A std::string
can not store unicode data. You will need a std::wstring for that.
I've never heard of UnicodeString before, but looking at the API here:
http://docwiki.embarcadero.com/Libraries/XE2/en/System.UnicodeString_Methods
It has a function called .c_str() which returns a wchar_t* which you can then use to construct a std::wstring
If you really need a std::string, then have a look at this answer.