Actually, I'm thinking that the fix may be easier than I thought after all - if I add a 2-arg "range-checked" constructor as an overload, then the iterator's constructor can validate the end-points of the underlying sequence during construction,
Doesn't that make construction of an iterator over N bytes an O(N) operation?
No, it only checks that each *end* of the sequence contains a valid multibyte sequence - effectly these can then act as sentinels - if there are invalid sequences within the range (not at the endpoints) then we can catch these anyway already.
That requires the underlying iterator to be bidirectional, right?
Correct - but that's what regex requires anyway (which is all these are used for for now). I guess a more sophisticated version could adapt to the traversal category of the underlying iterator - using relatively lightweight endpoint checking when it's possible and "check every increment" for forward/input iterators? John.