{

for (int cnt = 0; cnt < 40; ++cnt) {

while (lock.test_and_set(std::memory_order_acquire)) { // acquire lock

// Since C++20, it is possible to update atomic_flag's

// value only when there is a chance to acquire the lock.

// See also: https://stackoverflow.com/questions/62318642

#if defined(__cpp_lib_atomic_flag_test)

while (lock.test(std::memory_order_relaxed)) // test lock

#endif

; // spin

}

static int out{};

std::cout << n << ((++out % 40) == 0 ? '\n' : ' ');

lock.clear(std::memory_order_release); // release lock

}

{

std::vector<std::thread> v;

for (int n = 0; n < 10; ++n) {

v.emplace_back(f, n);

}

for (auto& t : v) {

t.join();

}