Custom Skip Parser with Boost::Spirit

Question

The standard ascii::space_type skipper does of course not skip my comments. The docs mention you can make your own skip parser but there is no example of actual

Answer 1

After some experimentation, I have found a way to specify a custom skipper and will outline it here:

template<typename Iterator>
struct pl0_skipper : public qi::grammar<Iterator> {

    pl0_skipper() : pl0_skipper::base_type(skip, "PL/0") {
        skip = ascii::space | ('{' >> *(qi::char_ - '}') >> '}');
    }
    qi::rule<Iterator> skip;
};

template<typename Iterator, typename Skipper = pl0_skipper<Iterator>>
struct pl0_grammar : public qi::grammar<Iterator, Skipper> {

    /* The rules use our skipper */
    qi::rule<Iterator, Skipper> start;
    qi::rule<Iterator, Skipper> block;
    qi::rule<Iterator, Skipper> statement;

};

The secret lies in the call of the parser. For some reason, when you want to parse this using parse_phrase, you have to give a skipper grammar object. I was not aware of this:

typedef std::string::const_iterator iterator_t;
typedef parser::pl0_grammar<iterator_t> grammar;
typedef parser::pl0_skipper<iterator_t> skipper;

grammar g;
skipper ws;

iterator_t iter = str.begin();
iterator_t end = str.end();
bool r = phrase_parse(iter, end, g, ws);

This works.