How to set max recursion in boost spirit - c++

Question

Using boost::spirit, if I have a recursive rule to parse parentheses
rule<std::string::iterator, std::string()> term;
term %= string("(") >> *term >> string(")");
how do I limit the maximum amount of recursion? For example, if I try to parse a million nested parentheses, I get a segfault because the stack size has been exceeded. To be concrete, here is a complete sample.
#include <iostream>
#include <string>
#include <boost/spirit/include/qi.hpp>
int main(void)
{
using namespace boost::spirit;
using namespace boost::spirit::qi;
const size_t string_size = 1000000;
std::string str;
str.resize(string_size);
for (size_t s=0; s<str.size()/2; ++s)
{
str[s]='(';
str[str.size() - s -1] = ')';
}
rule<std::string::iterator, std::string()> term;
term %= string("(") >> *term >> string(")");
std::string h;
parse(str.begin(), str.end(), term, h);
}
I compiled it with the command
g++ simple.cxx -o simple -std=c++11
It works fine if I set string_size to 1000 instead of 1000000.

Answer 1

Keep track of the depth in a qi::local<> or a phx::ref().
In this case an inherited attribute can take the role of the qi::local quite naturally:
qi::rule<std::string::const_iterator, std::string(size_t depth)> term;
qi::_r1_type _depth;
term %=
qi::eps(_depth < 32) >>
qi::string("(") >> *term(_depth + 1) >> qi::string(")");
term will now fail when depth exceeds 32.
Full Sample
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#include <iostream>
#include <string>
#include <boost/spirit/include/qi.hpp>
#include <boost/spirit/include/phoenix.hpp>
namespace qi = boost::spirit::qi;
int main(void) {
for (size_t n : { 2, 4, 8, 16, 32, 64 }) {
auto const str = [&n] {
std::string str;
str.reserve(n);
while (n--) { str.insert(str.begin(), '('); str.append(1, ')'); }
return str;
}();
std::cout << "Input length " << str.length() << "\n";
qi::rule<std::string::const_iterator, std::string(size_t depth)> term;
qi::_r1_type _depth;
term %=
qi::eps(_depth < 32) >>
qi::string("(") >> *term(_depth + 1) >> qi::string(")");
std::string h;
auto f = str.begin(), l = str.end();
bool ok = qi::parse(f, l, term(0u), h);
if (ok)
std::cout << "Ok: " << h << "\n";
else
std::cout << "Fail\n";
if (f != l)
std::cout << "Remaining unparsed: '" << std::string(f, std::min(f + 40, l)) << "'...\n";
}
}
Output:
Input length 4
Ok: (())
Input length 8
Ok: (((())))
Input length 16
Ok: (((((((())))))))
Input length 32
Ok: (((((((((((((((())))))))))))))))
Input length 64
Ok: (((((((((((((((((((((((((((((((())))))))))))))))))))))))))))))))
Input length 128
Fail
Remaining unparsed: '(((((((((((((((((((((((((((((((((((((((('...