Re: [boost] Boost.Iterator filter_iterator issues

Subject: Re: [boost] Boost.Iterator filter_iterator issues
From: Steven Watanabe (watanabesj_at_[hidden])
Date: 2009-09-26 22:34:59

AMDG

Christoph Heindl wrote:
> I think I've found an issue with boost::filter_iterator when
> decrementing a valid iterator without having any predicate evaluating
> to true on its way back.
>
> filter_iterator.hpp line# 93 states
>
> void decrement()
> {
>   while(!this->m_predicate(*--(this->base_reference()))){};
> }
>
> which will ?clearly? pass beyond the initial start iterator if no
> predicate evaluates to true.

It's undefined behavior if you try to decrement an iterator like that.

> Is it a requirement for the iterator
> being adapted by filter_iterator to catch decrements past the
> containers begin range?

Consider an unfiltered range.  e.g. [0, 1, 2].
If you have an iterator pointing to the 0, then it
is undefined behavior to decrement it, right?
That's just the way the STL works.  If you
use a filter_iterator with the predicate _1 > 1,
you get a range containing [2].  Again, it's
undefined behavior to decrement an iterator
pointing to the 2.  You can't decrement an
iterator that points to the beginning of a range.

In Christ,
Steven Watanabe