Re: [Boost-users] [xpressive] questions about semantic actions

Subject: Re: [Boost-users] [xpressive] questions about semantic actions
From: Eric Niebler (eric_at_[hidden])
Date: 2012-08-05 03:12:02

On 8/4/2012 11:57 PM, Nathan Ridge wrote:
> Hello,
> 
> I am trying to use semantic actions in Boost.Xpressive, and I have a
> couple of questions:
> 
> 1. When a semantic action is attached to a subexpression of a regex, what
>    is the meaning of _, s1, s2 etc. in the semantic action? For example,
>    suppose you have the following regex:
>       ((s1 = xpr1)[action]) >> (s2 = xpr2)
>    Inside 'action':
>      - is it valid to use '_'?
>         - if so does it refer to just the portion matched by xpr1?
>      - is it valid to use 's1'?
>      - is it valid to use 's2'?

Yes, this is all described in the docs.

http://www.boost.org/doc/libs/1_50_0/doc/html/xpressive/user_s_guide.html#boost_xpressive.user_s_guide.semantic_actions_and_user_defined_assertions

> 2. Is there a way to access the regex_id of the regex to which a semantic
>    action belongs from the semantic action? More generally, can a semantic
>    action be written as a custom function object that takes as argument
>    some sort of environment (perhaps a match_results<>)?

Please see the sections in the docs on refering to local and non-local
variables from within semantic actions. Non-local variables will do what
you want, I think.

http://www.boost.org/doc/libs/1_50_0/doc/html/xpressive/user_s_guide.html#boost_xpressive.user_s_guide.semantic_actions_and_user_defined_assertions.referring_to_non_local_variables

-- 
Eric Niebler
BoostPro Computing
http://www.boostpro.com