Spirit Qi First Parser - c++

Question

What did I mess up here? I'm getting 'start': undeclared identifier but I stuck pretty closely to the tutorial, so I'm not sure where I made a typo, or what I did wrong. Any hints? You all see the same thing, right?
#include <iostream>
#include <boost/spirit/include/qi.hpp>
#include <boost/spirit/include/phoenix_core.hpp>
#include <boost/spirit/include/phoenix_operator.hpp>
#include <boost/spirit/include/phoenix_object.hpp>
#include <iostream>
#include <string>
#include <boost/array.hpp>
#include <boost/fusion/include/io.hpp>
#include <boost/fusion/include/adapt_struct.hpp>
#include <boost/spirit/include/qi_no_skip.hpp>
#include <boost/spirit/include/phoenix.hpp>
namespace qi = boost::spirit::qi;
namespace ascii = boost::spirit::ascii;
using qi::lit;
using qi::int_;
using qi::double_;
using ascii::char_;
using boost::spirit::qi::phrase_parse;
using boost::spirit::qi::no_skip;
using qi::eoi;
struct LETTER
{
char hi;
// int fourtytwo;
// char mom;
};
BOOST_FUSION_ADAPT_STRUCT(
LETTER,
(char, hi)
// (int, fourtytwo)
// (char, mom)
)
template <typename Iterator>
struct LETTERParser : qi::grammar<Iterator, LETTER(), ascii::space_type>
{
LETTERParser(): LETTERParser::base_type(start)
{
start %= lit("LETTER") >> char_;
// >> char_
// >> int_
// >> char_
// >> eoi
// ;
}
};
const std::string wat("Z");
int main()
{
LETTERParser<std::string::const_iterator> f;
LETTER example;
phrase_parse(wat.begin(), wat.end(), f, no_skip, example);
return 0;
}

Answer 1

There are a number of issues, one of which is non obvious
where's no_skip? Why are you passing it to a grammar that requires ascii::space_type?
where is the start rule declared?
don't pollute global namespace - it creates hard problems in generic code
handle errors
the grammar starts with a mandatory character sequence, which doesn't match the input
the non-obvious one: single-element structs interfere in unfortunate ways in Spirit/Fusion land.
Simplify:
Fixing the above and modernizing (c++11) the fusion adaptation:
live On Coliru
#include <boost/fusion/include/adapt_struct.hpp>
#include <boost/spirit/include/qi.hpp>
#include <iostream>
namespace qi = boost::spirit::qi;
struct LETTER {
char hi;
int fourtytwo;
char mom;
};
BOOST_FUSION_ADAPT_STRUCT(LETTER, hi, fourtytwo, mom)
template <typename Iterator> struct LETTERParser : qi::grammar<Iterator, LETTER(), qi::ascii::space_type> {
LETTERParser() : LETTERParser::base_type(start) {
using qi::char_;
using qi::int_;
start = "LETTER" >> char_ >> int_ >> char_;
}
private:
qi::rule<Iterator, LETTER(), qi::ascii::space_type> start;
};
int main() {
const std::string input("LETTER Z 42m");
using It = std::string::const_iterator;
LETTERParser<It> parser;
LETTER example;
It f = input.begin(), l = input.end();
if (phrase_parse(f, l, parser, qi::ascii::space, example)) {
std::cout << "parsed: " << boost::fusion::as_vector(example) << "\n";
} else {
std::cout << "couldn't parse '" << input << "'\n";
}
if (f != l)
std::cout << "Remaining unparsed input: '" << std::string(f,l) << "'\n";
}
Prints
parsed: (Z 42 m)
Single Element:
You're in, luck it doesn't bite in your case:
Live On Coliru
Prints
parsed: (Z)
Remaining unparsed input: '42m'
as expected. If it strikes in the future, refer here e.g. Size of struct with a single element
Bonus
Consider encapsulating the choice of skipper. The caller should probably never be able to override it Live On Coliru - see also Boost spirit skipper issues