Re: [Boost-users] Using filter_iterator in a for loop

From: David Abrahams (dave_at_[hidden])
Date: 2007-01-03 11:17:32

"David Ward" <david.ward_at_[hidden]> writes:

> I want to use a boost::filter_iterator to iterate through a std::vector 
> using a for-style loop.  I haven't been able to find any examples of the 
> proper way to do this in the Boost documentation (or how to do this for any 
> other Boost iterator, for that matter).
>
>
>>From searching this newsgroup, I found that it is possible to do:
>
> for (boost::filter_iterator< ... > iter(myVector.begin(), myVector.end());
>     iter != boost::filter_iterator< ... >(myVector.end());
>     ++iter )
> { ... }
>
> However, iter is quite aware of when it has reached the end of the vector, 
> since that is the second parameter in its constructor.  Is it really 
> necessary to create a second filter_iterator, set it to the end of the 
> vector, and then test iter against it?  Can't I somehow test iter alone to 
> see if it has reached the end of the vector?

Nope.  It's one of the weaknesses of the STL iterator paradigm.
istream_iterator has the same problem.  It can be remedied by
expanding the STL range concept to allow an end iterator of a
different type.  However, that's beyond the scope of
boost::filter_iterator: it potentially requires changes to all
algorithms, etc.

> If I use std::for_each, it needs both a beginning and ending iterator as 
> parameters, leading to the same situation as above.
>
>
> I've also looked at the Boost Range library and BOOST_FOREACH (described at 
> http://www.boost.org/regression-logs/cs-win32_metacomm/doc/html/foreach.html). 
> However, from my understanding, I don't think either of these apply to or 
> work with Boost iterators.

Not so; they work perfectly well with all conforming STL iterators.

> Whatever method is appropriate, I would ask that someone please exapnd the 
> Boost iterator documentation to include an example of using Boost iterators 
> in a for-style loop.  I feel that this is a very basic and common usage of 
> iterators which was not addressed.  Thank you.

I don't think there's really anything to write here; they're just
iterators -- you use them the same way you'd use any other STL
iterator.

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