operator overloading for nested derived templates

Question

I've got some problem with operator overloading for derived and nested class. I've tried many ways but failed many times. Can anyone help to solve this? Here's the code:

class A:

template <typename T>
class A {
  public:
    virtual ~A() = default;
};

template <typename T>
ostream& operator<<(ostream& out, const A<T>& a) {
  out << "in A\n";
  return out;
}

class B:

template <typename T>
class B : public A<T> {};

template <typename T>
ostream& operator<<(ostream& out, const B<T>& b) {
  out << "in B\n";
  return out;
}

And test code:

A<int> *pa = new B<int>();
B<int> *pb = dynamic_cast<B<int> *>(pa);
cout << *pa << typeid(*pa).name() << endl;
cout << *pb << typeid(*pb).name() << endl;

And I got what I need from result:

in A

1BIiE

in B

1BIiE

Then I created a nested class which is also derived from class A:

class BB::B:

template <typename T>
class BB {
  public:
    class B : public A<T> {};
};

template <typename T>
ostream& operator<<(ostream& out, const typename BB<T>::B& b) {
  out << "in BB::B\n";
  return out;
}

But when I test this class with code:

A<int> *pa2 = new BB<int>::B();
BB<int>::B *pb2 = dynamic_cast<BB<int>::B *>(pa2);
cout << *pa2 << typeid(*pa2).name() << endl;
cout << *pb2 << typeid(*pb2).name() << endl;

I got only an unexpected result:

in A

N2BBIiE1BE

in A

N2BBIiE1BE

Seems that compiler didn't specialize overloaded operator<< for BB::B. What's the problem here?

Answer 1

template <typename T>
ostream& operator<<(ostream& out, const typename BB<T>::B& b);

The type of b is in a non-deduced context. It will never deduce T from this signature.

You can call it buy passing T explicitly. Or you could write a type map from BB<T>::B to B and get real fancy. But that isn't really worth it.

---

The easy option is a Koenig operator.

template <typename T>
class BB {
  public:
    class B : public A<T> {
      friend ostream& operator<<(ostream& out, const B& b){
        out << "in BB::B\n";
        return out;
      }
    };
};

this Koenig operator is a non-template function that is generated for each instantiation of BB<T>::B, and can be found only via ADL.