parse std::vector<int> from comma separated integers

Question

I'm trying to implement a very specific grammar, which requires me at a certain point to parse a list of comma separated integers. The qi rule looks like the following:

qi::rule<Iterator, ascii::space_type> ident;
qi::rule<Iterator, ascii::space_type> nlist;

...

ident = char_ >> nlist;
nlist = ("(" >> int_ % "," >> ")");

...

I need to pass the values up to the ident rule (The expression ident has to create a syntax tree node, where the parsed values from nlist are required for the constructor). I thought about creating and filling a std::vector and use the semantic action like _val = vector<int>.... What is now unclear to me is how do I create a vector of arbitrary length from this rule, since I do not make any assumptions on how long the input will be or using a predefined vector like the examples.

Is this even possible or does is there a better way to do it?

Answer 1

This is the bread and butter of Spirit Qi.

Just use any compatible attribute type and profit:

    using nlist_t = std::vector<int>;
    using ident_t = std::pair<char, nlist_t>;

    qi::rule<Iterator, ident_t(), qi::ascii::space_type> ident;
    qi::rule<Iterator, nlist_t(), qi::ascii::space_type> nlist;

Note: For std::pair attribute compatibility, include the relevant fusion header:

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#include <boost/fusion/adapted/std_pair.hpp>
#include <boost/spirit/include/qi.hpp>

namespace qi = boost::spirit::qi;

int main()
{
    using nlist_t = std::vector<int>;
    using ident_t = std::pair<char, nlist_t>;

    using Iterator = std::string::const_iterator;
    qi::rule<Iterator, ident_t(), qi::ascii::space_type> ident;
    qi::rule<Iterator, nlist_t(), qi::ascii::space_type> nlist;

    ident = qi::char_ >> nlist;
    nlist = '(' >> qi::int_ % ',' >> ')';

    for (std::string const input : { "a (1,2,3)", "+(881,-2,42)    \n", "?(0)" }) {

        ident_t data;
        if (qi::phrase_parse(input.begin(), input.end(), ident, qi::ascii::space, data)) {
            std::cout << "Parsed: " << data.first << "(";
            for (auto i : data.second) std::cout << i << ",";
            std::cout << ")\n";
        } else
            std::cout << "Parse failed: '" << input << "'\n";
    }
}

Prints

Parsed: a(1,2,3,)
Parsed: +(881,-2,42,)
Parsed: ?(0,)

BONUS

Version with imagined Ast type using phoenix::construct:

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#include <boost/fusion/adapted/std_pair.hpp>
#include <boost/spirit/include/qi.hpp>
#include <boost/phoenix.hpp>

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

namespace OoShinyAst {

    using MyName = char;
    using MyArgument = int;
    using MyArgumentList = std::vector<MyArgument>;

    struct MyIdent {
        MyName         name;
        MyArgumentList args;

        MyIdent() = default;
        MyIdent(MyName name, MyArgumentList args)
            : name(std::move(name)), args(std::move(args)) { }
    };
}

int main()
{
    using Iterator = std::string::const_iterator;
    qi::rule<Iterator, OoShinyAst::MyIdent(),        qi::ascii::space_type> ident;
    qi::rule<Iterator, OoShinyAst::MyArgumentList(), qi::ascii::space_type> nlist;

    nlist = '(' >> qi::int_ % ',' >> ')';
    ident = (qi::char_ >> nlist) [ qi::_val = px::construct<OoShinyAst::MyIdent>(qi::_1, qi::_2) ];

    for (std::string const input : { "a (1,2,3)", "+(881,-2,42)    \n", "?(0)" }) {

        OoShinyAst::MyIdent data;
        if (qi::phrase_parse(input.begin(), input.end(), ident, qi::ascii::space, data)) {
            std::cout << "Parsed: " << data.name << "(";
            for (auto i : data.args) std::cout << i << ",";
            std::cout << ")\n";
        } else
            std::cout << "Parse failed: '" << input << "'\n";
    }
}