Exception机制的不当使用示例

ExceptionHandling1OutlineIntroductionJavaExceptionsManchesterUniversity,Kung-KiuLau,CS3101源自http://www.cs.man.ac.uk/~kung-kiu/cs3101/JavaException机制旳不当使用示例C++Exceptions/~ingber/SNL/CS162/PowerPoint/Session12and13.pptEiffelExceptionHandlingSummary2简介问题何谓“异常（Exception）”?一种情况要异常到什么程度才算“异常”？为何要引入异常处理机制？Robustnessreadability?怎样进行异常处理？MandatoryorOptional不同旳认识，不同旳答案Java/C++/C#Eiffel3JavaExceptionMechanismsJava对Exception旳界定较宽因而需仔细区别不同类型旳ExceptionsJava旳Exception机制回忆try/catch/finally自定义ExceptionsJavaException与DesignbyContractException旳转换4UnderstandingJavaExceptions这部分使用ManchesterUniversity,Kung-KiuLau,CS3101旳PPTSlides源自http://www.cs.man.ac.uk/~kung-kiu/cs3101/5WhatareExceptions?Many“exceptional”thingscanhappenduringtherunningofaprogram,e.g.:

Bugintheactuallanguageimplementation

Usermis-typesinput

Webpagenotavailable

Filenotfound

Dividebyzero

Arrayindexoutofbounds

Methodcalledonanullobject

OutofmemoryExceptionsareunexpectedconditionsinprograms.checkeduncheckedsyserrors6Wecandistinguish3categories:

checkedexceptions—Problemstodowiththeprogram'sinteractionwith“theworld”.

uncheckedexceptions—Problemswithintheprogramitself(i.e.violationsofthecontract,orbugs).

systemerrors

—Problemswiththeunderlyingsystem.Theseareoutsideourcontrol.Theworldisunpredictable,sowewouldexpectthesethingstohappeninproductioncode,andsoneedtohandlethem.Theseshouldberemovedbytesting,andnotoccurinproductioncode.UncheckedExceptionsSystemErrorsProgramSystemTheWorldCheckedExceptions7CheckedvsUncheckedExceptionsUncheckedExceptionsSystemErrorsProgramSystemTheWorldCheckedExceptionsanimportantdistinction,whichtheJavaExceptionclasshierarchydoesmake,butinaratherconfusingwaywewouldnormallycheckforthese,anddealwiththemwhentheyoccur.it'snormaltoletthesejustcrashtheprogramsowecandebugit.Exceptionhandlingisthebusinessofhandlingthesethingsappropriately.8ExceptionHierarchyinJavaThrowableExceptionRunTimeExceptionError...UncheckedExceptionsSystemErrorsProgramSystemTheWorldCheckedExceptionsErrorRunTimeExceptionException

RunTimeException9Whatdowewantofexceptions?Ideally,alanguage(anditsimplementation)should:

Restrictthesetofpossibleexceptionsto“reasonable”ones

Indicatewheretheyhappened,anddistinguishbetweentheme.g.nopointerstodeallocatedmemoryandnotmapthemallto“buserror”etc.

Allowexceptionstobedealtwithinadifferentplaceinthecodefromwheretheyoccursonormalcasecodecanbewrittencleanlywithouthavingtoworryaboutthemsowethrowexceptionswheretheyoccur,andcatchthemwherewewanttodealwiththem.Ideally,wedon'twantnon-fatalexceptionstobethrowntoofar—thisbreaksupthemodularityoftheprogramandmakesithardtoreasonabout.10ExceptionsinJava—areminderInJava,thebasicexceptionhandlingconstructisto:Ifathrownexceptionisnotcaught,itpropagatesouttothecallerandsoonuntilmain.Ifamethodcangenerate(checked)exceptionsbutdoesnothandlethem,ithastoexplicitlydeclarethatitthrowsthemsothatclientsknowwhattoexpect.

finallydoanythingwewanttodoirrespectiveofwhathappenedbefore.

catch

anyexceptionsthatitgenerates,andIfitisnevercaught,itterminatestheprogram.

try

ablockofcodewhichnormallyexecutesok11ExamplepublicclassEtest{publicstaticvoidmain(Stringargs[]){//WhatweexpecttohappenSystem.out.println(x+"/"+y+"="+x/y);}try{intx=Integer.parseInt(args[0]);inty=Integer.parseInt(args[1]);catch(NumberFormatExceptione){//Thingswhichcangowrongcatch(IndexOutOfBoundsExceptione){System.out.println("Usage:Etest<int><int>");}System.out.println(e.getMessage()+"isnotanumber");}//Dothisregardlessfinally{System.out.println("That'sall,folks");}}//main}//Etest12publicclassEtest{publicstaticvoidmain(Stringargs[]){//WhatweexpecttohappenSystem.out.println(x+"/"+y+"="+x/y);}try{intx=Integer.parseInt(args[0]);inty=Integer.parseInt(args[1]);catch(NumberFormatExceptione){//Thingswhichcangowrongcatch(IndexOutOfBoundsExceptione){System.out.println("Usage:Etest<int><int>");}System.out.println(e.getMessage()+"isnotanumber");}//Dothisregardlessfinally{System.out.println("That'sall,folks");}}//main}//Etest>javaEtest994299/42=2That'sall,folks13publicclassEtest{publicstaticvoidmain(Stringargs[]){//WhatweexpecttohappenSystem.out.println(x+"/"+y+"="+x/y);}try{intx=Integer.parseInt(args[0]);inty=Integer.parseInt(args[1]);catch(NumberFormatExceptione){//Thingswhichcangowrongcatch(IndexOutOfBoundsExceptione){System.out.println("Usage:Etest<int><int>");}System.out.println(e.getMessage()+"isnotanumber");}//Dothisregardlessfinally{System.out.println("That'sall,folks");}}//main}//Etest>javaEtest99Usage:Etest<int><int>That'sall,folks14publicclassEtest{publicstaticvoidmain(Stringargs[]){//WhatweexpecttohappenSystem.out.println(x+"/"+y+"="+x/y);}try{intx=Integer.parseInt(args[0]);inty=Integer.parseInt(args[1]);catch(NumberFormatExceptione){//Thingswhichcangowrongcatch(IndexOutOfBoundsExceptione){System.out.println("Usage:Etest<int><int>");}System.out.println(e.getMessage()+"isnotanumber");}//Dothisregardlessfinally{System.out.println("That'sall,folks");}}//main}//Etest>javaEtest99fredfredisnotanumberThat'sall,folks15publicclassEtest{publicstaticvoidmain(Stringargs[]){//WhatweexpecttohappenSystem.out.println(x+"/"+y+"="+x/y);}try{intx=Integer.parseInt(args[0]);inty=Integer.parseInt(args[1]);catch(NumberFormatExceptione){//Thingswhichcangowrongcatch(IndexOutOfBoundsExceptione){System.out.println("Usage:Etest<int><int>");}System.out.println(e.getMessage()+"isnotanumber");}//Dothisregardlessfinally{System.out.println("That'sall,folks");}}//main}//Etest>javaEtestfredfredisnotanumberThat'sall,folks16publicclassEtest{publicstaticvoidmain(Stringargs[]){//WhatweexpecttohappenSystem.out.println(x+"/"+y+"="+x/y);}try{intx=Integer.parseInt(args[0]);inty=Integer.parseInt(args[1]);catch(NumberFormatExceptione){//Thingswhichcangowrongcatch(IndexOutOfBoundsExceptione){System.out.println("Usage:Etest<int><int>");}System.out.println(e.getMessage()+"isnotanumber");}//Dothisregardlessfinally{System.out.println("That'sall,folks");}}//main}//Etest>javaEtest990That'sall,folksjava.lang.ArithmeticException:/byzeroatEtest.main(Etest.java:8)17UsingfinallyforCleanupFinalizersaren'tmuchgoodforreleasingresourcesTogetguaranteedcleanupofnetworkconnectionsetc.usefinallyclauses,e.g.:becausewedon'tknowwhen(orevenif)theywillbecalledButthere'sasnag—thecalltoin.close()mayitselfthrowanexceptione.g.ifthenetworkgoesdownatthewrongmomentSoweactuallyneedatry

...

catchblockwithinthefinallyclauseSockets;InputStreamin;try{s=newSocket(...);...in=s.getInputStream();...catch(IOExceptione){...}}finally{if(in!=null)in.close());s.close();}

s.close();finally{try{if(in!=null)in.close());}}catch(IOExceptione){}18CreatingandThrowingYourOwnExceptionsYoucancreateyourownexceptionclassesbyinheritingfromthestandardones.classUnhappyWidgetExceptionextendsException{publicUnhappyWidgetException(Widgetw){super(w.getProblem());}E.g.Whentherelevantproblemisdetected,youcreateanexceptionandthrowit.Theconstructorshouldcallthesuperclassconstructorwithastringtoindicatewhatwentwrong19Ifit'snothandledwithinthesamemethod,youneedtodeclarethatthemethodthrowstheexceptionif(wig.isHappy()){...}elsethrow(newUnhappyWidgetException(wig));}NotethefreedomwhichGCgivesus:wheneverweneedanewthingy(UnhappyWidgetException)wejustmakeonewedon'thavetoworryaboutwhenwewillbeabletoreclaimthememory.andlikewiseformethodswhichcallthatmethod,andmethodswhichcallthosemethods...E.g.publicvoidwidgetTest(Widgetwig)throwsUnhappyWidgetException{20TheThrowableClassHierarchyThestandardAPIdefinesmanydifferentexceptiontypes

basiconesinjava.lang

othersinotherpackages,especiallyjava.ioThrowableExceptionRunTimeExceptionError...Toplevelclassisnot

ExceptionbutThrowable.21Ingeneral,youwanttocatchspecificsortsof(checked)exceptions,andsayingcatch(Exceptione)isbadstyle.ThrowableExceptionRunTimeExceptionError...ProblemswiththeunderlyingJavaplatform,e.g.VirtualMachineError.UsercodeshouldneverexplicitlygenerateErrors.ThisisnotalwaysfollowedintheJavaAPI,andyoumayoccasionallyneedtosaycatch(Throwablee)todetectsomebizarreerrorconditionnevermindThrowable!22ItwouldbemuchclearerifthiswascalledUncheckedExceptionTheseincludeouroldfriendsNullPointerExceptionandArrayIndexOutOfBoundsException.Virtuallyanymethodcaninprinciplethrowoneormoreofthese,sothere'snorequirementtodeclaretheminthrows

clauses.ThrowableExceptionRunTimeExceptionError...UncheckedexceptionsaresubclassesofRuntimeException23Ifyouwritecodewhichcanthrowoneofthese,youmusteithercatchitordeclarethatthemethodthrows

it.ItwouldbeniceiftherewasaclassCheckedException.ThrowableExceptionRunTimeExceptionError...AllotherexceptionsarecheckedexceptionsThemostcommononesareIOExceptionssuchasFileNotFoundException.Thecompilergoestoenormouslengthstoenforcethis,andrelatedconstraints,e.g.thatalloverriddenversionsofamethodhaveconsistentthrowsclauses.24ExceptionHandlingandDbCDbCisaboutstaticallydefiningtheconditionsunderwhichcodeissupposedtooperate.

UncheckedexceptionsareExceptionsareaboutdealingwiththingsgoingwrongatruntime.(Thetwoarenicelycomplementary.)

Checkedexceptionsareexpectedtohappenfromtimetotimee.g.ifapreconditionthatanarrayhasatleastoneelementisbroken,anArrayIndexOutOfBoundsException

willprobablyoccur.“whathappenswhenthecontractisbroken”soarenotcontractviolations.25

codewhichreliesontheinputbeingcorrecttoprocessitSoifwe'regoingtouseDbC,weoughttostructureourcodeinto:

codewhichdealswithTheWorld

codewhichisinsulatedfromTheWorld.E.g.anapplicationinvolvingform-fillingshouldhaveMethodsdoingthiswillhaveno(orweak)preconditions,andwilldealwithproblemsviaexceptionhandling.Thiscanhavestrongpreconditionsandwon'tthrowexceptions(apartfromuncheckedonesduringdebugging).

codewhichvalidatestheuserinputfortype-correctnessetc.Ifthesetwosortsofoperationsaremixedtogether,theapplicationwillprobablybehardertomaintainandmoreerror-prone.26ConvertingExceptionsGoodOOdesigndecouplesthings.E.g.inmostpatternsthere'saClient

classwhichdelegatesoperationstosome“blackbox”inthepattern.ClientBlackboxopWhathappensiftheblackboxthrowsa(checked)exception?someapplication-specificcodesomeapplication-independentlibrarymayhaveitsownexceptiontypes,buttheyneedtobeapplication-specific.exceptionsthrownfromwithinthelibrarywillbeapplication-independent.Thetrickistoconvertfromtheformertothelatter.27ApplicationSpecificExceptionwillthereforehaveaconstructorwhichtry{...obj.delegateOp(...);...}catch(ApplicationIndependentExceptione){thrownewApplicationSpecificException(e);}

extractstherelevantinformationandre-castsitinapplication-specificterms.

takesanApplicationIndependentException28ExampleConsideracompilerforanoolanguageIfsomethinggoeswronginbuildingorusingaHierarchyanInvalidHierarchyExceptionisthrown.Thekeyistheclassnameandthevalueisthedefinitionoftheclass.

Ifthisoccurswhiletheclasshierarchyisbeingbuilt,itmeanstheuserhasmis-speltaclassnameorwhatever.Insuchacompiler,it'susefultohaveanexplicitrepresentationoftheinheritancehierarchyofthesourceprogramasatree(orgraph).e.g.anodehasnoparent,orthere'sacycleTheInvalidHierarchyExcetpionisconvertedtoa(checked)IllegalInheritanceExceptionresultinginasuitableerrormessagebeingdisplayedtotheuser.

Ifithappenslater,an(unchecked)CompilerBrokenExpectionisthrowninstead,becauseitmeansthere'sabuginthecompiler.whichhasanapplication-independent

Hierarchy

classwhichrepresentsahierarchyofarbitrarykey-valuepairs.29Summary

Exceptionhandlingisanimportant,highlyintegrated,partoftheJavasystem,oneoftheReallyNeatThingsAboutJava.

Checkedexceptionshappenwhentheprogram'sinteractionwiththeworlddepartsfromthenormalcase.Makesureyouhandlethemappropriately.

Uncheckedexceptionsroutinelyoccurduringdebugging,andmakethatprocessmucheasier.Byproductshippingtime,theyshouldnolongeroccur.Inparticular,thinkhardaboutwheretohandleanexception,soastosimplifythenormal-casecode,butnottodothehandlingsofarawaythatmodularityiscompromised.

Application-independentexceptionsneedtobeconvertedtoapplication-specificones.30JavaExceptionJavaException机制旳不当使用：引自网络论坛：311

OutputStreamWriter

out

=

...

2

java.sql.Connection

conn

=

...

3

try

{

//

⑸

4

Statement

stat

=

conn.createStatement();

5

ResultSet

rs

=

stat.executeQuery(

6

"select

uid,

name

from

user");

7

while

(rs.next())

8

{

9

out.println("ID："

+

rs.getString("uid")

//

⑹

10

"，姓名："

+

rs.getString("name"));

11

}

12

conn.close();

//

⑶

13

out.close();

14

}

15

catch(Exception

ex)

//

⑵

16

{

17

ex.printStackTrace();

//⑴，⑷

18

}32问题丢弃异常不指定详细旳异常占用资源不释放不阐明异常旳详细信息过于庞大旳try块输出数据不完整33OutputStreamWriter

out

=

...

java.sql.Connection

conn

=

...

try

{

Statement

stat

=

conn.createStatement();

ResultSet

rs

=

stat.executeQuery(

"select

uid,

name

from

user");

while

(rs.next())

{

out.println("ID："

+

rs.getString("uid")

+

"，姓名:

"

+

rs.getString("name"));

}

}

catch(SQLException

sqlex)

{

out.println("警告：数据不完整");

throw

new

ApplicationException(

"读取数据时出现SQL错误",

sqlex);

}

catch(IOException

ioex)

{

throw

new

ApplicationException(

"写入数据时出现IO错误",

ioex);

}

finally

{

if

(conn

!=

null)

{

try

{

conn.close();

}

catch(SQLException

sqlex2)

{

System.err(this.getClass().getName()

+

".mymethod

-

不能关闭数据库连接:

"

+

sqlex2.toString());

}

}

if

(out

!=

null)

{

try

{

out.close();

}

catch(IOException

ioex2)

{

System.err(this.getClass().getName()

+

".mymethod

-

不能关闭输出文件"

+

ioex2.toString());

}

}

}34C++Exception简介这部分使用JeanineIngber旳PPTSlides源自/~ingber/SNL/CS162/PowerPoint/Session12and13.ppt该连接似乎实效了35WhatisanException?Anyeventoutsidetheboundsofroutinebehavior.Sometypicalexamples:Errorconditions(ieiterator/offsetoutofbounds)UnexpecteddataformatMemorymanagementfailures36HandlingProblemsReturnaspecialerrorvalueT*error=“someerrorcode”;if(iter<0||iter>=maxSize)return*error;returnmat[iter];Butwhathappenshere?x=v[2]+v[4]; //notsafe!37HandlingProblemsJustDieif(iter<0||iter>=maxSize)exit(1);returnmat[iter];DieGracefullyassert(iter<0&&iter>=maxSize)returnmat[iter];Outputwhenassertfails:Assert.C:7:failedassertion`iter<0&&iter>=5'Abort38WhyUseExceptionhandling?Manytimesyoudonotknowwhatshouldbedoneifanerroroccurs.Solution:Sendnotificationandletthecallerofyourroutinetakeresponsibility.39ExceptionHandlingInC++,ExceptionHandlingallowstheprogrammertonotifytheuserwhenthereisaproblem.Theproblemcanbehandledinthenotifyingroutine,orpassedalongtothecaller.40ExceptionHandlingFacilityTheexceptionhandlingfacilityconsistsoftwoprimarycomponents:Recognizingaproblemandthrowinganexception.(try-throw)Eventualhandlingoftheexception.(catch)41Simplisticoverview:intmain(intargc,char*argv[]){stringfilename;try{if(argc<=1) throw“nocommandlinearguments”;fin.open(argv[1]);}catch(strings){cout<<“enterinputfilename“;cin>>filename;fin.open(filename.c_str());}//processdata42ThrowinganExceptionAnexceptionisthrownusingathrowstatement.Example:throw42;throw“Panicmessage”;throwIteratorOverflow(iter);IteratorOverflowe(iter);throwe;

43throwStatementAnexception(ieexpressioninathrowstatement)isanobjectofsometype.Typicallyanexceptionisaclassobjectofanexplicitlyprovidedexception

classorexception

class

hierarchy.if(iter<0||iter>=maxSize)

throwIteratorOverflow(iter,maxSize); //classconstructorreturnmat[iter];44classIteratorOverflow{public: IteratorOverflow(intindex,intmax):offset(index),bound(max){}intgetOffset(){returnoffset;}intgetBound(){returnmaxSize;}voidreset(){offset=0;}voidmessage(ostream&os=cerr){os<<“Error:offset“<<offset<<“exceedsmaximumbound“<<bound;}private:intoffset,bound;};45NothingExceptionalAboutExceptionClassesAnexceptionclasshierarchyprovides:Storagefordatathatwewishtocarryacrossfunctioncalls.Methodsfordisplayingdataandmessages.Anexceptionobjectcallsitsmessagemethodtodisplayinformationfromwithinacatchblock.46catchBlocksAprogramcatchesexceptionsbytheirtypeusingasequenceofcatchblocks.Acatchblockconsistsofthreeparts:thekeywordcatchthedeclarationofasingletypewithinparenthesesastatementblock47Example:catch(IteratorOverflow&iof){iof.message(); //messagewritestocerriof.message(log); //logisiostreamobjectiof.reset() //orsomething…}continue;Thetypeoftheexceptionobjectthatwasthrowniscomparedagainsttheexceptiondeclarationofeachcatchblockinturn.Ifthetypesmatch,thestatementblockisexecuted.Theaboverepresentsacomplete

handlingoftheexception.Normalprogramexecutioncontinueswiththefirststatementfollowingthecatchblock.48Re-throwExpressionItisoftenthecasethatwecannotcompletelyhandleanexception.Ifthisisthecase,wecanre-throwtheexception.catch(IteratorOverflow&iof){iof.message(); //messagewritestocerriof.message(log); //logisiostreamobjectthrow; }Are-throwcanoccuronlywithinacatchblockThesearchforamatchingcatchblockcontinuesupthelineIfcompletehandlinginsomecatchblockdoesnotoccur,theprogramaborts.49CatchallBlockcatch(…) //catchanyandallexceptions{cerr<<“unknowexceptionthrown.GoodBye!“;//…exit(1);}50ExceptionClassHierarchyclassMatrixException{protected:

virtualvoiddiagnostic(){cout<<"Matrixerror\n";}};classSubscriptOutOfBoundsException:publicMatrixException{protected:voiddiagnostic(intr,intc){cout<<"outofbounds"<<r<<c<<endl;}};classSizeMismatchException:publicMatrixException{protected:voiddiagnostic(){cout<<"binaryopsizemis-match\n";}};51Handlerstry{//somecodethatusesMatrixbinaryops}catch(SizeMismatchException&e){e.diagnostics();}catch(SubScriptsOutOfBounds&e){e.diagnostics();}catch(MatrixException&e){e.diagnostics();}//matchesallMatrix..52tryBlocksAtryblockbeginswiththekeywordtryfollowedbyastatementblock.Zeroormorecatchblocksfollowtheendofthetrystatementblock.Ifnoexceptionisthrownwithinthetryblock,allcatchblocksareignored.Ifanexceptionisthrownwithinthetryblock,asearchforamatchingcatchblockbegins.Ifnomatchingcatchblockisfound,searchcontinuesupthecallchain.Programabortsifamatchingcatchisnotfound.53ExceptionHandling#include<iostream>#include"Matrix.h"#include"MatrixException.h"usingnamespacestd;//functionprototypesvoidfunc();voidfuncHandlesIt();voidfuncPassItOn();voidfuncCatchAll();intmain(){funcHandlesIt();funcPassItOn();funcCatchAll();func();cout<<"LeavingMain\n";return0;}54Example:Codenotawareofproblemvoidfunc() //funcdoesn’tcare{MatrixA(4,4),B(2,2);MatrixC(4,4);C=A+B; //Exceptionthrown//Controlneverreachesthispoint//Exceptionpropagates

//Searchformatchingcatchblock.//Nonefound.Programaborts//Destructorsnotcalled.}output:Constructing0xbffffbf0Constructing0xbffffc00Constructing0xbffffc10Abort55CallerHandlesExceptionvoidfuncHandlesIt(){Matrixtemp;try{func();}catch(MatrixException&e){e.diagnostic();}cout<<“LeavingfuncHandlesIt<<endl”;}output:Constructing0xbffffc20Constructing0xbffffb80Constructing0xbffffb90Constructing0xbffffba00xbffffba0destructingmatrix0xbffffb90destructingmatrix0xbffffb80destructingmatrix

binaryopsizemis-matchLeavingfuncHandlesIt0xbffffc20destructingmatrixLeavingmainExceptiondoesnotpropagate56CallerPassesitonvoidfuncPassItOn(){Matrixtemp;try{func();}catch(MatrixException&e){cout<<"Exceptionthrown\n";throw;//ExceptionPropagates}cout<<"LeavingfuncPassItOn\n";}Constructing0xbffffc20Constructing0xbffffb80Constructing0xbffffb90Constructing0xbffffba00xbffffba0destructingmatrix0xbffffb90destructingmatrix0xbffffb80destructingmatrixExceptionthrownAbortre-thrownexceptionnothandled57CatchAllvoidfuncCatchAll(){Matrixtemp;try{func();}catch(...){cout<<"Exceptionthrown.Stopshere\n";}cout<<"LeavingfuncCatchAll\n";}output:Constructing0xbffffc20Constructing0xbffffb80Constructing0xbffffb90Constructing0xbffffba00xbffffba0destructingmatrix0xbffffb90destructingmatrix0xbffffb80destructingmatrixExceptionthrown.StopshereLeavingfuncCatchAll0xbffffc20destructingmatrixLeavingMain58Reviewthrowstatementraisestheexceptioncontrolpropagates

backtofirstcatchblockthatmatchespropagationfollowsthecallchainobjectsonstackareproperlydestroyedthrowexp;throwsobjectformatchingthrow;re-throwstheexceptionbeinghandledvalidonlywithinacatchblock59TheStandardExceptions...60bad_allocExceptionMatrix*p=0;try{p=newMatrix;}catch(bad_alloc){ //catchtype,noobjectcout<<“heapexhausted\n”;//…cleanupandexit}61bad_allocExceptionMatrix*p=0;try{p=newMatrix;}catch(bad_alloc){ //catchtype,noobjectcout<<“heapexhausted\n”;//…cleanupandexit}output:heapexausted62exceptionClassexceptionisanabstractbaseclassexceptiondeclaresavirtualfunctionnamedwhat()Classedderivedfromexceptiondefinetheirowninstanceofwhat()classMatrixException:publicexception{…}63bad_allocExceptionMatrix*p=0;try{p=newMatrix;}catch(constbad_alloc&e){e.what(); //whatreturnsaconstchar*cout<<“heapexhausted\n”;//…cleanupandexit}output:***malloc:vm_allocate(size=2211577856)failedwith3***malloc[13737]:error:Can'tallocateregionheapexausted64BenefitsofInheritingexceptionMatrixExceptionscanbecaughtwithcatch(constexception&ex){cerr<<ex.what()<<endl;}NoneedtoretrofitexistingcodetoknowaboutMatrixExceptionsclass.Noneedtouseanonymouscatch-allMustinclude<exception>65EiffelException机制简介Eiffel观点：契约破坏才会有Exception机制旳设计基于此观点66ExceptionhandlingTheneedforexceptionsariseswhenthecontractisbroken.Twoconcepts:Failure:aroutine,orotheroperation,isunabletofulfillitscontract.Exception:anundesirableeventoccursduringtheexecutionofaroutine—asaresultofthefailureofsomeoperationcalledbytheroutine.67Theoriginalstrategy

r(...)is

require ...

do

op1

op2 ...

opi ...

opn

ensure ...

endFails,triggeringanexceptioninr(risrecipientofexception).68CausesofexceptionsAssertionviolationVoidcall(x.fwithnoobjectattachedtox)Operatingsystemsignal(arithmeticoverflow,nomorememory,interrupt...)69HandlingexceptionsproperlySafeexceptionhandlingprinciple:Thereareonlytwoacceptablewaystoreactfortherecipientofanexception:Concedefailure,andtriggeranexceptioninthecaller(OrganizedPanic).Tryagain,usingadifferentstrategy(orrepeatingthesamestrategy)(Retrying).70(Fro