1 | #include <iostream>
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2 | #include <boost/spirit/include/qi.hpp>
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3 | #include <boost/optional.hpp>
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4 | #include <boost/optional/optional_io.hpp>
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5 |
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6 | int main() {
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7 | using namespace std;
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8 |
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9 | // parsing parentheses that may or may contain integers or one of a couple other things
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10 |
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11 | using namespace boost::spirit::qi;
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12 | rule<string::iterator> nulltok = boost::spirit::qi::string("novalue");
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13 | rule<string::iterator> othertok = boost::spirit::qi::string("somethingelse");
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14 |
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15 | boost::optional<int> result;
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16 | std::string in("(novalue)");
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17 | auto beg = in.begin();
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18 | auto end = in.end();
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19 | phrase_parse(beg, end,
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20 | '(' > ((int_ | nulltok) ^ othertok) > ')', // <<<< uninit instead of "not present"
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21 | ascii::space,
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22 | result);
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23 | cout << result << "\n";
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24 | }
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