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

From: Markus Werle (numerical.simulation_at_[hidden])
Date: 2006-11-18 15:42:42

David Abrahams wrote:

>> 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.

Does that mean that I cannot create a standard conforming 
random access iterator for a class that behaves as a read-only 
proxy for values calculated on-the-fly when requested?

>> 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.

So the best way is to define

  distance(MyIterator, MyIterator);
  lower_bound...
  etc.
 
in namespace std?
 
>> 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 

I kind of fixed it by overloading operator- in order to avoid 
-distance_to. Now it "works". Is this dangerous?

> (and new-style) iterators.

So using std::size_t as index argument type is a bad idea
if I want to plug iterators? I mean: AFAIK it is not guaranteed
that I have a signed type that can hold std::size_t.
Correct me if I am wrong, please.

Markus