Boost.Spirit解析可选前缀 [英] Boost.Spirit Parsing Optional Prefix

问题描述

我正在尝试解析由空格分隔的，可选标记的字符串.例如

I'm attempting to parse a string of whitespace-delimited, optionally-tagged keywords. For example

descr:expense type:receivable customer 27.3

其中冒号前的表达式是标记，并且是可选的(即假定使用默认标记).

where the expression before the colon is the tag, and it is optional (i.e. a default tag is assumed).

我不能完全让解析器做我想做的事情.我对规范进行了一些小的改编示例，其目的是解析键/值对(非常类似于HTTP查询字符串).

I can't quite get the parser to do what I want. I've made some minor adaptations from a canonical example whose purpose it is to parse key/value pairs (much like an HTTP query string).

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，当标记被忽略时， optional< string> 并非为空/false.相反，它具有值的副本.该对的第二部分也具有该值.

However, when I parse it, when the tag is left out, the optional<string> isn't empty/false. Rather, it's got a copy of the value. The second part of the pair has the value as well.

如果untagged关键字不能是标签(语法规则，例如，具有小数点)，那么事情就会像我期望的那样工作.

If the untagged keyword can't be a tag (syntax rules, e.g. has a decimal point), then things work like I'd expect.

我做错了什么?这是PEG的概念性错误吗?

What am I doing wrong? Is this a conceptual mistake with the PEG?

推荐答案

相反，它具有值的副本.该对的第二部分也具有该值.

Rather, it's got a copy of the value. The second part of the pair has the value as well.

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

This is the common pitfall with container attributes and backtracking: use qi::hold, e.g. Understanding Boost.spirit's string parser

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

完整示例 在Coliru上直播

#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 failed\n";
        }

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

打印

==== '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'

这篇关于Boost.Spirit解析可选前缀的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！