std::condition_variable::notify_one

#include <iostream>
#include <condition_variable>
#include <thread>
#include <chrono>
 
std::condition_variable cv;
std::mutex cv_m;
int i = 0;
bool done = false;
 
void waits()
{
    std::unique_lock<std::mutex> lk(cv_m);
    std::cout << "Waiting... \n";
    cv.wait(lk, [](){return i == 1;});
    std::cout << "...finished waiting. i == 1\n";
    done = true;
}
 
void signals()
{
    std::this_thread::sleep_for(std::chrono::seconds(1));
    std::cout << "Notifying...\n";
    cv.notify_one();
 
    std::unique_lock<std::mutex> lk(cv_m);
    i = 1;
    while (!done) {
        lk.unlock();
        std::this_thread::sleep_for(std::chrono::seconds(1));
        lk.lock();
        std::cerr << "Notifying again...\n";
        cv.notify_one();
    }
}
 
int main()
{
    std::thread t1(waits), t2(signals);
    t1.join(); t2.join();
}