unhandled exception using Boost Spirit to parse grammar - c++

Question

I am trying to use Boost Spirit to parse the following grammar:
sentence:
noun verb
sentence conjunction sentence
conjunction:
"and"
noun:
"birds"
"cats"
verb:
"fly"
"meow"
parsing succeeds when the grammar only includes noun >> verb rule.
when grammar is modified to include sentence>>conjunction>>sentence rule and i supply an invalid input such as "birds fly" instead of "birdsfly" i get an unhandled exception when the program runs.
here is the code which is modified from examples found on boost doc
#define BOOST_VARIANT_MINIMIZE_SIZE
#include <boost/config/warning_disable.hpp>
#include <boost/spirit/include/qi.hpp>
#include <boost/spirit/include/lex_lexertl.hpp>
#include <boost/spirit/include/phoenix_operator.hpp>
#include <boost/spirit/include/phoenix_statement.hpp>
#include <boost/spirit/include/phoenix_container.hpp>
#include <iostream>
#include <string>
using namespace boost::spirit;
using namespace boost::spirit::ascii;
template <typename Lexer>
struct token_list : lex::lexer<Lexer>
{
token_list()
{
noun = "birds|cats";
verb = "fly|meow";
conjunction = "and";
this->self.add
(noun)
(verb)
(conjunction)
;
}
lex::token_def<std::string> noun, verb, conjunction;
};
template <typename Iterator>
struct Grammar : qi::grammar<Iterator>
{
template <typename TokenDef>
Grammar(TokenDef const& tok)
: Grammar::base_type(sentence)
{
sentence = (tok.noun>>tok.verb)
|
(sentence>>tok.conjunction>>sentence)>>eoi
;
}
qi::rule<Iterator> sentence;
};
int main()
{
typedef lex::lexertl::token<char const*, boost::mpl::vector<std::string>> token_type;
typedef lex::lexertl::lexer<token_type> lexer_type;
typedef token_list<lexer_type>::iterator_type iterator_type;
token_list<lexer_type> word_count;
Grammar<iterator_type> g (word_count);
std::string str = "birdsfly";
//std::string str = "birds fly"; this input caused unhandled exception
char const* first = str.c_str();
char const* last = &first[str.size()];
bool r = lex::tokenize_and_parse(first, last, word_count, g);
if (r) {
std::cout << "Parsing passed"<< "\n";
}
else {
std::string rest(first, last);
std::cerr << "Parsing failed\n" << "stopped at: \""
<< rest << "\"\n";
}
system("PAUSE");
return 0;
}

Answer 1

You have left-recursion in the second branch of the sentence rule.
sentence = sentence >> ....
will always recurse on sentence, so you're seeing a stackoverflow.
I suggest writing the rule like, e.g:
sentence =
(tok.noun >> tok.verb)
>> *(tok.conjunction >> sentence)
>> qi::eoi
;
Now the result reads
g++ -Wall -pedantic -std=c++0x -g -O0 test.cpp -o test
Parsing failed
stopped at: " fly"
(and the inevitable "sh: PAUSE: command not found" of course...)
PS. Don't using namespace please. Instead:
namespace qi = boost::spirit::qi;
namespace lex = boost::spirit::lex;
Here's a cleaned up version with some other stuff removed/fixed: http://coliru.stacked-crooked.com/view?id=1fb26ca3e8c207979eaaf4592c319316-e223fd4a885a77b520bbfe69dda8fb91
#define BOOST_VARIANT_MINIMIZE_SIZE
#include <boost/spirit/include/qi.hpp>
#include <boost/spirit/include/lex_lexertl.hpp>
// #include <boost/spirit/include/phoenix.hpp>
#include <iostream>
#include <string>
namespace qi = boost::spirit::qi;
namespace lex = boost::spirit::lex;
template <typename Lexer>
struct token_list : lex::lexer<Lexer>
{
token_list()
{
noun = "birds|cats";
verb = "fly|meow";
conjunction = "and";
this->self.add
(noun)
(verb)
(conjunction)
;
}
lex::token_def<std::string> noun, verb, conjunction;
};
template <typename Iterator>
struct Grammar : qi::grammar<Iterator>
{
template <typename TokenDef>
Grammar(TokenDef const& tok) : Grammar::base_type(sentence)
{
sentence =
(tok.noun >> tok.verb)
>> *(tok.conjunction >> sentence)
>> qi::eoi
;
}
qi::rule<Iterator> sentence;
};
int main()
{
typedef std::string::const_iterator It;
typedef lex::lexertl::token<It, boost::mpl::vector<std::string>> token_type;
typedef lex::lexertl::lexer<token_type> lexer_type;
typedef token_list<lexer_type>::iterator_type iterator_type;
token_list<lexer_type> word_count;
Grammar<iterator_type> g(word_count);
//std::string str = "birdsfly";
const std::string str = "birds fly";
It first = str.begin();
It last = str.end();
bool r = lex::tokenize_and_parse(first, last, word_count, g);
if (r) {
std::cout << "Parsing passed"<< "\n";
}
else {
std::string rest(first, last);
std::cerr << "Parsing failed\n" << "stopped at: \"" << rest << "\"\n";
}
}