boost::text::v1::to_upper
// In header: <boost/text/case_mapping.hpp> template<grapheme_range R, std::weakly_incrementable O> unspecified to_upper(R && r, O out, case_language lang = case_language::other);
Writes the code point sequence comprising the upper-case form of r to out, using language-specific handling as indicated by lang. The normalization of the result is undefined. Returns a case_mapping_result. Note that the type of the result's .in member may not be std::ranges::iterator_t<R>, since the result may be a new iterator that results from reconstituting unpacked iterators out of the input. The type of .in will model grapheme_iter. The .in member of the result is a borrowed_iterator_t in C++20 and later.