Swift Academy · Lesson

Mixed Value/Reference Graphs

Structs containing classes: the hidden reference semantics and how to handle them.

Lesson 3 of 412 steps

Mixed Value/Reference Graphs is a free Swift Academy lesson on CoddyKit. This is lesson 3 of 4. Any 3 lessons of this learning path are free to read in full — after that, CoddyKit PRO unlocks every lesson, plus hands-on practice with a built-in code editor and a 24/7 AI tutor. It is part of the Swift Academy learning path, and your progress syncs across the web and the CoddyKit app. The Swift Academy course includes 4 lessons in total.

Welcome

Structs can contain classes as stored properties. When they do, the struct behaves as a value type on the outside but shares the class reference internally — a subtle trap.

Struct Containing a Class

```swift class Storage { var data: [Int] = [] } struct Container { var storage = Storage() var label: String = "" } var a = Container() var b = a // copies label, but SHARES storage! a.storage.data.append(1) print(b.storage.data) // [1] — shared! ```

Only Value Properties Are Copied

When a struct is copied: • Value-type properties (`label: String`) are fully copied • Reference-type properties (`storage: Storage`) — only the pointer is copied This is called a *shallow copy*.

Making a Deep Copy

```swift struct Container { var storage: Storage func deepCopy() -> Container { let newStorage = Storage() newStorage.data = storage.data // copy values return Container(storage: newStorage, label: label) } } ```

COW to the Rescue

Swift's own collections avoid this by using COW: ```swift struct SafeContainer { private(set) var storage = Storage() mutating func append(_ n: Int) { if !isKnownUniquelyReferenced(&storage) { storage = storage.copy() // copy before mutating } storage.data.append(n) } } ```

Classes Containing Structs

```swift class ViewModel { var point = CGPoint(x: 0, y: 0) // value type inside reference type } let vm1 = ViewModel() let vm2 = vm1 // same ViewModel! vm1.point.x = 5 print(vm2.point.x) // 5 — shared because ViewModel is a class ```

Identity vs Equality

```swift class Dog { var name: String = "" } let x = Dog() let y = x print(x === y) // true — same object print(x !== y) // false let z = Dog() print(x === z) // false — different objects ```

Recursive Struct via Class

Structs can't reference themselves, but can contain a class that references the struct: ```swift struct Tree { var value: Int var children: [Tree] // OK — Array uses class internally } ```

Detecting Unexpected Sharing

Use `isKnownUniquelyReferenced` at runtime to detect sharing: ```swift var s = Storage() var r: AnyObject = s print(isKnownUniquelyReferenced(&s)) // true var s2 = s print(isKnownUniquelyReferenced(&s)) // false — shared ```

Design Guidelines

When a struct must hold mutable reference-type state: 1. Implement COW to restore value semantics 2. Or use an immutable class: `let` vs `var` for the class property 3. Or redesign — maybe the struct should be a class Document clearly when sharing is intentional.

Quick Check

When you copy a struct containing a class property, what happens to the class?

Answer it in the interactive lesson

Recap

Key takeaways: • Struct copy is shallow — class references are shared • `isKnownUniquelyReferenced` detects sharing at runtime • Implement COW to restore true value semantics • `===` / `!==` test object identity for classes • Recursive structures use Array (class-backed) as children Next: designing APIs for value semantics.

Free to start

Learn Swift with an AI tutor — free

Write and run real code in your browser, get instant help from a 24/7 AI tutor, and pick up where you left off on the web or in the app.

Courses: 122
Lessons: 409

Start learning free View the full course

Frequently Asked Questions

Is the “Mixed Value/Reference Graphs” lesson free?

Yes — any 3 lessons of the Swift Academy learning path, including “Mixed Value/Reference Graphs”, are free to read in full here on the web. After that, CoddyKit PRO unlocks every lesson, plus interactive practice with a built-in code editor and a 24/7 AI tutor. The Swift Academy course includes 4 lessons in total.

What will I learn in “Mixed Value/Reference Graphs”?

Structs containing classes: the hidden reference semantics and how to handle them. 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, so you can start here or from the beginning and move at your own pace. This is lesson 3 of 4.

How long does the “Mixed Value/Reference Graphs” 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

Stack vs Heap: Where Values Live
Mutation, Sharing and Unexpected Aliasing
Mixed Value/Reference Graphs
Designing APIs for Value Semantics

← Back to Swift Academy