[banachtarski]

It isn't idiomatic. If you've already locked the shared_ptr, you should extract it's boxed value and pass that by reference directly. Passing a shared_ptr by reference has fairly niche applications and allows the callee to do things like reset the shared_ptr, extract a weak_ptr from it, etc.

That said, if you are in need of the latter use case, it certainly should be passed by reference. It's not just a reference count! Shared pointers in C++ are threadsafe so there's a fair bit more going on under the hood that makes copying it (more) expensive.