class Fraction {
    ...
    public:
      virtual void printOn (std::ostream& strm) const;  // Вывод
      virtual void scanFrom (std::istream& strm);       // Ввод
      ...
};

std::ostream& operator << (std::ostream& strm, const Fraction& f)
{
    f.printOn(strm);
    return strm;
}

std::istream& operator >> (std::istream& strm, Fraction& f)
{
    f.scanFrom (strm);
    return strm;
}