>>21
It's not really a subclass persay as the string gets chars via a template parameter (basic_char or somesuch for the usual std::string if memory server). Importantly, and not mentioned by
>>19, the standard library also includes wstrings, which are basically just strings with 2-byte characters.
As far as handling unicode <i>correctly</i>, basically no library that I've heard of does. What with the code points and pages and ability to combine multiple memory-level "characters" into one glyph, it's often even questionable what the definition of a "character" is. Lots of libraries get it mostly right, though, and that's what you've got to go for.
As long as you're not doing anything specialized with the wstring, the standard library ought to suit you just fine. If you're doing specialized work with Unicode strings in nasty languages, though, you're going to have to go buy a library to handle it or write your own. This isn't any different from any other library in any language, of course. Standard libraries are widely useful, but not universally adequate, nor is it even possible that they could be.