Noncopyable types (~Copyable)

Swift language Senior Swift 5.9+ iOS 17+ Reviewed 31 Oct 2026 By Alok Singh

Pick the fix

A file wrapper must never be copied, so the descriptor can't be closed twice. Which design?

Optionsstruct File: ~Copyable with deinit { close(fd) } and consuming func close()A final class with deinitA struct with an isClosed flagAn enum with an open and closed case

Hint 1 of 1

Which option makes copying impossible?

Choose an answer, then open Show answer below to check it.

Show answer

Answer: A struct File: ~Copyable with deinit { close(fd) } and consuming func close()

Correct. One owner, and deinit runs exactly once.

Why not the others?

B A final class with deinit. It works, but references can be shared freely, so ownership is unclear.
C A struct with an isClosed flag. Each copy has its own flag, so two copies can both close.
D An enum with an open and closed case. Copies are still allowed and can disagree.

30-second answer

A type marked ~Copyable can't be copied, only moved or borrowed. That gives unique ownership: exactly one owner, and a deinit that runs once. It fits resources like file descriptors, locks and tokens that must not be duplicated. Swift 6 extended generics and Optional to support noncopyable types.

Practise saying it · 30 s. No recording, just a timer

Explain ~Copyable.

Say your answer out loud, then check which of these points you covered:

Can't be copied
Unique ownership and deinit
borrowing / consuming

Deep dive

Parameters are borrowing (read-only) or consuming (takes ownership).

A consuming func close() ends the value's life, so it can't be used afterwards.

Noncopyable structs can have deinit, unlike normal structs.

Mutex and Atomic in Synchronization are noncopyable.

Common mistakes

Using a class when unique ownership is the point.
Expecting noncopyable types to work in every generic API.

Senior take & follow-ups

Seniors connect ~Copyable to resource safety and mention borrowing and consuming.

Follow-up questions

Think of your answer first, then open the hint.

What does consuming mean?
Show hint
The function takes ownership; the caller can't use the value afterwards.
Why is Mutex noncopyable?
Show hint
Copying a lock would split its state.

Suggest a fix if something here is wrong or out of date.