Which value types in Swift supports copy-on-write?

Question

I read about copy-on-write implementation for Array in Swift here.

Arrays, like all variable-size collections in the standard library, use copy-on-write optimization. Multiple copies of an array share the same storage until you modify one of the copies. When that happens, the array being modified replaces its storage with a uniquely owned copy of itself, which is then modified in place. Optimizations are sometimes applied that can reduce the amount of copying.

I was wondering if you have any information about which structure supports copy-on-write.

Heartless

Answer 1

Copy-on write is supported for String and all collection types - Array, Dictionary and Set.

Besides that, compiler is free to optimize any struct access and effectively give you copy-on-write semantics, but it is not guaranteed.

Schilling

"not guaranteed" is one way to put it. If you want copy-on-write semantics, you use isKnownUniquelyReferenced(&ref) explicitly. (also you "must only call this function from mutating methods with appropriate thread synchronization"; note that text does not point to how the standard unsynchronized collections manage to avoid violating this. Nor does this official doc) marcosantadev.com/copy-write-swift-value-types – Spidery

I don´t think its a compiler feature. Nowhere says that. – Putdown