Global namespace friend class cannot access private member of named namespace class

Question

In a named namespace class, I declare a class (which is in the global namespace) as friend. However, the latter class cannot access the private member of the former class. Why is this? Is there any way around it?

Bob.h

namespace ABC {
    class Bob {
        friend class Joe;
        public:
            Bob ();
            int pub_number;
        private:
            int priv_number;
    };
}

Bob.cc

#include "Bob.h"

ABC::Bob::Bob () {
    pub_number=10;
    priv_number=6;
}

Joe.h

class Joe {
    Joe ( );
};

Joe.cc

#include "Joe.h"
#include <iostream>
#include "Bob.h"

Joe::Joe ( ) {
    ABC::Bob b;
    std::cout << b.pub_number << std::endl;
    std::cout << b.priv_number << std::endl;
}

The above code produces the following error when compiled:

Joe.cc:8:16: error: ‘int ABC::Bob::priv_number’ is private within this context
INFO: 1>     8 | std::cout << b.priv_number << std::endl;

If I do the same code as above, but without any namespace for the "Bob" class, then the code compiles.

I have attempted to forward declare the Joe class in Bob.h as follows:

class Joe; // This does nothing to help

class ::Joe // This produces compiler message "error: ‘Joe’ in namespace ‘::’ does not name a type"

Answer 1

You need add both an unscoped forward declaration in the global namespace, as well as use the scoping operator when declaring the friend:

class Joe;  // Forward declaration

namespace ABC {
    class Bob {
        friend class ::Joe;  // Use the Joe class from the global scope
        public:
            Bob ();
            int pub_number;
        private:
            int priv_number;
    };
}

Answer 2

Your friend declaration needs the :: prefix, too:

class Joe;

namespace ABC {
    class Bob {
        friend class ::Joe;
        //           ^^ here

        ...
     };
 }

Answer 3

In this class definition

namespace ABC {
    class Bob {
        friend class Joe;
        public:
            Bob ();
            int pub_number;
        private:
            int priv_number;
    };
}

the declaration of the friend class Joe introduces the name Joe in the scope of the namespace ABC because a prior declaration of the class Joe is not visible and there is used an unqualified name.

From the C++ Standard (10.3.1.2 Namespace member definitions)

3. ... If the name in a friend declaration is neither qualified nor a template-id and the declaration is a function or an elaborated-type-specifier, the lookup to determine whether the entity has been previously declared shall not consider any scopes outside the innermost enclosing namespace.

You need to place a declaration of the class Joe in the global namespace before the declaration of the class Bob and within the class Bob you have to use a qualified name of the friend class at least like

class Joe;

namespace ABC {
    class Bob {
        friend class ::Joe;
        public:
            Bob ();
            int pub_number;
        private:
            int priv_number;
    };
}

Or you could use a using declaration in the namespace ABC like

class Joe;

namespace ABC {
    using ::Joe;

    class Bob {
        friend class Joe;
        public:
            Bob ();
            int pub_number;
        private:
            int priv_number;
    };
}