JOAQUIN LOPEZ MU?Z skrev:
----- Mensaje original ----- De: David Abrahams <dave@boost-consulting.com> Fecha: Viernes, Noviembre 9, 2007 4:23 pm Asunto: Re: [boost] [range] range_size<T> deprecation, updated docs etc. Para: boost@lists.boost.org
on Fri Nov 09 2007, Thorsten Ottosen <nesotto-AT-cs.aau.dk> wrote: [...]
"template<class InputIterator> typename iterator_traits<InputIterator>::difference_type distance(InputIterator first, InputIterator last); 4 Effects: Returns the number of increments or decrements needed to get from first to last. 5 Requires: last shall be reachable from first."
How can one ever use decrements to get from first to last? When InputIterator is a random access iterator, last < first does not violate the precondition.
I'm afraid (last<first) violates the precondition even with random access iterators, according to 24.1.6 and as explicitly acknowledged at http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-closed.html#204 . This is IMHO an unfortunate decision by the committee and renders the "or decrements" bit in the effects clause of std::distance void and misleading.
Thanks for the link. I'm convinced that boost::size(Rng&) can assert the result to be positive and similar for boost::distance(Rng&). The Range concept requires "forward" reachability; otherwise [x,y) makes no sense. -Thorsten