operator<<(ostream&, X) for class X nested in a class template

Question

This one compiles and works like it should (non-nested template):

#include 

template class Z;

template          


        

Answer 1

Matthieu explained the problem very well, but a simple work-around can be used in this case. You can implement the friend function inside the class with the friend declaration:

#include 

template  class Z {
public:
  class ZZ {
    friend std::ostream& operator<< (std::ostream& os, const ZZ&) {
      return os << "ZZ!";
    }
  };
};


int main () {
  Z::ZZ zz;
  std::cout << zz << std::endl;
}