[Boost-users] [xpressive] Accessing named capture from another scope in the semantic action

Subject: [Boost-users] [xpressive] Accessing named capture from another scope in the semantic action
From: Roger (perrog_at_[hidden])
Date: 2014-01-19 14:23:04

Hi!

Is it possible access captured group from another scope in the
semantic action? Can I write a lazy function somehow and access
another scope?
The following code results in ”bad_lexical_cast” exception, likely due
to the named capture "Year" is an empty string in the "outer scope".

    const sregex yyyymmdd = (Year=_d>>_d>>_d>>_d) >> (Month= _d>>_d)
>> (Day= _d>>_d);

    sregex rx =     yyyymmdd [ tm_year(t) = as<int>(Year) - 1900 /*, …
*/ ] // bad lexical cast: source type value could not be interpreted
as target
                >>  hhmmss   [ tm_hour(t) = as<int>(Hour) /*, …*/ ]
            ;

In my case the regex has no recursion, no captured repeated groups, so
I can't understand really why I got nested result set.

Dumping the nested result tree indicates that the values are captured
in another ”scope”. But this is in the match_result<> set, and I can't
figure out if there is any equivalent set available inside the
semantic action.

20140119190000   #rx
day =
hour =
minute =
month =
second =
year =
    20140119   #yyyymmdd
     day = 19
     hour =
     minute =
     month = 01
     second =
     year = 2014
    190000   #hhmmss
     day =
     hour = 19
     minute = 00
     month =
     second = 00
     year =

I’m asking mostly for educational reason.
Thanks for any hints!
perrog

Code:
mark_tag Year(1),Month(2),Day(3),Hour(4),Minute(5),Second(6), Micro(7);
const sregex yyyymmdd = (Year=_d>>_d>>_d>>_d) >> (Month= _d>>_d) >>
(Day= _d>>_d);
const sregex hhmmss = (Hour=_d>>_d)>>(Minute=_d>>_d)>>(Second=_d>>_d);

bool is_datetime(std::string const& input)
{
    // …don’t need any semantic actions when validating…
    smatch ignore;
    sregex rx = yyyymmdd >> hhmmss;
    return regex_match(input, ignore, rx);
}

bool get_datetime(std::string const& input, struct tm *time)
{
    local<struct tm*> t(time);
    function<int tm::*>::type tm_year = { &tm::tm_year };
    function<int tm::*>::type tm_hour = { &tm::tm_hour };
    // …

    sregex rx =     yyyymmdd [ tm_year(t) = as<int>(Year) - 1900 /*, …
*/ ] // bad lexical cast: source type value could not be interpreted
as target
                >>  hhmmss   [ tm_hour(t) = as<int>(Hour) /*, …*/ ]
            ;

    smatch ignore;
    return regex_match(input, ignore, rx);
}