Episode 230 Stack vs Heap - Thread safety?

[grennis]

I was wondering if the Wrapper class is thread safe. It seems like the check for isKnownUniquelyReferenced and the code that acts on the result should be locked because another thread could create another reference in between those statements. This could affect the performance test.

I'm not sure this is actually an issue though?

[stephencelis]

@grennis There's a good thread on the thread safety of isKnownUniquelyReferenced here, particularly John McCall's reply: https://forums.swift.org/t/isknownuniquelyreferenced-thread-safety/22933/5

In short, we don't need to manually lock here because the inout nature of the input has a precondition against allowing concurrent writes.