Re: operator+ in derived classes

"Victor Bazarov" <>
Thu, 29 Mar 2007 23:11:44 -0400
Jim Langston wrote:

"Jim Langston" <> wrote in message

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).

#include <iostream>
#include <string>

class Base
   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() {}
   int x_;

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

Add here

      Derived(const Base& b) : Base(b), y_(0) {}

   int Y() { return y_; }
   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";

My bad, the following line is the one in error, not the one below it.
It's the operator+ that's causing the problem, not the X() and Y();

   Derived MyDerived2 = MyDerived + Derived( 5, 10 );
  // error C2440: 'initializing' : cannot convert from 'Base' to
  'Derived' // No constructor could take the source type, or

   std::cout << MyDerived.X() << " " << MyDerived.Y() << "\n";
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?

Add the constructor from Base.


Any thoughts?

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