Re: [boost] [fusion] Adapter for std::array

Subject: Re: [boost] [fusion] Adapter for std::array
From: Mateusz Loskot (mateusz_at_[hidden])
Date: 2013-07-07 17:53:19

On 6 July 2013 01:25, Jonathan Wakely <jwakely.boost_at_[hidden]> wrote:
> On 6 July 2013 00:47, Mateusz Loskot <mateusz_at_[hidden]> wrote:
>> On 5 July 2013 10:44, Jonathan Wakely <jwakely.boost_at_[hidden]> wrote:
>>> On 5 July 2013 00:57, Mateusz Loskot wrote:
>>>>
>>>> Unfortunately, I have hit t apparently well-known problem.
>>>> It is the Fusion begin/end ADL issues with C++0x range-based for
>>>> and such, as discussed and reported here:
>>>>
>>>> http://lists.boost.org/Archives/boost/2010/12/174199.php
>>>> https://svn.boost.org/trac/boost/ticket/4028
>>>>
>>>> There has been no feedback to the ticket #8241, so I'm not
>>>> sure what is the Fusion authors' view on std::array adoption.
>>>> So, a couple of questions:
>>>>
>>>> Do the Fusion authors consider any cure for the ADL issues?
>>>
>>> Stop using C++0x compilers and use C++11 compilers?  :)
>>>
>>> We fixed the standard via Option 5 from
>>> http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2011/n3257.pdf
>>
>> Jonathan,
>>
>> Thanks for this paper.
>>
>> It is still unclear to me if/how the option 4/5 might solve the ambiguous
>> calls to begin/end as here:
>>
>> std::array<int, 3> = {{1,2,3}};
>> boost::fusion::distance(begin(arr), end(arr)) == 3;
>>
>> as the std::array does provide begin/end members.
>>
>> AFAIU, solution that remains is to fully qualify:
>>
>> boost::fusion::distance(boost::fusion::begin(arr),
>> boost::fusion::end(arr)) == 3;
>>
>> So, the following rule kicks:
>>
>> "to call begin() and end() members if they exist, and if not then to
>> make unqualified calls as specified in the current draft."
>>
>> I'm missing something, am I not?
>
> No, I think I'm missing something:  I thought you meant the problem
> was with the range-based for loop, but it's with other uses of
> unqualified begin/end.

Yes, I didn't explained the issue in details, just lazily pointed to some
referencess.

> The standard was changed to *not* use
> unqualified begin/end when the type has begin/end members, because
> when there are multiple "greedy" overloads that can be found by ADL
> there is no simple way to prevent ambiguity.

Right, it makes perfect sense to me.

> So I agree that your best option might be to avoid those unqualified calls.

Thanks, that itself is is a very important opinion for me.

If Joel agrees, perhaps the std::array adaptor could be
added to Fusion with the restrictions documented.

Best regards,
--
Mateusz  Loskot, http://mateusz.loskot.net