Re: Problem with frien template class

From:
"Igor Tandetnik" <itandetnik@mvps.org>
Newsgroups:
microsoft.public.vc.language
Date:
Tue, 12 Feb 2008 15:00:16 -0500
Message-ID:
<#b5SkHbbIHA.3572@TK2MSFTNGP02.phx.gbl>
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,

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;
};

--
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

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