction.Noticethatwehavetoacccssdatamembersrealandimaginthenon-memberoperatorfunctionsoweputthesedata

membersintothepubliepartoftheclassdeclarationincomplex.h.OperatorFunctionsHowever，puttingdatamembersrealandimagintothepublicpartofclasscomplexviolatestheinformationhidingoftheclass.WecandefineaminusoperatorQ）asafriendfunction.OperatorFunctionsTherefore，overloadingoperatorsasanon-membercouldrequireadditionalmemoryandcomputationaltimetomakealocalcopyofthedata.Thus，forefficiencypurpose，whereverpossible，youshouldoverloadoperatorsasmemberfunctions.OperatorFunctions03BinaryandUnaryOperatorsIngeneral，arithmetic，suchas+，-，*，orrelation，suchas-，<=，representabinaryoperator.Thesebinaryoperatorscanbedefinedbyeitheranon-staticmemberfunetiontakingoneparameteroranon-member（orfriend）functiontakingtwoparameters.OverloadingBinaryOperatorsBinaryandUnaryOperatorsOverloadingUnaryOperatorsTheprocessofoverloadingunaryoperatorsissimilartotheprocessofoverloadingbinaryoperators.Theonlydifferenceisthatinthecaseofunaryoperators，theoperatorhasonlyoneparameter.BinaryandUnaryOperatorsTheexamplesabovepresenttheprefixinerementoperator（+）tobeoverloaded.Todistinguishbetweenpre-andpost-fixincrementoperatoroverloading，weuseadummyparameter（oftypeint）inthefunctiondeelarationoftheoperatorfunction.BinaryandUnaryOperators04OverloadingCombinatorialOperatorsThemeaningsofsomebuilt-inoperatorsaredefinedtobeequivalenttosome

combinationofotheroperatorsonthesamearguments.Forexample，ifaandnareintegers，a

+=nmeansa=a+n.Suchrelationsdonotholdforuser-definedoperatorsunlesstheuserhappenstodefinethemthatway.OverloadingCombinatorialOperatorsYoumaynoticethatthetwostatementsinLines9and15（inexample5_4.cpp）outputthedifferentresults.Thesetwostatementslookverysimilar.However，theyhavedifferentmeanings.ThestatementinLine9initializesanobjectc2bytheconstructedobjectclandinvokesthecopyconstructorofclasscomplex，doingamember-wisecopyofobjectc1.OverloadingCombinatorialOperatorsThe

outputisshowninLine3oftheResult.ThestatementinLine15invokestheassignment

operator（=）functionofclasscomplex，assigningobjeeteltoobjecte2.Theoutputisshownin

Line7oftheResult.OverloadingCombinatorialOperatorsThefirststatementinvokesadefaultcopyconstructortoinitializethec2objeetby++cl.Thesecondstatementinvokesadefaultassignmentoperatortoassignthenewdatafromc1++toc2.Thesituationwiththedefaultimplementationsisthatasimplecopyofthemembers（i.e.Shallowcopy）maynotbeappropriatetocloneanobject.OverloadingCombinatorialOperatorsThe

shallowcopyingthepointerisnotenoughbecausenowyou'llhavetwoobjectsthathavethesamepointervalue，andbothobjectswilltrytofreethememoryallocatedtothatpointerwhentheyaredestroyed.Inthiscase，wemustdefineacopyconstructorandanassignmentoperatorforcopyingobjects（i.e.DeepCopy）.OverloadingCombinatorialOperators05MixedArithmeticofUserDefinedTypesAnoperatorfunctionmusteitherbeamemberortakeatleastoneargumentofa

user-definedtype（thisisnotneededforthefunctionsredefiningthenewanddeleteoperators）.Thisruleensuresthatausercannotchangethemeaningofanexpressionunlesstheexpression

containsan

objectofauser-definedtype.MixedArithmeticofUserDefinedTypesInparticular，itisnotpossibletodefineanoperator

functionthatoperatesexclusivelyonpointers.ThisensuresthatCt+isextensiblebutnot

mutable（withtheexceptionofoperators=and&，forclassobjects）.MixedArithmeticofUserDefinedTypesSometimes.anoperatorfunctionintendstoacceptabuilt-intypeasanoperand.Thesecondlinestatesthatacomplexobjectcisaddedtotheinteger2：c+2can，witha

mberfunetion，whichcanbeinterpretedasc.MixedArithmeticofUserDefinedTypesHowever，the2+estatementinthethirdlinecannotbedonebecausethereisnoclassint

forwhichthedefinitionofoperator+means2.Inthiscase，theoperatorfunctionneedstobedefinedasanon-memberfunctionas

follows.MixedArithmeticofUserDefinedTypesEveniftherewere，twodifferentmemberfunctionswouldbeneededtocopewith2+candc+2.Becausethecompilerdoesnotknowthemeaningofauser-defined+，itcannotassumethatitiscommutativeandtheninterpret2+casc+2.MixedArithmeticofUserDefinedTypes06TypeConversionofUser-DefinedTypesMostprogramsprocessavarietyoftypes.Sometimesalltheoperations"staywithina

type".Forinstance，addinganintegertoanintegerproducesaninteger.Butitisoftennecessarytoconvertdataofonetypetodataofanothertype.TypeConversionofUser-DefinedTypesThiscanhappeninassignments，incalculations，inpassingvaluestofunetions，andinreturningvaluesfromfunctions.Thecompilerknowshowtoperformcertainconversionamongbuilt-intypes.Programmerscanforceconversionsamongbuilt-intypesbycasting.TypeConversionofUser-DefinedTypesConvertingaBuilt-InTypetoaUserDefinedTypeSuchconversioncanbeperformedbyaconstructor，calledconversionconstructors，namely，singleargumentconstructorsthatconvertobjectsofbuilt-intypesintoobjectsofaspecifiedclass.TypeConversionofUser-DefinedTypesConvertingaBuilt-InTypetoaUserDefinedTypeInLine5ofthemainfunetion，10.5isfirstconvertedtoanunnamedcomplexobjectbyinvokingtheconversionconstructor，plex（10.5），andthec2objectisthencreatedbytheunnamedobject.TypeConversionofUser-DefinedTypesAconversionoperator（alsocalledaeastoperator）canbeusedtoconvertanobjectofoneclassintoan

objectofanotherclassorintoanobjectofabuilt-intype.转换运算符（也称为cast运算符）可用于将个类的对象转换为另一个类的对象或转换为内置类型。ConvertingaUser-DefinedTypetoaBuilt-InTypeTypeC