Gabriel Dos Reis wrote:
David Abrahams <dave@boost-consulting.com> writes:
| Daniel Frey <daniel.frey@aixigo.de> writes: | | > Consider: | > | > // A strange way to retrieve my_container.begin(): | > boost::prior( my_container.end(), my_container.size() ); | > | > Is that legal? The second template parameter of boost::prior is | > deduced to an unsigned(!) type, which IMHO means that -n is undefined, | > right? Do we need to fix this? Maybe: | > | > template <class T> | > T prior(T x, typename T::difference_type n) | > { | > std::advance(x, -n); | > return x; | > } | | Well, that wouldn't work! | How would you call prior(p, 4) when p is int*?
I suppose he wanted to say
template<class T> T prior(T x, typename std::iterator_traits<T>::different_type n)
?
You are obviously correct, thank you (But it's still difference_type, not different_type).
| > or shall we document that it's illegal/undefined to call | > boost::prior() with an unsigned second parameter? | | Hmm, no I think if it's unsigned we should either deduce an | appropriate signed type or implement the "backward advance" | ourselves. We could use reverse_iterator to do that, FWIW.
Should we propose std::regress (the counter-part for std::advance) for standardization (via a DR)? The current definition of std::advance seems to limit the number of elements that can be skipped backwards to signed types, but the container can be larger (OK, in theory, never tried it myself :). This would also nicely solve the implementation issue for us. Since then, the reverse_iterator implementation would be this: template <class T, class Distance> T prior(const T& x, Distance n) { return boost::next( std::reverse_iterator<T>(x), n ).base(); } right? Although I wonder what happens if T is already a reverse iterator. Is it legal to reverse a reverse_iterator? Or do we need a trait and implement both cases separately? Regards, Daniel -- Daniel Frey aixigo AG - financial solutions & technology Schloß-Rahe-Straße 15, 52072 Aachen, Germany fon: +49 (0)241 936737-42, fax: +49 (0)241 936737-99 eMail: daniel.frey@aixigo.de, web: http://www.aixigo.de