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Reference variable



Definition

intrate=80;int&pt=rate;

⒈ptisareferencevariable,itmustbeinitializedwhendeclaringthereferencevariable.ThevalueofptandratePointtothesamememoryvariable.Themostusefuluseofreferenceisasafunctionparameter(structureandobject).Byreferencingavariableasaparameter,thefunctioncanusetheoriginaldatainsteadofcopying.

⒉ThecharacteristicsofreferencearecloserForaconstpointer,onceitisassociatedwithavariable,itwillalwaysbeloyaltoit.Thereferencecanbesetbyinitialization,butitcannotbesetbyassignment

intother=30;pt=other;

Atthistime,thevalueofrateandthevalueofpthavebeenchangedto30,buttheaddressisstilltheaddressoftheoriginalrate.Nottheaddressofother.

⒊Iftheprogramdoesnotwanttochangetheinformationpassedtoit,butIfyouwanttouseareferenceagain,youshoulduseaconstantreference

inttest(constint&p)//Whenyouchangethevalueofp,thecompilerwillgenerateanerrormessage

⒋IftheactualparameterIfitdoesnotmatchtheapplicationparameters,C++willgeneratetemporaryvariablesforthispurpose.Thisisonlyallowedwhentheparametersareconstreferences.Inthefollowingcases,temporaryvariableswillbegenerated,andthefunctionparameterswillreferencethevariables.

****Whentheparameterisnotaconstreference,ifitisnotanlvalue(andcannotgenerateatemporaryvariable),itwillcauseacompilationerror*****(emphasizedthethirdpoint,thatis,trytouseconst)

Thetypeoftheactualparameteriscorrect,butitisnotanlvalue

.Thetypeoftheactualparameterisincorrect,butitcanbeconvertedtothecorrecttype

Theconceptoflvalue:exceptforliteralconstantsandexpressionscontainingpolynomialsThedataobjectthattheformulacanbereferenced

⒌Trytodeclarethereferencereturnedbythefunctionasconsttopreventyoufrommodifyingitsvaluethroughthereturnedreference.Thereturnreferenceisactuallythealiasofthereferencedvariable.

constsysop&use(sysop&pref)sysoppref=();sysoptest=use(pref);

equivalentto

user(pref);test=pref;

⒍Thecharacteristicofinheritanceisthatthereferenceofthebaseclasscanpointtothederivedclassobjectwithoutforcedtypeconversion.Thisadvantageisthatyoucandefineafunctionthatacceptsthebaseclassreferenceseatparameter.Classobjectasaparameter,youcanalsouseaderivedclassasaparameter

If

int&ir=0

meansthattheaddressis0,notareferenceMeaning,therearevariableassignmentsinthedeclarationInthecaseofvalue,thevariablemustbeusedasthereferencedobject,otherwisethesystemcannotrecognizewhetheryourstatementisareferenceoranassignment!

Instructionsforuse

Ifint&ir=0,itmeansthattheaddressis0,notareference.Ifthereisavariableassignmentinthedeclaration,thevariablemustbeusedasareferenceTheobject,otherwisethesystemcannotrecognizewhetheryourstatementisareferenceoranassignment!

⑴&;Hereitisnotforaddresscalculation,butforidentification.

⑵Thetypeidentifierreferstothetypeofthetargetvariable.

(3)Whendeclaringthereference,itmustbeinitializedatthesametime.

⑷Afterthequoteisdeclared,itisequivalenttothetargetvariablenamehastwonames,namelytheoriginalnameofthetargetandthequotedname,andthequotednamecannotbeusedasanaliasforothervariablenames.

ra=1;isequivalenttoa=1;

⑸Declaringareference,notanewdefinitionofavariable,itonlymeansthatthereferencenameisanaliasofthetargetvariablename,Itisnotadatatypebyitself,sothereferenceitselfdoesnotoccupythestorageunit,andthesystemdoesnotallocatestorageunittothereference.Therefore:toseektheaddressofthereferenceistoseektheaddressofthetargetvariable.&raisequalto&a.

⑹Itispossibletoestablishareferencetoanarray.InC++,thesolutiontothepricereductionproblemofanarrayasaformalparameteristousethereferenceofanarray

⑺Thereferencetothereferencecannotbeestablished,andthepointertothereferencecannotbeestablished..Becauseareferenceisnotadatatype,thereisnoreferencetoareference,noreferencetoapointer.

Forexample:

intn;

int&&r=n;//error,thecompilersystemregards"int&"asone,Regard"&r"asawhole,thatis,areferenceisestablished,andthereferencedobjectshouldbeavariableofacertaindatatype

int&*p=n;//error,compilesystemRegarding"int&"asawhole,and"*p"asawhole,thatis,apointertoareferenceisestablished.Thepointercanonlypointtoavariableofacertaindatatype.

⑻Itisworthmentioningthat,Youcancreatepointerreferences

Forexample:

int*p;

int*&q=p;//Correct,thecompilersystemputs"int*"Thinkofitasone,and"&q"asone,thatis,createareferencetothepointerp,thatis,givethepointerpanaliasq.

Principlesofuse

Forfunctionsthatarepassedbyvaluewithoutmodification:

.Thedataobjectissmall,anditispassedbyvalue

.Ifthedataobjectisanarray,defineaconstpointer

.Ifthedataobjectisalargerstructure,youcanuseaconstpointerorcosntreference,

.Ifitisaclassobject,thenUseconstreference.

Forfunctionsthatmodifythevalue

.Ifitisbuilt-indata,usepointers

.Arrays,youcanonlyusepointers

.Structureusingpointerorreference

.Object.Usereference

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