On 11/15/2014 09:51 AM, Gabriel Dos Reis wrote:
> On Sat, Nov 15, 2014 at 6:33 AM, Larry Evans <cppljev...@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 <cppljev...@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:
!defined(USE_STATIC) && !defined(DUPLICATE_DECLARATION)
yet, with clang 3.5, it only compiles with both these
macros are define.
Is clang wrong?
>
> -- Gaby
>
>
>
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//Purpose:
// See if code using "variable templates" proposed here:
/*
http://isocpp.org/files/papers/N3651.pdf
*/
// will compile. In particular, the 'struct matrix_constants'
// in the reference seems essentially the same as code below.
//Result:
// No.
/*
variable_templates.cpp:16:44: error: non-static data member cannot be constexpr; did you intend to make it static?
std::integral_constant<Integral,Value> constexpr
^
static
1 error generated.
*/
//=======================================
//#define DUPLICATE_DECLARATION
//#define USE_STATIC
#include <type_traits>
struct matrix_constants
{
template<typename T>
using pauli = std::integral_constant<T, 2>;
template<typename T>
#ifdef USE_STATIC
static
#endif
pauli<T> constexpr
sigma1 = {};
};
#ifdef DUPLICATE_DECLARATION
template <typename T>
matrix_constants::
pauli<T> constexpr
matrix_constants::
sigma1;
#endif
#include <iostream>
int main()
{
std::cout<<"sigma1="<<matrix_constants::sigma1<int>()<<"\n";
return 0;
}
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