template <typename T>
class threadsafe_queue {
private:
  node* get_tail()
  {
    std::lock_guard<std::mutex> tail_lock(tail_mutex);
    return tail;
  }

  std::unique_ptr<node> pop_head()
  {
    std::unique_ptr<node> const old_head = std::move(head);
    head                                 = std::move(old_head->next);
    return old_head;
  }

  std::unique_lock<std::mutex> wait_for_data()
  {
    std::unique_lock<std::mutex> head_lock(head_mutex);
    data_cond.wait(head_lock, [&] { return head != get_tail(); });
    return std::move(head_lock);
  }

  std::unique_ptr<node> wait_pop_head()
  {
    std::unique_lock<std::mutex> head_lock(wait_for_data());
    return pop_head();
  }

  std::unique_ptr<node> wait_pop_head(T& value)
  {
    std::unique_lock<std::mutex> head_lock(wait_for_data());
    value = std::move(*head->data);
    return pop_head();
  }

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

  void wait_and_pop(T& value)
  {
    std::unique_ptr<node> const old_head = wait_pop_head(value);
  }
};