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Swift Academy · Lesson

Mutating Methods and Copy-on-Write Semantics

Why mutating is required and how Swift's COW optimization works.

Mutating Methods and Copy-on-Write Semantics is a free Swift Academy lesson on CoddyKit — lesson 4 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the Swift Academy learning path, one of 4 lessons in the course, and your progress syncs across the web and the CoddyKit app.

Welcome

Struct methods that modify stored properties must be marked `mutating`. Swift also uses Copy-on-Write (COW) to make value semantics efficient.

mutating Keyword

```swift struct Counter { var count = 0 mutating func increment() { count += 1 } mutating func reset() { count = 0 } } var c = Counter() c.increment() // count = 1 ``` Without `mutating`, the compiler rejects methods that change `self`.

Why mutating is Needed

Structs are value types. When you pass a struct, you pass a *copy*. The compiler needs `mutating` to know a method will change the original value: ```swift let c = Counter() c.increment() // ❌ cannot use mutating on let ```

Reassigning self in mutating

A `mutating` method can replace `self` entirely: ```swift struct Toggle { var isOn: Bool mutating func flip() { self = Toggle(isOn: !isOn) } } ```

Value Semantics: Copy Behaviour

```swift var a = Counter() var b = a // b is an independent copy a.increment() print(a.count) // 1 print(b.count) // 0 — unaffected ``` Assigning a struct copies all stored properties.

Copy-on-Write (COW)

Swift's built-in collections (Array, Dictionary, Set, String) use COW: ```swift var a = [1,2,3] var b = a // no copy yet — shared storage b.append(4) // NOW a copy is made — a is unchanged print(a) // [1,2,3] print(b) // [1,2,3,4] ```

Why COW Matters

Without COW, every struct assignment would deep-copy all contents. COW means copies are cheap (just a reference count increment) until mutation triggers a real copy. This makes value types as fast as reference types in most scenarios.

Implementing Custom COW

```swift struct Buffer { private var storage: NSMutableData private mutating func ensureUnique() { if !isKnownUniquelyReferenced(&storage) { storage = storage.mutableCopy() as! NSMutableData } } mutating func append(_ byte: UInt8) { ensureUnique() // safe to mutate storage now } } ```

mutating and Protocols

Protocol methods that mutate must be marked `mutating` in the protocol: ```swift protocol Resettable { mutating func reset() } struct Counter: Resettable { var count = 0 mutating func reset() { count = 0 } } ``` Classes implementing the protocol don't need `mutating` (classes are reference types).

Let Structs are Fully Immutable

```swift let pt = Point(x: 1, y: 2) pt.x = 3 // ❌ even though x is var inside Point ``` When a struct is stored in a `let`, none of its stored properties can be mutated.

Quick Check

When does Swift's Copy-on-Write actually perform a copy?

Recap

Key takeaways: • `mutating` marks methods that change struct properties • Cannot call `mutating` methods on `let` structs • Assignment copies the struct; COW makes this cheap for collections • COW triggers a real copy only on first mutation • Implement custom COW with `isKnownUniquelyReferenced` Course complete! Next: class inheritance.

Frequently asked questions

Is the “Mutating Methods and Copy-on-Write Semantics” lesson free?

Yes — the full text of “Mutating Methods and Copy-on-Write Semantics” is free to read here on the web, and the Swift Academy course includes 4 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the Swift Academy course, upgrade to CoddyKit PRO.

What will I learn in “Mutating Methods and Copy-on-Write Semantics”?

Why mutating is required and how Swift's COW optimization works. You practise Swift Academy with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.

Do I need any experience to start Swift Academy?

No prior experience is required. Swift Academy on CoddyKit is structured for beginners through advanced learners; this is — lesson 4 of 4, so you can start here or from the beginning and move at your own pace.

How long does the “Mutating Methods and Copy-on-Write Semantics” lesson take?

Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.

Can I write and run code in this Swift Academy lesson?

Yes. Every Swift Academy lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.

All lessons in this course

  1. Memberwise Initializers and Custom Inits
  2. Stored Properties and Property Observers
  3. Computed Properties in Structs
  4. Mutating Methods and Copy-on-Write Semantics
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