[Boost-users] Re: MPL iterator category

Aleksey Gurtovoy writes:

Writing an algorithm for _bidirectional_ iterators and making sure that it gets selected for random-access ones as well is somewhat more tricky:

template< typename Iterator > struct algorithm : if_c< iterator_category<Iterator>::type::value >= bidirectional_iterator_tag::value , bidirectional_algorithm<Iterator> , forward_algorithm<Iterator> > { };

but try to write this up assuming that _there is_ an implicit conversion between 'random_access_iterator_tag' and 'bidirectional_iterator_tag', and you'll see that you'll end up with something at least as complex as the above.

If there was a _form_ relationship among MPL iterator tags just like the _inheritance_ relationship among the STL ones, it would be easier to do this with partial specialization. E.g., typedef tag<> forward_iterator_tag; typedef tag > bidirectional_iterator_tag; typedef tag > > random_access_iterator_tag; Unfortunately, I don't know how to make this look right. A specialization on: template struct algorithm< T , tag // Bidirectional

{ ... }; doesn't really communicate well. Maybe some built-in SFINAE facilities would help? template struct algorithm; template<class Iterator> struct algorithm { ... }; -- Dave Abrahams Boost Consulting www.boost-consulting.com