Can someone explain the real-world difference between memory_order_relaxed and the default memory_order_seq_cst in C++ atomics? When is relaxed safe to use?

memory_order_seq_cst enforces a globally consistent execution order across all CPU cores with hardware memory barriers. memory_order_relaxed guarantees atomicity of the single variable itself, but permits the CPU and compiler to reorder surrounding reads and writes! Use relaxed for independent counters or flags where no other memory access depends on the order.

If you are implementing synchronization where one thread publishes data and another reads it, you need memory_order_release on write and memory_order_acquire on read!