<fstream>

namespace std {
    template<class E, class T = char_traits<E> >
        class basic_filebuf;
    typedef basic_filebuf<char> filebuf;
    typedef basic_filebuf<wchar_t> wfilebuf;
    template<class E, class T = char_traits<E> >
        class basic_ifstream;
    typedef basic_ifstream<char> ifstream;
    typedef basic_ifstream<wchar_t> wifstream;
    template<class E, class T = char_traits<E> >
        class basic_ofstream;
    typedef basic_ofstream<char> ofstream;
    typedef basic_ofstream<wchar_t> wofstream;
    template<class E, class T = char_traits<E> >
        class basic_fstream;
    typedef basic_fstream<char> fstream;
    typedef basic_fstream<wchar_t> wfstream;
    };

Include the iostreams standard header <fstream> to define several template classes that support iostreams operations on sequences stored in external files.