[Boost-users] [Xpressive] Placeholder for user defined assertion

Subject: [Boost-users] [Xpressive] Placeholder for user defined assertion
From: LOL (nwd95jag3k_at_[hidden])
Date: 2014-05-14 12:59:57

I am trying to use a placeholder object for the check function in an xpressive regex.
The error I usually get is "error C2064: term does not evaluate to a function taking 1 arguments"

Please see my example below that uses the three_or_six user-defined assertion as described at http://www.boost.org/doc/libs/1_51_0/doc/html/xpressive/user_s_guide.html#boost_xpressive.user_s_guide.semantic_actions_and_user_defined_assertions.user_defined_assertions.

I am using boost 1.51 and Visual Studio 2010 Express.

This is a contrived example; the real reason I am trying to do this is because my assertion has state that is not known at the time the regex is compiled. In the interim I have worked around it by using static variables instead of placeholders but that makes for more brittle code than I would like.

Also, is there any way to reuse the match_results objects that are created by the iterator syntax?

Thanks,
Michael Mitchell

//Inspired by https://groups.google.com/forum/#!topic/boost-list/hTBAT7KMgd4
#include <iostream>
#include <boost/xpressive/regex_actions.hpp>
#include <boost/xpressive/xpressive_static.hpp>

using namespace std;
using namespace boost::xpressive;

// A predicate that is true IFF a sub-match is
// either 3 or 6 characters long.
struct three_or_six
{
    bool operator()(ssub_match const &sub) const
    {
        return sub.length() == 3 || sub.length() == 6;
    }
};

int main(int argc, char* argv[])
{
        int cnt = 0;
        string a_str("a_aaaaa___a_aa_aaa_");

        // match words of 3 characters or 6 characters (this works).
        sregex expr1 = (bow >> +_w >> eow)[ check(three_or_six()) ] ;
       
        placeholder<three_or_six> tos_ph;
        sregex expr2 = (bow >> +_w >> eow)[ check(tos_ph) ] ;
        // The above does not work (error C2064: term does not evaluate to a function taking 1 arguments)

        placeholder<three_or_six*> tos_ptr_ph;
        sregex expr3 = (bow >> +_w >> eow)[ check(*tos_ptr_ph) ] ;
        // The above does not work (error 2676: binary '&&' : 'three_or_six' does not define this operator or a converstion to a type acceptable to the predefinied operator)

        sregex_iterator iter1(a_str.begin(), a_str.end(), expr1);

        three_or_six tos;
        sregex_iterator iter2(a_str.begin(), a_str.end(), expr2, let(tos_ph = tos));

        three_or_six* tos_ptr = new three_or_six();
        sregex_iterator iter3(a_str.begin(), a_str.end(), expr3, let(tos_ptr_ph = tos_ptr));

        return 0;

}