David Abrahams <dave@boost-consulting.com> writes:
Raoul Gough <yg-boost-users@m.gmane.org> writes:
I have a situation where I want to specialize a class template on the basis of iterator_category, but I don't see how to get one of the standard categories when passed a boost iterator adapter. [snip] BTW, isn't this non-standard iterator_category bound to cause problems? e.g. if I use std::distance (MyIter(), MyIter()), how does the compiler know that it should use the std::random_access_iterator_tag specialization of std::distance?
Does your standard library really have a *specialization*? Most libraries use tag dispatching to discriminate categories - it works nicely with the inheritance structure of existing categories.
Aha - I had even tested std::distance, but couldn't figure out how it knew that the boost category matched random_access_iterator_tag. For some reason, I didn't realise that it was using function overloading rather than specialization. Of course, a specialization wouldn't make any sense in this case. On the other hand, I actually do need a specialization for the python range class extensions (which is where the original problem arose)...
Technically, you are right - a standard library is allowed to use specialization. AFAIK, none of them do, and we hope that when (if) our proposal is accepted, tag convertibility will be an acceptable standards-compliant mechanism. So, while I suggest you avoid specialization and use tag dispatching instead, if you must do it:
[snip hairy MPL code]
Oops, I just lifted this code from boost/iterator/iterator_categories.hpp but it's in boost::detail::, so let your conscience be your guide... ;->
OK, I've written something slightly simpler using is_convertible, since I really only need to cover two cases - a forward iterator or better (I'm using is_convertible<Category, std::forward_iterator_tag>) and random_access_iterator_tag or better. I'm using mpl::if to select between base classes for iterator_range. Thanks for putting me back on the right track - I'd forgotton about the whole iterator_category inheritance hierarchy (now revised from Josuttis's /Tutorial and Reference/ :-) -- Raoul Gough "Let there be one measure for wine throughout our kingdom, and one measure for ale, and one measure for corn" - Magna Carta