Re: Trouble grokking iterator adaptors

From: David Abrahams (dave_at_[hidden])
Date: 2003-04-16 09:22:48

"Kris Braun" <kbraun_at_[hidden]> writes:

> Hi there,
>
> I could really use the features provided by iterator adaptors part of 
> the library, but I'm having a lot of trouble getting a nested set of 
> adaptors to compile. In one case, I'd like to return an iterator into 
> std::map< std::string, boost::shared_ptr< ApiOption > >, and have it 
> return references to options in which option->isTemplate( ) is false. 
> Conceptually, I'd like a filter_iterator->indirect_iterator-
>>projection_iterator chain.

> I've been using this typedef to project onto the second member of the 
> pair:
>     typedef boost::projection_iterator_pair_generator< 
>         select_second< std::string, boost::shared_ptr< ApiOption > >,
          ^^^^^^^^^^^^^
Where is select_second defined with two template parameters?

>         OptionMap::iterator,
>         OptionMap::const_iterator > OptionMapIterators;

Do you need both const and non-const iterators?  If not, the
pair_generator is probably overkill.  

> but I've been at a loss to add indirect_iterator and filter_iterator 
> into the mix. Can anyone help me get unstuck?

I can't really tell what's going on here, but it looks like you
haven't got step 1 right.  Maybe you should try to compile a test
program that just uses the projection you want.

Also, I don't think the existing iterator adaptor fulfills its promise
to work properly with smart pointers.  I am sure that you can do it
(and more easily too) using the sandbox iterator adaptors if that's
an option for you.

// untested
filter_iterator<
   std::logical_not<
       boost::mem_fun_ref_t<bool, &ApiOption::isTemplate>
   >
 , indirect_iterator<
      transform_iterator<
          select_second<boost::shared_ptr<ApiOption> >
        , OptionMap::iterator>
      >
   >
>

HTH,

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