Re: rule's attribute captures too much?

On 09/11/2016 4:44 PM, [email protected] wrote:
> Hello,
>
> Trying to parse "0/1/x" with
>
> qi::ascii::digit >> +(qi::char_('/') >> qi::ascii::digit)
>
> I had expected the synthesized attributes value to be "0/1".
>
> But he following code results in "0/1/", where the suffix is "/x". Is this the correct behavior?

It might not be intuitive, but I think it is the correct behavior. Qi
is greedy and will eat as much as it can. '/' is valid at that point,
while digit, after it, will fail and thus exit the + loop. The other
interpretation will be very inefficient requiring backtracking and
attribute saving and try-commit semantics.

If you want the behavior you expect, use syntactic predicates or
some other means to test first before committing the attribute.
Example:

   digit >> +(('/' >> &digit) >> attr('/') >> digit)

Regards,
-- 
Joel de Guzman
http://www.ciere.com
http://boost-spirit.com
http://www.cycfi.com/


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