#include <cassert>
#include <condition_variable>
#include <iostream>
#include <mutex>
#include <queue>
#include <thread>

int main() {
    std::mutex mutex;
    std::condition_variable changed;
    std::queue<int> queue;
    bool closed = false;
    std::thread producer([&] {
        for (int value = 1; value <= 3; ++value) {
            {
                std::lock_guard<std::mutex> lock(mutex);
                queue.push(value);
            }
            changed.notify_one();
        }
        {
            std::lock_guard<std::mutex> lock(mutex);
            closed = true;
        }
        changed.notify_all();
    });

    int sum = 0;
    for (;;) {
        std::unique_lock<std::mutex> lock(mutex);
        changed.wait(lock, [&] { return closed || !queue.empty(); });
        if (queue.empty()) break;
        const int value = queue.front();
        queue.pop();
        lock.unlock();
        sum += value;
    }
    producer.join();
    assert(sum == 6);
    assert(queue.empty() && closed);
    std::cout << sum << '\n';
}