I'm not sure they were suggesting using type-specific sentinel values (like null and nan), but to always use null or point to a value.

I like wrapping the information in a data structure exactly as you suggested, and if you never mutate the dereferenced pointer, it's equivalent to what they proposed. Big if, though.

The best is to do this generically with https://en.m.wikipedia.org/wiki/Option_type , since this pattern comes up regularly, not just with this one domain concept of "Filter"