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

Question

Please consider the code:

#include    

using namespace std;

extern  \"C\"
void    foo( void );

namespace   A
{
    template< int No >
           


        

Answer 1

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 2

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 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.