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Subject: Re: The issue about the wording "member of a class"
From: jim x (xmh970252187_at_[hidden])
Date: 2020-09-15 21:04:05


Hi, Andrew Schepler

Thanks to your response. After reading your answers, I agree with your
point. It seems to exist many defect in [temp.dep.type]/1, as you said.
Maybe use "enclosing classes" to cover the situation that define the member
of n-level nested class, any of these enclosing classes is a current
instantiation name(include template-id whose template-name is the name of
these enclosing classes).

Andrew Schepler <aschepler_at_[hidden]> 于2020年9月16日周三 上午7:14写道:

> In order to does not violate the rule "In the definition of a class or
>> class template, the scope of a dependent base class is not examined during
>> unqualified name lookup either at the point of definition of the class
>> template or member or during an instantiation of the class template or
>> member. "
>>
>
>
> The class `C` should be considered as a member of the enclosing template
>> class `A`.
>
>
> I disagree. Calling a member of a nested class of class X also a member of
> X would change the meaning of far too many other rules about members,
> beyond the "friend" example you noticed.
>
> On "direct member": A member of class X which is not a direct member of X
> is a direct member of some base class of X. Roughly speaking, members with
> names are the names which are valid for the class member lookup used by
> X::M and X_expr.M ([class.qual]
> <https://timsong-cpp.github.io/cppwp/class.qual>).
>
> Instead, I think possibly all four points in [temp.dep.type]/1 are missing
> phrasing to say that the rules for "the current instantiation" apply in a
> member of a nested class, and recursively if the enclosing class is itself
> a nested class. The Standard seems inconsistent on whether "nested class"
> must mean a member, or can be a member of a member, etc. I see multiple
> uses of the term which only make sense for actual members, and multiple
> uses of the term which only make sense including recursive members. Maybe
> defining "direct nested class", "indirect nested class", and "nested class"
> (and similar for "enclosing class") would be helpful?
>
> -- Andrew Schepler
>
> On Tue, Sep 15, 2020 at 4:00 AM jim x via Std-Discussion <
> std-discussion_at_[hidden]> wrote:
>
>> Hi, everyone.
>>
>> Please consider the following code
>> ````
>>
>> template<class T> struct A {
>> typedef int M;
>> struct B {
>> typedef void M;
>> struct C;
>> };};
>> template<class T> struct A<T>::B::C : A<T> {
>> M m; // OK, A<T>::M};
>>
>> ````
>>
>> It is an example written in [temp.dep.type]
>> <https://timsong-cpp.github.io/cppwp/n4659/temp.dep.type>. whether the
>> name `A<T>` is a current-instantiation name is determined by the following
>> rules:
>> >A name refers to the *current instantiation*
>> <https://timsong-cpp.github.io/cppwp/n4659/temp.dep.type#def:current_instantiation> if
>> it is
>> >> in the definition of a primary class template or *a member of a
>> primary class template*, the name of the class template followed by the
>> template argument list of the primary template (as described below)
>> enclosed in <> (or an equivalent template alias specialization),
>>
>> I have to say that the class `C` is a *direct member* of the class `B`
>> which in turn is the direct member of the class template `A`, however I
>> have not find any rule in the standard says that such a member of a nested
>> class is also be considered as a member of enclosing classes. I only find
>> the definition of the wording "direct member", that is:
>>
>> >A direct member of a class X is a member of X that was first declared
>> within the member-specification of X, including anonymous union objects
>> ([class.union.anon]) and direct members thereof.
>>
>> In order to does not violate the rule "In the definition of a class or
>> class template, the scope of a dependent base class is not examined during
>> unqualified name lookup either at the point of definition of the class
>> template or member or during an instantiation of the class template or
>> member. "
>>
>> The class `C` should be considered as a member of the enclosing template
>> class `A`.
>>
>
>> If this speculation is true, then there will be another question, that is
>> `friend`, the rule says:
>> >A friend of a class is a function or class that is given permission to
>> use the private and protected member names from the class.
>>
>> But, obviously, any function or class that befriended does not have
>> permission to access the private members of a nested class of which the
>> befriending class is an enclosing class.
>>
>> So, my questions are:
>>
>> Issue A:
>> Could members of a nested class be viewed as members of enclosing
>> classes?
>>
>> Issues B:
>> If the answers to A is "yes", should the rule for `friend` be changed
>> to:
>> >A friend of a class is a function or class that is given permission to
>> use the private and protected *direct member names *from the class.
>>
>> --
>> Std-Discussion mailing list
>> Std-Discussion_at_[hidden]
>> https://lists.isocpp.org/mailman/listinfo.cgi/std-discussion
>>
>



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