std::atomic_thread_fence

const int num_mailboxes = 32;
std::atomic<int> mailbox[num_mailboxes];

// The writer threads update non-atomic shared data and then update mailbox[i] as follows
 std::atomic_store_explicit(&mailbox[i], std::memory_order_release);

// Reader thread needs to check all mailbox[i], but only needs to sync with one
 for (int i = 0; i < num_mailboxes; ++i) {
    if (std::atomic_load_explicit(&mailbox[i],  std::memory_order_relaxed) == my_id) {
        std::atomic_thread_fence(std::memory_order_acquire); // synchronize with just one writer
        do_work(i); // guaranteed to observe everything done in the writer thread before
                    // the atomic_store_explicit()
    }
 }