Boost.Spirit解析可选前缀

Boost.Spirit Parsing Optional Prefix

本文关键字：前缀 Spirit Boost      更新时间：2023-10-16

我正在尝试解析一串以空格分隔的可选标记关键字。例如

descr:expense type:receivable customer 27.3

其中冒号前面的表达式是标签，它是可选的(即假定默认标签(。

我无法让解析器做我想做的事。我从一个规范的例子做了一些小的改编，其目的是解析键/值对(很像HTTP查询字符串(。

typedef std::pair<boost::optional<std::string>, std::string> pair_type;
typedef std::vector<pair_type> pairs_type;
template <typename Iterator>
struct field_value_sequence_default_field
  : qi::grammar<Iterator, pairs_type()>
{
    field_value_sequence_default_field()
      : field_value_sequence_default_field::base_type(query)
    {
        query =  pair >> *(qi::lit(' ') >> pair);
        pair  = -(field >> ':') >> value;
        field = +qi::char_("a-zA-Z0-9");
        value = +qi::char_("a-zA-Z0-9+-\.");
    }
    qi::rule<Iterator, pairs_type()> query;
    qi::rule<Iterator, pair_type()> pair;
    qi::rule<Iterator, std::string()> field, value;
};

但是，当我解析它时，当标签被省略时，optional<string>不是空/假的。相反，它有该值的副本。货币对的第二部分也有价值。

如果未标记的关键字不能是标签(语法规则，例如具有小数点(，那么事情就像我期望的那样工作。

我做错了什么？这是PEG的概念错误吗？

相反，它有该值的副本。货币对的第二部分也有价值。

这是容器属性和回溯的常见陷阱：使用qi::hold，例如理解Boost.spirit的字符串解析器

    pair  = -qi::hold[field >> ':'] >> value;

完整的样品在科里鲁直播

#include <boost/spirit/include/qi.hpp>
#include <boost/fusion/adapted/std_pair.hpp>
#include <boost/optional/optional_io.hpp>
#include <iostream>
namespace qi    = boost::spirit::qi;
typedef std::pair<boost::optional<std::string>, std::string> pair_type;
typedef std::vector<pair_type> pairs_type;
template <typename Iterator>
struct Grammar : qi::grammar<Iterator, pairs_type()>
{
    Grammar() : Grammar::base_type(query) {
        query =  pair % ' ';
        pair  = -qi::hold[field >> ':'] >> value;
        field = +qi::char_("a-zA-Z0-9");
        value = +qi::char_("a-zA-Z0-9+-\.");
    }
  private:
    qi::rule<Iterator, pairs_type()> query;
    qi::rule<Iterator, pair_type()> pair;
    qi::rule<Iterator, std::string()> field, value;
};
int main()
{
    using It = std::string::const_iterator;
    for (std::string const input : {
            "descr:expense type:receivable customer 27.3",
            "expense type:receivable customer 27.3",
            "descr:expense receivable customer 27.3",
            "expense receivable customer 27.3",
    }) {
        It f = input.begin(), l = input.end();
        std::cout << "==== '" << input << "' =============n";
        pairs_type data;
        if (qi::parse(f, l, Grammar<It>(), data)) {
            std::cout << "Parsed: n";
            for (auto& p : data) {
                std::cout << p.first << "t->'" << p.second << "'n";
            }
        } else {
            std::cout << "Parse failedn";
        }
        if (f != l)
            std::cout << "Remaining unparsed: '" << std::string(f,l) << "'n";
    }
}

印刷

==== 'descr:expense type:receivable customer 27.3' =============
Parsed: 
 descr  ->'expense'
 type   ->'receivable'
--  ->'customer'
--  ->'27.3'
==== 'expense type:receivable customer 27.3' =============
Parsed: 
--  ->'expense'
 type   ->'receivable'
--  ->'customer'
--  ->'27.3'
==== 'descr:expense receivable customer 27.3' =============
Parsed: 
 descr  ->'expense'
--  ->'receivable'
--  ->'customer'
--  ->'27.3'
==== 'expense receivable customer 27.3' =============
Parsed: 
--  ->'expense'
--  ->'receivable'
--  ->'customer'
--  ->'27.3'
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