How shall I implement operator overloading in base class for base class part of derive class object?

Question

How shall I implement operator overloading in base class for base class part of derive class object?

Please see this sample code and implement base class part to implement * operator on derive class object

class base {
     int x;
public:

};

class der : public base {
    int y;
public:
    const der operator*(const der& rh) {
        der d;
        d.y = y * rh.y;
        return d;
    }

};

Answer 1

class base {
     int x;
public:
     base & operator *=(const base &rh) {
         x*=rh.x;
         return *this;
     }
     base operator *(const base &rh) {
         base b = *this;
         b*=rh;
         return b;
     }
};

class der : public base {
    int y;
public:
    using base::operator*;
    der & operator*= (const der& rh) {
         base::operator*=(rh);
         y*=rh.y;
         return *this;
    }
    const der operator*(const der& rh) {
        der d = *this;
        d*=rh;
        return d;
    }

};

Answer 2

Implement a base::operator * as below:

class base {
protected:
  int x;
public:
  base operator * (const base &rh) {
    base b;
    b.x = x * rh.x;
    return b;
  }
};

And call it as,

der operator * (const der& rh) {
  der d;
  Base &b = d;  // <---- assign a temporary reference of 'base'
  b = static_cast<Base&>(*this) * rh; // <---- invoke Base::operator *
  d.y = y * rh.y;
  return d;
}