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

From: David Abrahams (dave_at_[hidden])
Date: 2006-11-17 10:15:10

Markus Werle <numerical.simulation_at_[hidden]> writes:

> Sebastian Redl <sebastian.redl <at> getdesigned.at> 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!

Patches welcomed.

> So I can use std::random_access_iterator_tag instead?

Not with your iterator definition; you'd be lying to the standard
library.  A standard random access iterator has a reference type of
value_type&.  Without that, your iterator is just an input iterator
from the STL point-of-view.

> I simply want an iterator that works with current STL as random
> iterator. 

Then use a real reference type.  Sorry, the STL iterator design took a
limited worldview and you have to conform to it if you want to get all
the expected optimizations.

> Class Demo uses a std::size_t as index type.
> How do I build my iterator with iterator_facade?

Use a signed distance type; that's a requirement of STL (and
new-style) iterators.

-- 
Dave Abrahams
Boost Consulting
www.boost-consulting.com