Zero-filled results for unused (but matching) rules

Question

I have a grammar similar to the following:

ruleFormat %= ruleComment | ruleSpec;
ruleFile %= ruleFormat % ruleNewline;

And rules are so declared:

rule<Iterator, void()> ruleComment;
rule<Iterator, int()> ruleSpec, ruleFormat;
rule<Iterator, std::vector<int>()> ruleFile;

(I am not actually using int, but a structure with some ints in it).

Well, the grammar seems to work properly: when ruleSpec matches an integer, the rule has that attribute which is in turn given to the vector.

The problem is when ruleComment matches in ruleFormat, because the data is set to be null, and a null value is placed inside the vector. The result is that the vector is filled with a mix of "right" values and "wrong" zeroes.

How can I prevent this from happening?

EDIT: as requested, here a sample code:

template <typename Iterator>
struct MyGrammar: public qi::grammar<Iterator, std::vector<int>()> {
    MyGrammar(): MyGrammar::base_type(ruleFile) {
        ruleComment = qi::lit('@');
        ruleSpec %= qi::int_;

        ruleFormat %= ruleComment | ruleSpec;
        ruleFile %= ruleFormat % ' ';

        ruleComment.name("COMMENT");
        ruleSpec.name("SPEC");
        ruleFormat.name("FORMAT");
        ruleFile.name("FILE");

        qi::debug(ruleComment);
        qi::debug(ruleSpec);
        qi::debug(ruleFormat);
        qi::debug(ruleFile);
    }
    qi::rule<Iterator, void()> ruleComment;
    qi::rule<Iterator, int()> ruleSpec, ruleFormat;
    qi::rule<Iterator, std::vector<int>()> ruleFile;
};

int main() {
    typedef std::string::const_iterator Iter;
    std::string subj = "0 @ @ 12 3 @ 4 @ 56 @ ";

    MyGrammar<Iter> gram;
    qi::parse(subj.cbegin(), subj.cend(), gram);
    return 0;
}

From the debug information, I am getting:

<attributes>[[0, 0, 0, 12, 3, 0, 4, 0, 56, 0]]</attributes>

while I'd like to obtain is avoid to place 0 in the vector when @ matches

<attributes>[[0, 12, 3, 4, 56]]</attributes>

Ok, I'm a bit short of time, but I'll try to provide a minimal working example. — AkiRoss, Jan 31 '14 at 15:56
Haha. We're also short on time. You didn't show the declaration of `ruleFormat`. This is the most crucially interesting bit. — sehe, Jan 31 '14 at 16:02
This describes the same situation: http://stackoverflow.com/questions/20038186/boost-spirit-qi-omit-element-in-kleene-star-parser — Mike M, Jan 31 '14 at 17:18
@AkiRoss thanks for updating with a SSCCE (module `#include` line). I agree with the MikeM, voting as a duplicate. — sehe, Jan 31 '14 at 23:24

Zero-filled results for unused (but matching) rules

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