template <typename T>
class lock_free_stack {
private:
  void increase_head_count(counted_node_ptr& old_counter)
  {
    counted_node_ptr new_counter;
    do {
      new_counter = old_counter;
      ++new_counter.external_count;
    } while (!head.compare_exchange_strong(old_counter, new_counter));
    old_counter.external_count = new_counter.external_count;
  }

public:
  std::shared_ptr<T> pop()
  {
    counted_node_ptr old_head = head.load();
    for (;;) {
      increase_head_count(old_head);
      node* const ptr = old_head.ptr;
      if (!ptr) {
        return std::shared_ptr<T>();
      }
      if (head.compare_exchange_strong(old_head, ptr->next)) {
        std::shared_ptr<T> res;
        res.swap(ptr->data);
        int const count_increase = old_head.external_count - 2;
        if (ptr->internal_count.fetch_add(count_increase) == -count_increase) {
          delete ptr;
        }
        return res;
      }
      else if (ptr->internal_count.fetch_sub(1) == 1) {
        delete ptr;
      }
    }
  }
};