Yeah If you're using narrow strings for UTF8 you're making a mistake. wstrings also are not a good representation because of the platform differences, unless you don't care about Windows in which case it's fine but still not great semantically.
In most type definitions you cannot convert UTF8 via simple iteration because the type generally represents a code point and not a character.
You can have a library where UTF8 characters are a native type and code points are a mostly-hidden internal element. But again, that's highly opinionated for C++.
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.
In most type definitions you cannot convert UTF8 via simple iteration because the type generally represents a code point and not a character.
You can have a library where UTF8 characters are a native type and code points are a mostly-hidden internal element. But again, that's highly opinionated for C++.