operator+ in derived classes More options

This is something someone was asking in irc. I really don't need to do this
right now, but may have to in the future. The following code is in error
(marked).

Code:

#include <iostream> 
#include <string> 


class Base 
{ 
public: 
    Base( const int x = 0): x_( x ) {} 
    Base operator+( const Base b ) { Base Temp( x_ ); Temp.x_ += b.x_; 
return Temp; } 
    int X() { return x_; } 
    virtual ~Base() {} 
private: 
    int x_; 



}; 


class Derived: public Base 
{ 
public: 
    Derived( const int x = 0, const int y = 0): Base( x ), y_( y ) {} 
    int Y() { return y_; } 
private: 
    int y_; 


}; 


int main() 
{ 

    Base MyBase(10); 
    std::cout << MyBase.X() << "\n"; 


    Base MyBase2 = MyBase + Base(5); 
    std::cout << MyBase2.X() << "\n"; 


    Derived MyDerived( 10, 20 ); 
    std::cout << MyDerived.X() << " " << MyDerived.Y() << "\n"; 


    Derived MyDerived2 = MyDerived + Derived( 5, 10 ); 
    // Following line gives compilation error 
    std::cout << MyDerived.X() << " " << MyDerived.Y() << "\n"; 
   //  error C2440: 'initializing' : cannot convert from 'Base' to 'Derived' 
   //      No constructor could take the source type, or constructor 
overload resolution was ambiguous 


    std::string wait; 
    std::getline( std::cin, wait ); 



} 

I understand the error. I am tryign to operator+ on derived, but the only operator + is on base.

How do people handle this?

What would be a nice solution, if possible, is to have a derived operator+ that calls the base operator+ for the addition of the x_'s, then add the y_'s in derived operator+ and return derived. I couldn't come up with a way to do this however, and think I would have to totally rewrite the operator+ for derived. In this case it's trivial, but I could see a complex class where it wouldn't be so trivial.

Any thoughts?