Read empty values with boost::spirit

Question

I want to read a CSV into a struct :

struct data 
{
   std::string a;
   std::string b;
   std::string c;
}

However, I want to read even empty

Answer 1

You just want to make sure you parse a value for "empty" strings too.

value = +(char_ - ',' - eol) | attr("(unspecified)");
entry = value >> ',' >> value >> ',' >> value >> eol;

See the demo:

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//#define BOOST_SPIRIT_DEBUG
#include <boost/fusion/adapted/struct.hpp>
#include <boost/spirit/include/qi.hpp>

namespace qi = boost::spirit::qi;

struct data {
    std::string a;
    std::string b;
    std::string c;
};

BOOST_FUSION_ADAPT_STRUCT(data, (std::string, a)(std::string, b)(std::string, c))

template <typename Iterator, typename skipper = qi::blank_type>
struct google_parser : qi::grammar<Iterator, data(), skipper> {
    google_parser() : google_parser::base_type(entry, "contacts") {
        using namespace qi;

        value = +(char_ - ',' - eol) | attr("(unspecified)");
        entry = value >> ',' >> value >> ',' >> value >> eol;

        BOOST_SPIRIT_DEBUG_NODES((value)(entry))
    }
  private:
    qi::rule<Iterator, std::string()> value;
    qi::rule<Iterator, data(), skipper> entry;
};

int main() {
    using It = std::string::const_iterator;
    google_parser<It> p;

    for (std::string input : { 
            "something, awful, is\n",
            "fine,,just\n",
            "like something missing: ,,\n",
        })
    {
        It f = input.begin(), l = input.end();

        data parsed;
        bool ok = qi::phrase_parse(f,l,p,qi::blank,parsed);

        if (ok)
            std::cout << "Parsed: '" << parsed.a << "', '" << parsed.b << "', '" << parsed.c << "'\n";
        else
            std::cout << "Parse failed\n";

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

Prints:

Parsed: 'something', 'awful', 'is'
Parsed: 'fine', '(unspecified)', 'just'
Parsed: 'like something missing: ', '(unspecified)', '(unspecified)'

However, you have a bigger problem. The assumption that qi::repeat(2) [ value ] will parse into 2 strings doesn't work.

repeat, like operator*, operator+ and operator% parse into a container attribute. In this case the container attribute (string) will receive the input from the second value as well:

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Parsed: 'somethingawful', 'is', ''
Parsed: 'fine(unspecified)', 'just', ''
Parsed: 'like something missing: (unspecified)', '(unspecified)', ''

Since this is not what you want, reconsider your data types:

• either don't adapt the struct but instead write a customization trait to assign the fields (see http://www.boost.org/doc/libs/1_57_0/libs/spirit/doc/html/spirit/advanced/customize.html)

• change the struct to contain a vector of std::string to match the exposed attributes

• or create an auto-parser generator:

The auto_ approach:

If you teach Qi how to extract a single value, you can use a simple rule like

entry = skip(skipper() | ',') [auto_] >> eol;

This way, Spirit itself will generate the correct number of value extractions for the given Fusion sequence!

Here's a quick an dirty approach:

CAVEAT Specializing for std::string directly like this might not be the best idea (it might not always be appropriate and might interact badly with other parsers). However, by default create_parser<std::string> is not defined (because, what would it do?) so I seized the opportunity for the purpose of this demonstration:

namespace boost { namespace spirit { namespace traits {
    template <> struct create_parser<std::string> {
        typedef proto::result_of::deep_copy<
            BOOST_TYPEOF(
                qi::lexeme [+(qi::char_ - ',' - qi::eol)] | qi::attr("(unspecified)")
            )
        >::type type;

        static type call() {
            return proto::deep_copy(
                qi::lexeme [+(qi::char_ - ',' - qi::eol)] | qi::attr("(unspecified)")
            );
        }
    };
}}}

Again, see the demo output:

Live On Coliru

Parsed: 'something', 'awful', 'is'
Parsed: 'fine', 'just', '(unspecified)'
Parsed: 'like something missing: ', '(unspecified)', '(unspecified)'

NOTE There was some advanced sorcery to get the skipper to work "just right" (see skip()[] and lexeme[]). Some general explanations can be found here: Boost spirit skipper issues

UPDATE

The Container Approach

There's a subtlety to that. Two actually. So here's a demo:

Live On Coliru

//#define BOOST_SPIRIT_DEBUG
#include <boost/fusion/adapted/struct.hpp>
#include <boost/spirit/include/qi.hpp>

namespace qi = boost::spirit::qi;

struct data {
    std::vector<std::string> parts;
};

BOOST_FUSION_ADAPT_STRUCT(data, (std::vector<std::string>, parts))

template <typename Iterator, typename skipper = qi::blank_type>
struct google_parser : qi::grammar<Iterator, data(), skipper> {
    google_parser() : google_parser::base_type(entry, "contacts") {
        using namespace qi;
        qi::as<std::vector<std::string> > strings;

        value = +(char_ - ',' - eol) | attr("(unspecified)");
        entry = strings [ repeat(2) [ value >> ',' ] >> value ] >> eol;

        BOOST_SPIRIT_DEBUG_NODES((value)(entry))
    }
  private:
    qi::rule<Iterator, std::string()> value;
    qi::rule<Iterator, data(), skipper> entry;
};

int main() {
    using It = std::string::const_iterator;
    google_parser<It> p;

    for (std::string input : { 
            "something, awful, is\n",
            "fine,,just\n",
            "like something missing: ,,\n",
        })
    {
        It f = input.begin(), l = input.end();

        data parsed;
        bool ok = qi::phrase_parse(f,l,p,qi::blank,parsed);

        if (ok) {
            std::cout << "Parsed: ";
            for (auto& part : parsed.parts) 
                std::cout << "'" << part << "' ";
            std::cout << "\n";
        }
        else
            std::cout << "Parse failed\n";

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

The subtleties are:

• adapting a single-element sequence hits edge cases with automatic attribute handling: Spirit Qi attribute propagation issue with single-member struct
• Spirit needs hand-holding in this particular case to treat the repeat[...]>>value as synthesizing a single container /atomically/. The as<T> directive solves that here