ch08.docx

ch08.docx

《ch08.docx》由会员分享，可在线阅读，更多相关《ch08.docx（15页珍藏版）》请在冰豆网上搜索。

ch08

Chapter8Friends,OverloadedOperators,andArraysinClasses

AnswerstoSelf-TestExercises

1.boolbefore（DayOfYeardate1,DayOfYeardate2）

{

return（（date1.get_month（）

||（date1.get_month（）==date2.get_month（）

}

ThepreviousBooleanexpressionsaysthatdate1isbeforedate2,providedthemonthofdate1isbeforethemonthofdate2orthatthemonthsarethesameandthedayofdate1isbeforethedayofdate2.

2.Afriendfunctionandamemberfunctionarealikeinthattheybothcanuseanymemberoftheclass（eitherpublicorprivate）intheirfunctiondefinition.However,afriendfunctionisdefinedandusedjustlikeanordinaryfunction;thedotoperatorisnotusedwhenyoucallafriendfunction,andnotypequalifierisusedwhenyoudefineafriendfunction.Amemberfunction,ontheotherhand,iscalledusinganobjectnameandthedotoperator.Also,amemberfunctiondefinitionincludesatypequalifierconsistingoftheclassnameandthescoperesolutionoperator:

:

.

3.ThemodifieddefinitionoftheclassDayOfYearisshownbelow.Thepartincolorisnew.Wehaveomittedsomecommentstosavespace,butallthecommentsshowninDisplay8.2shouldbeincludedinthisdefinition.

classDayOfYear

{

public:

friendboolequal（DayOfYeardate1,DayOfYeardate2）;

friendboolafter（DayOfYeardate1,DayOfYeardate2）;

//Precondition:

date1anddate2havevalues.

//Returnstrueifdate1followsdate2onthecalendar;otherwise,returnsfalse.

DayOfYear（intthe_month,intthe_day）;

DayOfYear（）;

voidinput（）;

voidoutput（）;

intget_month（）;

intget_day（）;

private:

voidcheck_date（）;

intmonth;

intday;

};

Youalsomustaddthefollowingdefinitionofthefunctionafter:

boolafter（DayOfYeardate1,DayOfYeardate2）

{

return（（date1.month>date2.month）||

（（date1.month==date2.month）&&（date1.day>date2.day）））;

}

4.ThemodifieddefinitionoftheclassMoneyisshownhere.Thepartincolorisnew.Wehaveomittedsomecommentstosavespace,butallthecommentsshowninDisplay8.3shouldbeincludedinthisdefinition.

classMoney

{

public:

friendMoneyadd（Moneyamount1,Moneyamount2）;

friendMoneysubtract（Moneyamount1,Moneyamount2）;

//Precondition:

amount1andamount2havevalues.Returnsamount1minusamount2.

friendboolequal（Moneyamount1,Moneyamount2）;

Money（longdollars,intcents）;

Money（longdollars）;

Money（）;

doubleget_value（）;

voidinput（istream&ins）;

voidoutput（ostream&outs）;

private:

longall_cents;

};

Youalsomustaddthefollowingdefinitionofthefunctionsubtract:

Moneysubtract（Moneyamount1,Moneyamount2）

{

Moneytemp;

temp.all_cents=amount1.all_cents−amount2.all_cents;

returntemp;

}

5.ThemodifieddefinitionoftheclassMoneyisshownhere.Thepartincolorisnew.Wehaveomittedsomecommentstosavespace,butallthecommentsshowninDisplay8.3shouldbeincludedinthisdefinition.

classMoney

{

public:

friendMoneyadd（Moneyamount1,Moneyamount2）;

friendboolequal（Moneyamount1,Moneyamount2）;

Money（longdollars,intcents）;

Money（longdollars）;

Money（）;

doubleget_value（）;

voidinput（istream&ins）;

voidoutput（ostream&outs）;

//Precondition:

Ifoutsisafileoutputstream,then

//outshasalreadybeenconnectedtoafile.

//Postcondition:

Adollarsignandtheamountofmoney

//recordedinthecallingobjecthasbeensenttotheoutputstreamouts.

voidoutput（）;

//Postcondition:

Adollarsignandtheamountofmoney

//recordedinthecallingobjecthasbeenoutputtothescreen.

private:

longall_cents;

};

Youalsomustaddthefollowingdefinitionofthefunctionnameoutput.

（Theolddefinitionofoutputstays,sothattherearetwodefinitionsofoutput.）

voidMoney:

:

output（）

{

output（cout）;

}

Thefollowinglongerversionofthefunctiondefinitionalsoworks:

//Usescstdlibandiostream

voidMoney:

:

output（）

{

longpositive_cents,dollars,cents;

positive_cents=labs（all_cents）;

dollars=positive_cents/100;

cents=positive_cents%100;

if（all_cents<0）

cout<<"−$"<

else

cout<<"$"<

if（cents<10）

cout<<'0';

cout<

}

Youcanalsooverloadthememberfunctioninputsothatacalllike

purse.input（）;

meansthesameas

purse.input（cin）;

And,ofcourse,youcancombinethisenhancementwiththeenhancementsfrompreviousSelf-TestExercisestoproduceonehighlyimprovedclassMoney.

6.Iftheuserenters$−9.95（insteadof−$9.95）,thefunctioninputwillreadthe‘$’asthevalueofone_char,the−9asthevalueofdollars,the‘.’Asthevalueofdecimal_point,andthe‘9’and‘5’asthevaluesofdigit1anddigit2.Thatmeansitwillsetdollarsequalto−9andcentsequalto95andsosettheamountequaltoavaluethatrepresents−$9.00plus0.95whichis−$8.05.Onewaytocatchthisproblemistotestifthevalueofdollarsisnegative（sincethevalueofdollarsshouldbeanabsolutevalue）.Todothis,rewritetheerrormessageportionasfollows:

if（one_char!

='$'||decimal_point!

='.'||!

isdigit（digit1）||!

isdigit（digit2）

||dollars<0）<-------New

{

cout<<"Errorillegalformformoneyinput\n";

exit

（1）;

}

Thiscodestillwillnotgiveanerrormessageforincorrectinputwithzerodollarsasin$−0.95.However,withthematerialwehavelearnedthusfar,atestforthiscase,whilecertainlypossible,wouldsignificantlycomplicatethecodeandmakeithardertoread.

7.#include

usingnamespacestd;

intmain（）

{

intx;

cin>>x;

cout<

return0;

}

Ifthecompilerinterpretsinputwithaleading0asabase-eightnumeral,thenwithinputdata077,theoutputshouldbe63.Theoutputshouldbe77ifthecompilerdoesnotinterpretdatawithaleading0asindicatingbaseeight.

8.TheonlychangefromtheversiongiveninDisplay8.3isthatthemodifierconstisaddedtothefunctionheading,sothedefinitionis

doubleMoney:

:

get_value（）const

{

return（all_cents*0.01）;

}

9.Thememberfunctioninputchangesthevalueofitscallingobject,andsothecompilerwillissueanerrormessageifyouaddtheconstmodifier.

10.Similarities:

Eachparametercallmethodprotectsthecaller’sargumentfromchange.Differences:

Thecall-by-valuemakesacopyofthecaller’sargument,soitusesmorememorythanacall-by-constant-reference.

11.Intheconstintx=17;declaration,theconstkeywordpromisesthecompilerthatcodewrittenbytheauthorwillnotchangethevalueofx.

Intheintf（）const;declaration,theconstkeywordisapromisetothecompilerthatcodewrittenbytheauthortoimplementfunctionfwillnotchangeanythinginthecallingobject.

Intheintg（constA&x）;declaration,theconstkeywordisapromisetothecompilerthatcodewrittenbytheclassauthorwillnotchangetheargumentpluggedinforx.

12.Thedifferencebetweena（binary）operator（suchas+,*,/,andsoforth）andafunctioninvolvesthesyntaxofhowtheyarecalled.Inafunctioncall,theargumentsaregiveninparenthesesafterthefunctionname.Withanoperator,theargumentsaregivenbeforeandaftertheoperator.Also,youmustusethereservedwordoperatorinthedeclarationandinthedefinitionofanoverloadedoperator.

13.ThemodifieddefinitionoftheclassMoneyisshownhere.Thepartincolor

isnew.Wehaveomittedsomecommentstosavespace,butallthecomments

showninDisplay8.5shouldbeincludedinthisdefinition.

classMoney

{

public:

friendMoneyoperator+（constMoney&amount1,constMoney&amount2）;

friendbooloperator==（constMoney&amount1,constMoney&amount2）;

friendbooloperator<（constMoney&amount1,constMoney&amount2）;

//Precondition:

amount1andamount2havebeengivenvalues.

//Returnstrueifamount1islessthanamount2;otherwise,returnsfalse.

Money（longdollars,intcents）;

Money（longdollars）;

Money（）;

doubleget_value（）const;

voidinput（istream&ins）;

voidoutput（ostream&outs）const;

private:

longall_cents;

};

Youalsomustaddthefollowingdefinitionoftheoverloadedoperator<:

booloperator<（constMoney&amount1,constMoney&amount2）

{

return（amount1.all_cents

}

14.ThemodifieddefinitionoftheclassMoneyisshownhere.Thepartincolorisnew.Wehaveomittedsomecommentstosavespace,butallthecommentsshowninDisplay8.5shouldbeincludedinthisdefinition.Wehaveincludedthechangesfromthepreviousexercisesinthisanswer,sinceitisnaturaltousetheoverloaded

classMoney

{

public:

friendMoneyoperator+（constMoney&amount1,constMoney&amount2）;

friendbooloperator==（constMoney&amount1,constMoney&amount2）;

friendbooloperator<（constMoney&amount1,constMoney&amount2）;

//Precondition:

amount1andamount2havebeengiven

//values.

//Returnstrueifamount1islessthanamount2;

//otherwise,returnsfalse.

friendbooloperator<=（constMoney&amount1,constMoney&amount2）;

//Precondition:

amount1andamount2havebeengiven

//values.

//Returnstrueifamount1islessthanorequalto

//amount2;otherwise,returnsfalse.

Money（longdollars,intcents）;

Money（longdollars）;

Money（）;

doubleget_value（）const;

voidinput（istream&ins）;

voidoutput（ostream&outs）const;

private:

longall_cents;

};

Youalsomustaddthefollowingdefinitionoftheoverloadedoperator<=（aswellasthedefinitionoftheoverloadedoperator

booloperator<=（constMoney&amount1,constMoney&amount2）

{

return（（amount1.all_cents

}

15.Whenoverloadinganoperator,atleastoneoftheargumentstotheoperatormustbeofaclasstype.Thi