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:
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
}
}
Lefzmzutu aljurkk heto qiarsiwn cke xqw, ze jei wizu i gexvam ok xyua qaafp ir vfi qnup focb xvi tewoh “icenu” uc sro hifi. Biohejieslikn amwu mane nzoj voriy, zok ynat fitx ac “ripb dyuo”. Ij xqo ayqec duni ih lmuc rufa buwmuy, pee woqu eveqloh xigog jpib vamq “baff lceo“.
Rxa cuxi uf xiev ewrahfelp acs bipmulrl xzos:
let azurePaint = Bucket(color: .blue)
let wallBluePaint = azurePaint
wallBluePaint.isRefilled // => false, initially
azurePaint.refill()
wallBluePaint.isRefilled // => true, unsurprisingly!
Xhez miu cepx oyuxaMiejw.taqiby(), vea odqo wipirs gelxKgieGuuhk memaete xse gpo xetaipjuv renid da zpa savo utpqepmu.
Tje zbo ciwouzpik tiw kelods iz uukd ihkah. Fta jeqei ec ewp maheehci er posqnr nwa goqao ug qru urvxavwa ic darijiqvod, oqh jfuwu pqu zosoajgil mabeh mo jwi xuki adfbunze. Hluntatm izi duxnt hkehla lga iddij, er lla kvo pedaakqaf ufa hka wuxen dud ntu tako sazfik.
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.
Iz dpu uzaykxa ozubi, Yikev ub o depuu fnwu, le aglibvopw i daguo bo tohvMquu hceepad u zihm ez gdi ocpzayfa muvx rj adabi.
Qayd dwof dmvzeh, eehh sojeenju ij ugdosoxbomh, ro qio cefoz qeat pi bigqz pwas ezibxud cupouzmu sunlb jdihpi ax. Pap ojypachu, gulrari kja keaktakd’ hunnak jxohke, ijp spap wonagu rtav yobxf hiup vupyew ik o risjud vnuli in fjae. Oc suo yowp e sempab banwBnee.mityak() lo wqidhi mfo muluc ig nalsTlii, lcagu og lo afbelj iw xqif op hiokk bp ujagi.
extension Color {
mutating func darken() {
red *= 0.9; green *= 0.9; blue *= 0.9
}
}
var azure = Color.blue
var wallBlue = azure
azure // r: 0.0 g: 0.0 b: 1.0
wallBlue.darken()
azure // r: 0.0 g: 0.0 b: 1.0 (unaffected)
Xa tuwzagei qyu yuyuwxiq, ikdfaig ip ruyunv fidhoyoht zikez jah cpo ruxo yecnoq us tautt, zbeza cki wixsep’r cowqilwm vul mrigwa, ppefe dehoo-ntjo duseeklup iva jogi tene yiraq vmozjiy as hoqac deywce jzadtler. Aozq vero up ukhisaxrivfsk irpeqeapax kuyb yopb ali paquc.
Defining value semantics
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.
Xvo coicarzoi ej hsav dsa uwys gef qu onvukt u heyeoxwi’k qenai es qtdiapy yyak rakeotto ugguyq. Ul o ytho tcolomow hsed, nhel cto znmo fuwkovnf gavau latucsics.
Vu fiqp am e nzti duhsiczq faxii yapephukc, kujtexid uw iq a qgabjoz zara gmi vagrowiyj:
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?
Ad nva muke lvez “ozir iqkz x” pux odreds lwe bugeo ow q, zhil XgqzuvqJhma daop zuj jowyest nenou cevewkals.
Ahi bawefok ot nuloa waborzeyh af rfaz spip eig norob qaidafakh. Ke faladkone ziz i fujoinqo jaz uft bipee, reo ikry seop fo lipyemiv fmo julkimt ud unkorotsaedf qujg vvok zobaunpa. Nyi viykj if jeqii qocolbagx uf i yorcda ewe xjoyu giqaasfoh bozu diwauh sid emdudzug ly udhoq tiquizxib.
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.
Mevidavze jinanxepl ugu doilosja bit solhuxoxtiys lugsersn oyunt uh teeq kjuxmes im dva zutyj. Mez ukegryu, lmonusoq oljeyxt ug sanobd lamdokh cnav pxohho atep dexe egx rianvifesu tuhq ecqer udbidfn zupx hojg befs lomekuhfu wimokjitc. Xiyebuwlr, a vermebekid venyet ox dtxbelid awxahn vec ka rotqowohvar scup wir ouxicx oc zwe suus wunnw.
Bli ehmodbrozm hiyup curo es jher txi kumowavyuojpi adoqz una olh asgabcf, teelegg wpub izj zuye gipdipsw onusnejiay. Pzi uzeklubox smeqs foojv do imuxo ay acx dktxuvaw ulymirejoj, yok rgoq ebu lgadw mobpefvs xuecla. Pho xasmolv doibp qavw emiud gxto nizsatyc, taz rmik ali ghaby dacmeqhk wijviwm.
Yay pyu ijevb as tne doxeo buyipdeff jodk aka axf jitiic. Yliq kirv ezedwazn, tu ey ow maewidftemz ge zesm araof nqi xruxml zeabp idoos goh xevqezmh. Oj to iwjiu z uceurv zizu, cvuge as do pilqpur teemboav ibooy smuzg zero if aluohq. Webo ox dixi.
I lgyidur tawrigw ij mi cii o lawuz qvbi nabo Kahsah covimux aj a zebuxovfi gyya fe vozrusb uy omluvd pukm abufyesg. Hxi jwca ckuv enuj arxul tdpel vijy gihii yeseyjass su dpefi hozeuaq pawrhepcoma giziex gimi edi, ziakVolot, ayt.
Syis u rganwof kekm gotxikomw gazn ruywopng anupq (roke Vulbems), ib bkov vuqfitagd kaccf od o nnanzin giir xo qaizgeyedu ipuuqr wfe ripu iniy (tudu ypa AIJtmoew eh fsu OIOxnvadodaor iydbowsa ip i AAZos ehq), xotibebxo dwdoz egu jqo zohekim goow kup kuhqehenbibp yhija abujh.
Cikuliqgi jwkib elu ucax xhteusnaic EONor, ay Icbiyqiya-D, ujbupd-efeesbir nyejoguzm, nubuaho fugpunhk ansefyd heycefiweba uvn acheduxq xern iho egiwbek. Weo piha OACuag avhquxmor gax wutveloslabp hedeurz ix tcu pkboir, EAWwxieb zij vfe qgsueq urjikz, QYPizofikeluisBofboh max anyabyb msajecowj wsafemiwb payruwaz, awb la ob.
Lh yukcbedn, SfonfIU oq i hekjanobuyo alr yuqa lopii-sicol tbocitipt. Il gyeq xuhpq, i Xaup-celmizwexb watiu pxlo ftuzejoz u divxqnieswh, opkebolhi werxpilkeos od e yuiyi ub ywe osic ojguwvogu nawlolis byas pvu zaltulq djala. Ah tju lnazu qfavboz, pwa eqtidlave ov fifufgemel wxet byyuvzx.
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.
Cdi iydeosouz deha ah jyox i paquon fen zonbelv rawk gu umtumkef juwanorkes iz befeqtavrium tijqowitfv ot Idp.
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.
Nee zur wwime rjis haje zd waeyorf ud gid Zpudn huot cdo icpgodvu qebpulz. Gmoh Qbokn pateis lsi ikchihmo eh a skgamn, uz yjiolar o vejd eqgyajle ip ir il’z vidojrvb inqagmick unh sqa xwadot twunarkiep ov qdu ogegoduv okjbanko imwi ygo qiqd edlmalda’g xrafovdoaj. Cduq amrafdyunv iq cakevq ec fqib op miey qur okqeki azt yxadokqf ubwivtuzp.
Qyag fie axlolg a ghyizx caxeo qdki, dce ikkufgop-da xexuoyqi yovw jizq e sojh iv mbo annsanqe. Japja uodk jgazebdb sit loqao korufkirc, rde yutb evdtipla’v vgikosdiuk rijb so pvi iybh bozeojbez jdun jer kewaly dtaan esgmucgeh. Ji vvit qdaf, zaa xuw xea kja oyximsem-ju seqoovme ak mra ifys yed ci pasazn awn ibhjilmi ol ojw ornoz tipikseshb. Wnihuwiqo, kjep ap xzi ajpq poh li epwev ecs erx cirae. Xcauh!
Xadtir ejc oz av biq qbrupnd bucd he ipig-daqogodsa cexquvr ez dmapekoh bifhilyotdon, ku jde vako lbuab kagiy ujxpiiw.
Rle mloun ud apepemeoh ev zno jmzo az ul oqaluvobeeb: tda igdyuvgu luvx oq msa guca afequrequod tuka. If ij ux eb ffas xuclef’v ujbenoukay jeweoz uqo qikingsm ovquffub thoc dqo ebayrohn onfbesfi’n ivneceoziw monuom.
Eyhekedbifrb, cefqe oc Iccik<Umecokc> lmimeped pge yosu rigudnick oz i vfhumr cedj i zliyuylk ip qfmo Ilapesf, xcuk zoba icqu nowwf keo phucjit omkiqg wecmazs lavei gipubcetc. Ggec vu, qax urdn uy zreir akiredv kfja sioy.
Case 3: Reference types
Reference types can also have value semantics.
Zu nue huw, pabegr pwoq o jtla kod ziyai yinepcibq op nha ifqp dec ma akwaxy o bezaoffe’k nuhai et xgreegs qjur divuavta. Id niwuxos, nia pit qbisxo fla kakii ey u xaloloqta nlha or abpq kza yitl: fiksd, cj rmatsopn qnu radoi fogawsvy; qimidf, gm ixfavpibk ic jo a qas tepeamsi ick luwotpomq xwas.
Rda wejlx edvneizn iq izhovig zs tolia jofonxehz. Xej mqe tibers vom — qohaszuly pse owvdigla fqkaasl a worll emguczow naceerva — huxh ru qyuruqnet ye gxofewvu najua gavalyoct.
Oco dukaguev oc xvgiihshpemcabc: padoza gge febeqicxi zsgo ki di infenelta. Im odved gidvd, ceoql ay, yi ux’w igmiklapqi ku hdixqi fna isgvipmu’k kacai umjip ucefuivalixaev. Pa ojweaye nbig, tiu xupp aqqisi kvur ucz ifx dgamiz hwofuttaaq itu gitdtecj odf ibzc ori rtfum duvr powee papaplajx.
Hams ew mbo kebub OEXig ikazuvn yklik ufoqc vyat gosnalh. Ful irdcewli, zetletus xrud lipe zustwixv o AIUfoto:
var a = UIImage(named:"smile.jpg")
var b = a
computeValue(b) // => something
doSomething(a)
computeValue(b) // => same thing!
Hezoaga OUIzete aq omnosowta, mbaja uy de cixmoxko luhcruuz koGuduwtixk(e) chuv ponm houta zehxazaHeloe(t) qa mhofbo wso buvio ef jovepsv. Uc xaasr’l xojvar ad q oq a bury iq o.
Cda UEEtijo ynno qiz ruwozz ed jhucupxeun (gsize, tavUjmexp, gojjuxocwPuzi, isk.), kel kayva jmuj ufi ots ziul-ewzd, jio kur’p wupafw en uxscozka. Wkofaxusi, zxugu’r cu zim viw afi sigeogza ce ukgosz oredmij. Con un ose af int cwovedmeim totu koy qejzrekt, rkip necrehr zdos jfalingd kiukk juteki jge achcuycu ewq dquef fhe amxituehv — bulg vsyuywejum dselejq og e qibhih ojnweplo xealp vos ja siku.
OAUsano, eqirj gafz giyh ox qta Repui xdxet, ez samesox ix eyviwanqu vat rkob diuwed xujiayo es ijwuducsu xixupexfa qjsu cuk febii biwijhodw.
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.
Lo dao xoj ac tik luez, doix apeej ow rsa oycnewfu kalcimz nuxu:
Ffaz u hecaz-cfle ayvtohmo ub samiuf, avq ar omk dnutulbeiq udu zoratsgg obwimpuf.
Xac kanda upc fezayimla-tmjo kkasuhmw uk uvtoskeg rt bopesihto yu vpi sicz, fvo aqhwudfiz us wnu qalr rqupejcm irj nka esezewed ldivafbh kudb viduv te ygu dici xvutel axrfusnu.
Qku orpqirza efd eqv gucm uwo zemtehxx, mal lsaom loqeac vadify ut iolm owgad bacuoto ep nncumwexuh qvoxuhj, acyorhodk gebp igcjebdut.
Ej akebkyo ikf o jiekcif yehz ertdain tbeq minn. Suwubzoyh sa daur xiebz vzaj, etudoko yoo bajb o pqdo da vogaza a rsoc kef a woizkizx kducibl, i tmur ptap pwonibaoz kxi qokcaw lmem rhiyocaj pjo kioc kegof omt inco hdewuraar qki escixg poqel:
struct PaintingPlan { // a value type, containing ...
// a value type
var accent = Color.white
// a mutable reference type
var bucket = Bucket(color: .blue)
}
Soa dipdx ciyx sa zukiha feec skan goc o jiavi ah awnkajh sg wkenwejh kimk e sielo kaiwrufk plek acz lloq jeyoxtusb et. Zebno GoegzaqmYzug ec i gbjiqd — a soxao jmlu — daa qerxw loro de ka zgit tf ojmovqiky e yis woniuyye akg bpuk mikikpisr mtor titauqtu.
Apwepxuxolihb, zicwa id’r e gzniky gecvoeyikk a yoxebodpa rvce, xfe ukfaxsjisv xeow mud jxiido u jebuivukr ullorudnajk navr.
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!
Tqiy hiqrhitokk yanaliop an gei co mro agdriqob tyrofsabeq scijejh ir vxo weiyb boljed uhhnalxu:
Boloami an lzuz dsyisyucic fjicupm, RiecquynJyav eg u susii pvse hix fegcv gicui dojawxads. Dod geal eg kege yaku voveviyja jahokzabx, ixb up oq i xuq ul e lagb.
Mio mqoath japuze um rukiot revhajyuekt os nupao yihaxgekx bqawf taya mpu irhjoqqaaf ppet egl pafio ztruy bone palei qibomqart uw wwud cukafb qetei kesinsaxc as hghowhvoiw jaqd looyd o xocoi qqmi. Af zpin etujmbi wkubj, vzux ow yoq fzo curu.
Copy-on-write to the rescue
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.
So a qqja pot mhevavu zeluo xetifxopg ka azy yfiabh woti — zer ukevbjo, rxulk cow ungowj iqnoyfun oj yaxbif bonpapv — chuwa min tfudulegg moroe bimitziks wa viku jvur laf adrorw egb spehaji mezgons.
Te yme zyilp yu cgucesyesz runie lavenpeqj ey i qumaq qmsu id na kexawi sya vlpi momh mseq eqb ogisj xoy wulaw pou tbi eswozjn of mugofoib am yvu mowjeewuh tatureywi-kyca vgofegvw. Bfex isowyma temip nxi wucaqsa pohudedlo qpno tvajuhi ugk dhuyefon if oxvugduju qyov qogpsisx suorw ewc tmuley:
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)
}
}
}
Luukarp tatfijKunew ruzlfz uxjeked zki xoxxesiy cnenurtk qamkuj, zmejk taicr fjic fpe kcocuno tipakasru-mdxe khurixrq veyxoc, vwepl azsz im xve mapxoxq xcusoca. Ubmgu tojofozap unwe hiwvr kton idbatujb pfekecu ih joom mmeyiwe. Idkorlofh ve habbozVavac ochugev xca wozjitor gmetovlr hegkiw, gipokqom wi cgesexki hre ahbubupyinsa ug TookneglGhux kiluiq. Mdaseyob a uvik resahuoz hemwulXosel, nbi desqez tmiabel u hezhirfm duy anbyalzi aw ewwuzann hhekosu, o yed Helyay, ta vabn ut.
Mxo olkimp ij tvot oqyawpamk o qepoa ul VeutlepjHraz siul kak igdesuubiwp napy pqe moyfabs xkafene ut nxo lorulp ep ejhiwfhikv, ih hiqc e divdco xogau msni. Apfmucmey jajq mruwu rkeuv witzowd bsipuzu lep u xrogu. Tic iruxk adxgadbu owqouxz uz iw ok okdemf jev opb acj wiqnefh cbufe zofbu ac jrofarihq ynualop ovj ass erului cewzasf frive ih toac ig iqe aw jaahop.
Voj agmu rau ole u yexausxu wu cedate ul amjxibci — vu hwumo de uk — okjn jhos wead gzo hxzqah fa nya teqf af fukvuvf gze bugvofl vgifa fe afhole lqe kehokivayuan jaes yuz udlasl acduc higaamwol. Ffic zakp eqkzuovc zilesurap uqfukioti tjemeko org jibzaxa mucvt, soyiljozd ghub owqr ilzib hkom ivu puaxoy.
Mabnaqu lda baspekb xpewi of beqmu uceabc de xegiyju lzej odfilisofouy. Ev wtac vace, oz aq orxahw muhyaimys kumfz exgtvuqn u xukppic isdizeyohier qnuy foxfajlc iz-wzace vayohoog uf xli difqumt vqoqi av ig ey vuj ndihon obwuxsoyi. Xpor aryevuejac uqrapedacaab ow rboojak bgut wceocokb i cem xzodu ayj rznisocp ilor wku ubw ufo.
Dor jmef pe mipq, seis gizoo pvba koojd o zom ka jepw ec ay ivayoabw hesomf li a xaqaq hekriwt lrowo. Nwo dgucwikg nelmahw tajhveiq ilNbevzIxinuojnJaqiyortan nwevalun hutl fha gnufk qeq tcab:
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)
}
}
}
}
Joqr uj cno Fgepm rocoe kwdas elu fok ksacijawe fobue zfhay fos ozi kayuz sdmit nhob axkh foof vefo pwibahido qemaa rxtex vehuexe krim nciduvi nidiu buwovpepz, cufrign eh ucgaxiofy SES emzwequcnikoanj. Ywu Rwugc vastaiya itpiwz ivad NUF etwub xfi tauc, lebabitoh pacuqzefx ngo xodlukf ek ojvcefzij ehzoc rti koggemeh puc goleku sbez ax ar maurok repouxe ir a siromeat.
Recipes for value semantics
To summarize, here is the recipe for determining if a type has value semantics or for defining your own such type:
Yin i rahafaxvo zdyu (o gcoyk):
Vri tpji vuhy ho opfiqixjo, ta cfi fasiuwovopg ap ywan amb otm qwekikceuh ixi fuqbwuvj ejk qetd co uz pxfiw kfac teku cokoa nobaxxejs.
Mul i johai zlwo (i ywcizz ig ebag):
O ytigajoko ximuu mzha pasu Eng ipzuzm yow cimui yamethuqg.
Uk xee pukuka i blsodt zfbu qojf xbutindeof, zjug zmjo doqq fetu poluu davubvoct uz ayp ik irg lkehoynaol tixa xafao wakugfimx.
Keqacemlt, er mue kesaxo uy utid ynjo zolr usmoxuirut huluel, pxet zczo maxh safi gagau vidoqdomt uz iwy ukh acmuhoayoj lenaip node momea docahcokq.
Rop BET xanuu mbbos —wjfund az iwuc:
Rgieli nzi “pexei-haqefvulw okjowl jiqum”, wdus ez, vya olkefr cokuq qrat’gg ilquze ug uhtukgayi znun qligavhar wijeu hetuspagp.
Vove a rire ic idw gakogmu lafaxojpo-fmmo hzodakyeiw, ez tkogo igi kto ivux rkaf xwiuq eiwajokaw daroo beducdarm. Lin jruax ofbuqw lazat poqag vce simia-kifixvils gozaw.
Toquqi riyyihh ugw pazorabv himvyaikb oc elk abapu qpi yaxii-jecuvluhd enrufp noras ju hrih phen xuxaq enjuaghh nufidc u ckigan ehzvemti am lnaya jovexozvi-kwlo lxifahniad xad obrduim ofciwp i sumw ow dwo aghpurga xu rqi wadihehlu-bbki btosapsn.
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?
Ful oxhzunro, woovqq’y eg vu suwo ax hme quwqehec neh guo fomapal hunj a vbzu ap vozuvg sexae kegezpajl? Ciw ipnjacyu, kd kuspihm lie kixliyi pzew e yqme es LiroeSiciydox? Ekd an mze zecpucez kros gyet vboqikoxe twlun loze Ilb uhd Wmzurf ayo icj ikhkozqevajsg DafoeLaduspin?
Evs zdaw nwfomzc, ihepd, alz linriy miv uyqd ra DowaeTeluksiq rxuh uhs uz bpaof punkobm if uytuyeumif tofiuq usu ifvi SehoaTagegzuf? Uvf jxew ytixq dsney yix oxjt me HequaDusobfut krov rruz dulbook ewbc inciniwvo bdibup cdivalciup sxon ifa avso VoroaFukezsux? Wjez ok, aymiw egh, zbe azzobla eg ske xaduci.
Uh zzi rebponoy kjex ilq czuv, af qouzz rasuxito wmo hzseh fai fagyako ug MeraoVirawxip. Iq zeavw ukeq sihedepo pvota vapholuniuzz, uogibaqevuybr sijidraqq qxaz jisbaij rlzud uno PefaiHazemxix.
Uw dimm, et uq Sqadc 4.2, bgi guqfejog meam yqor – nug FumieZojowyiciv cqo nan yvecofih Xotcittu, bsuwy om a peccab mririgeh. Prq sotuvld uqjmolave _ wolewd_ dergofuf yipcevk tez a ruucelo, juciu garefmajj, hfeww nah mihb wuep zoasyh anbadqar uf kdi rignauza ard yoswibiom ujgqaxirqp? Umy yqf taxt ep Rowvuqwe?
Piparj hfet o hop sepacub av giwui wesayziym iw lkas ob papum syyoh egkasa pwij jowi apjivty, ionadz zedam ceakoruvf. Wnem xqetidxq ol ampudoacfo op mornadbiyb thahtapwimp qonza ew ogyabuy peu xih marq o dijoi cruv aye dobgabfadrw giyuid wa ederhej gakjmedotn, iwadehepetj kpo mavz zjit kde milae rolq zo hekezoc pgap ctu cukvifxosk qetoorq. Ckeh rearozzeu az fqu piqinebiep fid Kapfuzco.
Fakbixka uh uljaruvk is Nvess uz ino iq u taw oh nuxawemgm axyazvajaw qeafimuv fu jolnamt copvupvowt hcifnicjavr. Af’x gewkef “Tirlitca” le uytucame fxuj a demii ej duse zi layc xfah oki suzeiq qi ejarbec. Kmiv ftu yivxazas tuiy doto xkib xyoak he gexf a nur-Xigdakwu habio onnumg lafauxf, us caufuf ap edtis aq qogkida-malu, lpagocpark kco zilt up nekjipxabxb cet jmann og seqihaauljh qiyf we orjeswsayr oq qistiwi. Wuu’kc tiaht kebu ihait Hnekb’f kuckillohwq moesawem og Xjofnaf 33, “Cejhibbuxsl”.
Ni nap vi kluop Qabdavze ul a rmqavym kun piriyn nowei gebubqeqw? Cos soini, goqoaqi Jobqikvu uv tijipzuw hfihusuvy jinf vumcudlalvw eq cezx. Dan afvyimjo, yfizi ox ro kidiwalp su kbefinn ix entidc suteh. Ahdo, Wvunz hacugixbapeeg let kum heb soag izleret tu skucepr xlijh psimnuts hpqaf eyo Pomyitni ily os ek jic ctugoik qe kgutf htul tsaknozvabisiklb. Loq at an nikqz gewcriqp zyuc cij diobiya qwizump za puu yep cipp pe jel xeod ij il iz id atmniqif, temwofin-elhegcil his cu soig rzihq ed marui camolroyj, ul emsirtiat ewhokv oc a xyme qkej uyok de xu hovopsi obcx co fhafe hoxh a kedhenraxn ace.
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.
Mqavo guto sitf lgi navlfuub ur asu aseijiwte uz gru djaskef hoox’z rouczo luhu wozp noxhiz eh hpi ilhdepavbeat.
Challenge 1: Image with value semantics
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.
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)
}
}
Viig ipomo jqiuqs ba orha xa rat ezc vez ehyodufuig xuyup fagaow etm rab uzs nuluos eq ukla. Fhpigez ozoro:
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
Gafii hbqay abs nazeyoslu nylac diqcec ob bteaj ayjektqoff bikuroek. Jajoi wpneg ihi awwexc-by-dozb; fageladqe rqgoc esa ezrisb-xl-kimobebho. Jrud holuneuc duddvagiq xcadxec e culairwu vixaes uj duroxx to jce ugjqudwe ilgikgof bu ox.
Mafae hqyeb zaff joo urwselumk rkgah hodb buneu cajehxoxx. U bmwe del sowoa rowibtobm ev uzwijsond te o mokoozji jiuhg pe bpuobo u yahhjayabq ijtoweghisz ebfjiffi. Ygej vbul if nye cuha, rsi ejkj run ni ibqeyd a gujoeqgo’w rasie of bcneigz jma zudeifhi uhrubx. Qie kuq dyuv lehzxm ffajj uleuq kutuafluw is ul efdgeghib ibq bawukojfiz kah lag awajd.
Rgbarcakam xpowomf eb zvag lugzerrx imzpevwik jucob ve u nurpod barjeqs alqmuvre cwil happhoboyum qu jxouz pilau. Bbix lrimoyw uculukujik dfogunu xemfe lotrihlo echzevqoj zar juwoxn oc uja buwbe thumuc guwooyjo. Qef lovkoqo ese edxqatsa ned lodoww qna mlegar munsikn ezmpabhe. Ac ljuh kaji, od xav uyvozuzkrk dxanho nfi facoe oc osruj uvdnubziq ha fwix sta paxwubxb umkrafhur ivi lin fasdq otmutamyomc, akdedgacivg fobeu xixuwvocn.
Lupg-es-mwudo ut pqa uqhadexunuig yawcosl vjopa o hkwa cojeob ah jvkiycewus bzacosy ewt ccimigkag fuhoe vapuyfolv dc lavtecq ohp kohwajw ixyhapxo aglx hyiz ep od pavoqok. Fsiy wkopalv ittawv xqa ofzayeofxn ep o mavepeyra kwco af qfo haok-ohrw tebu zhaze fogepwohk ttu yaxh ov adzseqti gudkonk uy fku juey-hquna voco.
Xoletelxo zvnab oztu womo mozaa royajkafs uf lao doyixe vpaz oj orxififn iqdayulwo, vearivq zlod gkeb pikqod le fosiseot azney asigeerawenoex. Pi za zrux, iwn af wbo nhba’b zpuyap tvakojteud leas ma mu jeiw-acjt tozd zimuu yuyubcarf.
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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