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 20:59:04

On 8/5/2012 1:21 PM, Nathan Ridge wrote:
> 
>>>>> 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<>)?
>>
>> <snip>
>>
>> The action does not receive the match_results. The only way to get data
>> into and out of an action is the way described in the docs. You will
>> have to use xpressive::value, xpressive::reference or
>> xpressive::placeholder to hold either the regex_id, a reference to the
>> regex_id, or a promise to return a regex_id respectively. You can use
>> those within the action.
> 
> All of these require providing the regex_id *manually*.
<snip>

Right.

> What I would like to do is this:
> 
> sregex rx = xpr[cout << _regex_id_];  // _regex_id_ is a special symbol
> 
> smatch what;
> // no need to provide anything here! _regex_id_ will be replaced by the
> // regex_id of rx
> regex_match(str, what, rx);
> 
> The reason this is important is because the action may be attached to a
> subregex of the regex on which the match is being done. For example:
> 
> placeholder<regex_id_type> _regex_id;
> sregex sub = xpr[cout << _regex_id];
> sregex rx = sub >> another_xpr;
> 
> smatch what;
> what.let(_regex_id = ???);  // sub.regex_id() needs to go here, but this
>                             // code knows only about rx, not about sub!
> regex_match(str, what, rx);
> 
> Is there no way to accomplish this?

Not at present, no. I don't think it would be hard to add. Could you
file a feature request?

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