#include <iostream>
#include <thread>
#include <mutex>

static std::once_flag g_once_flag;

void do_once() {
    std::call_once(g_once_flag, [](){ std::cout << "Do once: called once\n"; });
}

int main() {
    std::thread st1(do_once);
    std::thread st2(do_once);
    std::thread st3(do_once);
    std::thread st4(do_once);
    st1.join();
    st2.join();
    st3.join();
    st4.join();
}