How can memory_order_relaxed work for incrementing atomic reference counts in smart pointers?

Question

Consider the following code snippet taken from Herb Sutter\'s talk on atomics:

The smart_ptr class contains a pimpl object called control_block_ptr containing the refere

Answer 1

From C++ reference on std::memory_order:

memory_order_relaxed: Relaxed operation: there are no synchronization or ordering constraints imposed on other reads or writes, only this operation's atomicity is guaranteed

There is also an example below on that page.

So basically, std::atomic::fetch_add() is still atomic, even when with std::memory_order_relaxed, therefore concurrent refs.fetch_add(1, std::memory_order_relaxed) from 2 different threads will always increment refs by 2. The point of the memory order is how other non-atomic or std::memory_order_relaxed atomic operations can be reordered around the current atomic operation with memory order specified.