Re: [Boost-users] iterator_facade and distance()

Subject: Re: [Boost-users] iterator_facade and distance()
From: Dave Abrahams (dave_at_[hidden])
Date: 2011-04-16 19:54:21

At Sun, 17 Apr 2011 08:43:00 +0900,
Andrej van der Zee wrote:
> 
> Hi Dave,
> 
> Thanks for your email.
> 
> >
> > Not unless yours is a random access iterator. Â Is it?
> >
> 
> Its a forward iterator. Changing it to a random access iterator solves
> it indeed.
> 
> This is my first iterator that I am writing, but why is it not
> possible to implement a custom distance method for forward iterators?

The implementation of std::distance only has the iterator category to
go on when deciding whether to move by many steps or by one jump.
There isn't any other information it can use.

> Am I doing something conceptually wrong? 

Could be.  If you can move your iterator by N steps in one jump, maybe
it should be a random access iterator.

> Or should I specialize the
> std::distance() function?

You could specialize that function for specific iterator types.
There's no partial specialization of function templates, and you're
not allowed to add an overload to namespace std.

-- 
Dave Abrahams
BoostPro Computing
http://www.boostpro.com