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

On 6 July 2013 00:47, Mateusz Loskot wrote:

On 5 July 2013 10:44, Jonathan Wakely wrote:

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