This is the reason why I hope I never program in C++ again: you can't learn it (or re-learn it) by inspection.
Yeah, I suppose your example has something to do with the spiral rule (it's been many years), which comes from C (and C has its excuses). But if you come at that cold, it's impossible to reason it out.
No language is entirely learnable by inspection, it's a scale, and C++ is way on the wrong end.