If I was code reviewing this I would strongly suggest std::array instead of those raw array;) I’m also curious why you have page_id_to_frame_idx[] without a size. If you are using that paradigm to read off the end of the struct then it needs commenting heavily, if it’s a typo then std::array would have caught it.

I do understand that it’s just a snippet of code to show a style, so ignore this pedant!