StringView::SplitResult::Iterator does not conform to LegacyForwardIterator as claimed by the trait iterator_category = std::forward_iterator_tag; It's missing at least operator-> and postfix operator++ We should also consider upgrading the iterator to a bidirectional iterator. We could then do efficient checks for string formatting such as checking that an IPv4 address contains the proper number of octets (it.end() - it.begin() == 4) that don't require rolling your own loop.