Declaring "friend" functions fails under Visual Studio 2008

Question

I am trying to declare a global function as a "friend" of a class:

namespace first
{
    namespace second
    {
        namespace first
        {
            class Second
            {
                template <typename T> friend T ::first::FirstMethod();
            };
        }
    }
}

When I compile this code under Visual C++ 2008 I get:

error C3254: 'first::second::first::Second' : class contains explicit override 'FirstMethod' but does not derive from an interface that contains the function declaration
error C2838: 'FirstMethod' : illegal qualified name in member declaration

If I use template <typename T> friend T first::FirstMethod(); instead I get:

error C2039: 'FirstMethod' : is not a member of 'first::second::first'

What is the appropriate way of declaring friend functions?

Answer 1

You have hit my quiz by accident - the sequence T ::first:: ... is interpreted as a single name. You need to put some token in between the colons and T. Solution is presented in the linked question too.

Notice that in any case you first have to declare the function designated by a qualified name in its respective namespace, too.

---

Edit: There are different solutions for the syntax problem

 template <typename T> friend T (::first::FirstMethod)();
 template <typename T> T friend ::first::FirstMethod();

If you often need to refer to the outer namespace and have problems with this syntax, you can introduce a namespace alias

    namespace first
    {
        namespace outer_first = ::first;
        class Second
        {
            template <typename T> friend T outer_first::FirstMethod();
        };
    }