How to throw an expectation_failure from a function in Boost Spirit? - c++

Question

In Boost::Spirit, how can I trigger an expectation_failure from a function bound with Boost::Bind?
Background: I parse a large file that contains complex entries. When an entry is inconsistent with a previous entry I want to fail and throw an expectation_failure (containing proper parse position information). When I parse an entry I bind a function that decides if the entry is inconsistent with something seen before.
I made up a little toy example that shows the point. Here I simply want to throw an expectation_failure when the int is not divisible by 10:
#include <iostream>
#include <iomanip>
#include <boost/spirit/include/qi.hpp>
#include <boost/bind.hpp>
#include <boost/spirit/include/classic_position_iterator.hpp>
namespace qi = boost::spirit::qi;
namespace classic = boost::spirit::classic;
void checkNum(int const& i) {
if (i % 10 != 0) // >> How to throw proper expectation_failure? <<
std::cerr << "ERROR: Number check failed" << std::endl;
}
template <typename Iterator, typename Skipper>
struct MyGrammar : qi::grammar<Iterator, int(), Skipper> {
MyGrammar() : MyGrammar::base_type(start) {
start %= qi::eps > qi::int_[boost::bind(&checkNum, _1)];
}
qi::rule<Iterator, int(), Skipper> start;
};
template<class PosIter>
std::string errorMsg(PosIter const& iter) {
const classic::file_position_base<std::string>& pos = iter.get_position();
std::stringstream msg;
msg << "parse error at file " << pos.file
<< " line " << pos.line << " column " << pos.column << std::endl
<< "'" << iter.get_currentline() << "'" << std::endl
<< std::setw(pos.column) << " " << "^- here";
return msg.str();
}
int main() {
std::string in = "11";
typedef std::string::const_iterator Iter;
typedef classic::position_iterator2<Iter> PosIter;
MyGrammar<PosIter, qi::space_type> grm;
int i;
PosIter it(in.begin(), in.end(), "<string>");
PosIter end;
try {
qi::phrase_parse(it, end, grm, qi::space, i);
if (it != end)
throw std::runtime_error(errorMsg(it));
} catch(const qi::expectation_failure<PosIter>& e) {
throw std::runtime_error(errorMsg(e.first));
}
return 0;
}
Throwing an expectation_failure would mean that I get an error message like this on an int that is not divisible by 10:
parse error at file <string> line 1 column 2
'11'
^- here

Answer 1

You can use the _pass placeholder in phoenix to enforce a parse fail. Something like this should work.
bool myfunc(int i) {return i%10 == 0;}
...
_int [ _pass = phoenix::bind(myfunc,_1)]

Answer 2

Years late but anyway:
If you absolutely want to throw an exception and want the on_error to catch it, you have to throw the expectation_exception from the qi namespace because the error handler on_error does catch nothing else.
This might apply to a semantic action or a custom parser implementation.
Would look like:
boost::throw_exception(Exception(first, last, component.what(context)));
where Exception is a qi::expactation_exception and nothing else.
If you do not have a component at hand like in a semantic action you must provide your own qi::info object instead of component.what(..).
You can throw from everywhere inside a context guarded by on_error.