template<typename T>
void threadsafe_queue<T>::push(T new_value)
{
    std::shared_ptr<T> new_data(
        std::make_shared<T>(std::move(new_value)));
    std::unique_ptr<node> p(new node);
    {
        std::lock_guard<std::mutex> tail_lock(tail_mutex);
        tail->data=new_data;
        node* const new_tail=p.get();
        tail->next=std::move(p);
        tail=new_tail;
    }
    data_cond.notify_one();
}