Re: [Boost-users] [iterator] std::distance ignores random_access_traversal_tag

17 Nov 2006

      Sebastian Redl  writes:
...
Markus Werle wrote:
...
Hi!
This was detected by profiling. I expected the code below
to have a rather cheap std::distance call because my iterator
was tagged as random_access.
std::distance doesn't understand the new iterator categories,
Ouch! The tutorial is definitely too short!
So I can use std::random_access_iterator_tag instead?

I simply want an iterator that works with current STL as random
iterator. Class Demo uses a std::size_t as index type.
How do I build my iterator with iterator_facade?

Problem: If I use

template 
class signal_iter
	: public boost::iterator_facade
        ,
	ValueT,
	std::random_access_iterator_tag,
	ValueT>
{ [...] }

iterator_facade implementation applies unary minus to unsigned type
in its code:

      template 
      static typename Facade1::difference_type distance_from(
          Facade1 const& f1, Facade2 const& f2, mpl::true_)
      {
          return -f1.distance_to(f2);
      }

How can I circumvent this?

Markus