void MainLoop() {
        while(true) {
            Mutex&      mutex = condvar_.mutex();
            CondVar&    cv = condvar_.condvar();
            Message*    msg;

            // Process all enqueued messages,
            // waiting for next msg if the queue is empty.
            {
                // Lock the queue
                MutexLock lock(mutex);
                // Get a message if there is one
                msg = queue_.Pop();
                // If there was no msg in the queue
                // wait till we get one.
                // condvar may get a spurious wake,
                // thus we should wait again if the queue is still empty.
                while(!msg) {
                    cv.Wait();
                    msg = queue_.Pop();
                }
                // Unlock the queue when MutexLock goes out of scope
            }

            // Handle the message (a slow activity)
            ProcessMessage(msg);
        }
    }