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

[Gabriel Dos Reis]

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.
>

then don't put it in a class.


>
>   • 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.
>
> 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.
>
> In addition, the latest standard *draft*:
>
>   https://isocpp.org/files/papers/N3797.pdf
>
> contains the n3651 matrix_constants example on p. 313.
>
> Is there some more current standards document which requires the
> static qualifier on variable templates within a class?
>
> TIA.
>
> -regards,
> Larry
>
> > On Thu, Nov 13, 2014 at 8:38 PM, Larry Evans <cppljevans at suddenlink.net>
> > wrote:
> >
> >> The following code:
> >>
> >> //{=======================================
> >> #include <type_traits>
> >> struct variable_template
> >> {
> >>     template <int Value>
> >>     std::integral_constant<int,Value> constexpr
> >>   value{}
> >>     ;
> >> };
> >> #include <iostream>
> >> #include <iomanip>
> >> int main()
> >> {
> >>     std::cout<<
> >>       variable_template::
> >>       value<1>()<<std::endl;
> >>     return 0;
> >> }
> >> //}=======================================
> [snip]
>
>
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