Comment by _a_a_a_

>> If you write to cache, then depending on architecture that change has to be made visible to every other thread as well

> Only for atomics

I don't think cache coherency is aware of threads; they live at a level above it (IANAExpert though)

> where you use an atomic shared ptr to share across threads and then you downgrade it to a non atomic version in-thread (and can hand out another atomic copy at any time).

your idea of GC is very different from others', you are happy to do a ton of manual stuff. GC is generally about not doing a ton of manual stuff.