Boost自定义解析器无法使用qi::parse

#include <iostream>
#include <string>
#include <boost/spirit/include/qi.hpp>

namespace qi = boost::spirit::qi;

// 定义一个简单的解析规则
template <typename Iterator>
struct MyParser : qi::grammar<Iterator, int()> {
    MyParser() : MyParser::base_type(start) {
        start = qi::int_;
    }
    qi::rule<Iterator, int()> start;
};

int main() {
    std::string input = "123";
    auto iter = input.begin();
    auto end = input.end();

    MyParser<std::string::iterator> parser;
    int result;

    bool success = qi::parse(iter, end, parser, result);

    if (success && iter == end) {
        std::cout << "Parsed successfully: " << result << std::endl;
    } else {
        std::cout << "Parsing failed" << std::endl;
    }

    return 0;
}