std::basic_streambuf::seekoff

namespace std {
  template<class CharT, class Traits = char_traits<CharT>>
  class basic_streambuf {
  protected:
    virtual pos_type seekoff(off_type off, ios_base::seekdir way,
                             ios_base::openmode which = ios_base::in | ios_base::out);

    ……
  };
}