#include <iostream>
#include <boost/spirit/include/qi.hpp>
#include <boost/optional.hpp>
#include <boost/optional/optional_io.hpp>

int main() {
    using namespace std;

    // parsing parentheses that may or may contain integers or one of a couple other things

    using namespace boost::spirit::qi;
    rule<string::iterator> nulltok = boost::spirit::qi::string("novalue");
    rule<string::iterator> othertok = boost::spirit::qi::string("somethingelse");

    boost::optional<int> result;
    std::string in("(novalue)");
    auto beg = in.begin();
    auto end = in.end();
    phrase_parse(beg, end,   
                 '(' > ((int_ | nulltok) ^ othertok) > ')',   // <<<< uninit instead of "not present"
                 ascii::space,
                 result);
    cout << result << "\n";
}