What is the difference between struct, class, and actor?
Predict first
Does this compile?
final class Player { var name = "A" }
let p = Player()
p.name = "B"- Yes
- No — p is a let
Mental model. Struct = a photocopy. Class = a shared document. Actor = a shared document with a doorman who lets one person in at a time.
Struct — value semantics, no inheritance, members must be mutating to change self, no deinit.
Class — reference semantics, inheritance, deinit, identity via ===, participates in Objective-C runtime.
Actor — a reference type whose mutable state is isolated. Calls from outside must await and hop to the actor's executor, so only one task mutates its state at a time.
All three can conform to protocols. Actors do not support inheritance, and they are reference types — a fact people forget because they feel value-like at the call site.
For a class, let freezes the reference, not the object. With a struct, the same code would be an error.
| struct | class | actor | |
|---|---|---|---|
| Semantics | value | reference | reference |
| Inheritance | no | yes | no |
| deinit | no | yes | yes |
| Identity (===) | no | yes | yes |
| Mutating self | needs mutating | free | inside the actor only |
| Outside calls | sync | sync | await (isolated) |
| Data-race safety | by copying | your job | by isolation |
Common traps
- “Actors are value types.” No — they are reference types with isolated state. Passing an actor around shares the same instance.
- “Actors eliminate all data races.” They protect their own isolated state. Data leaked out of an actor, or captured by reference, is still racy.
They might follow up with
- Does an actor have
deinit? Yes — actors are reference types and may have adeinit, though it is nonisolated. - Can an actor conform to a protocol with synchronous requirements? Only if the requirement is
nonisolated, or the conformance is isolated to the actor.
Related questions
- 1.1.1 What is the difference between a value type and a reference type in Swift? When do you choose each?
- 1.1.4 When would you still use a class in a Swift-first codebase?
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