Boost.Spirit Parsing Optional Prefix

Question

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

descr:expense type:receivable customer 27.3

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

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;
};

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.

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

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

score 3 · Accepted Answer · answered Apr 13 '17 at 01:31

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

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;

Complete sample Live On Coliru

#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";
    }
}

Printing

==== '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'

Boost.Spirit Parsing Optional Prefix

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