解析字符转义时，Boost Spirit (x3) 未能使用最后一个令牌

Question

使用 boost spirit x3 来解析转义的 ascii 字符串我遇到了this answer，但我得到了一个期望异常。我已将原始中的 expectation 运算符更改为序列运算符，以禁用下面代码中的异常。运行代码，它会解析输入并将正确的值分配给属性，但返回 false 并且不使用输入。有什么想法我在这里做错了吗？

gcc 版本 10.3.0

提升 1.71

std = c++17

#include <boost/spirit/home/x3.hpp>
#include <string>
#include <iostream>


namespace x3 = boost::spirit::x3;
using namespace std::string_literals;

//changed expectation to sequence
auto const qstring = x3::lexeme['"' >> *(
             "\\n" >> x3::attr('\n')
           | "\\b" >> x3::attr('\b')
           | "\\f" >> x3::attr('\f')
           | "\\t" >> x3::attr('\t')
           | "\\v" >> x3::attr('\v')
           | "\\0" >> x3::attr('\0')
           | "\\r" >> x3::attr('\r')
           | "\\n" >> x3::attr('\n')
           | "\\"  >> x3::char_("\"\\")
           | "\\\"" >> x3::char_('"')
           | ~x3::char_('"')
       ) >> '"'];

int main(int, char**){

    auto const quoted = "\"Hel\\\"lo Wor\\\"ld"s;
    auto const expected = "Hel\"lo Wor\"ld"s;

    std::string result;
    auto first = quoted.begin();
    auto const last = quoted.end();
    bool ok = x3::phrase_parse(first, last, qstring, x3::ascii::space, result);
    std::cout << "parse returned " << std::boolalpha << ok << '\n';

    std::cout << result << " == " << expected << " is " << std::boolalpha << (result == expected) << '\n';

    std::cout << "first == last = " << (first == last) << '\n';
    std::cout << "first = " << *first << '\n';

    return 0;
}

Answer 1

您的输入没有以引号字符结束。将其写为原始字符串文字会有所帮助：

std::string const qinput   = R"("Hel\"lo Wor\"ld)";

应该是

std::string const qinput   = R"("Hel\"lo Wor\"ld")";

现在，剩下的就是普通的容器处理：在 Spirit 中，当规则失败时（也包括回溯分支时），容器属性不会回滚。参见例如boost::spirit::qi duplicate parsing on the output、Understanding Boost.spirit's string parser等

基本上，如果解析失败，您就不能依赖结果。这可能就是为什么原文有一个期望点：引发异常。

正确工作的完整演示：

Live On Coliru

#include <boost/spirit/home/x3.hpp>
#include <string>
#include <iostream>
#include <iomanip>

namespace x3 = boost::spirit::x3;

auto escapes = []{
    x3::symbols<char> sym;
    sym.add
        ("\\b", '\b')
        ("\\f", '\f')
        ("\\t", '\t')
        ("\\v", '\v')
        ("\\0", '\0')
        ("\\r", '\r')
        ("\\n", '\n')
        ("\\\\", '\\')
        ("\\\"", '"')
        ;
    return sym;
}();

auto const qstring = x3::lexeme['"' >> *(escapes | ~x3::char_('"')) >> '"'];

int main(){
    auto squote = [](std::string_view s) { return std::quoted(s, '\''); };
    std::string const expected = R"(Hel"lo Wor"ld)";

    for (std::string const qinput : {
        R"("Hel\"lo Wor\"ld)", // oops no closing quote
        R"("Hel\"lo Wor\"ld")",
        "\"Hel\\\"lo Wor\\\"ld\"", // if you insist
        R"("Hel\"lo Wor\"ld" trailing data)",
    })
    {
        std::cout << "\n -- input " << squote(qinput) << "\n";
        std::string result;

        auto first = cbegin(qinput);
        auto last  = cend(qinput);
        bool ok    = x3::phrase_parse(first, last, qstring, x3::space, result);

        ok &= (first == last);

        std::cout << "parse returned " << std::boolalpha << ok << "\n";

        std::cout << squote(result) << " == " << squote(expected) << " is "
                  << (result == expected) << "\n";

        if (first != last)
            std::cout << "Remaining input unparsed: " << squote({first, last})
                      << "\n";
    }
}

打印

 -- input '"Hel\\"lo Wor\\"ld'
parse returned false
'Hel"lo Wor"ld' == 'Hel"lo Wor"ld' is true
Remaining input unparsed: '"Hel\\"lo Wor\\"ld'

 -- input '"Hel\\"lo Wor\\"ld"'
parse returned true
'Hel"lo Wor"ld' == 'Hel"lo Wor"ld' is true

 -- input '"Hel\\"lo Wor\\"ld"'
parse returned true
'Hel"lo Wor"ld' == 'Hel"lo Wor"ld' is true

 -- input '"Hel\\"lo Wor\\"ld" trailing data'
parse returned false
'Hel"lo Wor"ld' == 'Hel"lo Wor"ld' is true
Remaining input unparsed: 'trailing data'