Boost Spirit Qi: changing tag of specialized uint_parser

Question

I've just implemented a basic parser in Qi to verify a specified TCP port range, e.g. 80-444.

  template<class It>
  struct port_range_grammar : qi::grammar<It, port_range_type()>
  {
    port_range_grammar()
      : port_range_grammar::base_type(start, "port_range")
    {
      using qi::lit;

      start = port > lit("-") > port;
    }

  private:
    qi::rule<It, port_range_type()> start;
    qi::uint_parser<uint16_t, 10, 2, 5> port;
  };

To make the error more descriptive I've attached an error handler to the start rule (in some upper grammar, which embeds this one), e.g.:

 // this how the code is attached to the start rule in the top level grammar:

  start = (tcp_endpoint | ipc_endpoint | inproc_endpoint)[_val=_1] > eoi;

  on_error<fail>
    ( start
    , pnx::bind
      ( [](auto const& what, auto begin, auto end)
          {
            ERROR_AC << "Expecting "
                     << what
                     << " here: '"
                     << std::string(begin, end)
                     << "'"
            ;
          }
      , _4
      , _3
      , _2
      )
    )
  ;

Everything works great with one minor exception, when I pass as port some invalid 16 bit unsigned number I see an error, but it is not descriptive enough:

Expecting <unsigned-integer> here: '74888'

Now the library's user can't understand that 74888 is invalid 16bit uint. unsiged-integer is a tag attached to the qi::uint_parser. Is there any way to change this tag?

Answer 1

I'd simply attach a name to a non-terminal rule:

        port = uint_parser<uint16_t, 10, 2, 5>();
        port.name("valid port number 10-65535");

See it Live On Coliru

#include <boost/fusion/adapted/std_pair.hpp>
#include <boost/spirit/include/qi.hpp>
#include <boost/spirit/include/phoenix.hpp>
namespace qi = boost::spirit::qi;
namespace pnx = boost::phoenix;

using port_range_type = std::pair<uint16_t, uint16_t>;

template<class It>
struct port_range_grammar : qi::grammar<It, port_range_type()>
{
    port_range_grammar()
        : port_range_grammar::base_type(start, "port_range")
    {
        using namespace qi;
        port = uint_parser<uint16_t, 10, 2, 5>();
        port.name("valid port number 10-65535");

        start = port > lit("-") > port;
        /*start = (tcp_endpoint | ipc_endpoint | inproc_endpoint)[_val=_1] > eoi;*/

        on_error<fail>
            ( start
              , pnx::bind
              ( [](auto const& what, auto begin, auto end)
                {
                    std::cerr << "Expecting "
                        << what
                        << " here: '"
                        << std::string(begin, end)
                        << "'\n"
                        ;
                } , _4 , _3 , _2
              )
            )
            ;
    }

  private:
    qi::rule<It, port_range_type()> start;
    qi::rule<It, uint16_t()> port;
};

int main() {
    using It = std::string::const_iterator;
    std::string const input = "11-1q0";

    It f = input.begin(), l = input.end();
    port_range_type range;
    bool ok = qi::parse(f, l, port_range_grammar<It>{}, range);

    if (ok) {
        std::cout << "Parsed port " << range.first << " to " << range.second << "\n";
    } else {
        std::cout << "Parse failed\n";
    }

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

Prints

Expecting <valid port number 10-65535> here: '1q0'
Parse failed
Remaining unparsed: '11-1q0'