Re: Undefined reference in G++.

Hi!

I'm writing a class that have a static function

class Servicios {
   public:
      static String * intToString(int value);
};

an another class String that I write to.

namespace DataTypes {
  class String {
     ......
   };

  ostream& operator<< (ostream&, const String &);
  istream& operator>>(istream&, String &)
}

the class String has two iostream functions implemented outside the
class.

I have to a main.cc that has something like this:

int main() {
   String * str = Servicios::intToString(100);
   cout << str;

}

but when I do a make from all this, I have the error

In main.cc: undefined reference to .............function
ostream....(....)

Questions:
  what is undefined reference? when ocurr? how to solve this problem?