Hi there,
dereference_iterator_adaptor uses iterator_adapter with a traversal/category type of boost::use_default. The way use_default behaves is to discover the category/traversal type of the iterator it's adapting (bit_aligned_pixel_iterator<...> in this case), then convert that into a traversal type. In this case, bit_aligned_pixel_iterator has a category of std::random_access_iterator_tag, which gets converted back into boost::random_access_traversal_tag. <snip>
I could not figure out how this use_default parameter behaves. With your explanation I think the conversion is done inside iterator_adaptor_base class when putting together a iterator_facade type. I think the compile time function ia_dflt_help is doing it. Please correct me when I'm wrong, but this function just compares against use_default and uses either iterator_traversal<Base> in case of use_default or does nothing. Now iterator_traversal<Base> is doing exactly what we are doing it's invoking std::iterator_traits<Iterator> internally. Though, I still don't quite understand where the boost::random_access_traversal_tag is coming from? But at the end of the day I understand your proposed fix and it works. Thanks a lot for your help, Christian