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());
  }
};