C++: What is std::atomic and when should you use it?

1#include <atomic>
2#include <thread>
3
4std::atomic<int> counter{0};
5
6void increment() {
7    for (int i = 0; i < 1000; i++) {
8        counter.fetch_add(1, std::memory_order_relaxed);
9    }
10}
11
12int main() {
13    std::thread t1(increment);
14    std::thread t2(increment);
15    t1.join(); t2.join();
16    std::cout << counter;  // 2000
17}
18
19// Memory ordering
20std::atomic<int> flag{0};
21
22// Producer
23flag.store(1, std::memory_order_release);
24
25// Consumer
26if (flag.load(std::memory_order_acquire) == 1) {
27    // Data guaranteed visible
28}
29
30// CAS loop
31int expected = 0;
32flag.compare_exchange_strong(expected, 1);