Re: [boost] [Fusion] begin/end ADL issues with C++0x range-based for

Subject: Re: [boost] [Fusion] begin/end ADL issues with C++0x range-based for
From: Christopher Schmidt (mr.chr.schmidt_at_[hidden])
Date: 2010-12-18 11:03:39

Michel MORIN schrieb:
> Hi,
> 
> In some cases, Fusion Sequence cannot be used in
> C++0x range-based for (available on GCC 4.6 Pre-Release).
> For example, this code
> 
>     #include <vector>
>     #include <boost/fusion/include/vector.hpp>
> 
> 
>     int main(int argc, char* argv[])
>     {
>         typedef boost::fusion::vector<int, int> sequence;
> 
>         for (auto x : std::vector<sequence>()) {}
> 
>         return 0;
>     }
> 
> does not compile.
> 
> The C++0x range-based for
>     for (auto x : std::vector<sequence>()) {}
> is equivalent to
>     {
>         auto&& rng = std::vector<sequence>();
>         for (
>             auto it_begin = begin(rng),
>                  it_end   =   end(rng);
>             it_begin != it_end;
>             ++it_begin
>         ) {}
>     }
> Note that rng has std and boost::fusion as its associated namespaces
> and the unqualified begin/end function call triggers ADL.
> 
> The reason for the compiler errors is that the call to begin/end
> is ambiguous between std::begin/end and boost::fusion::begin/end.
> (Also boost::fusion::begin/end tries to instanciate
>  result_of::begin/end::<std::vector<sequence> >
>  which leads to a compiler error.)
> 
> These compiler errors can be avoided by using SFINAE
> (lazy_enable_if with "is_sequence" metafunction)
> to boost::fusion::begin/end:
> 
>     // <boost/fusion/sequence/intrinsic/begin.hpp>
>     template <typename Sequence>
>     inline typename
>         lazy_enable_if<
>             traits::is_sequence<Sequence>
>           , typename result_of::begin<Sequence>
>         >::type const
>     begin(Sequence& seq)
>     {
>         return result_of::begin<Sequence>::call(seq);
>     }
> 
> I attached a patch (against trunk) and a test case.
> The test case does not compile before applying the patch,
> but compiles fine after applying the patch.
> Joel and Christopher, what do you think about this change?
> 
> Regards,
> Michel

Your patch makes perfect sense to me.
BTW., Mathias Gaunard created a ticket for this issue a while ago. He
also proposed disabling fusion::begin/fusion::end via SFINAE.

https://svn.boost.org/trac/boost/ticket/4028

-Christopher