Not related to C::B and therefore vioalting our forum rules.
Such a thread normally gets locked, so be warned.
Be aware C::b is "just" an IDE not a compiler.
Nevertheless:
iostream includes ostream,
ostream includes ios,
ios includes iosfwd,
iosfwd includes bits/stringfwd.h,
bits/stringfwd.h defines string in namespace std
that's the way gcc (at least 4.4) does it, Microsoft, Borland etc. compilers might do it in a different way.