C ++ namespaces, inner classes and operator resolution

In the C ++ namespace myspace

, I have a class Outer

that in turn has an inner class Inner

. While I can declare and define a global friends operator QDataStream& operator<<(QDataStream& s, const myspace::Outer& o)

, I cannot see how to declare a global friends operator QDataStream& operator<<(QDataStream& s, const myspace::Outer::Inner& o)

. The lines written represent a failed attempt. I can't see how to declare an inner class without defining the outer one.

namespace myspace {
    class Outer;
    //class Outer::Inner;
}

QDataStream& operator<<(QDataStream& s, const myspace::Outer& o);
//QDataStream& operator<<(QDataStream& s, const myspace::Outer::Inner& o);


namespace myspace {

    class Outer {

        friend QDataStream& (::operator <<)(QDataStream&, const Outer&);

        class Inner {
            //friend QDataStream& (::operator <<)(QDataStream&, const Inner&);
            int i;
        };

        int o;
    };

}

      

        
        
        
      

    

I've read Namespaces and Operator Resolution , C ++ Defining <Inner Class Operator , Accessing a Private Class in a <<Operator in a Namespace, and Operator Overloading, Name and Namespace Resolution , but none of them work.

If I uncomment these lines, the former throws the error "external.h: 7: error:" Inner "in" class myspace :: Outer "does not name the class type Outer :: Inner; ^" It seems to be a key. I cannot declare an inner class.

I am using C ++ 11.

This question is not a duplicate of the Forward nested types / classes declaration in C ++ if it can be resolved without direct reference.