how to make error handling work for boost::spirit

Question

in boost::spirit, I added error handling code based on example roman.

#include <boost/spirit/include/qi.hpp>
#include <boost/spirit/include/phoenix_operator.hpp>
#include <boost/spirit/include/phoenix_core.hpp>
#include <boost/spirit/include/phoenix_fusion.hpp>
#include <boost/spirit/include/phoenix_object.hpp>
#include <boost/foreach.hpp>

#include <iostream>
#include <fstream>
#include <string>
#include <vector>

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

template <typename Iterator>
struct roman : qi::grammar<Iterator>
{
  roman() : roman::base_type(start)
  {
    using qi::eps;
    using qi::lit;
    using qi::lexeme;
    using qi::_val;
    using qi::_1;
    using ascii::char_;

    // for on_error
    using qi::on_error;
    using qi::fail;
    using phoenix::construct;
    using phoenix::val;

    start = +(lit('M') )  >> "</>";

    on_error<fail>
    (
        start
      , std::cout
            << val("Error! Expecting ")
            // << _4                            // what failed?
            << val(" here: \"")
            // << construct<std::string>(_3, _2)   // iterators to error-pos, end
            << val("\"")
            << std::endl
    );
  }
  qi::rule<Iterator> start;
};

int
main()
{
    std::cout << "/////////////////////////////////////////////////////////\n\n";
    std::cout << "\t\tRoman Numerals Parser\n\n";
    std::cout << "/////////////////////////////////////////////////////////\n\n";
    std::cout << "Type a Roman Numeral ...or [q or Q] to quit\n\n";

    typedef std::string::const_iterator iterator_type;
    typedef roman<iterator_type> roman;

    roman roman_parser; // Our grammar

    std::string str;
    unsigned result;
    while (std::getline(std::cin, str))
    {
        if (str.empty() || str[0] == 'q' || str[0] == 'Q')
            break;

        std::string::const_iterator iter = str.begin();
        std::string::const_iterator end = str.end();
        //[tutorial_roman_grammar_parse
        bool r = parse(iter, end, roman_parser, result);

        if (r && iter == end)
        {
            std::cout << "-------------------------\n";
            std::cout << "Parsing succeeded\n";
            std::cout << "result = " << result << std::endl;
            std::cout << "-------------------------\n";
        }
        else
        {
            std::string rest(iter, end);
            std::cout << "-------------------------\n";
            std::cout << "Parsing failed\n";
            std::cout << "stopped at: \": " << rest << "\"\n";
            std::cout << "-------------------------\n";
        }
        //]
    }

    std::cout << "Bye... :-) \n\n";
    return 0;
}

my questions are:

1. the "on_error" isn't triggered, why?
2. I comment the "<< _4", if I want to print out the failure part, how to do that?

Answer 1

Three steps:

1. Qualify the placeholders:

   on_error<fail>(start, 
           std::cout
              << val("Error! Expecting ")
              << qi::_4
              << val(" here: \"")
              << construct<std::string>(qi::_3, qi::_2)
              << val("\"")
              << std::endl
       );
   

2. You'll also need to make sure you have expectation points to trigger the error handler.

   start = eps > +(lit('M') ) >> "</>";
   

   See e.g. Boost.Spirit.Qi - Errors at the beginning of a rule for explanation

3. (optionally) Name your rules

   start.name("start");
   

   Using BOOST_SPIRIT_DEBUG_NODE(S) is another way to implicit name your rules.

See it Live on Coliru (cleaned up and simplified in places)

Now it prints (input iv):

Error! Expecting <sequence>"M""</>" here: 'iv'
Parsing failed
stopped at: 'iv'

Full code

#include <boost/spirit/include/qi.hpp>
#include <boost/spirit/include/phoenix.hpp>

#include <iostream>
#include <fstream>

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

template <typename Iterator>
struct roman : qi::grammar<Iterator>
{
    roman() : roman::base_type(start)
    {
        using namespace qi;

        start = eps > +lit('M') >> "</>";
        start.name("start");

        on_error<fail>(start, 
                phx::ref(std::cout)
                   << "Error! Expecting "
                   << qi::_4
                   << " here: '"
                   << phx::construct<std::string>(qi::_3, qi::_2)
                   << "'\n"
            );
    }
    qi::rule<Iterator> start;
};

int main()
{
    typedef std::string::const_iterator iterator_type;
    roman<iterator_type> roman_parser; // Our grammar

    std::string str;
    while (std::getline(std::cin, str))
    {
        if (str.empty() || str[0] == 'q' || str[0] == 'Q')
            break;

        iterator_type iter = str.begin(), end = str.end();
        unsigned result;
        bool r = parse(iter, end, roman_parser, result);

        if (r && iter == end)
        {
            std::cout << "Parsing succeeded\n";
            std::cout << "result = " << result << std::endl;
        }
        else
        {
            std::string rest(iter, end);
            std::cout << "Parsing failed\n";
            std::cout << "stopped at: '" << rest << "'\n";
        }
    }
}

---

In addition to the comment: This is something I've been testing with - haven't exactly made it to work yet, but the error handler is getting invoked and eating input as it should. Maybe it could be of help?

static auto const at_eol = (*_1 == '\r') || (*_1 == '\n');
static auto const at_eoi = (_1 == _2);

on_error<retry>(start, 
    (
        (phx::ref(std::cout) << "rule start: expecting " << _4 << " here: '" << escape_(_3, _2) << "'\n"),
        phx::while_ (!at_eoi && !at_eol) [ ++_1, phx::ref(std::cout) << "\nadvance to newline\n" ],
        phx::while_ (!at_eoi && at_eol)  [ ++_1, phx::ref(std::cout) << "\neat newline\n" ],
        phx::if_ (at_eoi)                [ _pass = fail ]
    )
);

See also the note under Important in the documentation for multi_pass<>