I have noticed now that the MPL documentation actually says: "An iterator i is incrementable if there is a "next" iterator, that is, if next<i>::type expression is well-defined; past-the-end iterators are not incrementable." I guess this takes care of the recursion by cutting it off when it is no longer possible. It seems that such requirement can be implemented with Doug's technique that I linked to in my previous email. Cheers, Marcin On Tue, Sep 8, 2009 at 11:53 PM, Marcin Zalewski<marcin.zalewski@gmail.com> wrote:
I have been trying to implement a concept for mpl ForwardSequence, and besides the lack of const expr requirements in the latest version of concepts I have stumbled across the problem of concept recursion. One example of such recursion is the ForwardIterator [1] concept. The requirement on the next<i>::type type is that it must be a ForwardIterator. One could try to write a concept like that:
concept ForwardIteratorStatic<typename I> { typename deref; typename next;
requires ForwardIteratorStatic<next>; }
But for the above concept, the ConceptGCC compiler gives an error. Also, this should not compile according to the latest concepts proposal that states that a concept is not defined until the closing brace of the concept body. As for now, I think that one can try to apply technique described by Doug [2]:
concept IteratorStatic<typename I> { }
concept ForwardIteratorStatic<typename I> : IteratorStatic<I> { typename deref; typename next;
requires IteratorStatic<next>; }
But to get to my question finally, I wonder if the recursion in mpl is intentional and thought out, or is it an accident that should be re-examined. If the decision was indeed thought out, I really wonder what would be a reasonable semantics of concept recursion.
Cheers, Marcin
[1] http://www.boost.org/doc/libs/1_40_0/libs/mpl/doc/refmanual/forward-iterator... [2] http://www.osl.iu.edu/MailArchives/conceptgcc/2007/03/0084.php