Re: Problem with frien template class

From:

"Igor Tandetnik" <itandetnik@mvps.org>

Newsgroups:

microsoft.public.vc.language

Date:

Tue, 12 Feb 2008 20:58:05 -0500

Message-ID:

<uSMIpSebIHA.5900@TK2MSFTNGP02.phx.gbl>

"Ben Voigt [C++ MVP]" <rbv@nospam.nospam> wrote in message
news: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?

Yes, this does seem to be working:

template <template <typename> class C> class V;

template <typename T> class X {
friend class V< ::X>;
};

One has to be careful to use the correct namespace: if class X is
declared in some namespace, ::X won't work. Unfortunately, there's no
analog of file systems' .. operator ("go one scope up").

Another minor point: be careful to leave a space between opening angle
bracket '<' and a colon ':'. <: is a digraph for [ .
--
With best wishes,
Igor Tandetnik

With sufficient thrust, pigs fly just fine. However, this is not
necessarily a good idea. It is hard to be sure where they are going to
land, and it could be dangerous sitting under them as they fly
overhead. -- RFC 1925