C大學教程課后習題答案15

Chapter16

ExceptionHandling

1.SolutionstoandRemarksonSelectedProgrammingProblems

1Time

Writeaprogramtocovnertfrom24-hourtimeto12-hourtime.

Remarks:

Thetextgivessampledialog.ThecodeIgiveproducesaslightlydifferentdialogthat

diagnosesparticularinputerrorsbyprovidinganappropriatemessagetotheexception

objectatthepointtheerrorisdetectedandtheexceptionisthrown.Thissolutionisonly

slightlymorecomplexthanasolutionthatgivestherequesteddialog.

IusedtheclassDigtalTimefromChapter11asastartingpoint.Iremovedfunctionsnot

needed,andreplacedtheterminatingerrorreportingcodewithcallstotherequired

exception,TimeFormatError.Iprovidederrormessagesappropriatetotheparticularerrors

asdetected.Itheninsertedthetry-catchblocks,terminatingtheprogramattheerror.After

thiswasrunning,Iprovidedtheloop,torequestrepetition.

Bug(perhapsaquirk?):Ifaninputerrorcausesanexceptiontobethrown,thecode

diagnosestheerror,thenimmediatelyrequestsanother24hourtimeforconverstion

withoutrequestingapprovalfromtheuser.

//dtime.h

//TakenfromChapter11

//

//ThisistheHEADERFILEdtime.h.Thisisareductionof

1

Copyright?2008PearsonEducation,Inc.PublishingasPearsonAddison-Wesley

//theINTERFACEfortheclassDigitalTimefromChapter11for

//Chapter16ProgrammingProblem1.Theoperator==and

//AdvancemembersarenotneededsoIdeletedthem.Ihave

//addedtherequiredexceptionclassTimeFormatMistake.

//

//Valuesofthistypearetimesofday.Thevaluesare

//inputandoutputin24hournotationasin9:30for9:30

//AMand14:45for2:45PM.

#ifndefDTIME_H

?defineDT工ME_H

?include<string>

?include<iostream>

usingnamespacestd;

classTimeFormatMistake

(

public:

TimeFormatMistake(std::stringmesg)throw()

(

message=mesg;

)

stringwhat()throw()

returnmessage;

)

private:

std::stringmessage;

)；

classDigitalTime

(

public:

DigitalTime(intthe_hour,

intthe__minute)throw(TimeFormatMistake);

//Precondition:0<=the_hour<=23and

//0<=the_minute<=59.

//Initializesthetimevaluetothe_hourandthe_minute.

DigitalTime()throw();

//Initializesthetimevalueto0:00(whichismidnight).

voidTwelveHourTime();

//returnsthetimein12hourformat:hh:mmAMor

//hh:mmPM

friendstd::istream&

operator>>(std::istream&ins,

DigitalTimeSthe_object)throw(TimeFormatMistake);

//Overloadsthe>>operatorforinputvaluesoftype

//DigitalTime.

//Precondition:Ifinsisafileinputstream,theninshas

//alreadybeenconnectedtoafile.

friendstd::ostream&

operator<<(std::ostream&outs,

constDigitalTimeSthe_object);

//Overloadsthe<<operatorforoutputvaluesoftype

//DigitalTime.

//Precondition:Ifoutsisafileoutputstream,thenouts

//hasalreadybeenconnectedtoafile.

voidPrint12HourTime()throw();

private:

inthour;

intminute;

)；

#endif//DTIME_H

//file:dtime.cpp

//AdaptedfromChapter11.

//

//ThisistheIMPLEMENTATIONFILEforProgrammingProblem#1.

//Theoriginaldtime.cxxhasbeenmodifiedtoaddthrow

//declarations,replaceerrormessagereportingandexitcode

//withexceptionthrowstatements.

//Ihaveremoveddefinitionsofmembersnotusedinthis

//problem.TheinterfacefortheclassDigitalTimeisinthe

//headerfiledtime.h.

?include<iostream>

?include<cctype>

?include<cstdlib>

#include"dtime.hn

usingnamespacestd;

//ThesePROTOTYPESareforthedefinitionoftheoverloaded

//inputoperator>>:

voidread_hour(istream&ins,int&the_hour)

throw(TimeFormatMistake);

//Precondition:Nextinputinthestreaminsisatimein

//notation,like9:45or14:45.

//Postcondition:the_hourhasbeensettothehourpartof

//thetime.Thecolonhasbeendiscardedandthenextinput

//tobereadistheminute.

voidread_minute(istreamSins,int&the_minute);

//Readstheminutefromthestreaminsafterread_hourhas

//readthehour.

intdigit_to_int(charc)throw(TimeFormatMistake)

//Precondition:cisoneofthedigits*01through,91.

//Returnstheintegerforthedigit;e.g,,

//digit_to_int(*31)returns3.

(

if(。<=c&&c<=9)

returnc-101;

throwTimeFormatMistake(Hnondigitargfordigit_to_intH;

//return0;

//ANSIrequiresareturnstatement,evenifinaccessible.

}

//Usesiostream,cstdlibandstdexcept:

DigitalTime::

DigitalTime(intthe__hour,

intthe_minute)throw(TimeFormatMistake)

if(the_hour<0||the_hour>23||

the_minute<0||the_minute>59)

throwTimeFormatMistake(

nIllegalargumenttoDigitalTimeconstructor.n);

else

(

hour=the_hour;

minute=the_minute;

)

}

DigitalTime::DigitalTime()throw()

(

hour=0;

minute=0;

)

voidDigitalTime::Print12HourTime()throw()

(

enum{AM,PM}am_pm=PM;

inth,m;

if(hour<12)am_pm=AM;

if(hour==0||hour==12)

h=12;

elseif(hour<12)

h=hour;

else

h=hour-12;

cout<<h<<;

if(0<=minute&&minute<10)cout<<0<<minute;

elsecout<<minute;

if(am_pm==AM)cout<<”AM"；

elsecout<<"PM";

}

//Usesiostream:

istreamSoperator>>(istreamSins,DigitalTime&the_object)

throw(TimeFormatMistake)

(

read_hour(ins,the_object?hour);

read_minute(ins,the_object.minute);

returnins;

}

//Usesiostream:

ostream&operator<<(ostreamSouts,constDigitalTime&

th一_object)

(

outs<<the_object.hour<<1:1;

if(the_object.minute<10)

outs<<101;

outs<<the_object.minute;

returnouts;

)

//Usesiostream,cctype,cstdlib,andstdexcept:

voidread_hour(istream&ins,int&the_hour)

throw(TimeFormatMistake)

(

charcl,c2;

ins>>cl>>c2;

if(!(isdigit(cl)&&(isdigit(c2)||c2==1:*)))

throwTimeFormatMistake(

uNon-digitinputtoread_hourorbadseparator"”)；

if(isdigit(cl)&&c2==1:1)

the_hour=digit_to_int(cl);

)

else//(isdigit(cl)&&isdigit(c2))

(

the_hour=digit_to_int(cl)*10+digit_to_int(c2);

ins>>c2;//discard':,

if(c2!=一)

throwTimeFormatMistake(

“Errorillegalseparatingcharacter"”)；

}

if(the_hour<0||the_hour>23)

throwTimeFormatMistake(

“Errorillegalrangeforinputtoread_hour\nn);

}

//Usesiostream,cctype,stdlibandstdexcept:

voidread_minute(istreamSins,int&the_minute)

(

charclrc2;

ins>>cl>>c2;

if(!(isdigit(cl)&&isdigit(c2)))

throwTimeFormatMistake(uNon-digitinputto

read_minute\nu);

the_minute=digit_to_int(cl)*10+digit_to_int(c2);

if(the_minute<0||the_minute>59)

throwTimeFormatMistake(

“Errorillegalrangeforinputtoread_minute\nn);

)

//Chl6Prog01.cxx

//driverprogramforChapter16ProgrammingProblem1.

#include”dtime.h"

chargetAns()

(

charc;

cin,get(c);

while(c!='y，&&c!=1n1)

{

cout<<”y/n,please.\nn;

cin,get(c);

)

returnc;

)

intmain()

usingnamespacestd;

DigitalTimetime24;

charstuff[1002];

charans;

while(true)

(

cout<<"Entertimein24hourformat:hh:mm\nn;

try

(

cin>>time24;

cout<<"Thatisthesametimeas\nn;

time24.Print12HourTime();

cout<<endl;

cout<<"Again?(y/n)\nn;

ans=getAns();

if(ans==,n*)return0;

}

catch(TimeFormatMistaketfm)

(

cout<<tfm.what()<<flush;

cin.getline(stuff,1000);//discardallwaitinginput

}

)

return0;

TrialRun

Entertimein24hourformat:hh:mm

13:07

Thatisthesametimeas

1:07PM

Again?(y/n)

y/n,please.

y

Entertimein24hourformat:hh:mm

10:15

Thatisthesametimeas

10:15AM

Again?(y/n)

y/n,please.

y

Entertimein24hourformat:hh:mm

10:65

Errorillegalrangeforinputtoreadminute

Entertimein24hourformat:hh:mm

16:05

Thatisthesametimeas

4:05PM

Again?(y/n)

y/n,please.

n

bash-2.03$

*/

2.NoSolutionProvided.

//Chi6Proj3.cpp

//

//Thisprogramdrawsahistogramusinginputvaluesfrom1-10.

//Itcatchesanyinputthatisnotanumericvalueinthe

//properrange.

//*********************************************************************

#include<iostream>

#include<string>

usingnamespacestd;

//Exceptionclasses

classOutOfRange

()；

classNonDigits

()；

//FunctionPrototypes

boolisAllDigits(conststrings);

//====================

//isAllDigits:

//Returnstrueiftheinputstring

//containsonlydigits.

//====================

boolisAllDigits(conststrings)

inti;

for(i=0;i<s.length();i++)

{

charc=s[i];

if(!isdigit(c))returnfalse;

)

returntrue;

)

//====================

//mainfunction

//====================

constintMAX=10;

intmain()

(

inthistogram[MAX];

inti,num,temp;

strings;

//Initializehistogramtoallzerovalues

for(i=0;i<MAX;i++)

{

histogram[i]=0;

)

cout<<"Eachnumbermustbefrom1-10.Howmanynumberstoenter?1'<<

endl;

cin>>num;

i=0;

while(i<num)

{

//Inputeachnumber,throwingtwopossibleexceptionsbasedon

//errorconditions

try

{

cout<<"Enternumber"<<(i+1)<<<<endl;

cin>>s;

if(!isAHDigits(s))

{

throwNonDigits();

)

temp=atoi(s.c_str());//Convertstringtoan

integer

if(temp<l||temp>10)

{

throwOutOfRange();

)

histogram[temp-1]++;

i++；

)

catch(NonDigitse)

cout<<"Pleaseenteryourtextusingdigitsonly.Try

again.\nn;

)

catch(OutOfRangee)

|

cout<<"Thenumbermustbebetween1and10.Tryagain.

<<endl;

}

}

//Showhistogram

cout<<endl<<"Hereisthehistogram:n<<endl;

for(i=0;i<MAX;i++)

(

cout<<(i+1)<<”：*';

for(intj=0;j<histogram[i];j++)

cout<<

cout<<endl;

)

return0;

4-6.NoSolutionsProvided.

7.

//*********************************************************************

//Chi6Proj7.cpp

//

//ProgrammingProject7inChapter9describedatechniquetoemulatea

//2DarraywithwrapperfunctionsaroundaIDarray.Iftheindicesof

//adesiredentryinthe2Darraywereinvalid(e.g.zoutofrange)you

//wereaskedtoprintanerrormessageandexittheprogram.Modify

//thisprogram(ordoitforthefirsttime)toinsteadthrowan

//ArrayOutOfRangeErrorexceptionifeithertheroworcolumnindicesare

//invalid.YourprogramshoulddefinetheArrayOutOfRangeError

//exceptionclass.

/j

//FileName:twodimensional.cpp

//Author:

//EmailAddress:

//ProjectNumber:16.07

//Description:Functionsthatemulatea2-dimensionalarrayusinga

//1-dimensionalarray.Thisversionthrowsanexception

//whenaninvalidroworcolumnindexisused.

//LastChanged:October14,2007

#include<string>

#include<iostream>

usingnamespacestd;

//Exceptionclass

classArrayOutOfRangeError

public:

ArrayOutOfRangeError();

//Constructsanexceptionwithnomessage

ArrayOutOfRangeError(stringmsg,introws,intcols,

intdesired_rowAintdesired_col);

//Constructsanexceptionwiththegivenmessageandrow/column

//data

stringget_message();

//Returnstheexception1smessagestring

intget_rows();

//Returnsthenumberofrowsinthearray

intget_columns();

//Returnsthenumberofcolumnsinthearray

intget_desired_row();

//Returnstherequestedrownumber

intget_desired_column();

//Returnstherequestedcolumnnumber

voidwrite_error();

//Writestheerrormessagetocout

private:

stringmessage;

introws;

intcols;

intdesired_row;

intdesired_col;

)；

//Createsanemulatedtwo-dimensionalarraywiththegivennumberof

//rowsandcolumns.

int*create2DArray(introws,intcolumns);

//Storesvalinthedesiredpositioninanemulatedtwo-dimensional

array

voidset(int*arr,introws,intcolumns,

intdesired_rowrintdesired_columnzintval)

throw(ArrayOutOfRangeError);

//Retrievesthevalueinthedesiredpositioninanemulated

//two-dimensionalarray

intget(int*arrzintrows,intcolumns,intdesired_row,int

desired_column)

throw(ArrayOutOfRangeError);

//Helperfunctionfortestingrange-checking

voidtest_exception(stringmsg,int*arrzintrows,intcoluirms,

intdesired_rowzintdesired_column,intval);

intmain()

constintROWS=13;

constintCOLS=7;

intcount=0;

//Createa2Darray

int火table=create2DArray(ROWS^COLS);

//Populateit

for(intr=0;r<ROWS;r++)

(

for(intc=0;c<COLS;c++)

(

set(table,ROWS,COLS,r,c,count++)；

)

)

//Printthearray*scontents

cout<<"Arraycontents:“<<endl<<endl;

for(intr=0;r<ROWS;r++)

(

for(intc=0;c<COLS;C++)

(

intval=get(table,ROWS,COLS,r,c);

cout.width(4);

cout<<val;

)

cout<<endl;

)

//Trysomeboundarycases

test_exception(nColumntoolarge...”，table,ROWS,COLS,0,COLS,

400);

test_exception("Rowtoolarge...n,table,ROWS,COLS,ROWS,0,400);

test_exception(

"Rowandcolumntoolarge...”,table,ROWS,COLS,ROWS,COLS,400);

test_exception("Columntoosmall...,table,ROWS,COLS,0z-1,400);

n

test_exception("Rowtoosmall...,table,ROWS,COLS,-1,0z400);

test_exception(

n

"Rowandcolumntoosmall...rtable,ROWS,COLS,-1,-1,400);

return0;

)

//Helperfunctionfortestingrange-checking

voidtest_exception(stringmsg,int*tablezintrows,intcols,

intdesired_rowzintdesired_col,intval)

(

cout<<endl<<msg<<endl;

try

{

cout<<"Testingset(...):**;

set(table,rows,cols,desired_row,desired_colzval);

)

catch(ArrayOutOfRangeErrore)

e,write_error();

)

try

(

cout<<"Testingget(...):**;

intval=get(table,rows,cols,desired_rowzdesired_col);

)

catch(ArrayOutOfRangeErrore)

{

e?write_error();

)

)

//Createsanemulatedtwo-dimensionalarraywiththegivennumberof

//rowsandcolumns.

int*create2DArray(introws,intcolumns)

(

returnnewint[rows*columns];

)

//Utilitymethodthatchecksdesiredrow/columnargumentsforvalidity,

//returninganexplanationinthemessagestringifsomethingis

//wrong.

boolcheck_args(introws,intcolumns,intdesired_rowzint

desired_columnz

stringsmessage)

(

boolok=false;

if(desired_row<0)

(

message=ndesired_rowcannotbelessthan0n;

}

elseif(desired_coluinn<0)

(

message=ndesired_columncannotbelessthan0n;

)

elseif(desired_row>=rows)

(

message=ndesired_rowmustbelessthanrowcountn;

)

elseif(desired_column>=columns)

(

message=ndesired_columnmustbelessthancolumncountn;

)

else

(

ok=true;

}

returnok;

)

//Storesvalinthedesiredpositioninanemulatedtwo-dimensional

array

voidset(int*arrzintrows,intcolumns,

intdesired_row,intdesired_columnzintval)

throw(ArrayOutOfRangeError)

stringmsg;

if(check_args(rows,columns,desired_rowfdesired_columnrmsg))

(

arr[desired_row*columns+desired_column]=val;

}

else

(

throwArrayOutOfRangeError(

msg,rows,columns,desired_rowzdesired_column);

)

)

//Retrievesthevalueinthedesiredpositioninanemulated

//two-dimensionalarray

intget(int*arrzintrows,intcolumns,intdesired_row,int

desired_column)

throw(ArrayOutOfRangeError)

(

intresult;

stringmsg;

if(check_args(rows,columns,desired_row,desired_columnzmsg))

(

result=arr[desired_row*columns+desired_column];

)

else

(

throwArrayOutOfRangeError(

msg,rows,columns,desired_rowzdesired_column);

}

returnresult;

)

//Constructsanexceptionwithnomessage

ArrayOutOfRangeError::ArrayOutOfRangeError():

nn

message()trows(0)zcols(0),desired_row(0)zdesired_col(0)

(

)

//Constructsanexceptionwiththegivenmessage

ArrayOutOfRangeError::ArrayOutOfRangeError(

stringmsg,intr,intc,intd_row,intd_col):

message(msg),rows(r)rcols(c)tdesired_row(d_row)rdesired_col(d_col)

(

}

//Returnstheexception*smessagestring

stringArrayOutOfRangeError::get_message()

(

returnmessage;

)

//Returnsthenumberofrowsinthearray

intArrayOutOfRangeError::get_rows()

returnrows;

}

//Returnsthenumberofcolumnsinthearray

intArrayOutOfRangeError::get_columns()

(

returncols;

)

//Returnstherequestedrownumber

intArrayOutOfRangeError::get_desired_row()

(

returndesired_row;

)

//Returnstherequestedcolumnnumber

intArrayOutOfRangeError::get_desired_column()

(

returndesired_col;

)

//Writestheerrormessagetocout

voidArrayOutOfRangeError::write_error()

(

cout<<message<<”(rows=n<<rows<<",cols="<<cols

<<",desired_row=n<<desired__row<<”，desired_col=n

<<desired_col<<<<endl;

)

2.OutlineofTopicsintheChapter

16.1Exception-HandlingBasics

AToyExampleofExceptionHandling

DefininingyourownExceptionClasses

MultipleThrowsandCatches

ThrowinganExceptioninafunction

ExceptionSpecification

16.2ProgrammingTechniquesforExceptionHandling

WhentoThrowanException

ExceptionClassHierarchies

TestingforAvailableMemory

RethrowinganException

3.GeneralRemarksontheChapter

IntroductiontoExceptions

Mostprograms,thosewrittenbystudentsinparticular,arefrequentlywrittenunderthe

mostoptimisticofassumptions:Nothingwillevergoawry.Clearly,onlya"helloworld"

programissimpleenoughforthistobereasonable.Errorsthatmustbeplannedfor

abound.Inputvaluesmaybeoutofrange,ahardwareunitmayfail,orthedesired

removablemediummaynotbeinthedrive,astackmayoverfloworunderflow,oran

arrayindexmaytakeanillegalvalue.Mostprogrammersagreethattheseerrorsare

exceptionalevents.

However,torestrictexceptionaleventstoerrorconditionsonlyistoorestrictive.Itis

likelybettertothinkofexceptionalconditionsassituationsthat,whilerare,arenecessary

tomovingfromonephaseofaprogramtoanotherortoterminationoftheprogram.For

example,end-of-filemaysignalthatwearethroughenteringdata,andarestartingthe

computation.

Thereisnocleardividinglinebetweeneventsthatareerrorexceptionsandeventsthatare

normalexceptionalevents.

Anexceptioninprogrammingispartofafacilityformanagingexceptionalevents.An

"exceptionalevent"isanoccurrencethateithercannotbemanagedlocallyorisbetter

managedelsewhere.Whatan"exception"ismaybedefinedbytheprogramminglanguage

orbytheprogrammer.Theactionofbringingtheexceptionaleventtotheattentionofthe

invokingroutineiscalledraisingtheexception.Theresponseoftheinvokeriscalled

handlingtheexception.Thepurposeofan"exception"istocommunicateinformation

fromtheplacewheretheexceptionaleventoccurstothehandlerthatissufficientto

managetheerror.

Thedetectionandhandlingofexceptionaleventscanbedoneinanyprogramming

language.Somelanguageshavefacitilitiesfbrexceptionhandling,thatis,fordetecting

"exceptionaleventsandtakingspecificactioninresponsetotheseexceptions.Some

languagesimposedefaultactioninresponsetocertainexceptions.

AfewofthelanguagesthathaveanexceptionhandlingfacilitiesarePL/I,CLU,Ada,C++

andJava.Ifthelanguagedoesnotprovideamechanismfbrexceptionhandling,the

programmerhasthechoreofexceptionmanagement.

InhisChapter8ofTheC++ProgrammingLanguage3"edition,Stroustrupgives

calculatorexamples.Hisfirstexamplecalculatoruseseleganterrorhandlingthatisshorter

thanthesecondversionwhichusesexceptionserrormanagementcode.Thecostofthe

shorter,moreelegantcodeistightercouplingallpartsoftheprogram.Iurgetheinterested

readertoperuseChapter8andChapter14ofthatbook.

Weneedtodiscuss"exception"and"exceptionalevents"inC++.

InC++an''exception"maybeanyprimitivevalueoftypechar,anint,afloat,a

cstringliteral,oranobjectofaclassdefinedintheStandardC++Library,oranobjectofa

classderivedfromalibraryexceptionclass,oraclassdefinedbytheprogrammer.What

an"exceptionalevent1'iswillbedecidedinpartbythelanguageandinpartbythe

programmer.

Anexceptionaleventorconditionmaybedetectedanywheresuchaneventoccurs.An

exceptionaleventmaybehandledanywhereinthecodeafteritisdetected,butshouldbe

managedinapartofthecodethatunderstandstheexceptionalevent.Thisisusuallyinthe

callerofthefunctionwheretheexceptionisdetectedorsomewhereupthecallchain,

perhapsinthemainfunctionitself.

Iusuallygivemystudentsthesedefinitions:

Anexceptionalconditionisanyconditionthat

1)preventscompletionofanoperationthatinturndetectsthecondition,

2)eithercannotberesolvedinthelocalcontextoftheoperation,orisbetterresolved

elsewhere

3)andmustbebroughttotheattentionoftheoperation'sinvokingentity.

Anexceptionisadatastructurethatcommunicatesinformationtothehandler

sufficientforthehandlertomanagetheexceptionalsituation.

16.1ExceptionHandlingBasics

Ourauthorusesasimpleexampletoillustratedifficulttechnique.Hepointsoutthat

exceptionhandlingshouldbeusedsparingly,andinsituationsthataremoreinvolvedthat

isreasonableinanintroductoryexample.Consequently,theinitialexceptionexamplesare

justtoys,moreintheveinof“crackingawalnutwithasteamrollertolearntocontrolthe

steamroHer“thanreasonableuses.

Theauthor'sexampledealswithavariablethatonemightnotwanttobezero,testsit,and,

inthefirstexample,handlesitwithoutexceptionhandling.Inthesecondexample,the

variableistestedandwithinatryblock,thecodethrowsanexception,whichishandledin

alocalcatchblock.

Theprinciplesfromsecton16.1are:

Toraiseanexception,usethethrowkeywordthrowfollowedbyaninstanceofany

primitiveorclassobjectthatholdssufficientinformationthatthehandlercanmanagethe

exceptionalsituation.Ifyouwanttothrowaclassobjectexception,theeasiestandmost

commonwayistoinvokeaconstructorfr