c++ - 用qi和其他部分解析为结构

Question

我使用精神分析器已经很长时间了，但是现在我有一个我不太了解的问题。
我想将a，b-> c，d或a，b-> d解析为一个结构。如果输入是a，b-> c，d(规则的左侧)，则以下代码将执行此操作。但是，如果输入为a，b-> d(交替部分)，则产生aa，bb ,, d。因此，似乎替代解析器不会清除已经解析的部分。

struct Test
{
  std::string a;
  std::string b;
  std::string c;
  std::string d;
};

BOOST_FUSION_ADAPT_STRUCT(Test,
(std::string, a)
(std::string, b)
(std::string, c)
(std::string, d))
using namespace boost::spirit::qi;
using std::string;
using std::pair;
rule<const char *, Test()> r = (+alnum >> ',' >> +alnum >> "->" >> +alnum >> ',' >> +alnum) | (+alnum >> ',' >> +alnum >> "->" >> attr(string()) >> +alnum);
Test result;
//const char* s = "a,b->c,d"; //produces a Result with a,b,c,d
const char* s = "a,b->d"; // procudes a Result with aa,bb,,d
parse(s, s + strlen(s), r, result);

Answer 1

使其成为一个独立的副本:

Live On Wandbox

#include <boost/spirit/include/qi.hpp>
#include <boost/fusion/adapted.hpp>
#include <iomanip>
#include <iostream>

namespace qi = boost::spirit::qi;

struct Test { std::string a, b, c, d; };
BOOST_FUSION_ADAPT_STRUCT(Test, a,b,c,d)

int main() {
    qi::rule<const char *, Test()> static const r 
        = (+qi::alnum >> ',' >> +qi::alnum >> "->" >> +qi::alnum >> ',' >> +qi::alnum)
        | (+qi::alnum >> ',' >> +qi::alnum >> "->" >> qi::attr(std::string()) >> +qi::alnum);

    for (auto input : {
        "a,b->c,d",
        "a,b->d"
    }) {
        Test result;
        qi::parse(input, input + strlen(input), r, result);
        for (auto const& part: { result.a, result.b, result.c, result.d })
            std::cout << " " << std::quoted(part);
        std::cout << "\n";
    }
}

列印

 "a" "b" "c" "d"
 "aa" "bb" "" "d"

因此，您的问题类似于:

Understanding Boost.spirit's string parser

Use of optional parser in spirit qi

Rolling back changes in alternative parsers in qi spirit

常见的解决方案是 qi::hold[]，它有一些开销，但通常可以正常工作:

qi::rule<const char *, Test()> static const r 
    = qi::hold [+qi::alnum >> ',' >> +qi::alnum >> "->" >> +qi::alnum >> ',' >> +qi::alnum]
    | (+qi::alnum >> ',' >> +qi::alnum >> "->" >> qi::attr(std::string()) >> +qi::alnum);

哪个打印 Live On Wandbox

 "a" "b" "c" "d"
 "a" "b" "" "d"