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Showing content from https://cplusplus.com/reference/condition_variable/condition_variable/notify_all/ below:

public member function

<condition_variable>

std::condition_variable::notify_all
void notify_all() noexcept;

Notify all

Unblocks all threads currently waiting for this condition.

If no threads are waiting, the function does nothing.



Parameters none

Return value none

Example
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// condition_variable::notify_all
#include <iostream>           // std::cout
#include <thread>             // std::thread
#include <mutex>              // std::mutex, std::unique_lock
#include <condition_variable> // std::condition_variable

std::mutex mtx;
std::condition_variable cv;
bool ready = false;

void print_id (int id) {
  std::unique_lock<std::mutex> lck(mtx);
  while (!ready) cv.wait(lck);
  // ...
  std::cout << "thread " << id << '\n';
}

void go() {
  std::unique_lock<std::mutex> lck(mtx);
  ready = true;
  cv.notify_all();
}

int main ()
{
  std::thread threads[10];
  // spawn 10 threads:
  for (int i=0; i<10; ++i)
    threads[i] = std::thread(print_id,i);

  std::cout << "10 threads ready to race...\n";
  go();                       // go!

  for (auto& th : threads) th.join();

  return 0;
}

Possible output (thread order may vary):
10 threads ready to race...
thread 6
thread 2
thread 5
thread 3
thread 4
thread 1
thread 7
thread 0
thread 9
thread 8


Data races No data races (atomic operation).
Atomic operations on the object are ordered according to a single total order.

Exception safetyNo-throw guarantee: never throws exceptions.

See also
condition_variable::notify_one
Notify one (public member function)
condition_variable::wait
Wait until notified (public member function)

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