Re: Template friendship to nested template class
- From: Victor Bazarov <v.Abazarov@xxxxxxxxxxxx>
- Date: Tue, 15 Sep 2009 08:58:18 -0400
Mycroft Holmes wrote:
I remember that template friendship is a hot topic, because it's not
completely clear in the standard, but we recently discovered a piece
of code like this:
template <typename T>
class OUTER
{
int private_;
template <long A, long B>
struct INNER
{
int doIt(OUTER<T> x) {return x.private_; }
};
template <long A, long B>
friend struct INNER;
public:
int exec() { return INNER<3,4>().doIt(*this); }
};
int main()
{
OUTER<double> x;
return x.exec();
}
Both VC and gcc are happy with this code, but the intel compiler is
not, and in fact Comeau reports a detailed reason for the failure:
(btw, Comeau reports 3 errors, but only the first is meaningful; the
latest intel compiler v11 only reports the 2 irrelevant errors,
omitting the first...)
Comeau C/C++ 4.3.10.1 (Oct 6 2008 11:28:09) for
ONLINE_EVALUATION_BETA2
Copyright 1988-2008 Comeau Computing. All rights reserved.
MODE:strict errors C++ C++0x_extensions
"ComeauTest.c", line 14: error: template nesting depth does not match
the previous
declaration of class template "OUTER<T>::INNER [with
T=double]"
Probably you are missing an EXTRA template <Param> to declare
a nested friend,
where Param is not the same id used for the enclosing class
but matches it when instantiated
friend struct INNER;
^
detected during instantiation of class "OUTER<T> [with
T=double]" at
line 29
Can somebody explain what would be the "EXTRA template <Param> to
declare a nested friend"?
(note that we don't need a fix, we already reworked the code: we took
INNER outside OUTER and made "doIt" a template member function)
I think it's because the name referred to in a 'friend' declaration is looked up (or presumed) to exist at the namespace scope. So, if you consider that your 'OUTER' template is in the global namespace, then the 'friend' declaration you have refers to the template '::INNER', and not to '::OUTER::INNER'.
I vaguely recall that in early discussions on the defects of C++ Standard there was a mention that nested classes should be granted full access to the containing class since they are, in fact, members, and by definition 'private' is for *members* and friends only. I don't know what the resolution was, however. Perhaps a careful look at the current draft Standard for C++0x would reveal what to expect. I would think that if it was determined a defect, then in the situation like yours there would be no need for a 'friend' declaration - 'INNER' already has access to 'OUTER's private members.
V
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