template<typename T>
class threadsafe_queue
{
private:
    std::unique_ptr<node> try_pop_head()
    {
        std::lock_guard<std::mutex> head_lock(head_mutex);
        if(head.get()==get_tail())
        {
            return std::unique_ptr<node>();
        }
        return pop_head();
    }

    std::unique_ptr<node> try_pop_head(T& value)
    {
        std::lock_guard<std::mutex> head_lock(head_mutex);
        if(head.get()==get_tail())
        {
            return std::unique_ptr<node>();
        }
        value=std::move(*head->data);
        return pop_head();
    }

public:
    std::shared_ptr<T> try_pop()
    {
        std::unique_ptr<node> const old_head=try_pop_head();
        return old_head?old_head->data:std::shared_ptr<T>();
    }

    bool try_pop(T& value)
    {
        std::unique_ptr<node> const old_head=try_pop_head(value);
        return old_head;
    }

    void empty()
    {
        std::lock_guard<std::mutex> head_lock(head_mutex);
        return (head==get_tail());
    }
};