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

Larry Evans cppljevans at suddenlink.net
Sat Nov 15 09:13:51 PST 2014

On 11/15/2014 09:51 AM, Gabriel Dos Reis wrote:
> On Sat, Nov 15, 2014 at 6:33 AM, Larry Evans <cppljevans at suddenlink.net>
> wrote:
>> On 11/15/2014 06:59 AM, Gabriel Dos Reis 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.
>>> then don't put it in a class.
>> But then I don't understand why the problem was mentioned in n3651 on
>> pp. 2-3.  I thought one of the reasons for the proposal was to free
>> programmers from the need for "duplicate" declarations; however, the
>> solution:
>>   don't put it in a class
>> seems to indicate that the problem can be avoided simply by not
>> creating the problem.  IOW, the "duplicate" declaration in the
>> archetypical example on p. 2, the numeric_limits example, can be
>> avoided by not putting it in a template class (or struct in this
>> case).
>> In that case, I don't understand the rationale for proposal.
>> I must be missing something :(
>> -regards,
>> Larry
> If you hate (as I do) having to put a variable in a class template just so
> that you can abstract over its type but then you are forced to provide a
> "duplicate" declaration (the real definition) outside the class, then
> variable templates remove that pain: they do what they were designed for.

That's what I was hoping.

Maybe the problem is I'm not making my point clear enough.
If I understand your proposal, then the following code should
compile without error when:
yet, with clang 3.5, it only compiles with both these
macros are define.

Is clang wrong?

> -- Gaby
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