Re: [Boost-users] filter_iterator usage

From: David Abrahams (dave_at_[hidden])
Date: 2006-02-23 10:05:00

Nico Galoppo <nico_at_[hidden]> writes:

> Hi,
>
> I have a class with some custom iterators, I would like to add another custom 
> iterator that is a 'filtered' version of the custom iterator. Something like this:
>
> class A
> {
> public:
>    typedef boost::filter_iterator<predicate_type, custom_iterator> 
> filtered_iterator;
>
>    filtered_iterator begin()
>    {
>      predicate_type predicate = boost::bind(...);
>      return boost::make_filter_iterator(predicate, custom_begin(), custom_end());
>    }
> }
>
> Unfortunately, the following code doesn't compile:
>
> A a;
> A::filtered_iterator it = a.begin();
>
> Reason: cannot convert from 'boost::filter_iterator<Predicate,Iterator>' to 
> 'const boost::filter_iterator<Predicate,Iterator>'
>
>
> I can give more compiler output if needed.

Most likely the important output is in the following lines in a pair
of "with" clauses.  In the two clauses, you should expect to see that
either Predicate or Iterator don't match up.

I suggest you not use make_filter_iterator when you already know the
type of iterator you're creating anyway.  Instead, just use
constructor syntax:

      return filtered_iterator(predicate, custom_begin(),
      custom_end());

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