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Showing content from https://cplusplus.com/reference/mutex/call_once/ below:

public member function

<mutex>

template <class Fn, class... Args>  void call_once (once_flag& flag, Fn&& fn, Args&&... args);

Call function once

If another thread is already actively executing a call to call_once with the same flag, it causes a passive execution: Passive executions do not call fn but do not return until the active execution itself has returned, and all visible side effects are synchronized at that point among all concurrent calls to this function with the same flag.

If an active call to call_once ends by throwing an exception (which is propagated to its calling thread) and passive executions exist, one is selected among these passive executions, and called to be the new active call instead.

Note that once an active execution has returned, all current passive executions and future calls to call_once (with the same flag) also return without becoming active executions.

The active execution uses decay copies of the lvalue or rvalue references of fn and args, ignoring the value returned by fn.

flag

Object used by the function to track the state of invocations.
Using the same object for calls in different threads, results in a single call if called concurrently.

If flag has a state that is not valid, the call causes undefined behavior.

call_once is a specific type defined in header <mutex> to be used as argument to this function.

fn

A pointer to function, pointer to member, or any kind of move-constructible function object (i.e., an object whose class defines operator(), including closures and function objects).
The return value (if any) is ignored.

args...

Arguments passed to the call to fn. Their types shall be move-constructible.
If fn is a member pointer, the first argument shall be an object for which that member is defined (or a reference, or a pointer to it).

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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;
}

waiting for the first among 10 threads to count 1000 ms...
winner thread: 2

once_flag

Flag argument type for call_once (class)

thread

Thread (class)

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