Subject: Re: [Boost-users] Strange iterator_traits of iterator_facade
From: Swerts, Benjamin (Benjamin.Swerts_at_[hidden])
Date: 2010-02-26 04:46:56
> AMDG
>
> Swerts, Benjamin wrote:
> > I have a class that is templated on an iterator type. As it
> only works
> > with bidirectional and random access iterators, I thought I would
> > impose that as follows:
> >
> > template<typename T> class A
> > {
> > A() {
> checkIterator(std::iterator_traits<T>::iterator_category());
> > };
> >
> > template<typename Tag> void checkIterator( Tag );
> > void checkIterator( std::bidirectional_iterator_tag ) {};
> > void checkIterator( std::random_access_iterator_tag ) {};
> > void checkIterator( boost::random_access_traversal_tag ) {}; };
> >
> > This works when T is a standard iterator but not when it's
> one of my
> > iterator_facade subclasses. For example:
> >
> > class const_iterator : public
> iterator_facade<const_iterator, const B,
> > boost::random_access_traversal_tag, B>
> >
> > When using this class with A<const_iterator> I get a linker error
> > because the checkIterator function is called with:
> >
> > boost::detail::iterator_category_with_traversal<struct
> > std::input_iterator_tag,struct boost::random_access_traversal_tag>
> >
> > I certainly cannot add a specialization of checkIterator
> with this tag
> > to class A.
> > Could anyone shed some light on this problem? I'm working
> with Boost
> > 1.39.0 on MSVC2008SP1.
> >
>
> Try just having a single overload:
>
> void checkIterator(std::bidirectional_iterator_tag) {}
>
> In Christ,
> Steven Watanabe
Unfortunately that doesn't work. Now it complains also about regular
random access iterators. Tags are apparently not automatically converted
into each other.
Maybe I should keep the single templated checkIterator function and
determine the type of the tag using some MPL magic? I doubt I could come
up with such a solution myself as I never used it before. I made some
tests with boost::mpl::iterator_category but it looks like the iterator
should typedef 'category' instead of 'iterator_category'.
Thanks for the help so far.
Greets,
Ben