Boost Spirit Qi解析器無法正常工作時將其解析爲qi ::語法

Question

我正在使用Spirit解析器來解析像id 1234這樣的字符串。它適用於內聯start = qi::lit("id") >> qi::int_;，但如果我想將它放入單獨的基於qi :: grammar的結構體中，則不是。見情況1，2和3在下面的例子：

#include <boost/spirit/include/qi.hpp> 
#include <boost/spirit/include/phoenix.hpp> 

#include <iostream> 
#include <fstream> 

namespace qi = boost::spirit::qi; 
namespace phoenix = boost::phoenix; 

struct Context{}; 

template <typename Iterator, typename SkipParser> 
struct IdGrammar : qi::grammar<Iterator, SkipParser> 
{ 
    explicit IdGrammar(Context& out) : IdGrammar::base_type(start, "IdGrammar") 
    { 
     start = qi::lit("id") >> qi::int_; 
    } 

    qi::rule<Iterator, SkipParser> start; 
}; 

template <typename Iterator, typename SkipParser> 
struct MyGrammar : qi::grammar<Iterator, SkipParser> 
{ 
    explicit MyGrammar(Context& out) 
     : MyGrammar::base_type(start, "MyGrammar") 
    { 
     IdGrammar<Iterator, SkipParser> idGrammar(out); 
//  start = idGrammar >> *(',' >> idGrammar); // 1 = Parsing fails 
     start = idGrammar; // 2 = Parsing fails 
//  start = qi::lit("id") >> qi::int_; // 3 = Parsing succeeds 

     start.name("the start"); 
     qi::on_error<qi::fail>(
      start, 
      phoenix::ref(std::cout) << phoenix::val("Parsing error: expecting ") << qi::_4 // what failed? 
            << phoenix::val(" here: \"") 
            << phoenix::construct<std::string>(qi::_3, qi::_2) // iterators to error-pos, end 
            << phoenix::val("\"") 
            << std::endl); 
    } 

    qi::rule<Iterator, SkipParser> start; 
}; 

int main() 
{ 
    typedef std::string::const_iterator iterator_type; 
    Context ctx; 
    MyGrammar<iterator_type, qi::space_type> roman_parser(ctx); // Our grammar 

    std::string str = "id 5012"; 

    iterator_type iter = str.begin(), end = str.end(); 
    bool r = phrase_parse(iter, end, roman_parser, qi::space); 

    if (r && iter == end) 
    { 
     std::cout << "Parsing succeeded\n"; 
    } 
    else 
    { 
     std::string rest(iter, end); 
     std::cout << "Parsing failed\n"; 
     std::cout << "stopped at: \"" << rest << "\"\n"; 
    } 
} 

Run example on Coliru

輸出發生故障的情況下（1和2）：

Parsing failed 
stopped at: "id 5012" 

是什麼讓這裏的區別？請注意，我刪除了整數結果的任何賦值，以保持示例最小化 - 假設與問題無關。

Answer 1

idGrammar的生命週期必須比構造函數的作用域更長。 使其成爲會員變量：

template <typename Iterator, typename SkipParser> 
struct MyGrammar : qi::grammar<Iterator, SkipParser> 
{ 
    explicit MyGrammar(Context& out) 
     : MyGrammar::base_type(start, "MyGrammar") 
      , idGrammar(out) 
    { 
     start = idGrammar >> *(',' >> idGrammar); // 1 = Now parsing succeeds 
//  start = idGrammar; // 2 = Now parsing succeeds 
//  start = qi::lit("id") >> qi::int_; // 3 = Parsing succeeds 

     start.name("the start"); 
     qi::on_error<qi::fail>(
      start, 
      phoenix::ref(std::cout) << phoenix::val("Parsing error: expecting ") << qi::_4 // what failed? 
            << phoenix::val(" here: \"") 
            << phoenix::construct<std::string>(qi::_3, qi::_2) // iterators to error-pos, end 
            << phoenix::val("\"") 
            << std::endl); 
    } 

    qi::rule<Iterator, SkipParser> start; 
    IdGrammar<Iterator, SkipParser> idGrammar; 
};