Thinking in C++ 2nd ed Volume 2

5/3/2015 ThinkinginC++2ndedVolume2

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ThinkingInC++Volume2:PracticalProgramming

BruceEckel,President,MindView,Inc.ChuckAllison,UtahValleyStateCollege

BookHomePageAnnotatedSolutionGuide

ReportErrorsHere

CoverIntroduction

Part1:BuildingStableSystems1:Exceptionhandling2:Defensiveprogramming

Part2:TheStandardC++Library3:Stringsindepth4:Iostreams5:Templatesindepth6:Genericalgorithms7:Genericcontainers

Part3:SpecialTopics8:Runtimetypeidentification9:Multipleinheritance10:Designpatterns11:Concurrency

AppendixA:RecommendedreadingB:EtcIndex

Idliketocongratulatethebothofyouforaveryimpressivework!NotonlydidIfindyourbooktobeanenjoyableandrewardingreadIwasastoundedbytheaccuracybothintermsoftechnicalcorrectnessanduseofthelanguageIbelievethatyouhaveattainedalevelofcraftsmanshipthatissimplyoutstanding.

BjornKarlssonEditorialBoard,C/C++UsersJournal

Thisbookisatremendousachievement.Youoweittoyourselftohaveacopyonyourshelf.

AlStevensContributingEditor,DoctorDobbsJournal

Eckelsbookistheonlyonetosoclearlyexplainhowtorethinkprogramconstructionforobjectorientation.ThatthebookisalsoanexcellenttutorialontheinsandoutsofC++isanaddedbonus.

AndrewBinstockEditor,UnixReview

BrucecontinuestoamazemewithhisinsightintoC++,andThinkinginC++ishisbestcollectionofideasyet.IfyouwantclearanswerstodifficultquestionsaboutC++,buythisoutstandingbook.

GaryEntsminger

http://mindview.net:8080/TICPPV2/http://mindview.net/Books/TICPP/ThinkingInCPP2e.htmlhttp://mindview.net/Books/TICPPV2/Solutions



Author,TheTaoofObjects

ThinkinginC++patientlyandmethodicallyexplorestheissuesofwhenandhowtouseinlines,references,operatoroverloading,inheritanceanddynamicobjects,aswellasadvancedtopicssuchastheproperuseoftemplates,exceptionsandmultipleinheritance.TheentireeffortiswoveninafabricthatincludesEckelsownphilosophyofobjectandprogramdesign.AmustforeveryC++developersbookshelf,ThinkinginC++istheoneC++bookyoumusthaveifyouredoingseriousdevelopmentwithC++.

RichardHaleShawContributingEditor,PCMagazine



CIPDATAAVAILABLEVicePresidentandEditorialDirector,ECS:MarciaJ.HortonPublisher:AlanR.AptAssociateEditor:ToniDianneHolmEditorialAssistant:PatrickLindnerVicePresidentandDirectorofProductionandManufacturing,ESM:DavidW.RiccardiExecutiveManagingEditor:VinceOBrienManagingEditor:CamilleTrentacosteProductionEditor:IrwinZuckerDirectorofCreativeServices:PaulBelfantiCreativeDirector:CaroleAnsonCoverandInteriorDesigner:DanielWillHarrisCoverIllustrations:TinaJensenManufacturingManager:TrudyPisciottiManufacturingBuyer:LisaMcDowellMarketingManager:PamelaShaffer

2004MindView,Inc.PublishedbyPearsonPrenticeHallPearsonEducation,Inc.UpperSaddleRiver,NJ07458

Allrightsreserved.Nopartofthisbookmaybereproducedinanyformorbyanymeans,withoutpermissioninwritingfromthepublisher.PearsonPrenticeHallisatrademarkofPearsonEducation,Inc.

Theauthorsandpublisherofthisbookhaveusedtheirbesteffortsinpreparingthisbook.Theseeffortsincludethedevelopment,research,andtestingofthetheoriesandprogramstodeterminetheireffectiveness.Theauthorsand



publishermakenowarrantyofanykind,expressedorimplied,withregardtotheseprogramsorthedocumentationcontainedinthisbook.Theauthorsandpublishershallnotbeliableinanyeventforincidentalorconsequentialdamagesinconnectionwith,orarisingoutof,thefurnishing,performance,oruseoftheseprograms.PrintedintheUnitedStatesofAmerica

10987654321

ISBN0130353132

PearsonEducationLtd.,LondonPearsonEducationAustraliaPty.Ltd.,SydneyPearsonEducationSingapore,Pte.Ltd.PearsonEducationNorthAsiaLtd.,HongKongPearsonEducationCanada,Inc.,TorontoPearsonEducacindeMexico,S.A.deC.V.PearsonEducationJapan,TokyoPearsonEducationMalaysia,Pte.Ltd.PearsonEducation,Inc.,UpperSaddleRiver,NewJersey

DedicationToallthosewhohaveworkedtirelessly

todeveloptheC++language

WhatsinsideIntroduction1

Goals..........................................1Chapters.....................................2Exercises.....................................5

Exercisesolutions...............5Sourcecode.................................5Compilers....................................7Languagestandards.....................9Seminars,CDROMs&consulting.................................9Errors.......................................10Aboutthecover.........................10Acknowledgements.....................10

I:BuildingStableSystems13

1:ExceptionHandling15Traditionalerrorhandling.............16Throwinganexception................18Catchinganexception.................20

Thetryblock....................20Exceptionhandlers...........20Terminationandresumption................22

Exceptionmatching.....................23Catchinganyexception.....25Rethrowinganexception...26Uncaughtexceptions.........26

Cleaningup................................28Resourcemanagement.....30Makingeverythinganobject.........................32auto_ptr..........................35Functionleveltryblocks...36

Standardexceptions...................38Exceptionspecifications...............40



Betterexceptionspecifications?..................45Exceptionspecificationsandinheritance................46Whennottouseexceptionspecifications.....47

Exceptionsafety.........................48Programmingwithexceptions.......52

Whentoavoidexceptions..52Typicalusesofexceptions.54

Overhead...................................58Summary...................................60Exercises...................................61

2:DefensiveProgramming63Assertions.................................66Asimpleunittestframework........70

Automatedtesting............71TheTestSuiteFramework..75Testsuites.......................79Thetestframeworkcode...81

Debuggingtechniques.................87Tracemacros...................87Tracefile.........................88Findingmemoryleaks.......90

Summary...................................96Exercises...................................97

II:TheStandardC++Library101

3:StringsinDepth103Whatsinastring?....................104CreatingandinitializingC++strings.............................106Operatingonstrings.................109

Appending,inserting,andconcatenatingstrings......110Replacingstringcharacters......................112Concatenationusingnonmemberoverloadedoperators......117

Searchinginstrings..................117Findinginreverse...........123Findingfirst/lastofasetofcharacters..........124Removingcharactersfromstrings...................126Comparingstrings..........129Stringsandcharactertraits...............134

Astringapplication...................140Summary.................................145Exercises.................................146

4:Iostreams151Whyiostreams?........................151Iostreamstotherescue............156

Insertersandextractors..156Commonusage..............161



Lineorientedinput.........164Handlingstreamerrors..............165Fileiostreams...........................168

AFileProcessingExample........................169Openmodes...................171

Iostreambuffering....................173Seekinginiostreams.................175Stringiostreams.......................179

Inputstringstreams........180Outputstringstreams......182

Outputstreamformatting..........186Formatflags...................186Formatfields..................188Width,fill,andprecision..190Anexhaustiveexample...191

Manipulators.............................194Manipulatorswitharguments.....................196Creatingmanipulators.....199Effectors........................201

Iostreamexamples....................203Maintainingclasslibrarysourcecode...................204Detectingcompilererrors208Asimpledatalogger.......211

Internationalization...................216WideStreams.................216Locales..........................218

Summary.................................221Exercises.................................222

5:TemplatesinDepth227Templateparameters.................227

Nontypetemplateparameters.......228Defaulttemplatearguments.....................230Templatetemplateparameters....................232Thetypenamekeyword...238Usingthetemplatekeywordasahint...........240MemberTemplates.........242

Functiontemplateissues...........245Typedeductionoffunctiontemplatearguments........245Functiontemplateoverloading....................249Takingtheaddressofageneratedfunctiontemplate............251ApplyingafunctiontoanSTLsequence.........255Partialorderingoffunctiontemplates..........259

Templatespecialization...............260Explicitspecialization.......261PartialSpecialization.......263Apracticalexample........265Preventingtemplatecodebloat......................268

Namelookupissues..................273Namesintemplates........273Templatesandfriends.....279



Templateprogrammingidioms....285Traits.............................285Policies..........................291Thecuriouslyrecurringtemplatepattern.............294

Templatemetaprogramming.......297Compiletimeprogramming.................298Expressiontemplates......308

Templatecompilationmodels......315Theinclusionmodel........315Explicitinstantiation........316Theseparationmodel......319


6:GenericAlgorithms325Afirstlook...............................325

Predicates......................329Streamiterators.............331Algorithmcomplexity......333

Functionobjects.......................335Classificationoffunctionobjects..............336Automaticcreationoffunctionobjects..............338Adaptablefunctionobjects341Morefunctionobjectexamples.............343Functionpointeradaptors351Writingyourownfunctionobjectadaptors..358

AcatalogofSTLalgorithms.......362Supporttoolsforexamplecreation............365Fillingandgenerating......368Counting........................370Manipulatingsequences...372Searchingandreplacing..377Comparingranges..........385Removingelements........389Sortingandoperationsonsortedranges............393Heapoperations.............403Applyinganoperationtoeachelementinarange..405Numericalgorithms.........413Generalutilities..............417

CreatingyourownSTLstylealgorithms.................419Summary.................................420Exercises.................................421

7:GenericContainers429Containersanditerators............429

STLreferencedocumentation................431

Afirstlook...............................432Containersofstrings.......438InheritingfromSTLcontainers................440

Aplethoraofiterators...............442Iteratorsinreversiblecontainers.......445



Iteratorcategories..........446Predefinediterators........448

Thebasicsequences:vector,list,deque.....................454

Basicsequenceoperations454vector............................457deque............................465Convertingbetweensequences......................467Checkedrandomaccess.470list.................................471Swappingsequences.......477

set..........................................479Acompletelyreusabletokenizer..........482

stack.......................................487queue......................................491Priorityqueues.........................496Holdingbits..............................506

bitset.......................507vector.................511

Associativecontainers...............513Generatorsandfillersforassociativecontainers518Themagicofmaps.........521Multimapsandduplicatekeys................523Multisets........................527

CombiningSTLcontainers..........530Cleaningupcontainersofpointers...............534Creatingyourowncontainers.....536STLextensions.........................538NonSTLcontainers..................540Summary.................................546Exercises.................................546

III:SpecialTopics549

8:RuntimeTypeIdentification551Runtimecasts..........................551Thetypeidoperator..................557

Castingtointermediatelevels.........560voidpointers..................561UsingRTTIwithtemplates................562

Multipleinheritance....................563SensibleusesforRTTI...............564

Atrashrecycler..............565MechanismandoverheadofRTTI......................570Summary.................................570Exercises.................................571

9:MultipleInheritance573Perspective...............................573Interfaceinheritance..................575Implementationinheritance........579Duplicatesubobjects.................585Virtualbaseclasses...................589Namelookupissues..................599AvoidingMI..............................603



Extendinganinterface...............603Summary.................................608Exercises.................................609

10:DesignPatterns613Thepatternconcept..................613

Prefercompositiontoinheritance.................615

Classifyingpatterns...................615Features,idioms,patterns.........................616

SimplifyingIdioms.....................617Messenger.....................617CollectingParameter.......618

Singleton.................................619VariationsonSingleton....621

Command:choosingtheoperation...........................626

DecouplingeventhandlingwithCommand...............628

Objectdecoupling.....................631Proxy:frontingforanotherobject................632State:changingobjectbehavior..............634

Adapter...................................636TemplateMethod.......................639Strategy:choosingthealgorithmatruntime..................640ChainofResponsibility:tryingasequenceofstrategies...642Factories:encapsulatingobjectcreation.........................645

Polymorphicfactories......647Abstractfactories............651Virtualconstructors.........654

Builder:creatingcomplexobjects.......................660Observer..................................667

Theinnerclassidiom....671Theobserverexample....674

Multipledispatching...................679MultipledispatchingwithVisitor.....................683


11:Concurrency691Motivation................................692ConcurrencyinC++..................694

InstallingZThreads.........695DefiningTasks..........................696UsingThreads..........................698

Creatingresponsiveuserinterfaces...............700SimplifyingwithExecutors.......................702Yielding..........................706Sleeping........................707Priority..........................709

Sharinglimitedresources...........711Ensuringtheexistenceofobjects........711Improperlyaccessing



resources.......................715Controllingaccess...........719SimplifiedcodingwithGuards....................721Threadlocalstorage.......724

Terminatingtasks......................727Preventingiostreamcollision.........................727Theornamentalgarden...728Terminatingwhenblocked.................733Interruption....................735

Cooperationbetweenthreads.....741Waitandsignal...............742Producerconsumerrelationships...................747Solvingthreadingproblemswithqueues....................750Broadcast......................757

Deadlock..................................764Summary.................................770Exercises.................................773

A:RecommendedReading777GeneralC++............................777

Brucesbooks.................777Chucksbooks................779

IndepthC++..........................779DesignPatterns........................781

B:Etc783

Index791

IntroductionInVolume1ofthisbook,youlearnedthefundamentalsofCandC++.Inthisvolume,welookatmoreadvancedfeatures,withaneyetowardsdevelopingtechniquesandideasthatproducerobustC++programs.

WeassumeyouarefamiliarwiththematerialpresentedinVolume1.

GoalsOurgoalsinthisbookareto:

1.Presentthematerialasimplestepatatime,sothereadercaneasilydigesteachconceptbeforemovingon.

2.Teachpracticalprogrammingtechniquesthatyoucanuseonadaytodaybasis.

3.Giveyouwhatwethinkisimportantforyoutounderstandaboutthelanguage,ratherthaneverythingweknow.Webelievethereisaninformationimportancehierarchy,andtherearesomefactsthat95%ofprogrammerswillneverneedtoknow,butthatwouldjustconfusepeopleandaddtotheirperceptionofthecomplexityofthelanguage.TotakeanexamplefromC,ifyoumemorizetheoperatorprecedencetable(weneverdid)youcanwriteclevercode.Butifyoumustthinkaboutit,itwillconfusethereader/maintainerofthatcode.Soforgetaboutprecedenceanduse



parentheseswhenthingsarentclear.ThissameattitudewillbetakenwithsomeinformationintheC++language,whichismoreimportantforcompilerwritersthanforprogrammers.

4.Keepeachsectionfocusedenoughsothelecturetimeandthetimebetweenexerciseperiodsissmall.Notonlydoesthiskeeptheaudiencemindsmoreactiveandinvolvedduringahandsonseminar,butitgivesthereaderagreatersenseofaccomplishment.

5.WehaveendeavorednottouseanyparticularvendorsversionofC++.Wehavetestedthecodeonalltheimplementationswecould(describedlaterinthisintroduction),andwhenoneimplementationabsolutelyrefusedtoworkbecauseitdoesntconformtotheC++Standard,weveflaggedthatfactintheexample(youllseetheflagsinthesourcecode)toexcludeitfromthebuildprocess.

6.Automatethecompilingandtestingofthecodeinthebook.Wehavediscoveredthatcodethatisntcompiledandtestedisprobablybroken,sointhisvolumeweveinstrumentedtheexampleswithtestcode.Inaddition,thecodethatyoucandownloadfromhttp://www.MindView.nethasbeenextracteddirectlyfromthetextofthebookusingprogramsthatautomaticallycreatemakefilestocompileandrunthetests.Thiswayweknowthatthecodeinthebookiscorrect.

ChaptersHereisabriefdescriptionofthechapterscontainedinthisbook:

Part1:BuildingStableSystems

1.Exceptionhandling.Errorhandlinghasalwaysbeenaprobleminprogramming.Evenifyoudutifullyreturnerrorinformationorsetaflag,thefunctioncallermaysimplyignoreit.ExceptionhandlingisaprimaryfeatureinC++thatsolvesthisproblembyallowingyoutothrowanobjectoutofyourfunctionwhenacriticalerrorhappens.Youthrowdifferenttypesofobjectsfordifferenterrors,andthefunctioncallercatchestheseobjectsinseparateerrorhandlingroutines.Ifyouthrowanexception,itcannotbeignored,soyoucanguaranteethatsomethingwillhappeninresponsetoyourerror.Thedecisiontouseexceptionsaffectscodedesigninpositive,fundamentalways.

2.DefensiveProgramming.Manysoftwareproblemscanbeprevented.Toprogramdefensivelyistocraftcodeinsuchawaythatbugsarefoundandfixedearlybeforetheycandamageinthefield.Usingassertionsisthesinglemostimportantwaytovalidateyourcodeduringdevelopment,whileatthesametimeleavinganexecutabledocumentationtrailinyourcodethatrevealsyourthoughtswhileyouwrotethecodeinthefirstplace.Rigorouslytestyourcodebeforeyouletoutofyourhands.Anautomatedunittestingframeworkisanindispensabletoolforsuccessful,everydaysoftwaredevelopment.

Part2:TheStandardC++Library

3.StringsinDepth.Themostcommonprogrammingactivityistextprocessing.TheC++stringclassrelievestheprogrammerfrommemorymanagementissues,whileatthesametimedeliveringapowerhouseoftextprocessingcapability.C++alsosupportstheuseofwidecharactersandlocalesforinternationalizedapplications.

4.Iostreams.OneoftheoriginalC++librariestheonethatprovidestheessentialI/Ofacilityiscallediostreams.IostreamsisintendedtoreplaceCsstdio.hwithanI/Olibrarythatiseasiertouse,moreflexible,andextensibleyoucanadaptittoworkwithyournewclasses.ThischapterteachesyouhowtomakethebestuseoftheexistingiostreamlibraryforstandardI/O,fileI/O,andinmemoryformatting.

5.TemplatesinDepth.ThedistinguishingfeatureofmodernC++isthebroadpoweroftemplates.Templatesdomorethanjustcreategenericcontainers.Theysupportdevelopmentofrobust,generic,highperformancelibraries.Thereisalottoknowabouttemplatestheyconstitute,asitwere,asublanguagewithintheC++language,andgivetheprogrammeranimpressivedegreeofcontroloverthecompilationprocess.ItisnotanoverstatementtosaythattemplateshaverevolutionizedC++programming.

6.GenericAlgorithms.Algorithmsareatthecoreofcomputing,andC++,throughitstemplatefacility,supportsanimpressiveentourageofpowerful,efficient,andeasytousegenericalgorithms.Thestandardalgorithmsarealsocustomizablethroughfunctionobjects.Thischapterlooksateveryalgorithminthelibrary.(Chapters6and7coverthatportionoftheStandardC++librarycommonlyknownastheStandardTemplateLibrary,orSTL.)

7.GenericContainers&Iterators.C++supportsallthecommondatastructuresinatypesafemanner.Youneverneedtoworryaboutwhatsuchacontainerholds.Thehomogeneityofitsobjectsisguaranteed.Separatingthetraversingofacontainerfromthecontaineritself,anotheraccomplishmentoftemplates,ismadepossiblethroughiterators.Thisingeniousarrangementallowsaflexibleapplicationof



algorithmstocontainersusingthesimplestofdesigns.

Part3:SpecialTopics

8.Runtimetypeidentification.Runtimetypeidentification(RTTI)findstheexacttypeofanobjectwhenyouonlyhaveapointerorreferencetothebasetype.Normally,youllwanttointentionallyignoretheexacttypeofanobjectandletthevirtualfunctionmechanismimplementthecorrectbehaviorforthattype.Butoccasionally(likewhenwritingsoftwaretoolssuchasdebuggers)itishelpfultoknowtheexacttypeofanobjectwiththisinformation,youcanoftenperformaspecialcaseoperationmoreefficiently.ThischapterexplainswhatRTTIisforandhowtouseit.

9.Multipleinheritance.Thissoundssimpleatfirst:Anewclassisinheritedfrommorethanoneexistingclass.However,youcanendupwithambiguitiesandmultiplecopiesofbaseclassobjects.Thatproblemissolvedwithvirtualbaseclasses,butthebiggerissueremains:Whendoyouuseit?Multipleinheritanceisonlyessentialwhenyouneedtomanipulateanobjectthroughmorethanonecommonbaseclass.Thischapterexplainsthesyntaxformultipleinheritanceandshowsalternativeapproachesinparticular,howtemplatessolveonetypicalproblem.Usingmultipleinheritancetorepairadamagedclassinterfaceisdemonstratedasavaluableuseofthisfeature.

10.DesignPatterns.Themostrevolutionaryadvanceinprogrammingsinceobjectsistheintroductionofdesignpatterns.Adesignpatternisalanguageindependentcodificationofasolutiontoacommonprogrammingproblem,expressedinsuchawaythatitcanapplytomanycontexts.PatternssuchasSingleton,FactoryMethod,andVisitornowfindtheirwayintodailydiscussionsaroundthekeyboard.ThischaptershowshowtoimplementandusesomeofthemoreusefuldesignpatternsinC++.

11.ConcurrentProgramming.Peoplehavecometoexpectresponsiveuserinterfacesthat(seemto)processmultipletaskssimultaneously.Modernoperatingsystemsallowprocessestohavemultiplethreadsthatsharetheprocessaddressspace.Multithreadedprogrammingrequiresadifferentmindset,however,andcomeswithitsownsetofdifficulties.Thischapterusesafreelyavailablelibrary(theZThreadlibrarybyEricCrahenofIBM)toshowhowtoeffectivelymanagemultithreadedapplicationsinC++.

ExercisesWehavediscoveredthatsimpleexercisesareexceptionallyusefulduringaseminartocompleteastudentsunderstanding.Youllfindasetattheendofeachchapter.

Thesearefairlysimple,sotheycanbefinishedinareasonableamountoftimeinaclassroomsituationwhiletheinstructorobserves,makingsureallthestudentsareabsorbingthematerial.Someexercisesareabitmorechallengingtokeepadvancedstudentsentertained.Theyrealldesignedtobesolvedinashorttimeandareonlytheretotestandpolishyourknowledgeratherthanpresentmajorchallenges(presumably,youllfindthoseonyourownormorelikelytheyllfindyou).

ExercisesolutionsSolutionstoexercisescanbefoundintheelectronicdocumentTheC++AnnotatedSolutionGuide,Volume2,availableforanominalfeefromhttp://www.MindView.net.

SourcecodeThesourcecodeforthisbookiscopyrightedfreeware,distributedviathewebsitehttp://www.MindView.net.Thecopyrightpreventsyoufromrepublishingthecodeinprintmediawithoutpermission.

Inthestartingdirectorywhereyouunpackthecodeyouwillfindthefollowingcopyrightnotice:

//:!:CopyRight.txt(c)19952004MindView,Inc.Allrightsreserved.Sourcecodefilefromthebook"ThinkinginC++,2ndEdition,Volume2."Thefollowingpermissionsaregrantedrespectingthecomputersourcecode,whichiscontainedinthisfile:Permissionisgrantedtoclassroomeducatorstousethisfileaspartofinstructionalmaterialspreparedforclassespersonallytaughtorsupervisedbytheeducatorwho



usesthispermission,providedthat(a)thebook"ThinkinginC++"iscitedastheoriginoneachpageorslidethatcontainsanypartofthisfile,and(b)thatyoumaynotremovetheabovecopyrightlegendnorthisnotice.Thispermissionextendstohandouts,slidesandotherpresentationmaterials.Forpurposesthatdonotincludethepublicationorpresentationofeducationalorinstructionalmaterials,permissionalsoisgrantedtocomputerprogramdesignersandprogrammers,andtotheiremployersandcustomers,(a)touseandmodifythisfileforthepurposeofcreatingexecutablecomputersoftware,and(b)todistributeresultingcomputerprogramsinbinaryformonly,providedthat(c)youmaynotremovetheabovecopyrightlegendnorthisnoticefromretainedsourcecodecopiesofthisfile,and(d)eachcopydistributedinbinaryformhasembeddedwithinittheabovecopyrightnotice.Apartfromthepermissionsgrantedabove,thesoleauthorizeddistributionpointforadditionalcopiesofthisfileishttp://www.MindView.net(andofficialmirrorsites)whereitisavailable,subjecttothepermissionsandrestrictionssetforthherein.Thefollowingareclarificationsofthelimitedpermissionsgrantedabove:1.Youmaynotpublishordistributeoriginalsormodifiedversionsofthesourcecodetothesoftwareotherthaninclassroomsituationsdescribedabove.2.Youmaynotusethesoftwarefileorportionsthereofinprintedmediawithouttheexpresspermissionofthecopyrightowner.Thecopyrightownerandauthororauthorsmakenorepresentationaboutthesuitabilityofthissoftwareforanypurpose.Itisprovided"asis,"andallexpress,implied,andstatutorywarrantiesandconditionsofanykindincludinganywarrantiesandconditionsofmerchantability,satisfactoryquality,security,fitnessforaparticularpurposeandnoninfringement,aredisclaimed.Theentireriskastothequalityandperformanceofthesoftwareiswithyou.Innoeventwilltheauthorsorthepublisherbeliableforanylostrevenue,savings,ordata,orfordirect,indirect,special,consequential,incidental,exemplaryorpunitivedamages,howevercausedandregardlessofanyrelatedtheoryofliability,arisingoutofthislicenseand/ortheuseoforinabilitytousethissoftware,evenifthevendorsand/orthepublisherhavebeenadvisedofthepossibilityofsuchdamages.Shouldthesoftwareprovedefective,youassumethecostofallnecessaryservicing,repair,orcorrection.Ifyouthinkyouhaveacorrectionforanerrorinthesoftware,pleasesubmitthecorrectiontowww.MindView.net.(Pleaseusethesameprocessfornoncodeerrorsfoundinthebook.)Ifyouhaveaneedforpermissionsnotgrantedabove,pleaseinquireofMindView,Inc.,[email protected].///:~

Youmayusethecodeinyourprojectsandintheclassroomaslongasthecopyrightnoticeisretained.



CompilersYourcompilermaynotsupportallthefeaturesdiscussedinthisbook,especiallyifyoudonthavethenewestversionofyourcompiler.ImplementingalanguagelikeC++isaHerculeantask,andyoucanexpectthatthefeatureswillappearinpiecesratherthanallatonce.Butifyouattemptoneoftheexamplesinthebookandgetalotoferrorsfromthecompiler,itsnotnecessarilyabuginthecodeorthecompileritmaysimplynotbeimplementedinyourparticularcompileryet.

Weusedanumberofcompilerstotestthecodeinthisbook,inanattempttoensurethatourcodeconformstotheC++Standardandwillworkwithasmanycompilersaspossible.Unfortunately,notallcompilersconformtotheC++Standard,andsowehaveawayofexcludingcertainfilesfrombuildingwiththosecompilers.Theseexclusionsarereflectedinthemakefilesautomaticallycreatedforthepackageofcodeforthisbookthatyoucandownloadfromwww.MindView.net.Youcanseetheexclusiontagsembeddedinthecommentsatthebeginningofeachlisting,soyouwillknowwhethertoexpectaparticularcompilertoworkonthatcode(inafewcases,thecompilerwillactuallycompilethecodebuttheexecutionbehavioriswrong,andweexcludethoseaswell).

Herearethetagsandthecompilersthattheyexcludefromthebuild:

{dmc}WalterBrightsDigitalMarscompilerforWindows,freelydownloadableatwww.DigitalMars.com.Thiscompilerisveryconformantandsoyouwillseealmostnoneofthesetagsthroughoutthebook.

{g++}ThefreeGnuC++3.3.1,whichcomespreinstalledinmostLinuxpackagesandMacintoshOSX.ItisalsopartofCygwinforWindows(seebelow).Itisavailableformostotherplatformsfromgcc.gnu.org.

{msc}MicrosoftVersion7withVisualC++.NET(onlycomeswithVisualStudio.NETnotfreelydownloadable).

{bor}BorlandC++Version6(notthefreedownloadthisoneismoreuptodate).

{edg}EdisonDesignGroup(EDG)C++.Thisisthebenchmarkcompilerforstandardsconformance.Thistagoccursonlybecauseoflibraryissues,andbecausewewereusingacomplimentarycopyoftheEDGfrontendwithacomplimentarylibraryimplementationfromDinkumware,Ltd.Nocompileerrorsoccurredbecauseofthecompileralone.

{mwcc}MetrowerksCodeWarriorforMacintoshOSX.NotethatOSXcomeswithGnuC++preinstalled,aswell.

Ifyoudownloadandunpackthecodepackageforthisbookfromwww.MindView.net,youllfindthemakefilestobuildthecodefortheabovecompilers.WeusedthefreelyavailableGNUmake,whichcomeswithLinux,Cygwin(afreeUnixshellthatrunsontopofWindowsseewww.Cygwin.com),orcanbeinstalledonyourplatformseewww.gnu.org/software/make.(Othermakesmayormaynotworkwiththesefiles,butarenotsupported.)Onceyouinstallmake,ifyoutypemakeatthecommandlineyoullgetinstructionsonhowtobuildthebookscodefortheabovecompilers.

Notethattheplacementofthesetagsonthefilesinthisbookindicatesthestateoftheparticularversionofthecompileratthetimewetriedit.Itspossibleandlikelythatthecompilervendorhasimprovedthecompilersincethepublicationofthisbook.Itsalsopossiblethatwhilebuildingthebookwithsomanycompilers,wemayhavemisconfiguredaparticularcompilerthatwouldotherwisehavecompiledthecodecorrectly.Thus,youshouldtrythecodeyourselfonyourcompiler,andalsocheckthecodedownloadedfromwww.MindView.nettoseewhatiscurrent.

LanguagestandardsThroughoutthisbook,whenreferringtoconformancetotheANSI/ISOCstandard,wewillbereferringtothe1989standard,andwillgenerallyjustsayC.OnlyifitisnecessarytodistinguishbetweenStandard1989Candolder,preStandardversionsofCwillwemakethedistinction.WedonotreferenceC99inthisbook.

TheANSI/ISOC++CommitteelongagofinishedworkingonthefirstC++Standard,commonlyknownasC++98.WewillusethetermStandardC++torefertothisstandardizedlanguage.IfwesimplyrefertoC++,assumewemeanStandardC++.TheC++StandardsCommitteecontinuestoaddressissuesimportanttotheC++communitythatwillbecomeC++0x,afutureC++Standardnotlikelytobeavailableformanyyears.



Seminars,CDROMs&consultingBruceEckelscompany,MindView,Inc.,providespublichandsontrainingseminarsbasedonthematerialinthisbook,andalsoforadvancedtopics.Selectedmaterialfromeachchapterrepresentsalesson,whichisfollowedbyamonitoredexerciseperiodsoeachstudentreceivespersonalattention.Wealsoprovideonsitetraining,consulting,mentoring,anddesign&codewalkthroughs.Informationandsignupformsforupcomingseminarsandothercontactinformationisfoundathttp://www.MindView.net.

ErrorsNomatterhowmanytrickswritersusetodetecterrors,somealwayscreepinandtheseoftenleapoffthepageforafreshreader.Ifyoudiscoveranythingyoubelievetobeanerror,pleaseusethefeedbacksystembuiltintotheelectronicversionofthisbook,whichyouwillfindathttp://www.MindView.net.Yourhelpisappreciated.

AboutthecoverThecoverartworkwaspaintedbyLarryOBrienswife,TinaJensen(yes,theLarryOBrienwhowastheeditorofSoftwareDevelopmentMagazineforsomanyyears).Notonlyarethepicturesbeautiful,theyarealsoexcellentsuggestionsofpolymorphism.TheideaforusingtheseimagescamefromDanielWillHarris,thecoverdesigner(www.WillHarris.com),workingwithBruce.

AcknowledgementsVolume2ofthisbooklanguishedinahalfcompletedstateforalongtimewhileBrucegotdistractedwithotherthings,notablyJava,DesignPatternsandespeciallyPython(seewww.Python.org).IfChuckhadntbeenwilling(foolishly,hehassometimesthought)tofinishtheotherhalfandbringthingsuptodate,thisbookalmostcertainlywouldnthavehappened.TherearentthatmanypeoplewhomBrucewouldhavefeltcomfortableentrustingthisbookto.Chuckspenchantforprecision,correctnessandclearexplanationiswhathasmadethisbookasgoodasitis.

JamieKingactedasaninternunderChucksdirectionduringthecompletionofthisbook.Hewasanessentialpartofmakingsurethebookgotfinished,notonlybyprovidingfeedbackforChuck,butespeciallybecauseofhisrelentlessquestioningandpickingofeverysinglepossiblenitthathedidntcompletelyunderstand.Ifyourquestionsareansweredbythisbook,itsprobablybecauseJamieaskedthemfirst.Jamiealsoenhancedanumberofthesampleprogramsandcreatedmanyoftheexercisesattheendofeachchapter.ScottBaker,anotherofChucksinternsfundedbyMindView,Inc.,helpedwiththeexercisesforChapter3.

EricCrahenofIBMwasinstrumentalinthecompletionofChapter11(Concurrency).Whenwewerelookingforathreadspackage,wesoughtoutonethatwasintuitiveandeasytouse,whilebeingsufficientlyrobusttodothejob.WithEricwegotthatandthensomehewasextremelycooperativeandhasusedourfeedbacktoenhancehislibrary,whilewehavebenefitedfromhisinsightsaswell.

WearegratefultoPeteBeckerforbeingourtechnicaleditor.FewpeopleareasarticulateanddiscriminatingasPete,nottomentionasexpertinC++andsoftwaredevelopmentingeneral.WealsothankBjornKarlssonforhisgraciousandtimelytechnicalassistanceashereviewedtheentiremanuscriptwithshortnotice.

WalterBrightmadeHerculeaneffortstomakesurethathisDigitalMarsC++compilerwouldcompiletheexamplesinthisbook.Hemakesthecompileravailableforfreedownloadsathttp://www.DigitalMars.com.Thanks,Walter!

Theideasandunderstandinginthisbookhavecomefrommanyothersources,aswell:friendslikeAndreaProvaglio,DanSaks,ScottMeyers,CharlesPetzold,andMichaelWilkpioneersofthelanguagelikeBjarneStroustrup,AndrewKoenig,andRobMurraymembersoftheC++StandardsCommitteelikeNathanMyers(whowasparticularlyhelpfulandgenerouswithhisinsights),HerbSutter,PJPlauger,KevlinHenney,DavidAbrahams,TomPlum,RegCharney,TomPenello,SamDruker,UweSteinmueller,JohnSpicer,SteveAdamczyk,andDaveedVandevoordepeoplewhohavespokenintheC++trackattheSoftwareDevelopmentConference(whichBrucecreatedanddeveloped,andChuckspokein)ColleaguesofChucklikeMichaelSeaver,HustonFranklin,DavidWagstaff,andoftenstudentsinseminars,whoaskthequestionsweneedtoheartomakethematerialclearer.

Thebookdesign,typefaceselection,coverdesign,andcoverphotowerecreatedbyBrucesfriendDanielWillHarris,notedauthoranddesigner,whousedtoplaywithrubonlettersinjuniorhighschoolwhileheawaitedtheinventionofcomputersanddesktoppublishing.However,weproducedthecameraready



pagesourselves,sothetypesettingerrorsareours.MicrosoftWordXPwasusedtowritethebookandtocreatecamerareadypages.ThebodytypefaceisVerdanaandtheheadlinesareinVerdana.ThecodetypefaceisCourierNew.

WealsowishtothankthegenerousprofessionalsattheEdisonDesignGroupandDinkumware,Ltd.,forgivinguscomplimentarycopiesoftheircompilerandlibrary(respectively).Withouttheirexpertassistance,graciouslygiven,someoftheexamplesinthisbookcouldnothavebeentested.WealsowishtothankHowardHinnantandthefolksatMetrowerksforacopyoftheircompiler,andSandySmithandthefolksatSlickEditforkeepingChucksuppliedwithaworldclasseditingenvironmentforsomanyyears.GregComeaualsoprovidedacopyofhissuccessfulEDGbasedcompiler,ComeauC++.

Aspecialthankstoallourteachers,andallourstudents(whoareourteachersaswell).

EvanCofsky([email protected])providedallsortsofassistanceontheserveraswellasdevelopmentofprogramsinhisnowfavoritelanguage,Python.SharlynnCobaughandPaulaSteuerwereinstrumentalassistants,preventingBrucefrombeingwashedawayinafloodofprojects.

BrucessweetieDawnMcGeeprovidedmuchappreciatedinspirationandenthusiasmduringthisproject.Thesupportingcastoffriendsincludes,butisnotlimitedto:MarkWestern,GenKiyooka,KraigBrockschmidt,ZackUrlocker,AndrewBinstock,NeilRubenking,SteveSinofsky,JDHildebrandt,BrianMcElhinney,BrinkleyBarr,BillGatesatMidnightEngineeringMagazine,LarryConstantine&LucyLockwood,TomKeffer,GregPerry,DanPutterman,ChristiWestphal,GeneWang,DaveMayer,DavidIntersimone,ClaireSawyers,TheItalians(AndreaProvaglio,LauraFallai,MarcoCantu,Corrado,IlsaandChristinaGiustozzi),Chris&LauraStrand,TheAlmquists,BradJerbic,JohnKruth&MarilynCvitanic,HollyPayne(yes,thefamousnovelist!),MarkMabry,TheRobbinsFamilies,TheMoelterFamilies(&theMcMillans),TheWilks,DaveStoner,LaurieAdams,TheCranstons,LarryFogg,Mike&KarenSequeira,GaryEntsminger&AllisonBrody,ChesterAndersen,JoeLordi,Dave&BrendaBartlett,TheRentschlers,TheSudeks,Lynn&Todd,andtheirfamilies.Andofcourse,Mom&Dad,Sandy,James&Natalie,Kim&Jared,Isaac,andAbbi.

Part1:BuildingStableSystemsSoftwareengineersspendaboutasmuchtimevalidatingcodeastheydocreatingit.Qualityisorshouldbethegoalofeveryprogrammer,andonecangoalongwaytowardsthatgoalbyeliminatingproblemsbeforetheyhappen.Inaddition,softwaresystemsshouldberobustenoughtobehavereasonablyinthepresenceofunforeseenenvironmentalproblems.

ExceptionswereintroducedintoC++tosupportsophisticatederrorhandlingwithoutclutteringcodewithaninordinateamountoferrorhandlinglogic.Chapter1showshowproperuseofexceptionscanmakeforwellbehavedsoftware,andalsointroducesthedesignprinciplesthatunderlieexceptionsafecode.InChapter2wecoverunittestinganddebuggingtechniquesintendedtomaximizecodequalitylongbeforeitsreleased.Theuseofassertionstoexpressandenforceprograminvariantsisasuresignofanexperiencedsoftwareengineer.Wealsointroduceasimpleframeworktosupportunittesting.

1:ExceptionHandlingImprovingerrorrecoveryisoneofthemostpowerfulwaysyoucanincreasetherobustnessofyourcode.

Unfortunately,itsalmostacceptedpracticetoignoreerrorconditions,asifwereinastateofdenialabouterrors.Onereason,nodoubt,isthetediousnessandcodebloatofcheckingformanyerrors.Forexample,printf()returnsthenumberofcharactersthatweresuccessfullyprinted,butvirtuallynoonechecksthisvalue.Theproliferationofcodealonewouldbedisgusting,nottomentionthedifficultyitwouldaddin



readingthecode.

TheproblemwithCsapproachtoerrorhandlingcouldbethoughtofascouplingtheuserofafunctionmusttietheerrorhandlingcodesocloselytothatfunctionthatitbecomestooungainlyandawkwardtouse.

OneofthemajorfeaturesinC++isexceptionhandling,whichisabetterwayofthinkingaboutandhandlingerrors.Withexceptionhandling:

1.Errorhandlingcodeisnotnearlysotedioustowrite,anditdoesntbecomemixedupwithyournormalcode.Youwritethecodeyouwanttohappenlaterinaseparatesectionyouwritethecodetocopewiththeproblems.Ifyoumakemultiplecallstoafunction,youhandletheerrorsfromthatfunctiononce,inoneplace.

2.Errorscannotbeignored.Ifafunctionneedstosendanerrormessagetothecallerofthatfunction,itthrowsanobjectrepresentingthaterroroutofthefunction.Ifthecallerdoesntcatchtheerrorandhandleit,itgoestothenextenclosingdynamicscope,andsoonuntiltheerroriseithercaughtortheprogramterminatesbecausetherewasnohandlertocatchthattypeofexception.

ThischapterexaminesCsapproachtoerrorhandling(suchasitis),discusseswhyitdidnotworkwellforC,andexplainswhyitwontworkatallforC++.Thischapteralsocoverstry,throw,andcatch,theC++keywordsthatsupportexceptionhandling.

TraditionalerrorhandlingInmostoftheexamplesinthesevolumes,weuseassert()asitwasintended:fordebuggingduringdevelopmentwithcodethatcanbedisabledwith#defineNDEBUGfortheshippingproduct.Runtimeerrorcheckingusestherequire.hfunctions(assure()andrequire())developedinChapter9inVolume1andrepeatedhereinAppendixB.Thesefunctionsareaconvenientwaytosay,Theresaproblemhereyoullprobablywanttohandlewithsomemoresophisticatedcode,butyoudontneedtobedistractedbyitinthisexample.Therequire.hfunctionsmightbeenoughforsmallprograms,butforcomplicatedproductsyoullwanttowritemoresophisticatederrorhandlingcode.

Errorhandlingisquitestraightforwardwhenyouknowexactlywhattodo,becauseyouhaveallthenecessaryinformationinthatcontext.Youcanjusthandletheerroratthatpoint.

Theproblemoccurswhenyoudonthaveenoughinformationinthatcontext,andyouneedtopasstheerrorinformationintoadifferentcontextwherethatinformationdoesexist.InC,youcanhandlethissituationusingthreeapproaches:

1.Returnerrorinformationfromthefunctionor,ifthereturnvaluecannotbeusedthisway,setaglobalerrorconditionflag.(StandardCprovideserrnoandperror()tosupportthis.)Asmentionedearlier,theprogrammerislikelytoignoretheerrorinformationbecausetediousandobfuscatingerrorcheckingmustoccurwitheachfunctioncall.Inaddition,returningfromafunctionthathitsanexceptionalconditionmightnotmakesense.

2.UsethelittleknownStandardClibrarysignalhandlingsystem,implementedwiththesignal()function(todeterminewhathappenswhentheeventoccurs)andraise()(togenerateanevent).Again,thisapproachinvolveshighcouplingbecauseitrequirestheuserofanylibrarythatgeneratessignalstounderstandandinstalltheappropriatesignalhandlingmechanism.Inlargeprojectsthesignalnumbersfromdifferentlibrariesmightclash.

3.UsethenonlocalgotofunctionsintheStandardClibrary:setjmp()andlongjmp().Withsetjmp()yousaveaknowngoodstateintheprogram,andifyougetintotrouble,longjmp()willrestorethatstate.Again,thereishighcouplingbetweentheplacewherethestateisstoredandtheplacewheretheerroroccurs.

WhenconsideringerrorhandlingschemeswithC++,theresanadditionalcriticalproblem:TheCtechniquesofsignalsandsetjmp()/longjmp()donotcalldestructors,soobjectsarentproperlycleanedup.(Infact,iflongjmp()jumpspasttheendofascopewheredestructorsshouldbecalled,thebehavioroftheprogramisundefined.)Thismakesitvirtuallyimpossibletoeffectivelyrecoverfromanexceptionalconditionbecauseyoullalwaysleaveobjectsbehindthathaventbeencleanedupandthatcannolongerbeaccessed.Thefollowingexampledemonstratesthiswithsetjmp/longjmp:

//:C01:Nonlocal.cpp//setjmp()&longjmp().#include



#includeusingnamespacestdclassRainbow{public:Rainbow(){cout



}///:~

MyErrorisanordinaryclass,whichinthiscasetakesachar*asaconstructorargument.Youcanuseanytypewhenyouthrow(includingbuiltintypes),butusuallyyoullcreatespecialclassesforthrowingexceptions.

Thekeywordthrowcausesanumberofrelativelymagicalthingstohappen.First,itcreatesacopyoftheobjectyourethrowingand,ineffect,returnsitfromthefunctioncontainingthethrowexpression,eventhoughthatobjecttypeisntnormallywhatthefunctionisdesignedtoreturn.Anaivewaytothinkaboutexceptionhandlingisasanalternatereturnmechanism(althoughyoullfindyoucangetintotroubleifyoutakethatanalogytoofar).Youcanalsoexitfromordinaryscopesbythrowinganexception.Inanycase,avalueisreturned,andthefunctionorscopeexits.

Anysimilaritytoareturnstatementendstherebecausewhereyoureturnissomeplacecompletelydifferentfromwhereanormalfunctioncallreturns.(Youendupinanappropriatepartofthecodecalledanexceptionhandlerthatmightbefarremovedfromwheretheexceptionwasthrown.)Inaddition,anylocalobjectscreatedbythetimetheexceptionoccursaredestroyed.Thisautomaticcleanupoflocalobjectsisoftencalledstackunwinding.

Inaddition,youcanthrowasmanydifferenttypesofobjectsasyouwant.Typically,youllthrowadifferenttypeforeachcategoryoferror.Theideaistostoretheinformationintheobjectandinthenameofitsclasssothatsomeoneinacallingcontextcanfigureoutwhattodowithyourexception.

CatchinganexceptionAsmentionedearlier,oneoftheadvantagesofC++exceptionhandlingisthatyoucanconcentrateontheproblemyouretryingtosolveinoneplace,andthendealwiththeerrorsfromthatcodeinanotherplace.

ThetryblockIfyoureinsideafunctionandyouthrowanexception(oracalledfunctionthrowsanexception),thefunctionexitsbecauseofthethrownexception.Ifyoudontwantathrowtoleaveafunction,youcansetupaspecialblockwithinthefunctionwhereyoutrytosolveyouractualprogrammingproblem(andpotentiallygenerateexceptions).Thisblockiscalledthetryblockbecauseyoutryyourvariousfunctioncallsthere.Thetryblockisanordinaryscope,precededbythekeywordtry:

try{//Codethatmaygenerateexceptions}

Ifyoucheckforerrorsbycarefullyexaminingthereturncodesfromthefunctionsyouuse,youneedtosurroundeveryfunctioncallwithsetupandtestcode,evenifyoucallthesamefunctionseveraltimes.Withexceptionhandling,youputeverythinginatryblockandhandleexceptionsafterthetryblock.Thus,yourcodeisaloteasiertowriteandtoreadbecausethegoalofthecodeisnotconfusedwiththeerrorhandling.

ExceptionhandlersOfcourse,thethrownexceptionmustendupsomeplace.Thisplaceistheexceptionhandler,andyouneedoneexceptionhandlerforeveryexceptiontypeyouwanttocatch.However,polymorphismalsoworksforexceptions,sooneexceptionhandlercanworkwithanexceptiontypeandclassesderivedfromthattype.

Exceptionhandlersimmediatelyfollowthetryblockandaredenotedbythekeywordcatch:

try{//Codethatmaygenerateexceptions}catch(type1id1){//Handleexceptionsoftype1}catch(type2id2){//Handleexceptionsoftype2}catch(type3id3)//Etc...}catch(typeNidN)//HandleexceptionsoftypeN}//Normalexecutionresumeshere...

Thesyntaxofacatchclauseresemblesfunctionsthattakeasingleargument.Theidentifier(id1,id2,and



soon)canbeusedinsidethehandler,justlikeafunctionargument,althoughyoucanomittheidentifierifitsnotneededinthehandler.Theexceptiontypeusuallygivesyouenoughinformationtodealwithit.

Thehandlersmustappeardirectlyafterthetryblock.Ifanexceptionisthrown,theexceptionhandlingmechanismgoeshuntingforthefirsthandlerwithanargumentthatmatchesthetypeoftheexception.Itthenentersthatcatchclause,andtheexceptionisconsideredhandled.(Thesearchforhandlersstopsoncethecatchclauseisfound.)Onlythematchingcatchclauseexecutescontrolthenresumesafterthelasthandlerassociatedwiththattryblock.

Noticethat,withinthetryblock,anumberofdifferentfunctioncallsmightgeneratethesametypeofexception,butyouneedonlyonehandler.

Toillustratetryandcatch,thefollowingvariationofNonlocal.cppreplacesthecalltosetjmp()withatryblockandreplacesthecalltolongjmp()withathrowstatement:

//:C01:Nonlocal2.cpp//Illustratesexceptions.#includeusingnamespacestdclassRainbow{public:Rainbow(){cout



exceptionisgeneratedfrommanypoints.

ExceptionmatchingWhenanexceptionisthrown,theexceptionhandlingsystemlooksthroughthenearesthandlersintheordertheyappearinthesourcecode.Whenitfindsamatch,theexceptionisconsideredhandledandnofurthersearchingoccurs.

Matchinganexceptiondoesntrequireaperfectcorrelationbetweentheexceptionanditshandler.Anobjectorreferencetoaderivedclassobjectwillmatchahandlerforthebaseclass.(However,ifthehandlerisforanobjectratherthanareference,theexceptionobjectisslicedtruncatedtothebasetypeasitispassedtothehandler.Thisdoesnodamage,butlosesallthederivedtypeinformation.)Forthisreason,aswellastoavoidmakingyetanothercopyoftheexceptionobject,itisalwaysbettertocatchanexceptionbyreferenceinsteadofbyvalue. Ifapointeristhrown,theusualstandardpointerconversionsareusedtomatchtheexception.However,noautomatictypeconversionsareusedtoconvertfromoneexceptiontypetoanotherintheprocessofmatching.Forexample:

//:C01:Autoexcp.cpp//Nomatchingconversions.#includeusingnamespacestdclassExcept1{}classExcept2{public:Except2(constExcept1&){}}voidf(){throwExcept1()}intmain(){try{f()}catch(Except2&){cout



}catch(X::Big&){cout



Whenabort()iscalled,nocallstonormalprogramterminationfunctionsoccur,whichmeansthatdestructorsforglobalandstaticobjectsdonotexecute.Theterminate()functionalsoexecutesifadestructorforalocalobjectthrowsanexceptionwhilethestackisunwinding(interruptingtheexceptionthatwasinprogress)orifaglobalorstaticobjectsconstructorordestructorthrowsanexception.(Ingeneral,donotallowadestructortothrowanexception.)

Theset_terminate()functionYoucaninstallyourownterminate()functionusingthestandardset_terminate()function,whichreturnsapointertotheterminate()functionyouarereplacing(whichwillbethedefaultlibraryversionthefirsttimeyoucallit),soyoucanrestoreitlaterifyouwant.Yourcustomterminate()musttakenoargumentsandhaveavoidreturnvalue.Inaddition,anyterminate()handleryouinstallmustnotreturnorthrowanexception,butinsteadmustexecutesomesortofprogramterminationlogic.Ifterminate()iscalled,theproblemisunrecoverable.

Thefollowingexampleshowstheuseofset_terminate().Here,thereturnvalueissavedandrestoredsothattheterminate()functioncanbeusedtohelpisolatethesectionofcodewheretheuncaughtexceptionoccurs:

//:C01:Terminator.cpp//Useofset_terminate().Alsoshowsuncaughtexceptions.#include#includeusingnamespacestdvoidterminator(){cout



exceptionwasthrown.C++exceptionhandlingguaranteesthatasyouleaveascope,allobjectsinthatscopewhoseconstructorshavebeencompletedwillhavetheirdestructorscalled.

Heresanexamplethatdemonstratesthatconstructorsthatarentcompleteddonthavetheassociateddestructorscalled.Italsoshowswhathappenswhenanexceptionisthrowninthemiddleofthecreationofanarrayofobjects:

//:C01:Cleanup.cpp//Exceptionscleanupcompleteobjectsonly.#includeusingnamespacestdclassTrace{staticintcounterintobjidpublic:Trace(){objid=counter++cout



Thedifficultyisinallocatingresourcesinconstructors.Ifanexceptionoccursintheconstructor,thedestructordoesntgetachancetodeallocatetheresource.Thisproblemoccursmostoftenwithnakedpointers.Forexample:

//:C01:Rawp.cpp//Nakedpointers.#include#includeusingnamespacestdclassCat{public:Cat(){cout



Topreventsuchresourceleaks,youmustguardagainsttheserawresourceallocationsinoneoftwoways:

Youcancatchexceptionsinsidetheconstructorandthenreleasetheresource.

Youcanplacetheallocationsinsideanobjectsconstructor,andyoucanplacethedeallocationsinsideanobjectsdestructor.

Usingthelatterapproach,eachallocationbecomesatomic,byvirtueofbeingpartofthelifetimeofalocalobject,andifitfails,theotherresourceallocationobjectsareproperlycleanedupduringstackunwinding.ThistechniqueiscalledResourceAcquisitionIsInitialization(RAIIforshort)becauseitequatesresourcecontrolwithobjectlifetime.Usingtemplatesisanexcellentwaytomodifythepreviousexampletoachievethis:

//:C01:Wrapped.cpp//Safe,atomicpointers.#include#includeusingnamespacestd//Simplified.Yoursmayhaveotherarguments.templateclassPWrap{T*ptrpublic:classRangeError{}//ExceptionclassPWrap(){ptr=newT[sz]cout



UseResourcesur}catch(int){cout



}intmain(){auto_ptrpMyObject(newTraceHeap(5))cout



toreturntothecodethatcreatedit.Forthisreason,theonlysensiblethingtodoistothrowanexceptioninthefunctionlevelcatchclause.

Althoughitisnotterriblyuseful,C++alsoallowsfunctionleveltryblocksforanyfunction,asthefollowingexampleillustrates:

//:C01:FunctionTryBlock.cpp{bor}//Functionleveltryblocks.//{RunByHand}(Dontrunautomaticallybythemakefile)#includeusingnamespacestdintmain()try{throw"main"}catch(constchar*msg){cout



initialized.

logic_error Derivedfromexception.Reportsprogramlogicerrors,whichcouldpresumablybedetectedbyinspection.

runtime_error Derivedfromexception.Reportsruntimeerrors,whichcanpresumablybedetectedonlywhentheprogramexecutes.

Theiostreamexceptionclassios::failureisalsoderivedfromexception,butithasnofurthersubclasses.

Youcanusetheclassesinbothofthefollowingtablesastheyare,oryoucanusethemasbaseclassesfromwhichtoderiveyourownmorespecifictypesofexceptions.

Exceptionclassesderivedfromlogic_error

domain_error Reportsviolationsofaprecondition.

invalid_argument Indicatesaninvalidargumenttothefunctionfromwhichitisthrown.

length_error Indicatesanattempttoproduceanobjectwhoselengthisgreaterthanorequaltonpos(thelargestrepresentablevalueofcontextssizetype,usuallystd::size_t).

out_of_range Reportsanoutofrangeargument.

bad_cast Thrownforexecutinganinvaliddynamic_castexpressioninruntimetypeidentification(seeChapter8).

bad_typeid Reportsanullpointerpinanexpressiontypeid(*p).(Again,aruntimetypeidentificationfeatureinChapter8).

Exceptionclassesderivedfromruntime_error

range_error Reportsviolationofapostcondition.

overflow_error Reportsanarithmeticoverflow.

bad_alloc Reportsafailuretoallocatestorage.

ExceptionspecificationsYourenotrequiredtoinformthepeopleusingyourfunctionwhatexceptionsyoumightthrow.However,failuretodosocanbeconsidereduncivilizedbecauseitmeansthatuserscannotbesurewhatcodetowritetocatchallpotentialexceptions.Iftheyhaveyoursourcecode,theycanhuntthroughandlookforthrowstatements,butoftenalibrarydoesntcomewithsources.Gooddocumentationcanhelpalleviatethisproblem,buthowmanysoftwareprojectsarewelldocumented?C++providessyntaxtotelltheusertheexceptionsthatarethrownbythisfunction,sotheusercanhandlethem.Thisistheoptionalexceptionspecification,whichadornsafunctionsdeclaration,appearingaftertheargumentlist.



Theexceptionspecificationreusesthekeywordthrow,followedbyaparenthesizedlistofallthetypesofpotentialexceptionsthatthefunctioncanthrow.Yourfunctiondeclarationmightlooklikethis:

voidf()throw(toobig,toosmall,divzero)

Asfarasexceptionsareconcerned,thetraditionalfunctiondeclaration

voidf()

meansthatanytypeofexceptioncanbethrownfromthefunction.Ifyousay

voidf()throw()

noexceptionswhatsoeverwillbethrownfromthefunction(soyoudbetterbesurethatnofunctionsfartherdowninthecallchainletanyexceptionspropagateup!).

Forgoodcodingpolicy,gooddocumentation,andeaseofuseforthefunctioncaller,considerusingexceptionspecificationswhenyouwritefunctionsthatthrowexceptions.(Variationsonthisguidelinearediscussedlaterinthischapter.)

Theunexpected()functionIfyourexceptionspecificationclaimsyouregoingtothrowacertainsetofexceptionsandthenyouthrowsomethingthatisntinthatset,whatsthepenalty?Thespecialfunctionunexpected()iscalledwhenyouthrowsomethingotherthanwhatappearsintheexceptionspecification.Shouldthisunfortunatesituationoccur,thedefaultunexpected()callstheterminate()functiondescribedearlierinthischapter.

Theset_unexpected()functionLiketerminate(),theunexpected()mechanisminstallsyourownfunctiontorespondtounexpectedexceptions.Youdosowithafunctioncalledset_unexpected(),which,likeset_terminate(),takestheaddressofafunctionwithnoargumentsandvoidreturnvalue.Also,becauseitreturnsthepreviousvalueoftheunexpected()pointer,youcansaveitandrestoreitlater.Touseset_unexpected(),includetheheaderfile.Heresanexamplethatshowsasimpleuseofthefeaturesdiscussedsofarinthissection:

//:C01:Unexpected.cpp//Exceptionspecifications&unexpected(),//{msc}(Doesntterminateproperly)#include#includeusingnamespacestdclassUp{}classFit{}voidg()voidf(inti)throw(Up,Fit){switch(i){case1:throwUp()case2:throwFit()}g()}//voidg(){}//Version1voidg(){throw47}//Version2voidmy_unexpected(){cout



}catch(Fit){cout



//thecompilerdetectstheviolationandreports//anerror,soweputitinitsownfunction.voidt(){throwB()}voidf()throw(A){t()}voidg()throw(A,bad_exception){t()}intmain(){set_terminate(my_thandler)set_unexpected(my_uhandler1)try{f()}catch(A&){cout



classesotherwisetheexpectedisarelationshipbetweenderivedandbaseclassesisviolated.Sinceexceptionspecificationsarelogicallypartofafunctionsdeclaration,theytoomustremainconsistentacrossaninheritancehierarchy.Forexample,ifamemberfunctioninabaseclasssaysitwillonlythrowanexceptionoftypeA,anoverrideofthatfunctioninaderivedclassmustnotaddanyotherexceptiontypestothespecificationlistbecausethatwouldbreakanyprogramsthatadheretothebaseclassinterface.Youcan,however,specifyfewerexceptionsornoneatall,sincethatdoesntrequiretheusertodoanythingdifferently.YoucanalsospecifyanythingthatisaAinplaceofAinthederivedfunctionsspecification.Heresanexample.

//:C01:Covariance.cpp{xo}//Shouldcausecompileerror.{mwcc}{msc}#includeusingnamespacestdclassBase{public:classBaseException{}classDerivedException:publicBaseException{}virtualvoidf()throw(DerivedException){throwDerivedException()}virtualvoidg()throw(BaseException){throwBaseException()}}classDerived:publicBase{public:voidf()throw(BaseException){throwBaseException()}virtualvoidg()throw(DerivedException){throwDerivedException()}}///:~

AcompilershouldflagtheoverrideofDerived::f()withanerror(oratleastawarning)sinceitchangesitsexceptionspecificationinawaythatviolatesthespecificationofBase::f().ThespecificationforDerived::g()isacceptablebecauseDerivedExceptionisaBaseException(nottheotherwayaround).YoucanthinkofBase/DerivedandBaseException/DerivedExceptionasparallelclasshierarchieswhenyouareinDerived,youcanreplacereferencestoBaseExceptioninexceptionspecificationsandreturnvalueswithDerivedException.Thisbehavioriscalledcovariance(sincebothsetsofclassesvarydowntheirrespectivehierarchiestogether).(ReminderfromVolume1:parametertypesarenotcovariantyouarenotallowedtochangethesignatureofanoverriddenvirtualfunction.)

WhennottouseexceptionspecificationsIfyouperusethefunctiondeclarationsthroughouttheStandardC++library,youllfindthatnotasingleexceptionspecificationoccursanywhere!Althoughthismightseemstrange,thereisagoodreasonforthisseemingincongruity:thelibraryconsistsmainlyoftemplates,andyouneverknowwhatagenerictypeorfunctionmightdo.Forexample,supposeyouaredevelopingagenericstacktemplateandattempttoaffixanexceptionspecificationtoyourpopfunction,likethis:

Tpop()throw(logic_error)

Sincetheonlyerroryouanticipateisastackunderflow,youmightthinkitssafetospecifyalogic_errororsomeotherappropriateexceptiontype.ButtypeTscopyconstructorcouldthrowanexception.Thenunexpected()wouldbecalled,andyourprogramwouldterminate.Youcantmakeunsupportableguarantees.Ifyoudontknowwhatexceptionsmightoccur,dontuseexceptionspecifications.Thatswhytemplateclasses,whichconstitutethemajorityoftheStandardC++library,donotuseexceptionspecificationstheyspecifytheexceptionstheyknowaboutindocumentationandleavetheresttoyou.Exceptionspecificationsaremainlyfornontemplateclasses.

ExceptionsafetyInChapter7welltakeanindepthlookatthecontainersintheStandardC++library,includingthestackcontainer.Onethingyoullnoticeisthatthedeclarationofthepop()memberfunctionlookslikethis:



voidpop()

Youmightthinkitstrangethatpop()doesntreturnavalue.Instead,itjustremovestheelementatthetopofthestack.Toretrievethetopvalue,calltop()beforeyoucallpop().Thereisanimportantreasonforthisbehavior,andithastodowithexceptionsafety,acrucialconsiderationinlibrarydesign.Therearedifferentlevelsofexceptionsafety,butmostimportantly,andjustasthenameimplies,exceptionsafetyisaboutcorrectsemanticsinthefaceofexceptions.

Supposeyouareimplementingastackwithadynamicarray(wellcallitdataandthecounterintegercount),andyoutrytowritepop()sothatitreturnsavalue.Thecodeforsuchapop()mightlooksomethinglikethis:

templateTstack::pop(){if(count==0)throwlogic_error("stackunderflow")elsereturndata[count]}

Whathappensifthecopyconstructorthatiscalledforthereturnvalueinthelastlinethrowsanexceptionwhenthevalueisreturned?Thepoppedelementisnotreturnedbecauseoftheexception,andyetcounthasalreadybeendecremented,sothetopelementyouwantedislostforever!Theproblemisthatthisfunctionattemptstodotwothingsatonce:(1)returnavalue,and(2)changethestateofthestack.Itisbettertoseparatethesetwoactionsintotwoseparatememberfunctions,whichisexactlywhatthestandardstackclassdoes.(Inotherwords,followthedesignpracticeofcohesioneveryfunctionshoulddoonethingwell.)Exceptionsafecodeleavesobjectsinaconsistentstateanddoesnotleakresources.

Youalsoneedtobecarefulwritingcustomassignmentoperators.InChapter12ofVolume1,yousawthatoperator=shouldadheretothefollowingpattern:

1.Makesureyourenotassigningtoself.Ifyouare,gotostep6.(Thisisstrictlyanoptimization.)

2.Allocatenewmemoryrequiredbypointerdatamembers.

3.Copydatafromtheoldmemorytothenew.

4.Deletetheoldmemory.

5.Updatetheobjectsstatebyassigningthenewheappointerstothepointerdatamembers.

6.Return*this.

Itsimportanttonotchangethestateofyourobjectuntilallthenewpieceshavebeensafelyallocatedandinitialized.Agoodtechniqueistomovesteps2and3intoaseparatefunction,oftencalledclone().Thefollowingexampledoesthisforaclassthathastwopointermembers,theStringandtheInts:

//:C01:SafeAssign.cpp//AnExceptionsafeoperator=.#include#include//Forstd::bad_alloc#include#includeusingnamespacestd//AclassthathastwopointermembersusingtheheapclassHasPointers{//AHandleclasstoholdthedatastructMyData{constchar*theStringconstint*theIntssize_tnumIntsMyData(constchar*pString,constint*pInts,size_tnInts):theString(pString),theInts(pInts),numInts(nInts){}}*theData//Thehandle//Cloneandcleanupfunctions:staticMyData*clone(constchar*otherString,constint*otherInts,size_tnInts){



char*newChars=newchar[strlen(otherString)+1]int*newIntstry{newInts=newint[nInts]}catch(bad_alloc&){delete[]newCharsthrow}try{//Thisexampleusesbuiltintypes,soitwon't//throw,butforclasstypesitcouldthrow,sowe//useatryblockforillustration.(Thisisthe//pointoftheexample!)strcpy(newChars,otherString)for(size_ti=0itheString,otherData>theInts,otherData>numInts)}staticvoidcleanup(constMyData*theData){delete[]theData>theStringdelete[]theData>theIntsdeletetheData}public:HasPointers(constchar*someString,constint*someInts,size_tnumInts){theData=clone(someString,someInts,numInts)}HasPointers(constHasPointers&source){theData=clone(source.theData)}HasPointers&operator=(constHasPointers&rhs){if(this!=&rhs){MyData*newData=clone(rhs.theData>theString,rhs.theData>theInts,rhs.theData>numInts)cleanup(theData)theData=newData}return*this}~HasPointers(){cleanup(theData)}friendostream&operator



cout



theresourcesyourself.Basically,ifyoudontneedexceptions,yourenotforcedtousethem.

Newexceptions,oldcodeAnothersituationthatarisesisthemodificationofanexistingprogramthatdoesntuseexceptions.Youmightintroducealibrarythatdoesuseexceptionsandwonderifyouneedtomodifyallyourcodethroughouttheprogram.Assumingyouhaveanacceptableerrorhandlingschemealreadyinplace,themoststraightforwardthingtodoissurroundthelargestblockthatusesthenewlibrary(thismightbeallthecodeinmain())withatryblock,followedbyacatch(...)andbasicerrormessage).Youcanrefinethistowhateverdegreenecessarybyaddingmorespecifichandlers,but,inanycase,thecodeyoumustaddcanbeminimal.Itsevenbettertoisolateyourexceptiongeneratingcodeinatryblockandwritehandlerstoconverttheexceptionsintoyourexistingerrorhandlingscheme.

Itstrulyimportanttothinkaboutexceptionswhenyourecreatingalibraryforsomeoneelsetouse,especiallyifyoucantknowhowtheyneedtorespondtocriticalerrorconditions(recalltheearlierdiscussionsonexceptionsafetyandwhytherearenoexceptionspecificationsintheStandardC++Library).

TypicalusesofexceptionsDouseexceptionstodothefollowing:

Fixtheproblemandretrythefunctionthatcausedtheexception.

Patchthingsupandcontinuewithoutretryingthefunction.

Dowhateveryoucaninthecurrentcontextandrethrowthesameexceptiontoahighercontext.

Dowhateveryoucaninthecurrentcontextandthrowadifferentexceptiontoahighercontext.

Terminatetheprogram.

Wrapfunctions(especiallyClibraryfunctions)thatuseordinaryerrorschemessotheyproduceexceptionsinstead.

Simplify.Ifyourerrorhandlingschememakesthingsmorecomplicated,itispainfulandannoyingtouse.Exceptionscanbeusedtomakeerrorhandlingsimplerandmoreeffective.

Makeyourlibraryandprogramsafer.Thisisashortterminvestment(fordebugging)andalongterminvestment(forapplicationrobustness).

WhentouseexceptionspecificationsTheexceptionspecificationislikeafunctionprototype:ittellstheusertowriteexceptionhandlingcodeandwhatexceptionstohandle.Ittellsthecompilertheexceptionsthatmightcomeoutofthisfunctionsothatitcandetectviolationsatruntime.

Youcantalwayslookatthecodeandanticipatewhichexceptionswillarisefromaparticularfunction.Sometimes,thefunctionsitcallsproduceanunexpectedexception,andsometimesanoldfunctionthatdidntthrowanexceptionisreplacedwithanewonethatdoes,andyougetacalltounexpected().Anytimeyouuseexceptionspecificationsorcallfunctionsthatdo,considercreatingyourownunexpected()functionthatlogsamessageandtheneitherthrowsanexceptionorabortstheprogram.

Asweexplainedearlier,youshouldavoidusingexceptionspecificationsintemplateclasses,sinceyoucantanticipatewhattypesofexceptionsthetemplateparameterclassesmightthrow.

StartwithstandardexceptionsCheckouttheStandardC++libraryexceptionsbeforecreatingyourown.Ifastandardexceptiondoeswhatyouneed,chancesareitsaloteasierforyourusertounderstandandhandle.

Iftheexceptiontypeyouwantisntpartofthestandardlibrary,trytoinheritonefromanexistingstandardexception.Itsniceifyouruserscanalwayswritetheircodetoexpectthewhat()functiondefinedintheexception()classinterface.

NestyourownexceptionsIfyoucreateexceptionsforyourparticularclass,itsagoodideatonesttheexceptionclasseseitherinsideyourclassorinsideanamespacecontainingyourclass,toprovideaclearmessagetothereaderthatthisexceptionisonlyforyourclass.Inaddition,itpreventspollutionoftheglobalnamespace.

YoucannestyourexceptionsevenifyourederivingthemfromC++Standardexceptions.



UseexceptionhierarchiesUsingexceptionhierarchiesisavaluablewaytoclassifythetypesofcriticalerrorsthatmightbeencounteredwithyourclassorlibrary.Thisgiveshelpfulinformationtousers,assiststheminorganizingtheircode,andgivesthemtheoptionofignoringallthespecifictypesofexceptionsandjustcatchingthebaseclasstype.Also,anyexceptionsaddedlaterbyinheritingfromthesamebaseclasswillnotforceallexistingcodetoberewrittenthebaseclasshandlerwillcatchthenewexception.

TheStandardC++exceptionsareagoodexampleofanexceptionhierarchy.Buildyourexceptionsontopofitifyoucan.

Multipleinheritance(MI)AsyoullreadinChapter9,theonlyessentialplaceforMIisifyouneedtoupcastanobjectpointertotwodifferentbaseclassesthatis,ifyouneedpolymorphicbehaviorwithbothofthosebaseclasses.Itturnsoutthatexceptionhierarchiesareusefulplacesformultipleinheritancebecauseabaseclasshandlerfromanyoftherootsofthemultiplyinheritedexceptionclasscanhandletheexception.

Catchbyreference,notbyvalueAsyousawinthesectionExceptionmatching,youshouldcatchexceptionsbyreferencefortworeasons:

Toavoidmakinganeedlesscopyoftheexceptionobjectwhenitispassedtothehandler.

Toavoidobjectslicingwhencatchingaderivedexceptionasabaseclassobject.

Althoughyoucanalsothrowandcatchpointers,bydoingsoyouintroducemorecouplingthethrowerandthecatchermustagreeonhowtheexceptionobjectisallocatedandcleanedup.Thisisaproblembecausetheexceptionitselfmighthaveoccurredfromheapexhaustion.Ifyouthrowexceptionobjects,theexceptionhandlingsystemtakescareofallstorage.

ThrowexceptionsinconstructorsBecauseaconstructorhasnoreturnvalue,youvepreviouslyhadtwowaystoreportanerrorduringconstruction:

Setanonlocalflagandhopetheuserchecksit.

Returnanincompletelycreatedobjectandhopetheuserchecksit.

ThisproblemisseriousbecauseCprogrammersexpectthatobjectcreationisalwayssuccessful,whichisnotunreasonableinCbecausethetypesaresoprimitive.ButcontinuingexecutionafterconstructionfailsinaC++programisaguaranteeddisaster,soconstructorsareoneofthemostimportantplacestothrowexceptionsnowyouhaveasafe,effectivewaytohandleconstructorerrors.However,youmustalsopayattentiontopointersinsideobjectsandthewaycleanupoccurswhenanexceptionisthrowninsideaconstructor.

DontcauseexceptionsindestructorsBecausedestructorsarecalledintheprocessofthrowingotherexceptions,youllneverwanttothrowanexceptioninadestructororcauseanotherexceptiontobethrownbysomeactionyouperforminthedestructor.Ifthishappens,anewexceptioncanbethrownbeforethecatchclauseforanexistingexceptionisreached,whichwillcauseacalltoterminate().

Ifyoucallanyfunctionsinsideadestructorthatcanthrowexceptions,thosecallsshouldbewithinatryblockinthedestructor,andthedestructormusthandleallexceptionsitself.Nonemustescapefromthedestructor.

AvoidnakedpointersSeeWrapped.cppearlierinthischapter.Anakedpointerusuallymeansvulnerabilityintheconstructorifresourcesareallocatedforthatpointer.Apointerdoesnthaveadestructor,sothoseresourcesarentreleasedifanexceptionisthrownintheconstructor.Useauto_ptrorothersmartpointertypes forpointersthatreferenceheapmemory.

OverheadWhenanexceptionisthrown,theresconsiderableruntimeoverhead(butitsgoodoverhead,sinceobjectsarecleanedupautomatically!).Forthisreason,youneverwanttouseexceptionsaspartofyournormalflowofcontrol,nomatterhowtemptingandcleveritmayseem.Exceptionsshouldoccuronlyrarely,sotheoverheadispiledontheexceptionandnotonthenormallyexecutingcode.Oneoftheimportantdesigngoalsforexceptionhandlingwasthatitcouldbeimplementedwithnoimpactonexecutionspeedwhenitwasntusedthatis,aslongasyoudontthrowanexception,yourcoderunsasfastasitwouldwithout

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exceptionhandling.Whetherthisistruedependsontheparticularcompilerimplementationyoureusing.(Seethedescriptionofthezerocostmodellaterinthissection.)

Youcanthinkofathrowexpressionasacalltoaspecialsystemfunctionthattakestheexceptionobjectasanargumentandbacktracksupthechainofexecution.Forthistowork,extrainformationneedstobeputonthestackbythecompiler,toaidinstackunwinding.Tounderstandthis,youneedtoknowabouttheruntimestack.

Wheneverafunctioniscalled,informationaboutthatfunctionispushedontotheruntimestackinanactivationrecordinstance(ARI),alsocalledastackframe.Atypicalstackframecontainstheaddressofthecallingfunction(soexecutioncanreturntoit),apointertotheARIofthefunctionsstaticparent(thescopethatlexicallycontainsthecalledfunction,sovariablesglobaltothefunctioncanbeaccessed),andapointertothefunctionthatcalledit(itsdynamicparent).Thepaththatlogicallyresultsfromrepetitivelyfollowingthedynamicparentlinksisthedynamicchain,orcallchain,thatwevementionedpreviouslyinthischapter.Thisishowexecutioncanbacktrackwhenanexceptionisthrown,anditisthemechanismthatmakesitpossibleforcomponentsdevelopedwithoutknowledgeofoneanothertocommunicateerrorsatruntime.

Toenablestackunwindingforexceptionhandling,extraexceptionrelatedinformationabouteachfunctionneedstobeavailableforeachstackframe.Thisinformationdescribeswhichdestructorsneedtobecalled(sothatlocalobjectscanbecleanedup),indicateswhetherthecurrentfunctionhasatryblock,andlistswhichexceptionstheassociatedcatchclausescanhandle.Thereisspacepenaltyforthisextrainformation,soprogramsthatsupportexceptionhandlingcanbesomewhatlargerthanthosethatdont. Eventhecompiletimesizeofprogramsusingexceptionhandlingisgreater,sincethelogicofhowtogeneratetheexpandedstackframesduringruntimemustbegeneratedbythecompiler.

Toillustratethis,wecompiledthefollowingprogrambothwithandwithoutexceptionhandlingsupportinBorlandC++BuilderandMicrosoftVisualC++:

//:C01:HasDestructor.cpp{O}classHasDestructor{public:~HasDestructor(){}}voidg()//Forallweknow,gmaythrow.voidf(){HasDestructorhg()}///:~

Ifexceptionhandlingisenabled,thecompilermustkeepinformationabout~HasDestructor()availableatruntimeintheARIforf()(soitcandestroyhproperlyshouldg()throwanexception).Thefollowingtablesummarizestheresultofthecompilationsintermsofthesizeofthecompiled(.obj)files(inbytes).

Compiler\Mode WithExceptionSupport

WithoutExceptionSupport

Borland 616 234

Microsoft 1162 680

Donttakethepercentagedifferencesbetweenthetwomodestooseriously.Rememberthatexceptions(should)typicallyconstituteasmallpartofaprogram,sothespaceoverheadtendstobemuchsmaller(usuallybetween5and15percent).

Thisextrahousekeepingslowsdownexecution,butaclevercompilerimplementationavoidsthis.Sinceinformationaboutexceptionhandlingcodeandtheoffsetsoflocalobjectscanbecomputedonceatcompiletime,suchinformationcanbekeptinasingleplaceassociatedwitheachfunction,butnotineachARI.YouessentiallyremoveexceptionoverheadfromeachARIandthusavoidtheextratimetopushthemontothestack.Thisapproachiscalledthezerocostmodel ofexceptionhandling,andtheoptimizedstoragementionedearlierisknownastheshadowstack.

Summary

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Errorrecoveryisafundamentalconcernforeveryprogramyouwrite.ItsespeciallyimportantinC++whencreatingprogramcomponentsforotherstouse.Tocreatearobustsystem,eachcomponentmustberobust.

ThegoalsforexceptionhandlinginC++aretosimplifythecreationoflarge,reliableprogramsusinglesscodethancurrentlypossible,withmoreconfidencethatyourapplicationdoesnthaveanunhandlederror.Thisisaccomplishedwithlittleornoperformancepenaltyandwithlowimpactonexistingcode.

Basicexceptionsarenotterriblydifficulttolearnbeginusingtheminyourprogramsassoonasyoucan.Exceptionsareoneofthosefeaturesthatprovideimmediateandsignificantbenefitstoyourproject.

ExercisesSolutionstoselectedexercisescanbefoundintheelectronicdocumentTheThinkinginC++Volume2AnnotatedSolutionGuide,availableforasmallfeefromwww.MindView.net.

1.Writethreefunctions:onethatreturnsanerrorvaluetoindicateanerrorcondition,onethatsetserrno,andonethatusessignal().Writecodethatcallsthesefunctionsandrespondstotheerrors.Nowwriteafourthfunctionthatthrowsanexception.Callthisfunctionandcatchtheexception.Describethedifferencesbetweenthesefourapproaches,andwhyexceptionhandlingisanimprovement.

2.Createaclasswithmemberfunctionsthatthrowexceptions.Withinthisclass,makeanestedclasstouseasanexceptionobject.Ittakesasingleconstchar*asitsargumentthisrepresentsadescriptionstring.Createamemberfunctionthatthrowsthisexception.(Statethisinthefunctionsexceptionspecification.)Writeatryblockthatcallsthisfunctionandacatchclausethathandlestheexceptionbydisplayingitsdescriptionstring.

3.RewritetheStashclassfromChapter13ofVolume1sothatitthrowsout_of_rangeexceptionsforoperator[].

4.Writeagenericmain()thattakesallexceptionsandreportsthemaserrors.5.Createaclasswithitsownoperatornew.Thisoperatorshouldallocatetenobjects,andonthe

eleventhobjectrunoutofmemoryandthrowanexception.Alsoaddastaticmemberfunctionthatreclaimsthismemory.Nowcreateamain()withatryblockandacatchclausethatcallsthememoryrestorationroutine.Puttheseinsideawhileloop,todemonstraterecoveringfromanexceptionandcontinuingexecution.

6.Createadestructorthatthrowsanexception,andwritecodetoprovetoyourselfthatthisisabadideabyshowingthatifanewexceptionisthrownbeforethehandlerfortheexistingoneisreached,terminate()iscalled.

7.Provetoyourselfthatallexceptionobjects(theonesthatarethrown)areproperlydestroyed.8.Provetoyourselfthatifyoucreateanexceptionobjectontheheapandthrowthepointertothat

object,itwillnotbecleanedup.9.Writeafunctionwithanexceptionspecificationthatcanthrowfourexceptiontypes:achar,anint,

abool,andyourownexceptionclass.Catcheachinmain()andverifythecatch.Deriveyourexceptionclassfromastandardexception.Writethefunctioninsuchawaythatthesystemrecoversandtriestoexecuteitagain.

10.Modifyyoursolutiontothepreviousexercisetothrowadoublefromthefunction,violatingtheexceptionspecification.Catchtheviolationwithyourownunexpectedhandlerthatdisplaysamessageandexitstheprogramgracefully(meaningabort()isnotcalled).

11.WriteaGarageclassthathasaCarthatishavingtroubleswithitsMotor.UseafunctionleveltryblockintheGarageclassconstructortocatchanexception(thrownfromtheMotorclass)whenitsCarobjectisinitialized.ThrowadifferentexceptionfromthebodyoftheGarageconstructorshandlerandcatchitinmain().

2:DefensiveProgrammingWritingperfectsoftwaremaybeanelusivegoalfordevelopers,butafewdefensivetechniques,routinelyapplied,cangoalongwaytowardimprovingthequalityofyourcode.

Althoughthecomplexityoftypicalproductionsoftwareguaranteesthattesterswillalwayshaveajob,wehopeyoustillyearntoproducedefectfreesoftware.Objectorienteddesigntechniquesdomuchtocorralthedifficultyoflargeprojects,buteventuallyyoumustwriteloopsandfunctions.Thesedetailsof



programminginthesmallbecomethebuildingblocksofthelargercomponentsneededforyourdesigns.Ifyourloopsareoffbyoneoryourfunctionscalculatethecorrectvaluesonlymostofthetime,youreintroublenomatterhowfancyyouroverallmethodology.Inthischapter,youllseepracticesthathelpcreaterobustcoderegardlessofthesizeofyourproject.

Yourcodeis,amongotherthings,anexpressionofyourattempttosolveaproblem.Itshouldbecleartothereader(includingyourself)exactlywhatyouwerethinkingwhenyoudesignedthatloop.Atcertainpointsinyourprogram,youshouldbeabletomakeboldstatementsthatsomeconditionorotherholds.(Ifyoucant,youreallyhaventyetsolvedtheproblem.)Suchstatementsarecalledinvariants,sincetheyshouldinvariablybetrueatthepointwheretheyappearinthecodeifnot,eitheryourdesignisfaulty,oryourcodedoesnotaccuratelyreflectyourdesign.

ConsideraprogramthatplaystheguessinggameofHiLo.Onepersonthinksofanumberbetween1and100,andtheotherpersonguessesthenumber.(Wellletthecomputerdotheguessing.)Thepersonwhoholdsthenumbertellstheguesserwhethertheirguessishigh,loworcorrect.Thebeststrategyfortheguesserisabinarysearch,whichchoosesthemidpointoftherangeofnumberswherethesoughtafternumberresides.Thehighlowresponsetellstheguesserwhichhalfofthelistholdsthenumber,andtheprocessrepeats,halvingthesizeoftheactivesearchrangeoneachiteration.Sohowdoyouwritealooptodrivetherepetitionproperly?Itsnotsufficienttojustsay

boolguessed=falsewhile(!guessed){...}

becauseamalicioususermightresponddeceitfully,andyoucouldspendalldayguessing.Whatassumption,howeversimple,areyoumakingeachtimeyouguess?Inotherwords,whatconditionshouldholdbydesignoneachloopiteration?

Thesimpleassumptionisthatthesecretnumberiswithinthecurrentactiverangeofunguessednumbers:[1,100].Supposewelabeltheendpointsoftherangewiththevariableslowandhigh.Eachtimeyoupassthroughtheloopyouneedtomakesurethatifthenumberwasintherange[low,high]atthebeginningoftheloop,youcalculatethenewrangesothatitstillcontainsthenumberattheendofthecurrentloopiteration.

Thegoalistoexpresstheloopinvariantincodesothataviolationcanbedetectedatruntime.Unfortunately,sincethecomputerdoesntknowthesecretnumber,youcantexpressthisconditiondirectlyincode,butyoucanatleastmakeacommenttothateffect:

while(!guessed){//INVARIANT:thenumberisintherange[low,high]...}

Whathappenswhentheusersaysthataguessistoohighortoolowwhenitisnt?Thedeceptionwillexcludethesecretnumberfromthenewsubrange.Becauseoneliealwaysleadstoanother,eventuallyyourrangewilldiminishtonothing(sinceyoushrinkitbyhalfeachtimeandthesecretnumberisntinthere).Wecanexpressthisconditioninthefollowingprogram:

//:C02:HiLo.cpp{RunByHand}//PlaysthegameofHiLotoillustratealoopinvariant.#include#include#includeusingnamespacestdintmain(){cout



}intguess=(low+high)/2cout



cyclesneededtotestallassertionsatruntimemightbetoomuchofaperformancehitinthefield.Ifthatsthecase,youcanremovealltheassertioncodeautomaticallybydefiningthemacroNDEBUGandrebuildingtheapplication.

Toseehowthisworks,notethatatypicalimplementationofassert()lookssomethinglikethis:

#ifdefNDEBUG#defineassert(cond)((void)0)#elsevoidassertImpl(constchar*,constchar*,long)#defineassert(cond)\((cond)?(void)0:assertImpl(???))#endif

WhenthemacroNDEBUGisdefined,thecodedecaystotheexpression(void)0,soallthatsleftinthecompilationstreamisanessentiallyemptystatementasaresultofthesemicolonyouappendedtoeachassert()invocation.IfNDEBUGisnotdefined,assert(cond)expandstoaconditionalstatementthat,whencondiszero,callsacompilerdependentfunction(whichwenamedassertImpl())withastringargumentrepresentingthetextofcond,alongwiththefilenameandlinenumberwheretheassertionappeared.(Weused???asaplaceholderintheexample,butthestringmentionedisactuallycomputedthere,alongwiththefilenameandthelinenumberwherethemacrooccursinthatfile.Howthesevaluesareobtainedisimmaterialtoourdiscussion.)Ifyouwanttoturnassertionsonandoffatdifferentpointsinyourprogram,youmustnotonly#defineor#undefNDEBUG,butyoumustalsoreinclude.MacrosareevaluatedasthepreprocessorencountersthemandthususewhateverNDEBUGstateappliesatthepointofinclusion.ThemostcommonwaytodefineNDEBUGonceforanentireprogramisasacompileroption,whetherthroughprojectsettingsinyourvisualenvironmentorviathecommandline,asin:

myccDNDEBUGmyfile.cpp

MostcompilersusetheDflagtodefinemacronames.(Substitutethenameofyourcompilersexecutableformyccabove.)Theadvantageofthisapproachisthatyoucanleaveyourassertionsinthesourcecodeasaninvaluablebitofdocumentation,andyetthereisnoruntimepenalty.BecausethecodeinanassertiondisappearswhenNDEBUGisdefined,itisimportantthatyouneverdoworkinanassertion.Onlytestconditionsthatdonotchangethestateofyourprogram.

WhetherusingNDEBUGforreleasedcodeisagoodidearemainsasubjectofdebate.TonyHoare,oneofthemostinfluentialcomputerscientistsofalltime, hassuggestedthatturningoffruntimecheckssuchasassertionsissimilartoasailingenthusiastwhowearsalifejacketwhiletrainingonlandandthendiscardsitwhenhegoestosea. Ifanassertionfailsinproduction,youhaveaproblemmuchworsethandegradationinperformance,sochoosewisely.

Notallconditionsshouldbeenforcedbyassertions.Usererrorsandruntimeresourcefailuresshouldbesignaledbythrowingexceptions,asweexplainedindetailinChapter1.Itistemptingtouseassertionsformosterrorconditionswhileroughingoutcode,withtheintenttoreplacemanyofthemlaterwithrobustexceptionhandling.Likeanyothertemptation,usecaution,sinceyoumightforgettomakeallthenecessarychangeslater.Remember:assertionsareintendedtoverifydesigndecisionsthatwillonlyfailbecauseoffaultyprogrammerlogic.Theidealistosolveallassertionviolationsduringdevelopment.Dontuseassertionsforconditionsthatarenttotallyinyourcontrol(forexample,conditionsthatdependonuserinput).Inparticular,youwouldntwanttouseassertionstovalidatefunctionargumentsthrowalogic_errorinstead.

TheuseofassertionsasatooltoensureprogramcorrectnesswasformalizedbyBertrandMeyerinhisDesignbyContractmethodology. Everyfunctionhasanimplicitcontractwithclientsthat,givencertainpreconditions,guaranteescertainpostconditions.Inotherwords,thepreconditionsaretherequirementsforusingthefunction,suchassupplyingargumentswithincertainranges,andthepostconditionsaretheresultsdeliveredbythefunction,eitherbyreturnvalueorbysideeffect.

Whenclientprogramsfailtogiveyouvalidinput,youmusttellthemtheyhavebrokenthecontract.Thisisnotthebesttimetoaborttheprogram(althoughyourejustifiedindoingsosincethecontractwasviolated),butanexceptioniscertainlyappropriate.ThisiswhytheStandardC++librarythrowsexceptionsderivedfromlogic_error,suchasout_of_range. Iftherearefunctionsthatonlyyoucall,however,suchasprivatefunctionsinaclassofyourowndesign,theassert()macroisappropriate,sinceyouhavetotalcontroloverthesituationandyoucertainlywanttodebugyourcodebeforeshipping.

Apostconditionfailureindicatesaprogramerror,anditisappropriatetouseassertionsforanyinvariant

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atanytime,includingthepostconditiontestattheendofafunction.Thisappliesinparticulartoclassmemberfunctionsthatmaintainthestateofanobject.IntheMyVectorexampleearlier,forinstance,areasonableinvariantforallpublicmemberfunctionswouldbe:

assert(0



morerobust.

Whenyourcodepassesallyourtests,youknowthatifthesystemisntworking,yourcodeisprobablynottheproblem.ThestatementAllmytestspassisapowerfulargument.

AutomatedtestingSowhatdoesaunittestlooklike?Toooftendevelopersjustusesomewellbehavedinputtoproducesomeexpectedoutput,whichtheyinspectvisually.Twodangersexistinthisapproach.First,programsdontalwaysreceiveonlywellbehavedinput.Weallknowthatweshouldtesttheboundariesofprograminput,butitshardtothinkaboutthiswhenyouretryingtojustgetthingsworking.Ifyouwritethetestforafunctionfirstbeforeyoustartcoding,youcanwearyourtesterhatandaskyourself,Whatcouldpossiblymakethisbreak?Codeatestthatwillprovethefunctionyoullwriteisntbroken,andthenputonyourdeveloperhatandmakeithappen.Youllwritebettercodethanifyouhadntwrittenthetestfirst.

Theseconddangeristhatinspectingoutputvisuallyistediousanderrorprone.Mostanysuchthingahumancandoacomputercando,butwithouthumanerror.ItsbettertoformulatetestsascollectionsofBooleanexpressionsandhaveatestprogramreportanyfailures.

Forexample,supposeyouneedtobuildaDateclassthathasthefollowingproperties:

Adatecanbeinitializedwithastring(YYYYMMDD),threeintegers(Y,M,D),ornothing(givingtodaysdate).

Adateobjectcanyielditsyear,month,anddayorastringoftheformYYYYMMDD.

Allrelationalcomparisonsareavailable,aswellascomputingthedurati

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