boost::text::v1::stream_safe
// In header: <boost/text/stream_safe.hpp> template<code_point_iter I, std::sentinel_for< I > S> constexpr code_point_iter auto stream_safe(I first, S last);
Copies the stream-safe portion of [first, last) to the beginning of [first, last), and returns an iterator to the end of the copied range. Note that the type returned may not be I, since the result may be a new iterator that results from reconstituting unpacked iterators out of the input. The type returned will model code_point_iter.
See Also:
https://unicode.org/reports/tr15/#Stream_Safe_Text_Format