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// call_once example
#include <iostream> // std::cout
#include <thread> // std::thread, std::this_thread::sleep_for
#include <chrono> // std::chrono::milliseconds
#include <mutex> // std::call_once, std::once_flag
int winner;
void set_winner (int x) { winner = x; }
std::once_flag winner_flag;
void wait_1000ms (int id) {
// count to 1000, waiting 1ms between increments:
for (int i=0; i<1000; ++i)
std::this_thread::sleep_for(std::chrono::milliseconds(1));
// claim to be the winner (only the first such call is executed):
std::call_once (winner_flag,set_winner,id);
}
int main ()
{
std::thread threads[10];
// spawn 10 threads:
for (int i=0; i<10; ++i)
threads[i] = std::thread(wait_1000ms,i+1);
std::cout << "waiting for the first among 10 threads to count 1000 ms...\n";
for (auto& th : threads) th.join();
std::cout << "winner thread: " << winner << '\n';
return 0;
}
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