如何使用boost spirit x3 kleene star？

Question

我是 boost::spirit 的新手，正在尝试使用 x3 编写一个简单的解析器。我收到一个无法解释的错误。出于某种原因，我收到有关 boost::spirit::x3::unused_type 的错误消息。当我删除下面标记的行中的 kleene star 时，错误消失了。其他 kleene 星似乎还好（出于某种原因）。这是我的代码：

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

namespace x3 = boost::spirit::x3;

int main()
{
    std::string str = "qqq::qqq::qqq";

    auto begin = str.begin(), end   = str.end();
    bool r = x3::parse(begin, end,
        (        (x3::alpha | '_') >> *(x3::alnum | '_')) >>
        *                                         //this line is the problem
        ("::" >> (x3::alpha | '_') >> *(x3::alnum | '_'))
    ); // is supposed to match str

    if (r && std::distance(begin,end) < 1)
    {
        std::cout << "Parsing succeeded\n";
    }
    else
    {
        std::cout << "Parsing FAILED\n";
        std::cout << "r " << r << " d " << std::distance(begin,end) << "\n";
    }

    return 0;
}

这是错误：

/usr/include/boost/spirit/home/x3/core/detail/parse_into_container.hpp:254:22: error: ‘const struct boost::spirit::x3::unused_type’ has no member named ‘empty’
254 |             if (attr.empty())
    |                 ~~~~~^~~~~
/usr/include/boost/spirit/home/x3/core/detail/parse_into_container.hpp:259:22: error: ‘const struct boost::spirit::x3::unused_type’ has no member named ‘insert’
259 |                 attr.insert(attr.end(), rest.begin(), rest.end());
    |                 ~~~~~^~~~~~
/usr/include/boost/spirit/home/x3/core/detail/parse_into_container.hpp:259:34: error: ‘const struct boost::spirit::x3::unused_type’ has no member named ‘end’
259 |                 attr.insert(attr.end(), rest.begin(), rest.end());
    |                             ~~~~~^~~
/usr/include/boost/spirit/home/x3/core/detail/parse_into_container.hpp:259:46: error: ‘const struct boost::spirit::x3::unused_type’ has no member named ‘begin’
259 |                 attr.insert(attr.end(), rest.begin(), rest.end());
    |                                         ~~~~~^~~~~
/usr/include/boost/spirit/home/x3/core/detail/parse_into_container.hpp:259:60: error: ‘const struct boost::spirit::x3::unused_type’ has no member named ‘end’
259 |                 attr.insert(attr.end(), rest.begin(), rest.end());
    |                                                       ~~~~~^~~

我在 Ubuntu 18.04 使用 boost 1.65.1。我阅读了这篇文章 () post，其中描述了类似的问题。

有没有办法在不更改系统包的情况下解决或规避这个问题？

Answer 1

不，这只是一个已修复的错误。

1.65.1 https://wandbox.org/permlink/uUeqVJOsnSxk9aD1 损坏
1.66.0 https://wandbox.org/permlink/AhQ9G8Ubm9v0DeYj 损坏
1.67.0 https://wandbox.org/permlink/nh4wDJdhO8lSfBZo 还行

无论如何，像 x3::alpha | '_' 这样的语法结构不会公开您需要的内容，因为 literal '_' 不公开任何属性。

因此，在实践中您可能只想使用 raw[]:

来简化

auto ident = x3::raw [ x3::lexeme [ (x3::alpha | '_') >> *(x3::alnum | '_') ]

接下来，您可以匹配多个符合::的标识符：

x3::raw [ ident >> * ("::" >> ident) ]

顺便说一下，这完全等同于

x3::raw [ ident % "::") ]

见

1.65.1 https://wandbox.org/permlink/Jorllem5U7u3L04e 还行
1.67.0 https://wandbox.org/permlink/EXMjDTPhcZfu05Ot 也可以

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

namespace x3 = boost::spirit::x3;

int main() {
    std::string str = "foo::_bar::qux_1";

    auto begin = str.begin(), end = str.end();
    auto ident = x3::raw [ x3::lexeme [ (x3::alpha | '_') >> *(x3::alnum | '_') ] ];

    std::vector<std::string> qualified;
    bool r = x3::parse(begin, end, ident % "::", qualified);

    if (r && begin==end) {
        std::cout << "Parsing succeeded\n";
        for (auto& el : qualified) {
            std::cout << " element " << std::quoted(el) << "\n";
        }
    } else {
        std::cout << "Parsing FAILED\n";
    }

    std::cout << std::boolalpha << r << " " << std::quoted(std::string(begin, end)) << "\n";
}

打印：

Parsing succeeded
 element "foo"
 element "_bar"
 element "qux_1"
true ""

并且语法被简化为

std::vector<std::string> qualified;
bool r = x3::parse(begin, end, ident % "::", qualified);

这通常是个好兆头。

如何使用boost spirit x3 kleene star？

How to use boost spirit x3 kleene star?

c++

boost

boost-spirit

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