Re: [Boost-users] [spirit] how to get_position() from a multi_pass<position_iterator <const char*> >

Subject: Re: [Boost-users] [spirit] how to get_position() from a multi_pass<position_iterator <const char*> >
From: Peter Schueller (schueller.p_at_[hidden])
Date: 2009-08-23 07:10:02

On Fri, Aug 21, 2009 at 23:37, Hartmut Kaiser<hartmut.kaiser_at_[hidden]> wrote:
>> On Thu, Aug 20, 2009 at 22:42, Hartmut Kaiser<hartmut.kaiser_at_[hidden]>
>> wrote:
>> >> I use a position_iterator<const char*> for parsing with
>> boost::spirit.
>> >>
>> >> Because I need the regex_p feature, I have to wrap this iterator
>> >> using a multi_pass iterator.
>> >>
>> >> I call pt_parse and get a
>> >>
>> >> boost::spirit::tree_parse_info<multi_pass<position_iterator<const
>> >> char*> >, ...> parse_info.
>> >>
>> >> Now I have problems calling the get_position() method on
>> >> "parse_info.stop".
>> >>
>> >> I guess this is the case because multi_pass does not store my
>> >> iterator but wraps it using boost::iterator which loses the position
>> >> features of position_iterators and only keeps the const char*
>> pointer.
>> >>
>> >> Do you have an advice for me?
>> >
>> > It's difficult to tell from here as you didn't provide too much
>> information.
>> > Would you mind posting a small example exposing your problem?
>>
>> I have attached a minimal example.
>>
>> If you do not define MULTIPASSDBG, it builds but you are not allowed to
>> use regex_p in the parser.
>>
>> If you define it, you are allowed to use regex_p, but the get_position
>> does no longer compile and so the iterator becomes a bit useless.
>>
>> I read something about a "base()" method for wrapping iterators, but
>> could not find one in the multi_pass sources.
>
> Ok, I understand. But I think your whole approach is not viable. Regex_p requires a bi-directional iterator, while multi_pass exposes an input_iterator Only. So generally, even if it compiled it wouldn't work in the end.

I see, obviously I did not read the documentation closely enough, I
just found out that it states regex_p requires a bidirectional
iterator and multi_pass exposes a forward iterator.

Nevertheless the combination multi_pass/regex_p compiled and worked fine for me.

regex_p even worked fine with only the position_iterator (until you
changed the position_iterator iteration type) which made it no longer
compile with regex_p.

(I used regex_p in the first place because lexeme_p included leading
whitespace (which is really a nuisance to me), regex_p does not create
this issue).

> But to answer your original question, multi_pass doesn't allow to access the base iterator, which might be an oversight. I'll have a closer look and add it to the multi_pass iterator if possible.

Thank you for this.

I wonder if it would be possible to create some multi_pass like
iterator which allows regex_p to be used with a position_iterator
(both are very handy tools and it is quite of sad that they cannot be
used together).

Is there a solution to this in Spirit V2.1?

Nevertheless thank you a lot for your work on spirit, it is great! :)

Regards,
Peter