Question

//cannot declare operator<<(...) here:
//forward declarations:
class External;

template<class T, class Y>
class External::Internal;

template<class T, class Y>
std::ostream& operator<<(std::ostream& out, const External::Internal<T,Y>&);

class External
{
    template<class T, class Y>
    class Internal
    {};

    Internal data_;

    void print() {
        /*out is a std::ostream*/
        out << data_;
    }
};

template<class T, class Y>
std::ostream& operator<<(std::ostream& out, const External::Internal<T,Y>&)
{ }

I do want to implement operator<< for Internal but there is a problem when I try to use this operator call from External: It doesn’t see this operator when this operator is declared under the definition of this class, and there seems to be no way of declaring this operator above this class definition.

Answer 1

If you’re asking how to define Internal<>::operator<< as a friend, then:

class External
{
  template<class T, class Y>
  class Internal
  {
    friend std::ostream& operator <<(std::ostream& out, const Internal&)
    {
      // impl
      return out;
    }
  };

  Internal<Foo, Bar> data_;

public:
  void print() const
  {
    /*out is a std::ostream*/
    out << data_;
  }
};