boost::spirit optional parsing into a fusion adapted structure

Question

If there's a structure

struct price
{
   int chicken;
   int cow;
   bool in_stock;
   std::string place;
};

which is adapted using boost::fusion. If there's a need to parse it, but optionally for in_stock and place. e.g.

template <typename it, typename skipper = qi::space_type>
struct p : qi::grammar<it, price(), skipper>
{
    p() : p::base_type(p_instance)
    {
        using namespace qi;
        psr %= int_ > int_ > -bool_ > -('"' > char_ % ',' > '"');
    }
  private:
      qi::rule<it,price(),skipper> limit;
};

However, this does not work. There's an exception thrown, if an input is "2 3 \"Chili\"". What is a solution?

Answer 1

Firstly

I guess you're having a few too many > expectation points, which could/should be >> sequence operators.

I'd say, make your grammar more explicit, e.g. by saying

bool_ | attr(false)

to ensure that if bool_ fails, there will be a resulting attribute value of false exposed. This way, the exposed attribute is bool, instead of boost::optional<bool>, which is more compatible with the target struct (price).

Demo:

template <typename It, typename Skipper = qi::space_type>
    struct parser : qi::grammar<It, price(), Skipper>
{
    parser() : parser::base_type(start)
    {
        using namespace qi;

        in_stock = bool_ | attr(false);
        start    = int_ > int_ > in_stock > -('"' >> char_ % ',' > '"');
    }

  private:
    qi::rule<It, price(), Skipper> start;
    qi::rule<It, bool()> in_stock;
};

Secondly

Then,

('"' > char_ % ',' > '"')

will only ever match string like "a", "b,c,d" etc. If you wanted "Chili" to parse, you should probably change it to something like:

    place    = '"' >> *~char_('"') > '"';
    start    = int_ > int_ > in_stock > (place | attr(""));

Which means parse any string delimited by '"' (~char_("abc) means: any char except [abc]).

Sample solution

Here's a full demonstration showing:

• Fusion adaptation of the struct
• the above adjusted grammar
• using BOOST_SPIRIT_DEBUG to debug
• catching qi::expectation_failure to print diagnostics on parse failure
• using Spirit Karma to print the result of parsing in human readable form

Output of test program listed below:

Code

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

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

struct price
{
    int chicken;
    int cow;
    bool in_stock;
    std::string place;
};

BOOST_FUSION_ADAPT_STRUCT(price, 
        (int, chicken)
        (int, cow)
        (bool, in_stock)
        (std::string, place))

template <typename It, typename Skipper = qi::space_type>
    struct parser : qi::grammar<It, price(), Skipper>
{
    parser() : parser::base_type(start)
    {
        using namespace qi;

        in_stock = bool_ | attr(false);
#if 0
        start    = int_ > int_ > in_stock > -('"' >> char_ % ',' > '"');
#else
        place    = '"' >> *~char_('"') > '"';
        start    = int_ > int_ > in_stock > (place | attr(""));
#endif

        BOOST_SPIRIT_DEBUG_NODE(start);
        BOOST_SPIRIT_DEBUG_NODE(place);
        BOOST_SPIRIT_DEBUG_NODE(in_stock);
    }

  private:
    qi::rule<It, price(), Skipper> start;
    //
    qi::rule<It, bool()>            in_stock;
    qi::rule<It, std::string()>     place; // no skipper
};

bool doParse(const std::string& input)
{
    typedef std::string::const_iterator It;
    auto f(begin(input)), l(end(input));

    parser<It, qi::space_type> p;
    price data;

    try
    {
        bool ok = qi::phrase_parse(f,l,p,qi::space,data);
        if (ok)   
        {
            std::cout << "parse success: '" << input << "'\n";
            std::cout << "data: " << karma::format_delimited(karma::eps <<
                    "chicken:"  << karma::int_  << 
                    "cow:"      << karma::int_  << 
                    "in_stock:" << karma::bool_ << 
                    "place:"    << karma::auto_,
                    " ", data) << "\n";
        }
        else      std::cerr << "parse failed: '" << std::string(f,l) << "'\n";

        if (f!=l) std::cerr << "trailing unparsed: '" << std::string(f,l) << "'\n";
        return ok;
    } catch(const qi::expectation_failure<It>& e)
    {
        std::string frag(e.first, e.last);
        std::cerr << e.what() << "'" << frag << "'\n";
    }

    return false;
}

int main()
{
    doParse("1 2 true \"a,b,c,d,e,f\"");
    doParse("3 4 \"a,b,c,d,e,f\"");
    doParse("5 6");
    doParse("7 8 false");
}

Output

<start>
  <try>1 2 true "a,b,c,d,e,</try>
  <in_stock>
    <try>true "a,b,c,d,e,f"</try>
    <success> "a,b,c,d,e,f"</success>
    <attributes>[1]</attributes>
  </in_stock>
  <place>
    <try>"a,b,c,d,e,f"</try>
    <success></success>
    <attributes>[[a, ,, b, ,, c, ,, d, ,, e, ,, f]]</attributes>
  </place>
  <success></success>
  <attributes>[[1, 2, 1, [a, ,, b, ,, c, ,, d, ,, e, ,, f]]]</attributes>
</start>
parse success: '1 2 true "a,b,c,d,e,f"'
data:  chicken: 1 cow: 2 in_stock: true place: a,b,c,d,e,f 
<start>
  <try>3 4 "a,b,c,d,e,f"</try>
  <in_stock>
    <try>"a,b,c,d,e,f"</try>
    <success>"a,b,c,d,e,f"</success>
    <attributes>[0]</attributes>
  </in_stock>
  <place>
    <try>"a,b,c,d,e,f"</try>
    <success></success>
    <attributes>[[a, ,, b, ,, c, ,, d, ,, e, ,, f]]</attributes>
  </place>
  <success></success>
  <attributes>[[3, 4, 0, [a, ,, b, ,, c, ,, d, ,, e, ,, f]]]</attributes>
</start>
parse success: '3 4 "a,b,c,d,e,f"'
data:  chicken: 3 cow: 4 in_stock: false place: a,b,c,d,e,f 
<start>
  <try>5 6</try>
  <in_stock>
    <try></try>
    <success></success>
    <attributes>[0]</attributes>
  </in_stock>
  <place>
    <try></try>
    <fail/>
  </place>
  <success></success>
  <attributes>[[5, 6, 0, []]]</attributes>
</start>
parse success: '5 6'
data:  chicken: 5 cow: 6 in_stock: false place:  
<start>
  <try>7 8 false</try>
  <in_stock>
    <try>false</try>
    <success></success>
    <attributes>[0]</attributes>
  </in_stock>
  <place>
    <try></try>
    <fail/>
  </place>
  <success></success>
  <attributes>[[7, 8, 0, []]]</attributes>
</start>
parse success: '7 8 false'
data:  chicken: 7 cow: 8 in_stock: false place: