C++ Boost Spirit, parsing data and storing the maximum

Question

I'm trying the code sehe gave here : Boolean expression (grammar) parser in c++

I would like to create a string variable max, that would store the maximum variable encountered at each parsing (on the lexicographic order, for example).

I tried things like :

1. var_ = qi::lexeme[ +alpha ] [_val = _1, if_(phx::ref(m) < _1) [phx::ref(m) = _1]];, but there is a (really long) compilation error

2. var_ = qi::lexeme[ +alpha [_val = _1, if_(phx::ref(m) < _1) [phx::ref(m) = _1]]]; but with this one I only get the first caracter of a variable, which is restrincting.

I also tried to simplify things using integers instead of string for variables, but var_ = int_ [...] didn't work either, because int_ is already a parser (I think).

Do you have any ideas ?

Thanks in advance

Answer 1

I'd say that

start = *word [ if_(_1>_val) [_val=_1] ];

should be fine. However, due to a bug (?) Phoenix statements in a single-statement semantic action do not compile. You can easily work around it using a no-op statement, like e.g. _pass=true in this context:

start = *word [ if_(_1>_val) [_val=_1], _pass = true ];

Now, for this I assumed a

rule<It, std::string()> word = +alpha;

If you insist you can cram it all into one rule though:

start = *as_string[lexeme[+alpha]] [ if_(_1>_val) [_val=_1], _pass = true ];

I don't recommend that.

Demo

Live On Colir

#define BOOST_SPIRIT_USE_PHOENIX_V3
#include <boost/spirit/include/qi.hpp>
#include <boost/spirit/include/phoenix.hpp>

namespace qi  = boost::spirit::qi;
namespace phx = boost::phoenix;

template <typename It, typename Skipper>
struct max_parser : qi::grammar<It, std::string(), Skipper> {
    max_parser() : max_parser::base_type(start) {
        using namespace qi;
        using phx::if_;

#if 1
        word  = lexeme [ +alpha ];
        start = *word [ if_(_1>_val) [_val=_1], _pass = true ];
#else
        start = *as_string[lexeme[+alpha]] [ if_(_1>_val) [_val=_1], _pass = true ];
#endif
    }

  private:
    qi::rule<It, std::string(), Skipper> start, word;
};


int main() {

    std::string const input("beauty shall be in ze eye of the beholder");
    using It = std::string::const_iterator;
    max_parser<It, qi::space_type> parser;

    std::string data;
    It it = input.begin(), end = input.end();
    bool ok = qi::phrase_parse(it, end, parser, qi::space, data);

    if (ok) {
        std::cout << "Parse success: " << data << "\n";
    } else {
        std::cout << "Parse failed\n";
    }

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

Prints:

Parse success: ze