使用 boost::spirit 读取空值

Question

我想将 CSV 文件读入结构中：

struct data 
{
   std::string a;
   std::string b;
   std::string c;
}

但是，我什至想读取空字符串以确保所有值都在正确的位置。我将结构调整为 boost::fusion，因此以下工作：

// Our parser (using a custom skipper to skip comments and empty lines )
template <typename Iterator, typename skipper = comment_skipper<Iterator> >
  struct google_parser : qi::grammar<Iterator, addressbook(), skipper>
{
  google_parser() : google_parser::base_type(contacts, "contacts")
  {
    using qi::eol;
    using qi::eps;
    using qi::_1;
    using qi::_val;
    using qi::repeat;
    using standard_wide::char_;
    using phoenix::at_c;
    using phoenix::val;

    value = *(char_ - ',' - eol) [_val += _1];

    // This works but only for small structs
    entry %= value >> ',' >> value >> ',' >> value >> eol;
  }

  qi::rule<Iterator, std::string()> value;
  qi::rule<Iterator, data()> entry;
};

不幸的是，repeat 将所有非空值存储在向量中，因此属性值可能会混合在一起（即，如果 b 的字段为空，它可能包含来自c):

    entry %= repeat(2)[ value >> ','] >> value >> eol;

我想使用类似于 repeat 的简短规则，因为我的结构在实践中有 60 个属性！不仅写 60 条规则很乏味，而且 Boost 似乎不喜欢长规则...

Answer 1

您只想确保您也解析了 "empty" 个字符串的值。

value = +(char_ - ',' - eol) | attr("(unspecified)");
entry = value >> ',' >> value >> ',' >> value >> eol;

查看演示：

Live On Coliru

//#define BOOST_SPIRIT_DEBUG
#include <boost/fusion/adapted/struct.hpp>
#include <boost/spirit/include/qi.hpp>

namespace qi = boost::spirit::qi;

struct data {
    std::string a;
    std::string b;
    std::string c;
};

BOOST_FUSION_ADAPT_STRUCT(data, (std::string, a)(std::string, b)(std::string, c))

template <typename Iterator, typename skipper = qi::blank_type>
struct google_parser : qi::grammar<Iterator, data(), skipper> {
    google_parser() : google_parser::base_type(entry, "contacts") {
        using namespace qi;

        value = +(char_ - ',' - eol) | attr("(unspecified)");
        entry = value >> ',' >> value >> ',' >> value >> eol;

        BOOST_SPIRIT_DEBUG_NODES((value)(entry))
    }
  private:
    qi::rule<Iterator, std::string()> value;
    qi::rule<Iterator, data(), skipper> entry;
};

int main() {
    using It = std::string::const_iterator;
    google_parser<It> p;

    for (std::string input : { 
            "something, awful, is\n",
            "fine,,just\n",
            "like something missing: ,,\n",
        })
    {
        It f = input.begin(), l = input.end();

        data parsed;
        bool ok = qi::phrase_parse(f,l,p,qi::blank,parsed);

        if (ok)
            std::cout << "Parsed: '" << parsed.a << "', '" << parsed.b << "', '" << parsed.c << "'\n";
        else
            std::cout << "Parse failed\n";

        if (f!=l)
            std::cout << "Remaining unparsed: '" << std::string(f,l) << "'\n";
    }
}

打印：

Parsed: 'something', 'awful', 'is'
Parsed: 'fine', '(unspecified)', 'just'
Parsed: 'like something missing: ', '(unspecified)', '(unspecified)'

但是，你有一个更大的问题。 qi::repeat(2) [ value ] 将解析为 2 个字符串的假设不起作用。

repeat，像operator*、operator+和operator%解析成一个容器属性。在这种情况下，容器属性（字符串）也将接收来自第二个 value 的输入：

Live On Coliru

Parsed: 'somethingawful', 'is', ''
Parsed: 'fine(unspecified)', 'just', ''
Parsed: 'like something missing: (unspecified)', '(unspecified)', ''

因为这不是您想要的，请重新考虑您的数据类型：

要么不调整结构，而是编写自定义特征来分配字段（参见 http://www.boost.org/doc/libs/1_57_0/libs/spirit/doc/html/spirit/advanced/customize.html）
更改结构以包含 std::string 的向量以匹配暴露的属性
或者创建一个自动解析器生成器：

`auto_` 方法：

如果你教 Qi 如何提取单个值，你可以使用像

这样的简单规则

entry = skip(skipper() | ',') [auto_] >> eol;

这样，Spirit 本身将为给定的 Fusion 序列生成正确数量的值提取！

这是一个快速但肮脏的方法：

CAVEAT Specializing for std::string directly like this might not be the best idea (it might not always be appropriate and might interact badly with other parsers). However, by default create_parser<std::string> is not defined (because, what would it do?) so I seized the opportunity for the purpose of this demonstration:

namespace boost { namespace spirit { namespace traits {
    template <> struct create_parser<std::string> {
        typedef proto::result_of::deep_copy<
            BOOST_TYPEOF(
                qi::lexeme [+(qi::char_ - ',' - qi::eol)] | qi::attr("(unspecified)")
            )
        >::type type;

        static type call() {
            return proto::deep_copy(
                qi::lexeme [+(qi::char_ - ',' - qi::eol)] | qi::attr("(unspecified)")
            );
        }
    };
}}}

再次查看演示输出：

Live On Coliru

Parsed: 'something', 'awful', 'is'
Parsed: 'fine', 'just', '(unspecified)'
Parsed: 'like something missing: ', '(unspecified)', '(unspecified)'

NOTE There was some advanced sorcery to get the skipper to work "just right" (see skip()[] and lexeme[]). Some general explanations can be found here: Boost spirit skipper issues

更新

容器方法

其中有一个微妙之处。实际上是两个。所以这是一个演示：

Live On Coliru

//#define BOOST_SPIRIT_DEBUG
#include <boost/fusion/adapted/struct.hpp>
#include <boost/spirit/include/qi.hpp>

namespace qi = boost::spirit::qi;

struct data {
    std::vector<std::string> parts;
};

BOOST_FUSION_ADAPT_STRUCT(data, (std::vector<std::string>, parts))

template <typename Iterator, typename skipper = qi::blank_type>
struct google_parser : qi::grammar<Iterator, data(), skipper> {
    google_parser() : google_parser::base_type(entry, "contacts") {
        using namespace qi;
        qi::as<std::vector<std::string> > strings;

        value = +(char_ - ',' - eol) | attr("(unspecified)");
        entry = strings [ repeat(2) [ value >> ',' ] >> value ] >> eol;

        BOOST_SPIRIT_DEBUG_NODES((value)(entry))
    }
  private:
    qi::rule<Iterator, std::string()> value;
    qi::rule<Iterator, data(), skipper> entry;
};

int main() {
    using It = std::string::const_iterator;
    google_parser<It> p;

    for (std::string input : { 
            "something, awful, is\n",
            "fine,,just\n",
            "like something missing: ,,\n",
        })
    {
        It f = input.begin(), l = input.end();

        data parsed;
        bool ok = qi::phrase_parse(f,l,p,qi::blank,parsed);

        if (ok) {
            std::cout << "Parsed: ";
            for (auto& part : parsed.parts) 
                std::cout << "'" << part << "' ";
            std::cout << "\n";
        }
        else
            std::cout << "Parse failed\n";

        if (f!=l)
            std::cout << "Remaining unparsed: '" << std::string(f,l) << "'\n";
    }
}

细微之处在于：

通过自动属性处理调整单元素序列遇到边缘情况：Spirit Qi attribute propagation issue with single-member struct
在这种特殊情况下，Spirit 需要亲力亲为才能将 repeat[...]>>value 视为/原子/合成单个容器。 as<T> directive solves that here

使用 boost::spirit 读取空值

Read empty values with boost::spirit

c++

boost

boost-spirit

`auto_` 方法：

更新

容器方法

使用 boost::spirit 读取空值

Read empty values with boost::spirit

c++

boost

boost-spirit

auto_ 方法：

更新

容器方法

`auto_` 方法：