Swiftのstructとイミュータビリティ

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Swi$ͷ struct ͱ ΠϛϡʔλϏϦςΟ @koher

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ࣗݾ঺հ • @koher • ΤϯδχΞʢQonceptʣ • ΧϯϑΝϨϯεొஃ • try! Swi0 Tokyo: 2016 • iOSDC Japan: 2017, 2018, 2019, 2020, 2021, 2022, 2024 • Swi0 Zoomin' ओ࠵ • Heart of Swi0 ࣥච

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࣭໰ struct Foo { var value: Int } ͜ͷ struct ͸ϛϡʔλϒϧͰ͔͢ʁΠϛϡʔλϒϧͰ͔͢ʁ

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struct ͸ϛϡʔλϒϧͰ΋ ຊ࣭తʹ͸ΠϛϡʔλϒϧΫϥεͱಉ͡

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var ϓϩύςΟΛ࣋ͭ struct struct Foo { var value: Int }

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var ϓϩύςΟΛ࣋ͭ struct struct Foo { var value: Int } extension Foo { mutating func increment() { value += 1 } }

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var ϓϩύςΟΛ࣋ͭ struct var foo: Foo = .init(value: 0) foo.increment() print(foo.value) // 1

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let ϓϩύςΟΛ࣋ͭ struct struct Foo { let value: Int }

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let ϓϩύςΟΛ࣋ͭ struct struct Foo { let value: Int } extension Foo { mutating func increment() { value += 1 // ⛔ ίϯύΠϧΤϥʔ } }

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let ϓϩύςΟΛ࣋ͭ struct struct Foo { let value: Int } extension Foo { mutating func increment() { self = Foo(value: value + 1) // ͜ΕͳΒOK } }

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ϝιουͱ͸ // ϝιουͷ৔߹ extension Foo { // ҉໧ͷୈ1Ҿ਺ self Λ࣋ͭ func bar() -> Int { self.value * self.value } }

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ϝιουͱ͸ // ϝιουͷ৔߹ extension Foo { // ҉໧ͷୈ1Ҿ਺ self Λ࣋ͭ func bar() -> Int { self.value * self.value } } let foo: Foo = .init(value: 3) print(foo.bar()) // 9

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ϝιουͱ͸ // ؔ਺ͷ৔߹ // ໌ࣔతୈ1Ҿ਺ self Λ࣋ͭ func bar(_ self: Foo) -> Int { self.value * self.value } let foo: Foo = .init(value: 3) print(bar(foo)) // 9

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mutating ͱ͸ struct Foo { var value: Int } extension Foo { mutating func increment() { self.value += 1 // ✅ } }

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mutating ͱ͸ struct Foo { var value: Int } extension Foo { func increment() { self.value += 1 // ⛔ } }

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mutating ͱ͸ struct Foo { var value: Int } func increment(_ self: Foo) { self.value += 1 // ⛔ }

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mutating ͱ͸ struct Foo { var value: Int } func increment(_ self: inout Foo) { self.value += 1 // ✅ }

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mutating ͱ͸ struct Foo { var value: Int } extension Foo { mutating func increment() { self.value += 1 // ✅ } }

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let ϓϩύςΟΛ࣋ͭ struct struct Foo { let value: Int } extension Foo { mutating func increment() { self = Foo(value: self.value + 1) // } }

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let ϓϩύςΟΛ࣋ͭ struct struct Foo { let value: Int } func increment(_ self: inout Foo) { self = Foo(value: self.value + 1) // }

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let ϓϩύςΟΛ࣋ͭ struct struct Foo { let value: Int } extension Foo { mutating func increment() { self = Foo(value: self.value + 1) // } }

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let ϓϩύςΟΛ࣋ͭ struct var foo: Foo = .init(value: 0) foo.increment() // ✅ Foo.value ͕ let Ͱ΋ OK print(foo.value) // 1

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ϓϩύςΟͷએݴ͕ var ͔ let ͔͸ struct ͷϛϡʔλϏϦςΟΛܾΊͳ͍

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ม਺ʢఆ਺ʣએݴͷ var / let ͕ struct ͷϛϡʔλϏϦςΟΛܾΊΔ

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ม਺ʢఆ਺ʣએݴͱϛϡʔλϏϦςΟ var foo: Foo = .init(value: 0) foo.increment() // ✅ print(foo.value)

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ม਺ʢఆ਺ʣએݴͱϛϡʔλϏϦςΟ let foo: Foo = .init(value: 0) foo.increment() // ⛔ print(foo.value)

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஋ܕͷϛϡʔλϏϦςΟΛ ࢀরܕͱಉ͡Α͏ʹߟ͑ͯ͸͍͚ͳ͍

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஋ܕͷΠϯελϯε͸ ม਺ͷͨΊʹ֬อ͞ΕͨϝϞϦྖҬͱҰମ

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ΠϛϡʔλϒϧΫϥε final class Foo { let value: Int }

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ΠϛϡʔλϒϧΫϥε final class Foo { let value: Int } extension Foo { // ⛔ ࢒೦ͳ͕Β͜Ε͸Swiftͷߏจͷ໰୊ͰͰ͖ͳ͍ mutating func increment() { self = Foo(value: self.value + 1) } }

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ΠϛϡʔλϒϧΫϥε final class Foo { let value: Int } // ✅ ͜ΕͳΒOK func increment(_ self: inout Foo) { self = Foo(value: self.value + 1) }

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ΠϛϡʔλϒϧΫϥε var foo: Foo = .init(value: 0) increment(&foo) // ✅ ͜ΕͳΒ foo ΛมߋՄ print(foo.value) // 1

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struct ͱΠϛϡʔλϒϧΫϥε͸ ຊ࣭తʹಉ͜͡ͱ͕Ͱ͖Δ

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ΠϛϡʔλϒϧΫϥε͸Πϯελϯεͷ ϝϞϦྖҬΛมߋͰ͖ͳ͍

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ࢀরܕͰ͸ͳ͘஋ܕͷ ʢม਺ଆͷʣϛϡʔλϏϦςΟͰߟ͑Ε͹ ΠϛϡʔλϒϧΫϥε͸ struct ͱಉ͡

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ϛϡʔλϒϧΫϥε final class Foo { var value: Int }

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ϛϡʔλϒϧΫϥε final class Foo { var value: Int } extension Foo { func increment() { value += 1 } }

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ϛϡʔλϒϧΫϥε let foo: Foo = .init(value: 0) foo.increment() print(foo.value) // 1

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ϛϡʔλϒϧΫϥε let foo: Foo = .init(value: 0) foo.increment() print(foo.value) // 1

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ϛϡʔλϒϧΫϥε let foo: Foo = .init(value: 0) let foo2 = foo foo.increment() print(foo.value) // 1

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ϛϡʔλϒϧΫϥε let foo: Foo = .init(value: 0) let foo2 = foo foo.increment() print(foo.value) // 1 print(foo2.value) // 1

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struct var foo: Foo = .init(value: 0) var foo2 = foo foo.increment() print(foo.value) // 1 print(foo2.value) // 0

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ΠϛϡʔλϒϧΫϥε var foo: Foo = .init(value: 0) var foo2 = foo increment(&foo) print(foo.value) // 1 print(foo2.value) // 0

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struct ͱΠϛϡʔλϒϧΫϥε͸ shared mutable stateΛ࣋ͨͳ͍

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Sendable ४ڌ final class User: Sendable { // var name: String } final class User: Sendable { // let name: String } struct User: Sendable { // ✅ var name: String }

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struct Λ࢖͑͹ϛϡʔλϒϧͰ΋ ΠϛϡʔλϒϧΫϥεಉ༷ͷརӹΛڗडՄೳ

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·ͱΊ • struct ͸ຊ࣭తʹΠϛϡʔλϒϧΫϥεͱಉ͡ • struct Λ࢖͑͹ϛϡʔλϒϧͰ΋ΠϛϡʔλϒϧΫϥεͱಉ ͡རӹΛڗडͰ͖Δ