I'm not 100% sure what you mean by narrow string, but if you refer to std::string vs std::wstring, then std::string is perfectly fine for encoding UTF8, as that

I'm not 100% sure what you mean by narrow string, but if you refer to std::string vs std::wstring, then std::string is perfectly fine for encoding UTF8, as that uses 8 bit code units which are guaranteed to fit in a char. On the other hand, std::wstring would be a bizarre choice for UTF8 on any platform.