[JonChesterfield]

Deep copy on passing between threads. Interesting pros and cons to the performance of that vs sharing pointers.

[mgaunard]

There is no need to copy objects for multiple threads to refer to them.

It's just that there is a single owner, and the other threads have a non-owning view only.

You need strict control on the thread lifetime, which is enforced through the same single ownership system, in order to enforce this.