Sentinel values should have constexpr names.

[rpatters1]

Right now it seems like sentinel values, in particular -1, are hard-coded. It would be nice to have one or more constants defined that eliminated the need for that hard coding, e.g.,

constexpr int INDEX_UNSPECIFIED = -1;

If we wanted to, there could be a constant defined per use case.

EDIT: alternatively we could move towards std::optional, which is the C++ way but has its drawbacks.