How to skip (not output) tokens in Boost Spirit? - c++

Question

I'm new to Boost Spirit. I haven't been able to find examples for some simple things. For example, suppose I have an even number of space-delimited integers. (That matches *(qi::int_ >> qi::int_). So far so good.) I'd like to save just the even ones to a std::vector<int>. I've tried a variety of things like *(qi::int_ >> qi::skip[qi::int_]) https://godbolt.org/z/KPToo3xh6 but that still records every int, not just even ones.
#include <stdexcept>
#include <fmt/format.h>
#include <fmt/ranges.h>
#include <boost/spirit/include/qi.hpp>
namespace qi = boost::spirit::qi;
// Example based off https://raw.githubusercontent.com/bingmann/2018-cpp-spirit-parsing/master/spirit1_simple.cpp:
// Helper to run a parser, check for errors, and capture the results.
template <typename Parser, typename Skipper, typename ... Args>
void PhraseParseOrDie(
const std::string& input, const Parser& p, const Skipper& s,
Args&& ... args)
{
std::string::const_iterator begin = input.begin(), end = input.end();
boost::spirit::qi::phrase_parse(begin, end, p, s, std::forward<Args>(args) ...);
if (begin != end) {
fmt::print("Unparseable: \"{}\"\n", std::string(begin, end));
}
}
void test(std::string input)
{
std::vector<int> out_int_list;
PhraseParseOrDie(
// input string
input,
// parser grammar
*(qi::int_ >> qi::skip[qi::int_]),
// skip parser
qi::space,
// output list
out_int_list);
fmt::print("test() parse result: {}\n", out_int_list);
}
int main(int argc, char* argv[])
{
test("12345 42 5 2");
return 0;
}
Prints
test() parse result: [12345, 42, 5, 2]

Answer 1

You're looking for qi::omit[]:
*(qi::int_ >> qi::omit[qi::int_])
Note you can also implicitly omit things by declaring a rule without attribute-type (which make it bind to qi::unused_type for silent compatibility).
Also note that if you're making an adhoc, sloppy grammar to scan for certain "landmarks" in a larger body of text, consider spirit::repository::qi::seek which can be significantly faster and more expressive.
Finally, note that Spirit X3 comes with a similar seek[] directive out of the box.
Simplified Demo
Much simplified: https://godbolt.org/z/EY4KdxYv9
#include <fmt/ranges.h>
#include <boost/spirit/include/qi.hpp>
// Helper to run a parser, check for errors, and capture the results.
void test(std::string const& input)
{
std::vector<int> out_int_list;
namespace qi = boost::spirit::qi;
qi::parse(input.begin(), input.end(), //
qi::expect[ //
qi::skip(qi::space)[ //
*(qi::int_ >> qi::omit[qi::int_]) > qi::eoi]], //
out_int_list);
fmt::print("test() parse result: {}\n", out_int_list);
}
int main() { test("12345 42 5 2"); }
Prints
test() parse result: [12345, 5]
But Wait
Seeing your comment
// Parse a bracketed list of integers with spaces between symbols
Did you really mean that? Because that sounds a ton more like:
'[' > qi::auto_ % +qi::graph > ']'
See it live: https://godbolt.org/z/eK6Thzqea
//#define BOOST_SPIRIT_DEBUG
#include <fmt/ranges.h>
#include <boost/spirit/include/qi.hpp>
#include <boost/spirit/include/qi_auto.hpp>
//#include <boost/fusion/adapted.hpp>
// Helper to run a parser, check for errors, and capture the results.
template <typename T> auto test(std::string const& input) {
std::vector<T> out;
using namespace boost::spirit::qi;
rule<std::string::const_iterator, T()> v = auto_;
BOOST_SPIRIT_DEBUG_NODE(v);
phrase_parse( //
input.begin(), input.end(), //
'[' > -v % lexeme[+(graph - ']')] > ']', //
space, out);
return out;
}
int main() {
fmt::print("ints: {}\n", test<int>("[12345 USD 5 PUT]"));
fmt::print("doubles: {}\n", test<double>("[ 1.2345 42 -inf 'hello' 3.1415 ]"));
}
Prints
ints: [12345, 5]
doubles: [1.2345, -inf, 3.1415]