C常見英文面試筆試題

C/C++ProgramminginterviewquestionsandanswersBySatishShetty,July14th,2004Whatisencapsulation??Containingandhidinginformationaboutanobject,suchasinternaldatastructuresandcode.Encapsulationisolates(使間隔)theinternalcomplexityofanobject'soperationfromtherestoftheapplication.Forexample,aclientcomponentaskingfornetrevenue（利潤）fromabusinessobjectneednotknowthedata'sorigin.Whatisinheritance?Inheritanceallowsoneclasstoreusethestateandbehaviorofanotherclass.Thederivedclassinheritsthepropertiesandmethodimplementationsofthebaseclassandextendsitbyoverridingmethodsandaddingadditionalpropertiesandmethods.WhatisPolymorphism??Polymorphismallowsaclienttotreatdifferentobjectsinthesamewayeveniftheywerecreatedfromdifferentclassesandexhibit（顯現(xiàn)）differentbehaviors.Youcanuseimplementation（實現(xiàn)）inheritancetoachievepolymorphisminlanguagessuchasC++andJava.Baseclassobject'spointercaninvoke（調用）methodsinderivedclassobjects.YoucanalsoachievepolymorphisminC++byfunctionoverloadingandoperatoroverloading.Whatisconstructororctor?Constructorcreatesanobjectandinitializesit.Italsocreatesvtable表？forvirtualfunctions.Itisdifferentfromothermethodsinaclass.

變量列Whatisdestructor?Destructorusuallydeletesanyextraresourcesallocatedbytheobject.Whatisdefaultconstructor?Constructorwithnoargumentsoralltheargumentshasdefaultvalues.Whatiscopyconstructor?Constructorwhichinitializestheit'swithanotherobjectofthesameclass.

objectmembervariables(byshallowcopying)Ifyoudon'timplementoneinyourclassthencompilerimplementsoneforyou.forexample:BooObj1(10);Membertomembercopy(shallowcopy)Whatareallthefunctions

theimplicitmemberfunctionswhichcompilerimplementsfor

oftheusif

class?wedon't

Orwhatarealldefineone.??defaultctorcopyctorassignmentoperatordefaultdestructoraddressoperatorWhatisconversionconstructor?constructorwithasingleargumentmakesthatconstructorasconversionctoranditcanbeusedfortypeconversion.forexample:classBoo{public:Boo(inti);};BooBooObject=10;forexample:classBoo{doublevalue;public:Boo(inti)operatordouble( ){returnvalue;}};BooBooObject;doublei=BooObject;nowconversionoperatorgetscalledtoassignthevalue.Whatisdiffbetweenmalloc( )/free( )andnew/delete?mallocallocatesmemoryforobjectinheapbutdoesn'tinvokeobject'sconstructortoinitiallizetheobject.newallocatesmemoryandalsoinvokesconstructortoinitializetheobject.malloc( )andfree( )donotsupportobjectsemanticsDoesnotconstructanddestructobjectsstring*ptr=(string*)(malloc(sizeof(string)))ArenotsafeDoesnotcalculatethesizeoftheobjectsthatitconstructReturnsapointertovoidint*p=(int*)(malloc(sizeof(int)));int*p=newint;Arenotextensiblenewanddeletecanbeoverloadedinaclass"delete"firstcallstheobject'sterminationroutine.itsdestructor)andthenreleasesthespacetheobjectoccupiedontheheapmemory.Ifanarrayofobjectswascreatedusingnew,thendeletemustbetoldthatitisdealingwithanarraybyprecedingthenamewithanempty[]:-Int_t*my_ints=newInt_t[10];...delete[]my_ints;whatisthediffbetween"new"and"operatornew"?"operatornew"workslikemalloc.Whatisdifferencebetweentemplateandmacro??Thereisnowayforthecompilertoverifythatthemacroparametersareofcompatibletypes.Themacroisexpandedwithoutanyspecialtypechecking.Ifmacroparameterhasapost-incrementedvariable(likec++),theincrementisperformedtwotimes.Becausemacrosareexpandedbythepreprocessor,compilererrormessageswillrefertotheexpandedmacro,ratherthanthemacrodefinitionitself.Also,themacrowillshowupinexpandedformduringdebugging.forexample:Macro:#definemin(i,j)(i<j?i:j)template:template<classT>Tmin(Ti,Tj){returni<j?i:j;}WhatareC++storageclasses?autoregisterstaticexternauto:thedefault.Variablesareautomaticallycreatedandinitializedwhentheyaredefinedandaredestroyedattheendoftheblockcontainingtheirdefinition.Theyarenotvisibleoutsidethatblockregister:atypeofautovariable.asuggestiontothecompilertouseaCPUregisterforperformancestatic:avariablethatisknownonlyinthefunctionthatcontainsitsdefinitionbutisneverdestroyedandretains=keepitsvaluebetweencallstothatfunction.Itexistsfromthetimetheprogrambeginsexecutionextern:astaticvariablewhosedefinitionandplacementisdeterminedwhenallobjectandlibrarymodulesarecombined(linked)toformtheexecutablecodefile.Itcanbevisibleoutsidethefilewhereitisdefined.WhatarestoragequalifiersinC++?Theyare..constvolatilemutableConstkeywordindicatesthatmemoryonceinitialized,shouldnotbealteredbyaprogram.volatilekeywordindicatesthatthevalueinthememorylocationcanbealteredeventhoughnothingintheprogramcodemodifiesthecontents.forexampleifyouhaveapointertohardwarelocationthatcontainsthetime,wherehardwarechangesthevalueofthispointervariableandnottheprogram.Theintentofthiskeywordtoimprovetheoptimizationabilityofthecompiler.mutablekeywordindicatesthatparticularmemberofastructureorclasscanbealteredevenifaparticularstructurevariable,class,orclassmemberfunctionisconstant.structdata{charname[80];mutabledoublesalary;}constdataMyStruct={"SatishShetty",1000};prependingvariablewith"&"symbolmakesitasreference.forexample:inta;int&b=a;&讀ampWhatispassingbyreference?Methodofpassingargumentstoafunctionwhichtakesparameteroftypereference.forexample:voidswap(int&x,int&y){inttemp=x;x=y;y=temp;}inta=2,b=3;swap(a,b);Basically,insidethefunctioninsteadtheyrefertooriginalargumentsanditismoreefficient.

therewon'tbeanycopyofthearguments"x"and"y"variablesaandb.sonoextramemoryneededtopassWhendouse"const"referenceargumentsinfunction?a)Usingconstprotectsyouagainstprogrammingerrorsthatinadvertently不經(jīng)意的alterdata.b)Usingconstallowsfunctiontoprocessbothconstandnon-constactualarguments,whileafunctionwithoutconstintheprototypecanonlyacceptnonconstantarguments.Usingaconstreferenceallowsthefunctiontogenerateanduseatemporaryvariableappropriately.WhenaretemporaryvariablescreatedbyC++compiler?Providedthatfunctionparameterisa"constreference",compilergeneratestemporaryvariableinfollowing2ways.Theactualargumentisthecorrecttype,butitisn'tLvaluedoubleCube(constdouble&num){num=num*num*num;returnnum;}doubletemp=;doublevalue=cube+temp);classparent{voidShow( ){cout<<"i'mparent"<<endl;}};classchild:publicparent{voidShow( ){cout<<"i'mchild"<<endl;}};parent*parent_object_ptr=newchild;parent_object_ptr->show( ).classparent{virtualvoidShow( ){cout<<"i'mparent"<<endl;}};classchild:publicparent{voidShow( ){cout<<"i'mchild"<<endl;}};parent*parent_object_ptr=newchild;parent_object_ptr->show( )Thisbaseclassiscalledabstractclassandclientwon'tabletoinstantiateanobjectusingthisbaseclass.Youcanmakeapurevirtualfunctionorabstractclassthisway..classBoo{voidfoo( )=0;}BooMyBoo;Soapointerwithtwobytealignmenthasazerointheleastsignificantbit.Andapointerwithfourbytealignmenthasazeroinboththetwoleastsignificantbits.Andsoon.Morealignmentmeansalongersequenceofzerobitsinthelowestbitsofapointer.Whatproblemdoesthenamespacefeaturesolve?Multipleprovidersoflibrariesmightusecommonglobalidentifierscausinganamecollisionwhenanapplicationtriestolinkwithtwoormoresuchlibraries.Thenamespacefeaturesurroundsalibrary'sexternaldeclarationswithauniquenamespacethateliminates除去space[identifier]{namespace-body}Anamespacedeclarationidentifiesandassignsanametoadeclarativeregion.Theidentifierinanamespacedeclarationmustbeuniqueinthedeclarativeregioninwhichitisused.Theidentifieristhenameofthenamespaceandisusedtoreferenceitsmembers.Whatistheuseof'using'declaration?Ausingdeclarationmakesitpossibletouseanamefromanamespacewithoutthescope范圍operator.WhatisanIterator迭代器class?Aclassthatisusedtotraversethrough穿過theobjectsmaintainedbyacontainerclass.Therearefivecategoriesofiterators:inputiterators,outputiterators,forwarditerators,bidirectionaliterators,randomaccess.Aniteratorisanentitythatgivesaccesstothecontentsofacontainerobjectwithoutviolatingencapsulationconstraints.Accesstothecontentsisgrantedonaone-at-a-timebasisinorder.Theordercanbestorageorder(asinlistsandqueues)orsomearbitraryorder(asinarrayindices)oraccordingtosomeorderingrelation(asinanorderedbinarytree).Theiteratorisaconstruct,whichprovidesaninterfacethat,whencalled,yieldseitherthenextelementinthecontainer,orsomevaluedenotingthefactthattherearenomoreelementstoexamine.Iteratorshidethedetailsofaccesstoandupdateoftheelementsofacontainerclass.Somethinglikeapointer.Whatisadangling懸掛pointer?Adanglingpointerariseswhenyouusetheaddressofanobjectafteritslifetimeisover.Thismayoccurinsituationslikereturningaddressesoftheautomaticvariablesfromafunctionorusingtheaddressofthememoryblockafteritisfreed.WhatdoyoumeanbyStackunwinding?Itisaprocessduringexceptionhandlingwhenthedestructoriscalledforalllocalobjectsinthestackbetweentheplacewheretheexceptionwasthrownandwhereitiscaught.拋出異樣與棧睜開（stackunwinding）拋出異樣時，將暫停目前函數(shù)的履行，開始查找般配的catch子句。第一檢查throw自己是否在try塊內(nèi)部，假如是，檢查與該try有關的catch子句，看能否能夠辦理該異樣。假如不可以辦理，就退出目前函數(shù)，而且開釋目前函數(shù)的內(nèi)存并銷毀局部對象，連續(xù)到上層的調用函數(shù)中查找，直到找到一個能夠辦理該異樣的catch。這個過程稱為棧睜開（stackunwinding）。當辦理該異樣的catch結束以后，緊接著該catch以后的點連續(xù)履行。1.為局部對象調用析構函數(shù)如上所述，在棧睜開的過程中，會開釋局部對象所占用的內(nèi)存并運轉類種類局部對象的析構函數(shù)。但需要注意的是，假如一個塊經(jīng)過new動向分派內(nèi)存，而且在開釋該資源以前發(fā)生異樣，該塊因異樣而退出，那么在棧睜開時期不會開釋該資源，編譯器不會刪除該指針，這樣就會造成內(nèi)存泄漏。2.析構函數(shù)應當從不拋出異樣在為某個異樣進行棧睜開的時候，析構函數(shù)假如又拋出自己的未經(jīng)辦理的另一個異樣，將會以致調用標準庫terminate函數(shù)。平常terminate函數(shù)將調用abort函數(shù)，以致程序的非正常退出。所以析構函數(shù)應當從不拋出異樣。3.異樣與結構函數(shù)假如在結構函數(shù)對象時發(fā)生異樣，此時該對象可能不過被部分結構，要保證能夠合適的撤掉這些已結構的成員。4.未捕捉的異樣將會停止程序不可以不辦理異樣。假如找不到般配的catch，程序就會調用庫函數(shù)terminate。Nametheoperatorsthatcannotbeoverloaded??sizeof,.,.*,.->,::,?:Whatisacontainerclass?Whatarethetypesofcontainerclasses?Acontainerclassisaclassthatisusedtoholdobjectsinmemoryorexternalstorage.Acontainerclassactsasagenericholder.Acontainerclasshasapredefinedbehaviorandawell-knowninterface.Acontainerclassisasupportingclasswhosepurposeistohidethetopologyusedformaintainingthelistofobjectsinmemory.Whenacontainerclasscontainsagroupofmixedobjects,thecontaineriscalledaheterogeneous不平均的多樣的container;whenthecontainerisholdingagroupofobjectsthatareallthesame,thecontaineriscalledahomogeneous單調的平均的container.Whatisinlinefunction??The__inlinekeywordtellsthecompilertosubstitute代替thecodewithinthefunctiondefinitionforeveryinstanceofafunctioncall.However,substitutionoccursonlyatthecompiler'sdiscretion靈巧性.Forexample,thecompilerdoesnotinlineafunctionifitsaddressistakenorifitistoolargetoinline.使用預辦理器實現(xiàn)，沒有了參數(shù)壓棧，代碼生成等一系列的操作,所以，效率很高，這是它在C中被使用的一個主要原由Whatisoverloading??WiththeC++language,youcanoverloadfunctionsandoperators.Overloadingisthepracticeofsupplyingmorethanonedefinitionforagivenfunctionnameinthesamescope.Anytwofunctionsinasetofoverloadedfunctionsmusthavedifferentargumentlists.-Overloadingfunctionswithargumentlistsofthesametypes,basedonreturntypealone,isanerror.WhatisOverriding?Tooverrideamethod,asubclassoftheclassthatoriginallydeclaredthemethodmustdeclareamethodwiththesamename,returntype(orasubclassofthatreturntype),andsameparameterlist.Thedefinitionofthemethodoverridingis:Musthavesamemethodname.Musthavesamedatatype.·Musthavesameargumentlist.Overridingamethodmeansthatreplacingamethodfunctionalityimplyoverridingfunctionalityweneedparentandchildclasses.youdefinethesamemethodsignatureasonedefinedintheparentclass.

inchildclass.ToInthechildclassWhatis"this"pointer?Thethispointerisapointeraccessibleonlywithinthememberfunctionsofaclass,struct,oruniontype.Itpointstotheobjectforwhichthememberfunctioniscalled.Staticmemberfunctionsdonothaveathispointer.Whenanon-staticmemberfunctioniscalledforanobject,theaddressoftheobjectispassedasahiddenargumenttothefunction.Forexample,thefollowingfunctioncall(3);canbeinterpreted解說thisway:setMonth(&myDate,3);Theobject'saddressisavailablefromwithinthememberfunctionasthethispointer.Itislegal,thoughunnecessary,tousethethispointerwhenreferringtomembersoftheclass.Whathappenswhenyoumakecall"deletethis;"??Thecodehastwobuilt-inpitfalls圈套/誤區(qū).First,ifitexecutesinamemberfunctionforanextern,static,orautomaticobject,theprogramwillprobablycrashassoonasthedeletestatementexecutes.Thereisnoportablewayforanobjecttotellthatitwasinstantiatedontheheap,sotheclasscannotassertthatitsobjectisproperlyinstantiated.Second,whenanobjectcommitssuicidethisway,theusingprogrammightnotknowaboutitsdemise死亡/轉讓.Asfarastheinstantiatingprogramisconcerned有關的,theobjectremainsinscopeandcontinuestoexisteventhoughtheobjectdiditselfin.Subsequent以后的dereferencing間接引用（dereferencingpointer重引用指針，dereferencingoperator取值運算符）ofthepointercanandusuallydoesleadtodisaster不幸.Youshouldneverdothis.Sincecompilerdoesnotknowwhethertheobjectwasallocatedonthestackorontheheap,"deletethis"couldcauseadisaster.Howvirtualfunctionsareimplemented履行C++?Virtualfunctionsareimplementedusingatableoffunctionpointers,calledthetableiscreatedbytheconstructoroftheclass.Whenaderivedclassisconstructed,itsbaseclassisconstructedfirstwhichcreatesthevtable.Ifthederivedclassoverridesanyofthebaseclassesvirtualfunctions,thoseentriesinthevtableareoverwrittenbythederivedclassconstructor.Thisiswhyyoushouldnevercallvirtualfunctionsfromaconstructor:becausethevtableentriesfortheobjectmaynothavebeensetupbythederivedclassconstructoryet,soyoumightendupcallingbaseclassimplementationsofthosevirtualfunctionsWhatisnamemanglinginC++??Theprocessofencodingtheparametertypeswiththefunction/methodnameintoauniquenameiscallednamemangling.Theinverseprocessiscalleddemangling.ForexampleFoo::bar(int,long)constismangledas`bar__C3Fooil'.Foraconstructor,themethodnameisleftout.ThatisFoo::Foo(int,long)constismangledas`__C3Fooil'.Whatisthedifferencebetweenapointerandareference?Areferencemustalwaysrefertosomeobjectand,therefore,mustalwaysbeinitialized;pointersdonothavesuchrestrictions.Apointercanbereassignedtopointtodifferentobjectswhileareferencealwaysreferstoanobjectwithwhichitwasinitialized.Howareprefixandpostfixversionsofoperator++( )differentiated?Thepostfixversionofoperator++( )hasadummyparameteroftypeint.Theprefixversiondoesnothavedummyparameter.Whatisthedifferencebetweenconstchar*myPointerandchar*constmyPointer?Constchar*myPointerisanonconstantpointertoconstantdata;whilechar*constmyPointerisaconstantpointertononconstantdata.HowcanIhandleaconstructorthatfails?throwanexception.Constructorsdon'thaveareturntype,soit'snotpossibletousereturncodes.Thebestwaytosignalconstructorfailureisthereforetothrowanexception.HowcanIhandleadestructorthatfails?Writeamessagetoalog-file.Butdonotthrowanexception.TheC++ruleisthatyoumustneverthrowanexceptionfromadestructorthatisbeingcalledduringthe"stackunwinding"processofanotherexception.Forexample,ifsomeonesaysthrowFoo( ),thestackwillbeunwoundsoallthestackframesbetweenthethrowFoo( )andthe}catch(Fooe){willgetpopped.Thisiscalledstackunwinding.Duringstackunwinding,allthelocalobjectsinallthosestackframesaredestructed.Ifoneofthosedestructorsthrowsanexception(sayitthrowsaBarobject),theC++runtimesystemisinano-winsituation:shoulditignoretheBarandendupinthe}catch(Fooe){whereitwasoriginallyheaded?ShoulditignoretheFooandlookfora}catch(Bare){handler?Thereisnogoodanswer--eitherchoicelosesinformation.SotheC++languageguaranteesthatitwillcallterminate( )atthispoint,andterminate( )killstheprocess.Bangyou'redead.WhatisVirtualDestructor?Usingvirtualdestructors,youcandestroyobjectswithoutknowingtheirtype-thecorrectdestructorfortheobjectisinvokedusingthevirtualfunctionmechanism.Notethatdestructorscanalsobedeclaredaspurevirtualfunctionsforabstractclasses.ifsomeonewillderivefromyourclass,andifsomeonewillsay"newDerived",where"Derived"isderivedfromyourclass,andifsomeonewillsaydeletep,wheretheactualobject'stypeis"Derived"butthepointerp'stypeisyourclass.Canyouthinkofasituationwhereyourprogramwouldcrashwithoutreachingthebreakpointwhichyousetatthebeginningofmain( )?C++allowsfordynamicinitializationofglobalvariablesbeforemain( )isinvoked.Itispossiblethatinitializationofglobalwillinvokesomefunction.Ifthisfunctioncrashesthecrashwilloccurbeforemain( )isentered.NametwocaseswhereyouMUSTuseinitializationlistasopposedtoassignmentinconstructors.Bothnon-staticconstdatamembersandreferencedatamemberscannotbeassignedvalues;instead,youshoulduseinitializationlisttoinitializethem.Canyouoverloadafunctionbasedonlyonwhetheraparameterisavalueorareference?No.Passingbyvalueandbyreferencelooksidenticaltothecaller.WhatarethedifferencesbetweenaC++structandC++class?Thedefaultmemberandbaseclassaccessspecifiers表記符aredifferent.TheC++structhasallthefeaturesoftheclass.Theonlydifferencesarethatastructdefaultstopublicmemberaccessandpublicbaseclassinheritance,andaclassdefaultstotheprivateaccessspecifierandprivatebaseclassinheritance.Whatdoesextern"C"intfunc(int*,Foo)accomplish實現(xiàn)/達到/達成?Itwillturnoff"namemangling"forfuncsothatonecanlinktocodecompiledbyaCcompiler.Howdoyouaccessthestaticmemberofaclass?<ClassName>::<StaticMemberName>Whatismultipleinheritance(virtualinheritance)?Whatareitsadvantagesanddisadvantages?MultipleInheritanceistheprocesswhereby經(jīng)過achildcanbederivedfrommorethanoneparentclass.Theadvantageofmultipleinheritanceisthatitallowsaclasst