Use of optional parser in spirit qi

Question

I'm trying to parse either an additive expression of the form "A+C", or "A" alone. After a few tests I realized that the problem is apparently my use of the optional parser, so to exemplify:

qi::rule<string::iterator, string()> Test;

Test =
(
    qi::string("A")[qi::_val= qi::_1]
    >> -(
            qi::string("B")[qi::_val += qi::_1]
            >> qi::string("C")[qi::_val += qi::_1]
        )
)
;

string s1, s2;
s1 = "AB";
bool a= qi::parse(s1.begin(), s1.end(), Test, s2);

The idea is to parse 'A' or "ABC", but if the s1 value is "AB" without 'C', the value of a is true. I believe that although I put parenthesis after the operator '-' and then I use the ">>" operator, the 'C' part is considered optional, and not the B>>C as a whole. Any ideas?

score 2 · Accepted Answer · edited May 23 '17 at 12:29

Container attributes are not backtracked.

That's a performance choice. You need to explicitly control propagation using e.g. qi::hold:

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#include <boost/spirit/include/qi.hpp>

namespace qi = boost::spirit::qi;

int main() {
    using It = std::string::const_iterator;
    qi::rule<It, std::string()> Test;

    Test =
        (
         qi::char_('A')
         >> -qi::hold [
                qi::char_('B')
            >> qi::char_('C')
         ]
        )
        ;

    for (std::string const input : { "A", "AB", "ABC" })
    {
        std::cout << "-------------------------\nTesting '" << input << "'\n";
        It f = input.begin(), l = input.end();

        std::string parsed;
        bool ok = qi::parse(f, l, Test, parsed);
        if (ok)
            std::cout << "Parsed success: " << parsed << "\n";
        else
            std::cout << "Parsed failed\n";

        if (f != l)
            std::cout << "Remaining unparsed: '" << std::string(f,l) << "'\n";
    }
}

Prints:

-------------------------
Testing 'A'
Parsed success: A
-------------------------
Testing 'AB'
Parsed success: A
Remaining unparsed: 'B'
-------------------------
Testing 'ABC'
Parsed success: ABC

Note I have made a number of simplifications.

Use of optional parser in spirit qi

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