C++ global extern “C” friend can't reach private member on namespaced class

Question

Please consider the code:

#include    <iostream>

using namespace std;

extern  "C"
void    foo( void );

namespace   A
{
    template< int No >
    class   Bar
    {
    private:
        friend  void    ::foo( void );

        static void private_func( int n );
    };

    template< int No >
    void    Bar< No >::private_func( int n )
    {
        cout << "A:Bar< " << No << ">::private_func( " << n << " )" << endl;
    }
}

extern  "C"
void    foo( void )
{
    A::Bar< 0 >::private_func( 1 );
}

int main( )
{
    cout << " ---- " << endl;
    foo( );
}

G++ gives:

> g++ -Wall -o extern_c extern_c.cpp
extern_c.cpp: In function ‘void foo()’:
extern_c.cpp:20:7: error: ‘static void A::Bar<No>::private_func(int) [with int No = 0]’ is private
extern_c.cpp:29:31: error: within this context

If I comment the namspace A, it will compile and run correctly.

What am I missing?

I looked related topics, but could not find any that fits in my problem.

• C++: namespace conflict between extern "C" and class member
• Why this friend function can't access a private member of the class?

Thanks people.

---

EDIT:

I am now convinced that extern "C" has nothing to do with the problem. Please ignore it.

Answer 1

I don't know the explanation, but if you put foo( ) into a namespace, it works.

#include    <iostream>

using namespace std;

namespace C
{
    extern  "C"
    void    foo( void );
}

namespace   A
{
    template< int No >
    class   Bar
    {
    private:
        friend  void    C::foo( void );

        static void private_func( int n );
    };

    template< int No >
    void    Bar< No >::private_func( int n )
    {
        cout << "A::Bar< " << No << ">::private_func( " << n << " )" << endl;
    }
}


namespace C
{
    extern  "C"
    void    foo( void )
    {
        A::Bar< 0 >::private_func( 1 );
    }
}

int main( )
{
    cout << " ---- " << endl;
    C::foo( );
}

And the result:

bbcaponi@bbcaponi friends]$ g++ -Wall namespace_friend.cpp -o namespace_friend
[bbcaponi@bbcaponi friends]$ ./namespace_friend
 ----
A::Bar< 0>::private_func( 1 )

Answer 2

It's a g++ bug. Exists in 4.4, fixed in 4.6.

UPD: It seems that it's triggered by a combination of template and namespace. extern "C" is not relevant, as it may be commented out and the error remains.

Answer 3

You need to declare the friend as extern "C". Your current friend declaration finds a friend foo in the global namespace that has C++ name mangling.