Parsing nested lists with Boost.Spirit - c++

Question

I am trying to parse nested lists of numbers with Boost.Spirit. This is what I have so far:
//define a test string
std::string string = "[[\t1 , 2 ], [3, 4, 65.4]]";
auto it = string.begin();
//parse the string
std::vector<std::vector<double>> vector;
auto listRule = "[" >> (qi::double_ % ",") >> "]";
auto list2Rule = "[" >> (listRule % ",") >> "]";
bool match = qi::phrase_parse(it, string.end(), list2Rule, ascii::space, vector);
//check if we have a match
std::cout << "matched: " << std::boolalpha << match << '\n';
if (it != string.end())
std::cout << "unmatched part: " << std::string{it, string.end()} << '\n';
//print the result
std::cout << "result\n";
for (const auto& v : vector) {
std::cout << "[";
for (double i : v)
std::cout << i << ",";
std::cout << "]\n";
}
The above works wonderfully and prints:
matched: true
result
[1,2,]
[3,4,65.4,]
The problem I am facing is that it does not accept empty lists. For example, by changing the string like so:
std::string string = "[[\t1 , 2 ], [3, 4, 65.4], []]";
Then I have no match (that is match == false and it == string.begin()). The vector still gets populated, apparently, but the last empty list is missing. Can anyone provide an explanation on why this is the case, and how to fix it?

Answer 1

You're using auto for proto expression templates in the Qi domain - that's undefined behaviour 99.9% of the time:
undefined behaviour somewhere in boost::spirit::qi::phrase_parse
Now, while you fix that, also make the list body optional:
Live On Coliru
#include <boost/spirit/include/qi.hpp>
namespace qi = boost::spirit::qi;
int main() {
using It = std::string::const_iterator;
using Skipper = qi::space_type;
for(std::string const input : { "[[\t1 , 2 ], [3, 4, 65.4]]", "[[\t1 , 2 ], [3, 4, 65.4], []]", "[]" })
{
std::cout << " ---- '" << input << "' ----\n";
auto it = input.begin();
//parse the string
using doubles = std::vector<double>;
using vectors = std::vector<doubles>;
qi::rule<It, doubles(), Skipper> doubles_ = "[" >> -(qi::double_ % ",") >> "]";
qi::rule<It, vectors(), Skipper> vectors_ = "[" >> -(doubles_ % ",") >> "]";
vectors data;
bool match = qi::phrase_parse(it, input.end(), vectors_, qi::space, data);
//check if we have a match
std::cout << "matched: " << std::boolalpha << match << '\n';
if (it != input.end())
std::cout << "unmatched part: " << std::string{it, input.end()} << '\n';
//print the result
std::cout << "result\n";
for (const auto& v : data) {
std::cout << "[";
for (double i : v)
std::cout << i << ",";
std::cout << "]\n";
}
}
}
Prints
---- '[[ 1 , 2 ], [3, 4, 65.4]]' ----
matched: true
result
[1,2,]
[3,4,65.4,]
---- '[[ 1 , 2 ], [3, 4, 65.4], []]' ----
matched: true
result
[1,2,]
[3,4,65.4,]
[]
---- '[]' ----
matched: true
result