I believe the source isn't easy to follow on this, there are multiple macros depending on 

what platform and config macro you use.

Also, I believe the atomic "lockfree" counting happen in the shared_count type, not directly into the shared_ptr class.

Nevertheless, all documentation I found through years suggest that it is mandatory that shared_ptr counting is thread-safe

because it makes sharing between threads dangerous otherwise.

Unfortunately I don't have the standard available to check if std::shared_ptr is HAVE TO be implemented this way.

Joel Lamotte