How to use boost spirit x3 kleene star?

Question

I am new to boost::spirit and trying to write a simple parser using x3. I got an error that I can't explain. For some reason I get an error about boost::spirit::x3::unused_type. The error disappears when I remove the kleene star in the line I marked below. The other kleene stars seem to be ok (for some reason). This is my code:

#include <string>
#include <iostream>
#include <boost/spirit/home/x3.hpp>

namespace x3 = boost::spirit::x3;

int main()
{
    std::string str = "qqq::qqq::qqq";

    auto begin = str.begin(), end   = str.end();
    bool r = x3::parse(begin, end,
        (        (x3::alpha | '_') >> *(x3::alnum | '_')) >>
        *                                         //this line is the problem
        ("::" >> (x3::alpha | '_') >> *(x3::alnum | '_'))
    ); // is supposed to match str

    if (r && std::distance(begin,end) < 1)
    {
        std::cout << "Parsing succeeded\n";
    }
    else
    {
        std::cout << "Parsing FAILED\n";
        std::cout << "r " << r << " d " << std::distance(begin,end) << "\n";
    }

    return 0;
}

This is the error:

/usr/include/boost/spirit/home/x3/core/detail/parse_into_container.hpp:254:22: error: ‘const struct boost::spirit::x3::unused_type’ has no member named ‘empty’
254 |             if (attr.empty())
    |                 ~~~~~^~~~~
/usr/include/boost/spirit/home/x3/core/detail/parse_into_container.hpp:259:22: error: ‘const struct boost::spirit::x3::unused_type’ has no member named ‘insert’
259 |                 attr.insert(attr.end(), rest.begin(), rest.end());
    |                 ~~~~~^~~~~~
/usr/include/boost/spirit/home/x3/core/detail/parse_into_container.hpp:259:34: error: ‘const struct boost::spirit::x3::unused_type’ has no member named ‘end’
259 |                 attr.insert(attr.end(), rest.begin(), rest.end());
    |                             ~~~~~^~~
/usr/include/boost/spirit/home/x3/core/detail/parse_into_container.hpp:259:46: error: ‘const struct boost::spirit::x3::unused_type’ has no member named ‘begin’
259 |                 attr.insert(attr.end(), rest.begin(), rest.end());
    |                                         ~~~~~^~~~~
/usr/include/boost/spirit/home/x3/core/detail/parse_into_container.hpp:259:60: error: ‘const struct boost::spirit::x3::unused_type’ has no member named ‘end’
259 |                 attr.insert(attr.end(), rest.begin(), rest.end());
    |                                                       ~~~~~^~~

I am on Ubuntu 18.04 using boost 1.65.1. I read this (https://stackoverflow.com/a/49121776/2359966) post where a similar problem was described.

Is there a way to fix or circumvent this problem without changing system packages?

score 3 · Accepted Answer · answered Oct 14 '18 at 15:28

No there isn't it's just a bug that has been fixed.

1.65.1 https://wandbox.org/permlink/uUeqVJOsnSxk9aD1 broken
1.66.0 https://wandbox.org/permlink/AhQ9G8Ubm9v0DeYj broken
1.67.0 https://wandbox.org/permlink/nh4wDJdhO8lSfBZo ok

Regardless, the grammar construct like x3::alpha | '_' will not expose what you need, because the literal '_' exposes no attribute.

So, in practice you might just want to simplify using raw[]:

auto ident = x3::raw [ x3::lexeme [ (x3::alpha | '_') >> *(x3::alnum | '_') ]

Next up, you can match multiple identifiers qualified with :::

x3::raw [ ident >> * ("::" >> ident) ]

Which incidentally is completely equivalent to just

x3::raw [ ident % "::") ]

See

1.65.1 https://wandbox.org/permlink/Jorllem5U7u3L04e ok
1.67.0 https://wandbox.org/permlink/EXMjDTPhcZfu05Ot also ok

Bonus:

Assigning to attributes

Live On Coliru

#include <boost/spirit/home/x3.hpp>
#include <iostream>
#include <iomanip>
#include <string>

namespace x3 = boost::spirit::x3;

int main() {
    std::string str = "foo::_bar::qux_1";

    auto begin = str.begin(), end = str.end();
    auto ident = x3::raw [ x3::lexeme [ (x3::alpha | '_') >> *(x3::alnum | '_') ] ];

    std::vector<std::string> qualified;
    bool r = x3::parse(begin, end, ident % "::", qualified);

    if (r && begin==end) {
        std::cout << "Parsing succeeded\n";
        for (auto& el : qualified) {
            std::cout << " element " << std::quoted(el) << "\n";
        }
    } else {
        std::cout << "Parsing FAILED\n";
    }

    std::cout << std::boolalpha << r << " " << std::quoted(std::string(begin, end)) << "\n";
}

Prints:

Parsing succeeded
 element "foo"
 element "_bar"
 element "qux_1"
true ""

And the grammar got simplified to

std::vector<std::string> qualified;
bool r = x3::parse(begin, end, ident % "::", qualified);

That's usually a good sign.

How to use boost spirit x3 kleene star?

1 Answers1

Bonus: