template <typename T>
class lock_free_queue {
private:
  struct node {
    std::atomic<T*> data;
    std::atomic<node_counter> count;
    std::atomic<counted_node_ptr> next;
  };

public:
  std::unique_ptr<T> pop()
  {
    counted_node_ptr old_head = head.load(std::memory_order_relaxed);
    for (;;) {
      increase_external_count(head, old_head);
      node* const ptr = old_head.ptr;
      if (ptr == tail.load().ptr) {
        return std::unique_ptr<T>();
      }
      counted_node_ptr next = ptr->next.load();
      if (head.compare_exchange_strong(old_head, next)) {
        T* const res = ptr->data.exchange(nullptr);
        free_external_counter(old_head);
        return std::unique_ptr<T>(res);
      }
      ptr->release_ref();
    }
  }
};