void swap(queue_type &q) {
m_mutex.lock();
q.swap(m_queue);
if (m_queue.empty() && sleeping_on_empty) {
m_empty_conditional.signal();
}
m_mutex.unlock();
}
void swap(queue_type &q) {
m_mutex.lock();
q.swap(m_queue);
m_mutex.unlock();
}