Cmpt127 Lab 9

14/05/2015 CMPT127Lab9

http://www.cs.sfu.ca/CourseCentral/127/vaughan/lab/9/ 1/10

Lab9:C++classesclassesandobjects,constructorsanddestructors,refactoring

Goals

Afterthislabyouwillbeableto

1. Useclassesinplaceofstructs.2. Writeconstructorsanddestructorsforclasses.3. Usetwodimensionalarrays.4. Refactorcode.

Apartfromteachingyoutouseclassesand2darrays,thislabisdesignedtohaveyouworkrepeatedlyonthesamecode.Bynowyoushouldbereadingandwritingcodemuchmorequicklythanatthebeginningoftheclass.Youshouldalsousingyourtexteditormoreexpertly,forexampleusingsearchandreplace,movingaroundquicklyinthefile,cuttingandpastinglinesusingkeystrokes,etc.Youwillspendmanyhoursinyourtexteditorduringyourdegreeandbeyond:youshouldmasterit.

Refactoringcode(Task3)isanactivitythatexercisesyoureditorkungfu,andwillnottaketoolongifyouusetheeditorwell.

Setup

Notethattheseinstructionsnolongerincludeallsteps.Youshouldknowhowtodothesethingsbynow.Ifindoubt,refertopreviouslabinstructions.

Intheterminal:

1. Addthenewdirectory'9'totherootdirectoryofyourrepository.Thiswillbetheworkingdirectoryforalltheinstructionsbelow.

2. FetchthenewmaterialforLab9,storedinacompressedtarball:

$wgethttp://www.cs.sfu.ca/~vaughan/lab9.tgz

3. Expandthearchiveintotheworkingdirectory:

$tarxzvflab9.tgz

IntroducingC++C++extendstheClanguagewithsupportforclassestosupportObjectOrientedProgramming,andtemplatestosupportGenericProgramming.Thisweekwewilllookatclasses,butfirstasummaryoftheworkflowchangesyouneedtouseC++.

1. C++sourcecodefileshavetheconventionalsuffix".cpp"or".cc",insteadofC's".c".2. C++headerfilesmayhavethesuffix".hpp"or".hh",orstickwithC's".h".Wewilluse

".hpp"tomakeitclearwhenwehaveaC++header.3. TheGNUC++compilerprogramiscalled"g++",insteadof"gcc".

Notethattheintructionsbelowdonotprovidecompilecommandsforyoutocut'n'paste.Youwillneedtoreadthenotesaboveandconstructyourowncompilecommands.

AC++primerforCprogrammers

HereisaverygoodandconciseintroductiontoC++forCprogrammers.byProf.TomAndersonoftheUniversityofWashington.

Prof.AndersonhasstrongopinionsabouttheusefulandlessusefulpartsofC++,anddrawsyourattentiontothemostimportantpartsinlessthan12pages.Thedocumentisdatedinsomeways,butmostoftheadviceisexcellent.

Thingstodo:

1. ReadSections1through3carefully:theywillhelpwithyourTasksthisweek.

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2. ReadSections4and5atyourleisure,andrememberthatopinionsvarywildlyonthissubject,andthingschangeovertime.

3. NotethattheStandardTemplateLibrary(STL)isnowmatureandsupportedbymostcompilers,andyoucanandshoulduseitwithconfidence.WewillshowyouthebasicsoftheSTLinalaterlab.

Notethatyoureallyareexpectedtoreadthedocument,andthefollowinginstructionsmaynotbeenoughbythemselvestohelpyousolvethetasks.Anothergoalofthislab(andtheclass)istoencourageyoutoreadaroundyoursubjectsandfeelpersonalresponsibilityforlearning.

**STOP**READTHEPRIMERDOCUMENTBEFORECONTINUING**

Task1:ImageclassWewilldevelopaC++program,usingideasfamiliarfrompreviouslabsplusnewsyntaxforcreatingclasses.

Requirements

1. ReadtheGuidesectiononclasses,constructorsanddestructorsbelow.2. WriteanewC++sourcefile"image.cpp"thatcontainsacompleteimplementationofthe

ImageclassdeclaredintheprovidedC++headerfile"image.hpp".3. Yourfilemustinclude"image.hpp"withoutanymodifications.4. Writeyourownmain()functionfortestingyourclass,butputitinanothersourcefile:do

notdefinemain()in"image.cpp"asthiswillinterferewiththeservertest.

Submission

Addandcommitthesinglefile"image.cpp"toyourrepo.

Guide

CtoC++

HereisaninterfacedefinitionforanimagedatatypeinC,extendedfromtheonewedevelopedinanearlierlab:

typedefstructimage{unsignedintcols;unsignedintrows;uint8_t*pixels;}img_t;

/*Returnsanimg_tstructurecontaining0x0pixels.*/img_timg_create(void);

/*Freesallmemoryallocatedforimg*/voidimg_destroy(img_t*img);

/*Changesthesizeofanimage,allocatingmemoryasnecessary,andsettingallpixelstofillcolor.Returns0onsuccess,oranonzeroerrorcode.*/intimg_resize(img_t*img,unsignedintwidth,unsignedintheight,uint8_tfillcolor);

/*Setsthecolorofthepixelat(x,y)tocolor.Returns0onsuccess,elseanonzeroerrorcode.If(x,y)isnotavalidpixel,thecallfailsandtheimagedoesnotchange.*/intimg_set_pixel(img_t*img,unsignedintx,unsignedinty,uint8_tcolor);

/*Getsthecolorofthepixelat(x,y)andstoresattheaddresspointedtobypcolor.Returns0onsuccess,elseanonzeroerrorcode.*/intimg_get_pixel(img_t*img,unsignedintx,unsignedinty,uint8_t*pcolor);

WecouldusethesedefinitionsasisinaC++program,sinceCisalmostasubsetofC++.However,animplementationinC++stylewouldhavethefollowingdifferences:

1. Thestructurewouldbeaclass

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2. Thefunctionsoperatingonimagestructureswouldbemethodsoftheimageclass,andwouldnotneedthe"img_"prefix.

3. Imageclassmethodsdononeedapointertotheimagedatastructure,sincetheyautomaticallyhaveathispointertotheobjectonwhichthemethodwascalled.

4. Thefieldsofthestructwouldnotbeaccessiblefromoutsidetheclass:animagecouldonlybemodfifiedusingitsmethods.

5. img_create()wouldbeaconstructor6. img_destroy()wouldbeadestructor

HereistheC++version:

classImage{

private:unsignedintcols;unsignedintrows;uint8_t*pixels;

public:/*Constructsanimageof0x0pixels.*/Image();

/*Freesallmemoryallocatedforimg*/~Image();

/*Changesthesizeofanimage,allocatingmemoryasnecessary,andsettingallpixelstofillcolor.Returns0onsuccess,oranonzeroerrorcode.*/intresize(unsignedintwidth,unsignedintheight,uint8_tfillcolor);

/*Setsthecolorofthepixelat(x,y)tocolor.Returns0onsuccess,elseanonzeroerrorcode.If(x,y)isnotavalidpixel,thecallfailsandtheimagedoesnotchange.*/intset_pixel(unsignedintx,unsignedinty,uint8_tcolor);

/*Getsthecolorofthepixelat(x,y)andstoresattheaddresspointedtobycolorp.Returns0onsuccess,elseanonzeroerrorcode.*/intget_pixel(unsignedintx,unsignedinty,uint8_t*colorp);};

Heretheyareagainwithoutthecommentssothefunctionalpartsareeasiertocompare:

C:

typedefstructimage{unsignedintcols;unsignedintrows;uint8_t*pixels;}img_t;

img_timg_create(void);voidimg_destroy(img_t*img);intimg_resize(img_t*img,unsignedintwidth,unsignedintheight,uint8_tfillcolor);intimg_set_pixel(img_t*img,unsignedintx,unsignedinty,uint8_tcolor);intimg_get_pixel(img_t*img,unsignedintx,unsignedinty,uint8_t*colorp);

C++:

classImage{private:unsignedintcols;unsignedintrows;uint8_t*pixels;

public:Image();~Image();intresize(unsignedintwidth,unsignedintheight,uint8_tfillcolor);intset_pixel(unsignedintx,unsignedinty,uint8_tcolor);

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intget_pixel(unsignedintx,unsignedinty,uint8_t*colorp);};

Examplesofusage,firstinC:

img_timg=img_create();

if(img_resize(&img,640,480,0)!=0){exit(1);//quit}for(inti=0;i

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Image::Image():cols(0),//initializationlistrows(0),pixels(NULL){//Noworktodoinconstructorbodyinthisexample://theintializationlisttookcareofit.//Allmembersnowhavesafeinitialvalues.}

AweakpointoftheC++designisthathandlingerrorsinconstructorsisquitecomplicated,sincetheyhavenoreturnvalue.Itisagoodideatoavoiddoinganythinginaconstructorthatcouldfail,forexampleanysystemcallormemoryallocation.Thisiswhyinourexampleweconstructonly0x0pixelimages,anddoourpixelallocationinImage::resize()Ifanynewfails,yourprogramwillbeterminatedimmediately.Youcanchangethisbehaviourusingexceptionsbutthatisbeyondthescopeofthisclass.(SomepeoplethinkexceptionsareaBadIdea,forexampleGoogledoesnotallowtheminanyC++code).

Destructors

Wheneveranobjectisdestroyed,eitherbygoingoutofscopeorwhendeleteiscalledonitsaddress,thedestructorisautomaticallycalled.Youputmemorydeallocationandcleanupcodeinthedestructorsoyoucannotforgettorecycleresourceswhenyouarefinishedwithanobject.Afterthedestructorfinishes,thedestructorsofanydatamembersarecalled.

Destructorshavethesamenameastheirclass,prefixedwitha'~'andtheyhavenoreturntypeorvalue.

//containstheclassdeclarationforImage#include"image.hpp"

Image::~Image(){//freeeverythingthatcouldhavebeenallocatedinsidethisobjectif(pixels!=NULL){delete[]pixels;}}

Creatinganddestroyingobjects

Objectscanbecreatedonthestack,justbydeclaringthem,andtheirmethodscalledusingdotsyntax:

Imageimg;img.resize(100,100,255);//whitesquareimage

Whenastackallocatedobjectgoesoutofscope,itsdestructoriscalledautomatically.

Objectscanalsobeallocatedontheheapusingthenewkeywordfollowedbytheclassname.newreplacesmalloc()formostpurposesinC++.newreturnsapointertoaninstanceoftheclass,somembersandmethodsareaccessedusingarrowsyntax.

Unfortunately,keepingtrackofpointerstodestructobjectsautomaticallyisquitecostly,soheapallocatedobjectsmustbedestroyedmanually:

Image*img=newImage;img>resize(100,100,255);//whitesquareimage//(useimage,thenwhendone)deleteimg;

OtherlanguagessuchasJavaandC#cankeeptrackofheapallocationsandsaveyouthetroubleofdeletingthings.Thisgarbagecollectionprocesscanbeverygoodforcoderobustness,butslowsthingsdownabit.

InC++thebasicmemorysanitationstrategyis(i)writethoroughdestructorsand(ii)usestack

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variableswheneverpossible.Rememberyourstackissmallsobigthingsgoontheheap.Agoodapproachistohaveasmallstackallocatedwrapper/headerobjectforeachofyourheapallocatedbigthings,sothatwhenthewrapperobjectgoesoutofscopeitsdestructorcleansupproperly.TheImageclassfollowsthispattern.

Defaultconstructors

Aconstructorthattakesnoargumentsiscalledadefaultconstructor.Ifyoudonotspecifyanyconstructorforyourclass,adefaultconstructorisgeneratedautomaticallythatsimplycallstheconstructorsofalldatamembersoftheclass.Thebuiltintypes(int,float,etc)havedefaultconstructorsthatallocatespaceonly:theirvaluesareleftinitialized,likeinC.

Creatingarraysofobjects

Newanddeletehaveconvenientarrayforms,whichcanbeusedwithbuiltintypesandanyobjectthathasadefaultconstructor.

int*arr=newint[100];//(useintarray,thenwhendone)delete[]arr;

Forobjects,thedefaultconstructoriscalledforeveryiteminthearray.Similarlydelete[]callsthedestructoroneachiteminthearraybeforefreeingtheentirearray.

Bubble*bubbles=newBubble[100];//produces100bubbles,allthesame//(usebubblearray,thenwhendone)delete[]bubbles;

Teachthecontroversy

Noteveryoneisafanofnewanddelete,andthis"rant"byStanfordprofessorDavidMaziresdiscussessomeoftheissuesthatbotherpeople.Followingtheauthor'sargumentwilltestandboostyourunderstandingofhowtheywork.

Methods

Classmethodsarelikenormalfunctions,buttheycanonlybecalledthroughanobjectoftherighttype.Akeyadvantageofthisisthatyoucannotpassinthewrongkindofpointer,oranullpointer.

Methodsaredefinedbyprefixingthemethodnamewiththenameoftheclass:

#include"image.hpp"

intImage::resize(unsignedintwidth,unsignedintheight,uint8_tfillcolor){//dotheworkhere//...return0;}

intImage::set_pixel(unsignedintx,unsignedinty,uint8_tcolor){//dotheworkhere//...return0;}

//(etc)

Insideaclass'smethods,allthemembersoftheclassareavailableusingjusttheirnames:

intImage::set_pixel(unsignedintx,unsignedinty,uint8_tcolor){//x,y,andcolorarelocalvariables//pixelsandcolsaremembersofthisobjectpixels[y*cols+x]=color;

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return0;}

Incaseamethodneedstoknowwhichobjectitwascalledon,apointercalledthisalwaysexists.thispointstotheobjectonwhichthemethodwascalled.Sothiscodeisidenticaltotheabove:

intImage::set_pixel(unsignedintx,unsignedinty,uint8_tcolor){//x,y,andcolorarelocalvariables//pixelsandcolsaremembersofthisobject//socanbeaccessedthroughthe'this'pointerthis>pixels[y*this>cols+x]=color;return0;}

Insideanobject'smethods,itsothermethodsarealsoavailableusingtheirnamealone,orthroughthethispointer:

voidImage::set_grey_all_over(void){for(inty=0;yrowsfor(intx=0;xcolsset_pixel(x,y,128);//this>set_pixel()}

Onceanobjectisconstructed,itsdatamembersandmethodsareaccessedusingdotorarrowsyntax,justlikestructures,exceptthatnowwecanaccessanobject'smethodstooperateonit:

Imagei();i.resize(640,480,0);i.set_pixel(100,100,255);

Image*j=newImage();j>resize(640,480,0);j>set_pixel(100,100,255);

Task2:Save&LoadExtendyourImageclassbyaddingsave()andload()methods.Thefileformatisuptoyou.Itcanbetextorbinary,aslongastheload()loadsthesave()'dformatcorrectly.

Requirements

1. WriteanewC++sourcefile"image2.cpp"thatcontainsacompleteimplementationoftheImageclassdeclaredintheprovidedC++headerfile"image2.hpp".

2. Yourfilemustinclude"image2.hpp"withoutanymodifications.3. Writeyourownmain()functionfortestingyourclass,butputitinanothersourcefile:do

notdefinemain()in"image2.cpp"asthiswillinterferewiththeservertest.

Submission

Addandcommitthesinglefile"image2.cpp"toyourrepo.

Guide

SeeLab6forfileI/Otutorialandexamples.

Task3:Alternateimplementationusing2DarraysChangetheinternalrepresentationoftheimageinyourImageclass,withoutchangingtheexternalinterface.

Wewilluseamultidimensionalarraytostorepixelsinsteadoftheonedimensionalpixelarraywehaveusedsofar.

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Theprocessofrevisingyourimplementationwithoutchangingtheinterfaceorexternalbehaviouriscalledrefactoringandcanbeveryimportantforkeepingcodemaintainable,particularlyafteraperiodofaddingalotoffeaturesorbugfixes.Tidyingupamessyimplementationbeforeitgetstoocomplicatedformortalstounderstandisusefulwork.Ontheotherhandyoushouldresisttinkeringforthesakeofit:youriskintroducingbugs.

Requirements

Thesuppliedfile"image3.hpp"differsfrom"image2.hpp"onlyinthatitdeclarestheprivatepixeldatapointertobeoftypeuint8_t**.YoumustreimplementtheImageclassusinga2Darray.Thiswillmakememoryallocationcodealittlemorecomplex(seetheGuidebelow),butmakeaccessingpixelseasierandarguablymakethecodemorereadable.Certainimageoperationsyouimplementedinanearlierlabcouldbefasteroreasierusing2Darrays:youdonothavetoimplementthemhere,butyoushouldthinkaboutwhichoneswouldbenefit.

1. WriteanewC++sourcefile"image3.cpp"thatcontainsacompleteimplementationoftheImageclassdeclaredintheprovidedC++headerfile"image3.hpp".

2. Yourfilemustinclude"image3.hpp"withoutanymodifications.3. Notethatsincetheinterfaceoftheclass(i.e.thepublicmethods)didnotchange,youcan

usethesamemain()programtotestitasfortheprevioustasks.Makesurenomain()functionexistsin"image3.cpp".

Caution

save()andload()needsomecarefulthought.Favourcorrectnessoverspeed,buttrynottomaketoomanysystemcalls:theyareslow.

Submission

Addandcommitthesinglefile"image3.cpp"toyourrepo.

Guide

CandC++supportmultidimensionalarrays.Thesyntaxisverysimplebuttheunderlyingmodelisalittlecomplex.Considerthesearrays,wherea2Darrayisdeclaredandusedinthewayyoumightguessbyextrapolatingfromthe1Dcase:

intA[16];//1Darrayof16elementsintB[4][4];//2Darrayof4x4elements

//setseveryelementinAfor(inta=0;a

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'A'isoftypearrayofint.'B'isoftypearrayofarrayofint.Youcanseethatthereissomeoverheadinusingthe2Darray,aswerequiremorememoryspaceandatwosteplookuptofindtheintegerofinterest.

Moredimensions

Thearraydeclarationandaccesssyntaxworksforarbitrarynumbersofdimensions:

intC[3][3][3];//3Darrayof9intsintD[20][5][10][4];//4Darrayof20*5*10*4ints

C[0][0][0]=99;D[101][2][1][0]=99;

Inpractice,arraysofmorethan3dimensionsarerarelyused.

Building2Darraysontheheap

Theexamplesaboveshowmultidimensionalarraysallocatedonthestack.Theycanalsobeallocatedmanuallyontheheap.ThisexamplebuildsarraysAandBontheheap(omittingerrorchecking)inC:

int*A=malloc(16*sizeof(int));

int**B=malloc(4*sizeof(int*));

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for(inti=0;i

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Cmpt127 Lab 9