<streambuf>

namespace std {
    template<class E, class T = char_traits<E> >
        class basic_streambuf;
    typedef basic_streambuf<char, char_traits<char> > streambuf;
    typedef basic_streambuf<wchar_t, char_traits<wchar_t> > wstreambuf;
    };

Include the iostreams standard header <streambuf> to define template class basic_streambuf, which is basic to the operation of the iostreams classes. (This header is typically included for you by another of the iostreams headers. You seldom have occasion to include it directly.)