C++面向对象程序设计双语教程（第3版）课件 ch02Basic Facilities-Shifting from C to C++Programs

BasicFacilities—ShiftingfromCtoC++ProgramsC++面向对象程序设计双语教程(第3版)Chapter201C++ProgramStructureTheC++programconsistsofidentifiers，declarations，variables，constants，expressions，statements，andcomments.Acommonstructureforasimple，onefile，C++programincludesthefollowing：Comments.Theydescribeprogramfeature，author，andsoon.C++ProgramStructureIncludestatements.Theyspecifytheheaderfilesforlibraries.Usingnamespacestatement.Typically，thisisonly“usingnamespacestd”.Globaldeclarations（constants，types，variables，…）.Avoidglobalvariablesifpossible.C++ProgramStructureFunctionprototypes（declarations）.Mainfunctiondefinition.Functiondefinitions.C++ProgramStructureThefollowingexampleisusedtoillustrateaC++programstructure.Donotbetoo

concernedwiththedetailsintheprogram-justobserveitsover-alllookandstructure.C++ProgramStructureAllC++programscompriseoneormorefunctions，whicharealogicalgroupingofone

ormorestatements.Afunctionisidentifiedbyafunctionnameandafunctionbody.C++ProgramStructureAfunctionnameisidentifiedbyawordfollowedbyparentheses.InExample2-1，mainisafunctionname.Allprogramsmusthaveafunctioncalledmain.Theexeeutionofaprogrambeginswiththemainfunction.Thekeywordintalongwiththefunctionnamesignifiesthatthefunctionreturnsanintegervalue.C++ProgramStructureAfunctionbodyissurroundedbycurlybraces（{}）.Thebracesdelimitablockof

programstatements.Everyfunctionmusthaveapairofbraces.C++ProgramStructure02Input/Output

StreamsAprogramperformsthreebasicoperations：itgetsdata，itmanipulatesthedata，anditoutputtheresults.BecausewritingprogramsforI/Oisquitecomplex，C++offersextensivesupportforI/Ooperations.InC++，I/Oisasequenceofcharacters，calledastream，fromthe

sourcetothedestination.Input/OutputStreamsAstreamisasequenceofcharactersfromthesourcetothedestination.Inputstreamisasequenceofcharactersfromaninputdevice（e.g.keyboard）tothecomputer.Outputstreamisasequenceofcharactersfromthecomputertoanoutputdevice（e.g.monitor）.Input/OutputStreamsThestandardlibraryoffersistreamforinput.Theistreamdealswiththecharacterstring

representationsofbuilt-intypesandcaneasilybeextendedtocopewithuser-definedtypes.Withinputstreams，theextractionoperator（>>）isusedtoremovevaluesfromthestream.Thismakessense：whentheuserpressesakeyonthekeyboard，thekeycodeisplacedinan

inputstream.Yourprogramthenextractsthevaluefromthestreamsoitcanbeused.InputStreamInput/OutputStreamsThe>>operator（“getfrom”）isusedasaninputoperator；ein（readsee-in）isoneofthestandardInput/Outputstreams，i.e.iostream.Thetypeoftheright-handoperandof>>determineswhatinputisacceptedandwhatisthetargetoftheinputoperation.Input/OutputStreamsOutputStreamWithoutputstreams，theinsertionoperator（<<）isusedtoputvaluesinthestream.Thisalsomakessense：youinsertyourvaluesintothestream，andthedataconsumer（e.g.monitor）usesthem.Input/OutputStreamsOutputStreamThe<<operator（“putin”）areusedasanoutputoperation.cout（readsee-out）isalsooneofthestandardInput/Outputstreams，i.e.iostream.Bydefault，valuesoutputtocoutareconvertedtoasequenceofcharacters.Input/OutputStreams03ConstantsC++offerstheconceptofauser-definedconstant，aconst，toexpressthenotionthatthevalueofavariablecannotbechangeddirectly.Thisisusefulinseveralcontexts.Forexample，manyvariablesdonotactuallyhavetheirvalueschangedafterinitialization；symbolic

constantsleadtomoremaintainablecodethandoliteralsembeddeddirectlyincode；pointers

areoftenreadthroughbutneverwrittenthrough，andmostfunctionparametersarereadbut

notwrittento.ConstantsTheconstkeywordspecifiesthatavariable'svalueisconstantandtellsthecompilertopreventtheprogrammerfrommodifyingit.Itcanbeaddedtothedeclarationofavariabletodeclarethevariableasaconstant.ConstantsGiventhis，thecompilerknowsthevaluesofclandc2sothattheycanbeusedinconstantexpressions.Sincethevaluesofc3andc4arenotknownatcompiletime，storagemustbeallocatedfore3andc4.Becausetheaddressofc2istaken（andpresumablyusedsomewhere），storagemustbeallocatedforc2.ConstantsPointersandConstantsWhenusingapointer，twovariablesareinvolved：thepointeritselfandthevariablepointedto."Prefixing³thedeclarationofapointerwithconstmakesthevariable，butnotthepointer，aconstant.Todeclareapointeritself，ratherthantheobjectpointedto，tobeaconstant，weusethedeclaratoryoperator*constinsteadofplain*.Constants04FunctionsLetuslookbacktheprograminExample1-3ofChapter1.Whenyouworkonaprogram，itcanbedividedintoafewblockscalledfunetionbody.Sometimes，itisnotpracticaltoputtheentireprogramintoonefunction，e.g.themainfunction，asyouseeinExample1-3.Youmustlearntobreaktheproblemintomanageablepieces.FunctionsFunctionDeclarationsAfunctiondeclarationestablishesthenameofthefunction，thetypeofdatareturnedby

esofitsparameters.FunctionsAfunctiondeclarationconsistsofareturntype，aname，andaparameterlist.Inaddition，afunctiondeclarationmayoptionallyspecifythefunction'slinkage.InC++，thedeclarationcanalsospecifyanexceptionspecification，aconst-qualification，oravolatile-qualification.FunctionsAdeclarationinformsthecompileroftheformatandexistenceofafunctionpriortoitsuse.Afunctioncanbedeclaredseveraltimesinaprogram，providedthatallthedeclarationsarecompatible.Animplicitdeclarationofafunctionisnotallowed：everyfunctionmustbeexplicitlydeclaredbeforeitcanbecalled.FunctionsFunctionDeclarationsAfunctionargument（i.e.actualparameter）isanexpressionoravariablethatyouusewithinthe

parenthesesofafunctioncall.Afunetionparameter（i.e.formalparameter）isanobjeetorreferencedeclared

withintheparenthesisofafunctiondeclarationordefinition.FunctionsFunctionprototypesarerequiredforcompatibilitybetweenCandC++.Thenon-prototypeformofafunctionthathasanemptyparameterlistmeansthatthefunctiontakesanunknownnumberofparametersinC，whereas，inC++，itmeansthatittakesno

parameters.FunctionsFunctionDefinitionsAfunctiondefinitionconsistsofitsname，itsreturntype，itsparameterlistandbody.Thefunctionheaderspecifiesthefunction'sreturntype，nameandparameters.Thefunetionsignatureconsistsofthefunctionnameandtheparameterlist.FunctionsEveryfunctioninaprogrammustbedefinedsomewhere（onceonly）beforeitisinvoked.Afunctiondefinitionisafunctiondeclarationinwhichthebodyofthefunctionispresented.FunctionDefinitionsFunctionsDefaultArgumentsAgeneralfunctionoftenneedsmoreparametersthatarenecessarytohandlesimplecases.Inparticular，functionsthatconstructobjectsoftenprovideseveraloptionsforflexibility.FunctionsAdefaultargumentmeansthatthedefaultvaluesarepassedtotheparameterswhena

functionisinvokedwithoutthearguments.Ofcourse，theusercanchangethedefaultvalueif

heorsheprovidesavalueforthedefaultparameter.FunctionsInlineFunctionsAninlinefunctionisoneforwhichthecompilercopiesthecodefromthefunctiondefinitiondirectlyintothecodeofthecallingfunctionratherthancreatingaseparatesetofinstruetionsinmemory.Insteadoftransferringcontroltoandfromthefunctioncodesegment，amodifiedcopyofthefunctionbodymaysubstitutethefunctiondirectlyforthefunctioncall.Inthisway，theperformaneeoverheadofafunetioncallisavoided.FunctionsFunctionOverloadingMostoften，itisagoodideatogivedifferentfunctions

differentnames，butwhensomefunctionsconceptuallyperformthesametaskonobjectsofdifferenttypes，itcanbemofeconvenienttogivethemthesamename.Usingthesamenameforoperationsondifferenttypesiscalledoverloading.Functions05ReferencesReferenceDefinitionAreferenceisasimplereferencedatatype.Areferenceisanaliasoralternativenameforanobject.ThismeansthatC++allowsyoutocreateasecondnamefortheobjectthatyoucanusetoreadormodifytheoriginaldatastoredintheobject.ReferencesParametersprovideacommunicationlinkbetweenthecallingfunction（suchasmain）andthecalledfunction.Theyenablefunctionstomanipulatedifferentdataeachtimetheyarecalled.Ingeneral，therearetwotypesofformalparameters：valueparametersandreferenceparameters.ReferenceVariablesasParametersReferencesReferencesasReturningValuesReturningvaluesfromafunctiontoitscallerbyvalue，address，orreferenceworksalmost

the

same way

aspassingparameterstoafunction.Theyhavethesameadvantages

and

disadvantages.Theprimarydifferencebetweenthetwoissimplytheoppositedirectionsofdataflow.However，thereisonemoreaddedbitofcomplexity-becauselocalvariablesinafunctiongooutofscopewhenafunctionreturns，weneedtoconsidertheeffectofthisoneachretumtype.ReferencesReturningbyreferenceistypicallyusedtoreturnargumentspassedbyreferencetothe

functionbacktothecallerusedontheright-handsideofanassignmentoperator.Ifthefunctioncallcanappearontheleft-handsideofanassignmentoperator，whatwillhappen？ReferenceasLeft-HandValuesReferences06Namespaces