Overload operator<< for nested class template

Question

I have the following setup:

template< class T >
struct Foo {

  struct Bar {
    Bar ( const T &t ) : otherT_( t ) {}

    T otherT_;
  };

  Foo ( const T &t ) : myT_( t ) {}

  T myT_;
};

Now, I want to make instances of Foo< T >::Bar streamable to std::cout and friends. I tried this:

template< class T >
std::ostream& operator<< ( std::ostream &os, 
                           const typename Foo< T >::Bar &bar ) {
  os << "<bar: " << bar.otherT_ << ">";
  return os;
}

But the following code does not compile:

  Foo< int > foo( 5 );
  Foo< int >::Bar bar( 7 );

  std::cout << bar << std::endl;

I guess that the compiler is not able to deduce the type T or something. Is there a way to make such instances of the nested class behave well with operator<<?

Thank you!

Answer 1

Yep, the easy way is to put operator<< inside Bar:

struct Bar {
  Bar ( const T &t ) : otherT_( t ) {}

  T otherT_;

  friend std::ostream& operator<< ( std::ostream &os, const Bar &bar ) 
  {
    os << "<bar: " << bar.otherT_ << ">";
    return os;
  }
};

I am digging the other way ...