> when you do that analysis your answer is going to be acquire-release and only for some edge cases, in many places the relaxed atomic ordering is fine.

Why would shared_ptr refcounting need anything other than relaxed? Acq/rel are for implementing multi-variable atomic protocols, and shared_ptr refcounting simply doesn't have other variables.

It's because you're not solely managing the refcount here. Other memory locations have a dependence on the refcount, given that you're also deleting the object after the refcount reaches zero. That means you need all writes to have completed at that point, and all reads to observe that. Otherwise you might destroy an object while it's in an invalid state, or you might release the memory while another thread is accessing it.

It's extremely difficult to see in real C++ standard library source because of the layers of obfuscating compiler workaround hacks, but eventually they are in fact using acquire-release ordering, but only for decrementing the reference count. Does that help you figure out why we want acquire-release, or do you need more help ?

You need it to avoid a use after free.