Zstring makeUpperCopy(const Zstring& str) { const size_t len = str.size(); Zstring output; output.resize(len); std::transform(str.begin(), str.end(), output.begin(), [](char c) { return static_cast<char>(::toupper(static_cast<unsigned char>(c))); }); //locale-dependent! //result of toupper() is an unsigned char mapped to int range, so the char representation is in the last 8 bits and we need not care about signedness! //this should work for UTF-8, too: all chars >= 128 are mapped upon themselves! return output; }