Re: Problem with frien template class

From:

"Ben Voigt [C++ MVP]" <rbv@nospam.nospam>

Newsgroups:

microsoft.public.vc.language

Date:

Tue, 12 Feb 2008 18:41:24 -0600

Message-ID:

<Ol$$xjdbIHA.5892@TK2MSFTNGP06.phx.gbl>

Igor Tandetnik wrote:

Tamas <t@v.com> wrote:

Please help me find the problem with the code below. It is working
until I try to set the friendship between RingElem and Ring. I get
the following error message in VC2008:
Error 1 error C3200: 'RingElem<T,u>' : invalid template argument for
template parameter 'RE', expected a class template

template <typename T, int u, template <typename, int> class RE> class
Ring;
template <typename T, int u = 0>
class RingElem
{
friend Ring <T, u, RingElem>; // error C3200
protected:
T* __next;
};

The problem is that, inside the definition of RingElem, the identifier
'RingElem' refers to a particular instantiation RingElem<T, u> of the
template, not the template as a whole. This allows you to write, for
example,

Have you tried using "::RingElem", which ought to refer to the whole family
of templates?

Does it work with Comeau?

template <typename T, int u = 0>
class RingElem
{
   void f() {
       RingElem r; // same as RingElem<T, u> r;
   }
};

But RingElem<T, u>, being a class and not a template, can't be used as
an argument for template template parameter.

I don't see any way out of this quandary, short of declaring all
instantiations of Ring to be friends of RingElem:

template <typename T, int u = 0>
class RingElem
{
   template <typename U, int v, template <typename, int> class RE>
   friend class Ring;
};