Re: [Boost-users] BGL defining my own iterator?

Subject: Re: [Boost-users] BGL defining my own iterator?
From: Jeremiah Willcock (jewillco_at_[hidden])
Date: 2012-01-24 16:17:14

On Tue, 27 Dec 2011, Anders Wallin wrote:

>> In your second test, begin and end have the same edge, so they compare equal
>> using your iterator's == operator, breaking the loop immediately. Does the
>> do loop you use in your first test case work with iterators?  If so, you
>> will need some special-case way to create an end iterator; I know
>> Boost.Intrusive has a circular linked list class (slist), so checking that
>> code might show how to set up the iterators.
>
> I now got both a do-while and a BOOST_FOREACH loop working, but only
> when building in debug mode.
> When built in release mode (NDEBUG and -O3) I the iterator gives
> incorrect results in both cases.
> Output along with a compilation warning is here: http://pastebin.ca/2096568
>
> updated code is here:
> https://github.com/aewallin/sandbox/blob/master/bgl_iterator/main.cpp
>
> what causes this difference in behaviour with NDEBUG and -O3 ?

Sorry about not responding to this earlier.  The code you posted will not 
work for an empty iterator range (due to the check for m_inc) and it seems 
like comparisons of arbitrary edge_iterators (rather than just a 
comparison to the end) are not supported.  I looked at how 
boost::intrusive::slist does their iterators, and they have a dummy node 
in the circular list that is used to represent the end.  It looks like 
CGAL doesn't try to provide the iterator-pair interface to their circular 
lists at all.  Maybe you could store a pointer to the last element of the 
list (or the first) in each iterator, and special-case incrementing onto 
that element to act like a non-circular list would (using an invalid list 
element).  Would that make sense for you?  Are you going to be changing 
these edge cycles often?

-- Jeremiah Willcock