std::atomic_ref::notify_all

#include <iostream>
#include <atomic>
#include <thread>

int main()
{
  bool x = false;

  auto f = [&x] {
    std::atomic_ref r{x};
    while (true) {
      r.wait(false);
      if (r.load() == true) {
        break;
      }
    }
  };

  std::thread t1 {f};
  std::thread t2 {f};

  std::atomic_ref{x}.store(true);
  std::atomic_ref{x}.notify_all();

  t1.join();
  t2.join();
}