template<typename T>
class msg_queue
{
public:
    // preventing implicit generation of move constructor
    ~msg_queue() {}

    void post(T&& msg)
    {
        std::lock_guard<std::mutex> lock(mtx);
        queue.push_front(std::move(msg));
        cond.notify_one();
    }

    bool wait_for(T& msg, std::chrono::milliseconds time)
    {
        std::unique_lock<std::mutex> lock(mtx);
        bool b = cond.wait_for(lock, time, [this]{return !queue.empty(); });
        if (b)
        {
            msg = std::move(queue.back());
            queue.pop_back();
            return true;
        }
        else
        {
            return false;
        }
    }

    unsigned int size()
    {
        std::lock_guard<std::mutex> lock(mtx);
        return queue.size();
    }

    void clear()
    {
        std::lock_guard<std::mutex> lock(mtx);
        queue.clear();
    }

private:
    std::deque<T> queue;
    std::condition_variable cond;
    std::mutex mtx;
};