[cfe-users] variable template within class produces non-static data member cannot be constexpr

[Gabriel Dos Reis]

Good.

-- Gaby

On Mon, Nov 24, 2014 at 9:01 AM, Larry Evans <cppljevans at suddenlink.net>
wrote:

> On 11/19/2014 10:32 AM, Larry Evans wrote:
> > On 11/14/2014 01:01 PM, David Blaikie wrote:> On Fri, Nov 14, 2014 at
> > 4:23 AM, Larry Evans <cppljevans at suddenlink.net>
> >> wrote:
> >>
> >>> On 11/14/2014 12:48 AM, David Blaikie wrote:
> >>>> My guess is that the proposal was written assuming a certain
> >>> implementation
> >>>> of constexpr that never panned out.
> >>>>
> >>>> It looks like both GCC and Clang expect constexpr member variables to
> be
> >>>> explicitly marked static:
> >>>>
> >>>> const.cpp:5:19: error: non-static data member 'f' declared 'constexpr'
> >>>>    constexpr foo f{};
> >>>>                    ^
> >>>>
> >>>> (is GCC 4.9's diagnostic - for a simple non-template constexpr member
> >>> variable)
> >>>>
> >>> That's sad because, as n3651 says on pp. 2-3:
> >>>
> >>>   The main problems with “static data member” are:
> >>>
> >>>   • they require “duplicate” declarations: once inside the class
> >>>     template, once outside the class template to provide the “real”
> >>>     definition in case the constants is odr-used.
> >>>
> >>>   • programmers are both miffed and confused by the necessity of
> >>>     providing twice the same declaration. By contrast, “ordinary”
> >>>     constant declarations do not need duplicate declarations.
> >>>
> >>
> >> I don't think this is referring to the need to write "static" in a
> static
> >> member variable template declaration.
> >>
> >> This is referring to the workaround (In the absence of static
> > variables) of
> >> using a class template with a nested static data member - pointing out
> > that
> >> when you write it that way (see the example near this workaround
> >> description) you have to write the class, the variable declaration,
> > and the
> >> variable definition. Whereas with a plain non-member variable template
> you
> >> don't need separate declaration/definition - the variable template
> (like a
> >> function template) is the declaration+definition all in one.
> >>
> >
> > So, if the keyword, static, *is* used, a separate definition *outside* of
> > the class (IOW, in, I think the terminology is, "in namespace scope")
> > is needed. In contrast, if keyword static *is not* used, the
> > separate definition is not required ( even though the
> > variable is still a static member) ?
> >
> >>
> >>> Also, the clang status page:
> >>>
> >>>   http://clang.llvm.org/cxx_status.html
> >>>
> >>> reference n3651 below the section title:
> >>>
> >>>   C++14 implementation status
> >>>
> >>> Hence, I thought that's what clang implemented.
> >>>
> >>
> >> Ah, here's the wording that's important:
> >>
> >> "A variable template at class scope is a static data member template."
> in
> >> 14 paragraph 1.
> >>
> >> (sorry, I'd been looking in the static member parts of the spec, not the
> >> variable template parts, which have this special case)
> >>
> >> It looks like this just hasn't been implemented - Larisse (CC'd) did the
> >> initial variable template implementation. Perhaps she can tell us if
> that
> >> was a more recent change, whether it's planned to be fixed, or a bug
> > in the
> >> spec etc.
> >>
> > The reponse to my comment here:
> >
> >   http://en.cppreference.com/w/Talk:cpp/language/variable_template
> >
> > says:
> >
> >   the examples in the standard have an error, mentioned
> >   _here_
> >
> > where _here_ links to:
> >
> >
> >
> http://stackoverflow.com/questions/21474111/variable-template-at-class-scope/21482264?noredirect=1#comment32627331_21482264
> >
> > which is, essentially, the question posed in my OP to this
> > thread.  In addition, the comments to that SO post contain:
> >
> >   _The committee is aware that the wording for variable
> >   templates needs some help in various places_
> >
> > which links to:
> >
> >
> >
> https://groups.google.com/a/isocpp.org/forum/#!topic/std-proposals/ZTw3W3vjWWs
> >
> > which I'm still trying to digest :(, but it seems to
> > indicate the variable template meaning is ambiguous; hence,
> > one can't say clang has a bug in its implementation
> > of variable templates.
> >
> My last response to the talk:
>
> http://en.cppreference.com/w/Talk:cpp/language/variable_template
>
> contains:
>
>   the definition outside the class definition is ...
>   *only* optional if a reference is not made to the template variable.
>
> and a reference to the standard's 9.4.2p3 is made to justify that
> conclusion.
>
> Hence, AFAICT, clang is correct in requiring the outside class defintion
> when the template variable is referenced.
>
> -regards,
> Larry
>
>
>
>
>
>
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