On 12/21/2010 5:33 AM, Christopher Schmidt wrote: [...]
Let me throw another possibility into the mix. This problem - a possible ambiguity between fusion::begin and std::begin/boost::begin is uncommon. In fact, only adapted boost::array's and std::array's are both fusion sequences and ranges. To get ADL kicking in we'd actually need an array of one of the inbuilt fusion container...
We could disable fusion::begin/fusion::end for those few types that may collide via a new fusion extension metafunction.
Say:
namespace boost{namespace fusion { namespace extension {template<typename>struct disable_begin_end_impl;}
namespace result_of {template<typename>struct begin;};
template<typename Seq> typename lazy_enable_if< mpl::and_< traits::is_sequence<Seq> , mpl::not_<extension::disable_begin_end_impl< typename traits::tag_of<Seq>::type >> > , result_of::begin<Seq> >::type begin(Seq&& seq); }}
That way we'd just cripple the fusion interface for adapted arrays and probably a minority of user-defined fusion sequences. Ultimately, to make such types full-blown fusion sequences again, we could offer a new (meta-)function, for example fusion::pack/fusion::result_of::pack, which takes such a 'reduced' fusion sequence and returns an implementation-specific object/type that encapsulates the passed sequence and offers a full-blown fusion interface. To do so, that wrapper forwards its fusion implementation to the implementation of the underlying type - but always returns mpl::false_ for extension::disable_begin_end_impl.
It's not pretty - but this approach does not break much and we'd keep ADL, even for fusion::begin/fusion::end, in most cases.
Do I make sense?
I would like to see a situation where finding boost::fusion::begin/end via ADL is actually useful (i.e., couldn't be replaced by explicit qualification). Until then, I believe preventing boost::fusion::begin/end (and, possibly, boost::begin/end) from being found via ADL is a better solution; further, it explicitly discourages any future code from relying on ADL (assuming that relying on ADL *should* be discouraged). - Jeff