Hi Jeffrey, Jeffrey Lee Hellrung, Jr. wrote:
Just to be clear, as you seemed to indicate something slightly different in another post: Do you propose to prevent boost::begin/end from being picked up by ADL; to prevent boost::fusion::begin/end from being picked up by ADL; or both...?
For boost::begin/end, when I started this thread, I managed not to break existing codes and so boost::begin/end can be picked by ADL. But after I read your post in the Fusion thread, I was beginning to think that cooperation between boost::begin/end and ADL is not important and that solving the ambiguity call problem by ADL barrier is a very simple and acceptable way. For boost::fusion::begin/end, when I started the Fusion thread, I also managed not to break existing codes and so boost::fusion::begin/end can be picked by ADL. But after I read your post in the Fusion thread, I agreed with your solution.
Do either of these begin/end combinations actually *need* to be found via ADL (i.e., is that a desirable feature, regardless of it possibly breaking existing code)?
Now, my answer is no.
Ranges adapted to work with Boost.Range (by defining range_begin/range_end) will generally have to be re-adapted to work with the range-based for loops, so I wouldn't think relying on boost::begin/end to be found via ADL to be too useful...
Agreed.
Also, again to be clear: What are the tradeoffs between defining Boost.Fusion's begin/end in another namespace and bringing them into the boost::fusion namespace with a using declaration; and defining the data structures in another namespace and bringing them into the boost::fusion namespace with a using declaration; or both?
Oh, I remembered the ADL barrier idiom wrongly; I was thinking only the former is the idiom (and I didn't know that a using directive should be used in the former case). Thank you for pointing this out, Jeffrey. The correct idiom is: - Define Fusion's begin/end in another namespace and bringing them into the boost::fusion namespace with a using directive (not with a using declaration). or - Define the data type in another namespace and bringing them into the boost::fusion namespace with a using declaration/directive. The former prevents boost::fusion::begin/end from being picked up by ADL. The latter prevents any ADL for the data type, except ADL triggered by data type's template arguments and base classes. I cannot tell which is more suitable. Do you have any idea? Regards, Michel