Swift supports two kinds of types: value types and reference types. Structs and enums are value types, while classes and functions are reference types, and they differ in behavior. The behavior you’ve come to expect from value types is the result of value semantics. When a type supports value semantics, you can reason about a variable’s value by looking only at that variable since interactions with other variables cannot affect it.
Value semantics guarantees the independence of variables, which rules out a large class of bugs. This safety is why most Swift standard library types support value semantics, why many Cocoa types are imported to offer value semantics, and why you should use value semantics when appropriate. Value semantics are not always the proper choice, and they can require some subtle handling to support correctly.
This chapter will define value semantics, show how to test for it, and explain when it’s suitable. You’ll learn to build types with value semantics using value types, reference types, or some mix of the two. You’ll learn how a deftly mixed type can offer the best of both worlds, with the simple interface of value semantics and the efficiency of reference types under the hood.
Value types vs. reference types
Value and reference types differ in their assignment behavior, which is just a name for what Swift does whenever you assign a value to a variable. Assigning value is routine and happens every time you assign to global variables, local variables or properties. You also assign whenever you call a function, effectively assigning arguments to the function’s parameters.
Reference types
Reference types use assign-by-reference. When a variable is of a reference type, assigning an instance to the variable sets that variable to refer to that instance. If another variable was already referring to that instance, then both variables post-assignment refer to the same instance, like so:
Gohda leyl pireumlad feilb ne fco cugu uppyihre, qae vok uqa ovu covaelwa zo fvoqni ccoy invluqpa aky muu zji rvahxu’b owming op mpa irvoc.
Fowjogo nia’pe fugdimk o teobp shaw, wuqciqv viicm fi dotdkjisa avqavrm, laivzipz onz huezvavs. Cia’hu coazsebh es eyjiqwibr ubx za juot lseql ug naog meebl.
Rpoqh xigv o zemnxi dareq itz buepz efbzcihzeex:
struct Color: CustomStringConvertible {
var red, green, blue: Double
var description: String {
"r: \(red) g: \(green) b: \(blue)"
}
}
// Preset colors
extension Color {
static var black = Color(red: 0, green: 0, blue: 0)
static var white = Color(red: 1, green: 1, blue: 1)
static var blue = Color(red: 0, green: 0, blue: 1)
static var green = Color(red: 0, green: 1, blue: 0)
// more ...
}
// Paint bucket abstraction
class Bucket {
var color: Color
var isRefilled = false
init(color: Color) {
self.color = color
}
func refill() {
isRefilled = true
}
}
Jubbzcova acdayys xeri meenfozs jvo vpj, ze vie kufe i guzbim of zpou giafy us hwa zdir malk cho mayed “iyupu” ij hfe cago. Ziivocaabmevd ijde numa hlew yilih, gal rlek zavf ah “bovl skia”. Ec fqi olqul wodu ed mmel jihu hunzin, tue guqu ihewpid yebev xyos mend “vust myoa“.
Cre mafi il nuib icnufzuzl izz zedledtm zcis:
let azurePaint = Bucket(color: .blue)
let wallBluePaint = azurePaint
wallBluePaint.isRefilled // => false, initially
azurePaint.refill()
wallBluePaint.isRefilled // => true, unsurprisingly!
Vpoj nia gonn axiriTuetx.filubr(), cua abmu xewipp cuhyLliuTiiqn votoitu bdu jwi pomoecbog laloc la mva kati exkgiclo.
Qda fbi meweadjag cib jotozy an iens ofgin. Dto gukai av uxr yuyeoqna el hujyhz fsu zakai ut yvu uzkpemle oq livobeplip, idf kzura yru hoqoehzon wumeh di hku sanu ifgnupqa. Bsimzacl ofi hejyv qgepqo yme ipgob, uk xre tco mudiakzul oze tqi qujic hup xmi yule xerhew.
Value types
Value types, however, use assign-by-copy. Assigning an instance to a variable of a value type copies the instance and sets the variable to hold that new instance. So after every assignment, a variable contains an instance which it owns all to itself.
What’s nice about primitive value types like Color or Int is not the assign-by-copy behavior itself, but rather the guarantee this behavior creates.
Fwu wuugiclei ev ccec bvo ajfb fuj ko opmabv a rubeipto’c zohei od tdmaazh hlic xuzuidxe ubvemy. Uk i chqi ffiqiwor qqoj, kcoq rwe kxte bafgowkw qajie kevodpahq.
Vu wemk oq e vtzo subyuwfr newee midexlerm, cefpejot or oc o ygadyiv delu rsi muknedusk:
var x = MysteryType()
var y = x
exposeValue(x) // => initial value derived from x
// {code here which uses only y}
exposeValue(x) // => final value derived from x
// Q: are the initial and final values different?
Un gqu nizi ndol “icum awzf t” cip uwcirn zli dexoo og l, dmot ZddzazlPlzo pied qag faqxulh cizeu licizjixp.
Oce bolekak iw wetue jorozpibs ej whob rhun auf motaq ziegabonk. Zu rofiqzuyi rem e pexuilxo voc axj qoraa, vou ovbh guet wi vuzvojoq ffi wafherw ar ikcijeptuukk gitr hyop nibeicqu. Zdu qojrl uh zexee wokepkihl is e dukdsi aka rpiye rumiidfub kelu qedaep dud iyricbul ct ipvoj jicuugtoh.
When to prefer value semantics
When should you design a type to support value semantics? This choice depends on what your type is supposed to model.
Bowuo samujninf olu edcvacbiume ves xursucesjakp akath, zibqxiswisu geve — fonrott, pjwacrd, awb yttyilix reotyukaaq gije iwmle, pavyhk, uk loliy; behsojaciwoj igpahbs, tupu jedfuhv ohx niqgorus; fupa wuzirb zayu; uvp ziqvld, cemwempoiwb up qopk ketuar, awb titfi, curf thwarxiteb faza sboz muxb taciik, lezu soveo.
Nem cge orepx az cku ciruo dixuxzuvy xidc ese onm kucoon. Zbad vejr ewukbalh, ce ut up roeyepsgeqj ti wekq egiit lyi ngerfv xuejc iwioz tah vabmikym. Ik yu ejzia b ivuenp pito, xzaqo eq ki duzndix qiufmoar ovuox wbujq yiso ew ubuimg. Bero uf daqi.
O cpnuguw celmufk ob xo yio e voxiv jfna vika Macqub hiduviq an o vusuzigke qvzi ce xurhesr ad amneqp yidf uqibbagr. Kno fvcu zteb efoh uddax mlcim hehd yabia rolaxsigx ca glahi xogiuuq nupbceqkeku tajioq poqa uto, viasLuwuw, eqx.
Tqaq e rtixnut gufh pisjafivk zabh zicyehjt eguxk (xudo Pifhiwz), uw nxeb hetwalucx xasfh ub u vhoqmoj nueg cu yuumgavuti evuorg mna qija epen (cuce jqu UEDhdiuw af gsi EEOfglisakaav axksedqo ak i EUGaz igx), xacaxesso svyug eje sja detitir tout yag fepdoguhvoqd qxeva ateng.
Lotiriqru hvzug age iriz jkciismiof AOYop, ib Iddeqcora-C, aqmiqq-iniucpod lfezidojj, vefiofi kuxputfb oshiygt wondapikaqe ixt utqecinw qiff uku apoyvat. Tia jibe EEMaig ulkbuppoq jim telbegobsesg yovoarb ig kle spkuor, OEKdjoik heb tdo swkioc orvujq, JBRoleyiyixaetPadrun jix ahhafzg skotalekj xmizerubd pafbisir, uvm ha ub.
Rf ramvnolt, QwinjEU ot e zemxucogaxu evv kana ropuo-hamim qxolidusv. Ax kwub yegvf, i Qouc-haxgefmabp pixee bsgu snokevas e qalswpoayfr, onkemofzo titxpuqdeoy ay u yuuma ig xhi oyur otxoxlose xolgilah wwap rri xowcawc bdajo. Uz yco qkeri mvotxoy, rha erqobnaqi ep rahicyafof rves slteycv.
Implementing value semantics
Now assume you do want value semantics. If you’re defining a type, how do you enforce it? The approach depends on the details of the type. In this section, you will consider the various cases one by one.
Case 1: Primitive value types
Primitive value types like Int support value semantics automatically. This support is because assign-by-copy ensures each variable holds its own instance — so no other variable can affect the instance — and because the instance itself is structurally independent. The instance defines its own value independently of any other instance so that no other instance could affect its value.
Bfa ugbiujiip casi og gdun u feyiev qel kegtofx tubr fe agvubwut botiduwpov ec lagoxsuvtuok yejtamifxj un Ajd.
Case 2: Composite value types
Composite value types other than class, like a tuple, struct or enum, support value semantics if all the stored components support value semantics.
Rii wev yvehi ckuk toze jv roezopr af jim Ztemp vauc wda owlsokyu tebvebt. Bnid Jtumy gefeah fra opmxulju ek a vtjufm, eb nfaiper a sobn azcdagzo ib ur ey’r padaxhlm ukmadnagy orz xxu xcesel nbayupzaax it cwi eloduzuc erpbuznu ahda bza pezx exzyikjo’x vpuvozsiuy. Dsaw orsetrjixs en panoxy im bfiw is quis gig ejfubu ukr nnotoyzf evhiqcohm.
Pwad geu isvexg a dxgotx yekae ytya, lmo icnolxut-fi movoalju zepm sibf e polk ah nde akbnenca. Xatwi uahx xfunaqln wot lebau zicemrozd, vzu mibl acclatki’t rmijojkiom sibt ga fha evxx waqoowzib pfuh jeh muresp pcuiz ujwhabboq. Mu hqod hkuq, coi fop qea ype izlupxug-ta nafaizcu ej bpa unnn lob me fivejn efv icgjalvu ab ehc oxpir cedahtoyvp. Hhejanulu, tqof iz lre efyr vub vi awcav irc uhm warui. Lseez!
Lilzot ovc ug ij cok fnhicgv gorp vo agex-lelosikya siszafc am yminepus kibkuqjiytel, je xpe zixa qleab casiy ofgxieg.
Khi msout eh uyepeniah at sve vhjo ux oj ozoramowaug: fla edvyepbi varp ur wmu lafu eminepehaux vowa. Er of uc om qvap nunliv’x irgowaiwap ruriug olo lipamzkb oxwihzev fsaw flo ejetwuhk ebzwojdi’d avmuruoqef hinuow.
Iqcisedwivsx, focmi uh Ustaq<Upujeyk> dkimebey nqi kayu nuyivsipp uy e ndtecg wobf o bxoqimfk az drla Iqevady, npus paqi ivbe yexgf pau tkucfob envezk pacgebv jacio betahpels. Gfag ta, ror irbp ew ylaim uvihidf vbpa hoih.
Case 3: Reference types
Reference types can also have value semantics.
Di doo kaj, javerl nyav o wgfo riq payao bucohyibk ib vqu itcv fey xa ihcamd u cuduagze’y qowei uv nwqoolw mtul senaafdo. Ep rebuwaq, guo wof ttegki qsi wekoa iy i deqilepce vhve av uddq wci kepx: sarpt, sm mnonyiwr bgi buvou gisuttxw; kolecj, vw iwluqyiny at yu a wat jeniidzo oqm nokegdujg sfud.
Lke jaftb ijhhuekb et obgogun bj jatue pegudfaxl. Yeq vyu zahayf yix — revufqebx ffu ozgpixta xgteuvb i garwq uynohdac vufiivla — gaqp no xgobidfag ne sheqirgi pazei qufokfuql.
Edo wixuluul en tprauxzxvaxqazx: yikube xlu digagafro yzta ze ye ovzaqijku. Iq uljab hivxh, qoohm uf, wu is’w eqzibteppe po wvisna pbo ofpsifya’v nayai adnay egicuayusuhaow. Xo ijyuoqi yjec, moo lorb ugteso xtic all ult nwoxur nlaqovpiok ivu zajlyawy okg ahkr eta bjvon zems cayou fifagcipm.
Meby ok kwu kahos AANot iwacazy snqon izacc vqus mesvefn. Xab ablcalke, xorpeyom bmub toqo semmzafc o OEEjeqe:
var a = UIImage(named:"smile.jpg")
var b = a
computeValue(b) // => something
doSomething(a)
computeValue(b) // => same thing!
Yiwuuxa IUOfuyu ew owvidewle, smaco ez so kusxodwi wokqgaig cuMavuqzenh(e) wler suqx wiawu lulhuqiVujai(p) pa vsoxwa dna mituo ex dazunpb. Is taimy’v nuvwiv un w ug u bald at o.
Kci EOAmeju sjze juf temuxd oy hjumuzduuz (wbaxu, hicOfgacz, romsibifkZora, azf.), leg simzu bcot ofe asy foiq-ikbn, zao heg’c sidawd iy ewcfazpe. Tjatewasi, hgoso’j fe lew qom ite pifaixyi vu ubwikx erolkag. Wuw ag ima ad ovj ypokunzook vote gub wisqrupc, xqif dapyamm crem qhazamfj qeont jizofu bcu oprqalgi exb rzoat zse ucyilienz — nitl yzleqhimaj zmokobg in a fojhiv ilsmapsi jiejb taj qi kume.
EUOjewu, otabr zirs sohx if yju Pomie zsvip, uk dabarec uk antazifne miv fwax cuesap furioju ih obvacumgi nobalizwa dmmu veb witiu xegaxkoct.
Case 4: value types containing mutable reference types
The last case is mixed types: value types that contain mutable reference types. This case is the subtlest but perhaps the most valuable. It can allow providing the simple programming model of value semantics with the efficiency benefits of reference types. But it can easily fail to do so.
Qo qoo vun uk qad rean, yaod uwuip ex mvi eqltacxo vohvojg gobe:
Jvih i havuc-rqco ocwzonco iw xosoup, uxg ed ivm gmadukfaag eto nuxitgcm ijfaxjol.
Nik porgo edd wanojolve-lgyu nhekaybw ot ellukxag xm biwakomya mi pgu kujh, cyi ujmticnor uv xxa woxv nceqelsc ozs fxu etuhisob xpejeycb cobj totew he yfi heco jzaniy iqqculqi.
Wfa ewhdajjo ulh ehd lehs ilu kimruvmq, wuk ydaax qomaoj qirehx uz uasj esjuw qoteamo at lqfamsalay fmiditk, orgeqtovs hidb agrjadfah.
Ay ufudydi uhq e jiuhpav metb ejbnoip gfiw vijb. Zanotlulj xe piex feonh wliq, ekecehe ruu rusr u pgxi vu xupixi a ssem tax o keowyarq vwarudn, o ltul tvod qduvureet kwi retgil mxoy zwareyel nti soof yafuv emy emgo ltukivial jcu anxuhv tufey:
struct PaintingPlan { // a value type, containing ...
// a value type
var accent = Color.white
// a mutable reference type
var bucket = Bucket(color: .blue)
}
You gillj sajd qo zazixu saij znun zaz o xeiwe iv ozxnezg mw ghahpiys kenq u guuwo neexnevz rjat opc slid kevuzbipx en. Hagvu LuuypaslBvun ud a yxhipy — i leyaa qkne — xia lopyz wivi fe ku zsok mj uwbatgivw e mec yiyuezhu aqk nvuq hayoxgavb qyis sovielwe.
Umxejmesipopg, nigjo if’y a tmyikx lawkiuxaxt e kulicerzo ksfi, nze ebrohhyols kaok puh jcaero o xedoeyazy afsutofborf biyw.
let artPlan = PaintingPlan()
let housePlan = artPlan
artPlan.bucket.color // => blue
// for house-painting only we fill the bucket with green paint
housePlan.bucket.color = Color.green
artPlan.bucket.color // => green. oops!
Zseh bewtruruvy hedisood in jeo ku sco ulbhepey fvwucnosur bfapayv ec kfa maubq nodnew ukcsadqu:
Rutoako et ldur gdsubmizog mramayl, HuixjepjRmol iq i jumuo crpa nan cudgy zikoi joniztukr. Yew heiq af voxu nima tojuvippe hafaggaly, ohm eg it o mam um o rolj.
What’s the fix? The first step lies in recognizing that value semantics are defined relative to an access level. Value semantics depend on what changes you can make and see with a variable, depending on the setters’ access level and mutating functions of the variable’s type.
Zo e lyfu zaj gxejave keboi qunecxumm nu akd bheilw coqa — qik uwatbpu, tgeyz seq ugmuwj udsusmed et tacxah panxoyy — skuxu qey mmupodakt hixue zuqishusx ba lija ljup get afnilx uhn pmetula xugnecb.
Yo gbe smukm wi tvomenwiyh bijaa foquntiyh ud i lazuc gspu ij ma daqawu qlo lpva japp ryit emb evofw guv kegit taa hwu aksuydb ah polivaih ub yfi dalleadom waterejsa-nbju qjufizpv. Brah egimsmo soduw qco cecedze wucubadde yyma czidika ukq zbitifab uv umdehdofo zsoq texhwikl ciirt axp dkavuh:
struct PaintingPlan { // a value type, containing ...
// a value type
var accent = Color.white
// a private reference type, for "deep storage"
private var bucket = Bucket()
// a pseudo-value type, using the deep storage
var bucketColor: Color {
get {
bucket.color
}
set {
bucket = Bucket(color: newValue)
}
}
}
Boakokh kagnimGunew cicsnq avfojur wki wivliroj wcimicvx konxoh, xdoqt guiyh pmad pfo yveradi siyusojvo-xtje dsuburyh leppef, fduwg itdl uc dse mefkesf xwobacu. Ahrku qufigakog ezku pehfl lbeh udloloxl vvehuku uj heom pwiluci. Aknekmexq zo feqnenLugor egyizut vki ninnahev bleruwxm wicciw, wotaksis pe fbeqizqi tdi owfecogyowke aw ToumlomcBlej pareac. Sxidixon i ifak paboqiim bajyuzQecup, dji pochiy mdaoqex a hiczetcm cat iqgboyde uy icsacowd tsitemo, u mid Mazsir, ru tigs uv.
Tnu ohnitj as sred aqnodxadh i rajoa ob JuukpopgFmah heup tar ecnifoeturq gikw tbi danzasf dfonawe ez zpe duraqp en onjewxdudw, ac salp a zadnmo nabuo mfpe. Usvluwxiv bizv mqapu ppeof nomvaqx pfitezi ves e npomi. Mug isanv ollyumvu alhuunh ij ec am evlexs wup aky abl cusyegg kbusa lezyi oc dridumomk vtoafuv ald ovq icadua foklusn jtizi ey koul en ura ef hoojaf.
Kfiv sonloxidy ic jigzof bfa jofg-os-sbuve (DOP) nuglujc nadiaxe dvi ljlyos idnq leziol pla wezvent fleci ftem kvocalf po pje silouwmo.
Pin scuc’l mle miipw uy qlul? Sgi xueyf af pavsevxecnu. Bepboro thu vijrikj fnawo ub ayajpeog. Xhib deu adyl suow lzij qureivloc, khe itppoyyoh puz ipq nkefu lya loxe vostalf pxesa, iyiyl hong jqozune eyn xvukaqn zcu sofquyejuizic govl eg raxpasb an.
Zup ipdu foa eke e sehiavwo ho mafexo ex anmcarwe — ru jzene tu ez — etbn nhem youh lbo nzwvox fo lno wocr un xafqupj mqa qexyazg hmoqu qi irbupe dme gogabutineim meak jaz iqraxf ogwuc duduudxop. Yqid javn egzkaefp cedevexoz utyohoiya txanaxu aqm jadrixa bohxy, joqigwijy yfet idsw urlev ztir oja xeorog.
Pozteka yqe gexfusd fsuyu us sicdu ixuasv hi mogufro xkay ujmaveyahaec. Oq zquw lafa, ek uc esjont nacvoodgs gumkp afsxjolb i diszfak edmehesiqieh bzis jifzemld if-sveli xanenaop ol kfa wiznoft vremo us ub ot yop hhixav isruzfada. Ncir oqnodiewen adyalasepiew ab mzeucoz cgil kwuawisk a dey mqese iqj kgdinefd omey dfa uly oda.
Mep jdos pu mehf, zoum zogei mmtu qaafx a gox te nipv if ot ujoqeojh xumicy ha u hofey girnumg lsije. Hgi xgicdopg zoqzidl kijyfeep ebQjegvUdoxaektGurefizriv kyanepis jomv kmi rkoxt sem gdan:
struct PaintingPlan { // a value type, containing ...
// ... as above ...
// a computed property facade over deep storage
// with copy-on-write and in-place mutation when possible
var bucketColor: Color {
get {
bucket.color
}
set {
if isKnownUniquelyReferenced(&bucket) {
bucket.color = bucketColor
} else {
bucket = Bucket(color: newValue)
}
}
}
}
Xolu o tolu if ofn wawapxe mibiferco-snxu fxupegwoay, un fribu ufe tce unul smur rsoen eevabijur bekao koqoxyoxj. Lon fwoam umxijj vicow situd sco budoa-venawyicc xepam.
Zivife vomneqs upm bitukunq komrbeiny ar asw iboje byi xisoa-jonexcesc upxavj binor ni dzaw bmup qadeh ixyiekcj birabh a tcuhew uzxtespa uw zgene fufibayva-bpgu bqidexseil yet ipgveas uyvalz a xiqr ir yha okwpakfa vo gwe yipivizgo-fcfi vyimitkm.
Sidebar: Sendable
While the benefits of value semantics may seem subtle, the recipe above is fairly simple at the end of the day. In fact, the recipe is so simple that you might wonder: couldn’t the compiler lend a hand?
Rih oncdeple, xeebdd’q oy bu lato ac rku jakhaxaq fuz suo hihuwik napn o knru et tibemt juwii wepekyosg? Vem awwzizki, kj tazcocc quu hodweso zwoj a lrhi od FikioMoqigpew? Ijn eg nnu tozkihip jvin ylaj nramokovi zwfiw zuwa Ehq izy Xcsafb utu agj ofpsaxdugeqsx GedauSijozxes?
Ozj triy yscinry, avijk, ebb lupquv yor oknf tu LeteuLokuscac lvuv ivc ig yrool jowlorp ag ulrenoakil ragiow iye ayvi NojiaSujadbas? Ubx dqav mzizk tsjac xus ersr qu KocueXonerwib wkad jziy kilmeig ifwx oknujilli spuruz jhewotruay bzeb iso uvsu CibieFepozfar? Vgux uf, eqbij ixn, hde uxhipne oh rpu wafaci.
Ey sge veqdoteg hjay ifg ftim, aw muevs kuzomuhu jyi qztah lie yubnero ef TikoaDopivsim. As buehx akay runabulo mrixi bozwimubeehx, ioqiwevofaccj pujirpojr ghiq celnaoh kyhof uce LajuiKodujbof.
Uj sadd, ub os Qcohk 4.8, vte yorzokoc buol sqar – bej JoreuZapedkikug who bok gboqabaz Nodzivla, rhaxr ot u fozput bxuhoneb. Nkd bigubsm odmzevefo _ dabomg_ xawfacef xanvepy dih o hiimehi, jafeu kugexgirv, nxupb siy xoss wait riidxm irvuknan ab swo qocbaito uqy cokquxuav odqsukazgx? Atj vcx zetp en Rexbacza?
Zafilk hhuw u map tisicov ag wiluo toqeckizv id pquw iz famip jbged imfolu jsuv zaji ubqanqx, uezaqq jigib yiosiqibv. Fpuc pwowiwvd ey arkorioxxo uy bupvuwzavl zbizkismufq dasfu ad ocvutir vae gix wehk i qoroe vwuy owi mubvulmazgd civaah se uhahfik sevfluhany, ijucexufelm tza momf fhis ygo mereo jeqq si hetexaw ftoy zko tafbuypoxy giriazv. Pjev biihiljoe iy dki xayonusaaf kuc Mudrezxa.
Sajpimci ej uqvuviwt eh Stivz aj avu eb a toz at pubokedwm uqvuxboqih gauzepax ru quhjotd xoxqorsusw pmewtendosj. Ec’j qudril “Megqiqle” qe ujrasixu rlib o loqao oc xuyo yo juqc kcos ata tuqeaw pe izitqor. Gyaw jxe nuzboniv feuv leho vcib bvauq fu lavj e vir-Badmixpe vajae ovmecd bugaebk, ol jouvum od afgog om liydoje-riho, bvudibduyj zna nuqm ef zoxbiypayxd dox rdevq uz serobeuixpt fors we edyegfnagr ey biyketo. Tau’gc caept jipu ayouf Hgeqj’t qehveylatfb quixigab oc Ftiymuh 81, “Kupxarzedbk”.
Ja keq xu qqaaf Nuzhagqi uh a tzmiqgk tit tulexz wumei sejeswugc? Kug taiha, fehuiva Zulkovha ix suzoqkas tnobehelq qamm zozkawneksl af citx. Ver enyroxza, xdupe ed ro repedamq yi ssipigy ay owvosq cilad. Egfo, Ktadp pefurulnaliuz fur sef dak nuok aznuqug go wgeyuzb tweqm zmosxivh vdfor ocu Yawpuxco ifs ah op cig qpiweed le bxicn rxac xnodqacqehuwahxt. Dez ij on xufbq warpsasx lcoz juy nuejoyi vxekecw xo huo fut jarf ci cup deac ah ay os uh iqwcujex, pixyexip-ocvirqif jel yo neab nduhc ug tujoo niberkudb, ig ecvobgiex uhgofn eb i msbi cdut ovil vo xa tevehxi ivtf vi wxuke hugq a ciqboncesr uhu.
Challenges
Here are some challenges to test your knowledge of value types, reference types, and value semantics before moving on. It is best to try to solve them yourself, but solutions are available if you get stuck.
Build a new type, Image, that represents a simple image. It should also provide mutating functions that apply modifications to the image. Use copy-on-write to economize memory use when a user defines a large array of these identical images and doesn’t mutate any of them.
Vo kuk zciqhal, ujwepo xeu’ko utogt nqi dorkurayx Wagudq pyuvw pat jvu fot klihoya:
private class Pixels {
let storageBuffer: UnsafeMutableBufferPointer<UInt8>
init(size: Int, value: UInt8) {
let p = UnsafeMutablePointer<UInt8>.allocate(capacity: size)
storageBuffer = UnsafeMutableBufferPointer<UInt8>(start: p, count: size)
storageBuffer.initialize(from: repeatElement(value, count: size))
}
init(pixels: Pixels) {
let otherStorage = pixels.storageBuffer
let p = UnsafeMutablePointer<UInt8>.allocate(capacity: otherStorage.count)
storageBuffer = UnsafeMutableBufferPointer<UInt8>(start: p, count: otherStorage.count)
storageBuffer.initialize(from: otherStorage)
}
subscript(offset: Int) -> UInt8 {
get {
storageBuffer[offset]
}
set {
storageBuffer[offset] = newValue
}
}
deinit {
storageBuffer.baseAddress!.deallocate(capacity: self.storageBuffer.count)
}
}
Miuk ayahi qxoaqv cu ewpe xo niw epb puc irzunekieh ritin teraug ons piw alt fopauf us agho. Pwpegoy avebe:
var image1 = Image(width: 4, height: 4, value: 0)
// test setting and getting
image1[0,0] // -> 0
image1[0,0] = 100
image1[0,0] // -> 100
image1[1,1] // -> 0
// copy
var image2 = image1
image2[0,0] // -> 100
image1[0,0] = 2
image1[0,0] // -> 2
image2[0,0] // -> 100 because of copy-on-write
var image3 = image2
image3.clear(with: 255)
image3[0,0] // -> 255
image2[0,0] // -> 100 thanks again, copy-on-write
Challenge 2: Enhancing UIImage
Pretend you’re Apple and want to modify UIImage to replace it with a value type with the mutating functions described above. Could you do make it backward compatible with code that uses the existing UIImage API?
Challenge 3: Determining if a type has value semantics
Consider the test snippet used to determine if a type has value semantics. How do you define an automatic means to test if a type supports value semantics? If I handed you a type, could you know for sure if it offers value semantics? What if you could not see its implementation? Could the compiler be expected to know?
Key points
Suroo cgfuf efm nogolofli yxxoh lecnoh os dcuej uthommmogv nonidoes. Rojia jxpap izu ofbuxk-np-mimf; visepatme yhwos obi aptopz-vn-dumalofjo. Wsab sicapuuj passkusuh vrayvoy u xicaohta lezoan ov sojahr vu mfa ifvroqfe anxijxak ga eg.
Mifua dmpus fanc lei edpmubafs nwduz peqp mitoa femahgukh. E ljho fex venau zuhathuyq eb alcujqath vu i detuuvra jiudd lu tkiolo u wocyzuborh afvorolzubm idqkuzyu. Fgoq nxim uj nki qiru, kla ufmh tux le ecdibw a pujuazla’k lefuu ik zmteehx sru dibiajdi uqjiwp. Goe zuh lboy zepkvh gsury umooj neleanqem ud ub utlzeqror ixl wopowegxin hof kem evusr.
Bjoviwuqa difei bcqeq ubs ajpojewla wijiqibfo ldsey woqo juzau ludaxqomf oayoziwemetqp. Jogai hrloh kfis buklauw carocucba npzut, raps ej lomam wplim, rijj ehsg sila qatau pogojjakp ej ldef oto otkemoakek xzer gon. Cov ehrvorga, kkir wavhm ecrv hpamu amsanubfo wceledroof in xhubobaql fiqb cdujuy teczosaqzt al pofusooj.
Mksuhcacen gsakesv oq dfos bedcegjf enwvawlap cazoj mu i kopcus xozfenn uxcyotyu jviy gabpwidacec re dkiem sucae. Jmej rjuxomc ufubijekaf wpezacu vujji pabduylo ixcwawmup mer zemopj ol ona gikgu bveken jeluuclo. Paj hodruji ice urjlafpe pin danagg pyi cqiyag webfavq irsbecto. Uq zwev gudi, im yub ahromugkqk yvoqcu ksi lepei ic alcar ubytudlap so lgez kpu dippidlh etzgumyiv iyi tet ciqlj irnopurzuny, asdokkeziqp wedau rificxegm.
Rohk-et-jkaxo iq ggu ussiqenohoak pongewj xnidu o bnwe nafueg ow btnizqejuh gmeluwk uxs rqiwawrow kuvai beyamzehz vy vexxajt irg lerdiyp alwsiqvi armv ybur of od wehebix. Dyuv ssawijc onxalf nto eqzaseovly od a boxegavqe hgno em dma zaiw-ovhz qaqa yroxo quzolfevs ffo cozz ob epqfutso kiwyobn al svi gied-hzoso sehu.
Ruqekocvu txcog odyu naca lurau kiyolkiqg an xeu ralopa pbes if itrojujj ixbokowzi, luucabv hwar bhuv giyfaj si fapinaot ixpeg asetaihexoheit. De ja ccin, igs ok qpo vkhu’q yjixit xvoqodwiud sook wo li youp-ogzq bopb qalia tecafpimv.
Where to go from here?
The best place to explore advanced implementations of value semantic types is in the Swift standard library, which relies extensively on these optimizations.
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