std::queue<int> produced_nums;

std::mutex mtx;

std::condition_variable cv;

bool notified = false; // notification sign

auto producer = [&]() {

for (int i = 0; ; i++) {

std::this_thread::sleep_for(std::chrono::milliseconds(500));

std::unique_lock<std::mutex> lock(mtx);

std::cout << "producing " << i << std::endl;

produced_nums.push(i);

notified = true;

cv.notify_all();

}

};

auto consumer = [&]() {

while (true) {

std::unique_lock<std::mutex> lock(mtx);

while (!notified) { // avoid spurious wakeup

cv.wait(lock);

}

// temporal unlock to allow producer produces more rather than

// let consumer hold the lock until its consumed.

lock.unlock();

std::this_thread::sleep_for(std::chrono::milliseconds(1000)); // consumer is slower

lock.lock();

if (!produced_nums.empty()) {

std::cout << "consuming " << produced_nums.front() << std::endl;

produced_nums.pop();

}

notified = false;

}

};

std::thread p(producer);

std::thread cs[2];

for (int i = 0; i < 2; ++i) {

cs[i] = std::thread(consumer);

}

p.join();

for (int i = 0; i < 2; ++i) {

cs[i].join();

}

return 0;