How to use the qi :: hold [] directive Parser. (Issue with attribute type for boost :: swap)

Hold is not used for this.

Think about it: how would that help? This will not cause the integer branch to not match, so it will still not parse and hold

return nothing.

hold[]

mostly useful for container attributes¹, where partial parses can change the attribute. Containers usually have a swap already. You were barking the wrong tree here.

¹ including strings, see for example
_{• Understanding the parsing of the Boost.spirit string
• boost :: spirit :: qi re-parsing on output
• Reconstructing the parsing
• Optional Boost Spirit parser and rollback}

Solution to the problem:

You can use a strict real-world policy to parse only real values ​​as doubles.

value =  strict_double | qi::int_ | qi::lexeme[+(qi::char_ - qi::eol)] ;

// with
qi::real_parser<double, qi::strict_real_policies<double> > strict_double;

      

        
        
        
      

    

See also int or double parameter using boost spirit (longest_d) for some unit tests

Live On Coliru

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

namespace qi    = boost::spirit::qi;
namespace ascii = boost::spirit::ascii;

typedef boost::variant<int, double, std::string> value_t;
typedef std::map<std::string, value_t> namevalue;
typedef std::pair<std::string, value_t> namevaluepair;

template <typename Iterator>
struct keys_and_values2
    :   qi::grammar<Iterator, namevalue(), ascii::blank_type>
{
    keys_and_values2() : keys_and_values2::base_type(start)
    {
        start %= query >> qi::eoi;
        query =  +pair;
        value =  strict_double | qi::int_ | qi::lexeme[+(qi::char_ - qi::eol)] ;
        pair  =  key >> qi::lit(':') >> value >> qi::eol;
        key   =  qi::char_("a-zA-Z_") >> *qi::char_("a-zA-Z_0-9");

        BOOST_SPIRIT_DEBUG_NODES((start)(query)(value)(pair)(key))
    }
  private:
    qi::real_parser<double, qi::strict_real_policies<double> > strict_double;
    qi::rule<Iterator, namevalue(),     ascii::blank_type> start;
    qi::rule<Iterator, namevalue(),     ascii::blank_type> query;
    qi::rule<Iterator, namevaluepair(), ascii::blank_type> pair;
    qi::rule<Iterator, std::string(),   ascii::blank_type> key;
    qi::rule<Iterator, value_t(),       ascii::blank_type> value;
};

int main() {
    typedef boost::spirit::istream_iterator It;
    It f(std::cin >> std::noskipws), l;

    keys_and_values2<It> g;
    namevalue data;
    bool ok = qi::phrase_parse(f,l,g,ascii::blank,data);

    if (ok) {
        std::cout << "Parse succeeded:\n";
        for(auto& p : data)
            std::cout << "\t'" << 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

Parse succeeded:
    'Briley'    -> -40.9054
    'Clara' -> -92.4086
    'Jamiya'    -> Jelly Drop
    'Janelle'   -> 132349223
    'Sarai' -> 52.7312

      

        
        
        
      

    

And debug info (if enabled)

<start>
<try>Sarai    :    52.731</try>
<query>
    <try>Sarai    :    52.731</try>
    <pair>
    <try>Sarai    :    52.731</try>
    <key>
        <try>Sarai    :    52.731</try>
        <success>:    52.731199473801</success>
        <attributes>[[S, a, r, a, i]]</attributes>
    </key>
    <value>
        <try>    52.731199473801\n</try>
        <success>\nJamiya   :    Jelly</success>
        <attributes>[52.7312]</attributes>
    </value>
    <success>Jamiya   :    Jelly </success>
    <attributes>[[[S, a, r, a, i], 52.7312]]</attributes>
    </pair>
    <pair>
    <try>Jamiya   :    Jelly </try>
    <key>
        <try>Jamiya   :    Jelly </try>
        <success>:    Jelly Drop\nClar</success>
        <attributes>[[J, a, m, i, y, a]]</attributes>
    </key>
    <value>
        <try>    Jelly Drop\nClara</try>
        <success>\nClara    :    -92.4</success>
        <attributes>[[J, e, l, l, y,  , D, r, o, p]]</attributes>
    </value>
    <success>Clara    :    -92.40</success>
    <attributes>[[[J, a, m, i, y, a], [J, e, l, l, y,  , D, r, o, p]]]</attributes>
    </pair>
    <pair>
    <try>Clara    :    -92.40</try>
    <key>
        <try>Clara    :    -92.40</try>
        <success>:    -92.40860586988</success>
        <attributes>[[C, l, a, r, a]]</attributes>
    </key>
    <value>
        <try>    -92.408605869885</try>
        <success>\nJanelle  :    13234</success>
        <attributes>[-92.4086]</attributes>
    </value>
    <success>Janelle  :    132349</success>
    <attributes>[[[C, l, a, r, a], -92.4086]]</attributes>
    </pair>
    <pair>
    <try>Janelle  :    132349</try>
    <key>
        <try>Janelle  :    132349</try>
        <success>:    132349223\nBrile</success>
        <attributes>[[J, a, n, e, l, l, e]]</attributes>
    </key>
    <value>
        <try>    132349223\nBriley</try>
        <success>\nBriley   :    -40.9</success>
        <attributes>[132349223]</attributes>
    </value>
    <success>Briley   :    -40.90</success>
    <attributes>[[[J, a, n, e, l, l, e], 132349223]]</attributes>
    </pair>
    <pair>
    <try>Briley   :    -40.90</try>
    <key>
        <try>Briley   :    -40.90</try>
        <success>:    -40.90535249560</success>
        <attributes>[[B, r, i, l, e, y]]</attributes>
    </key>
    <value>
        <try>    -40.905352495602</try>
        <success>\n</success>
        <attributes>[-40.9054]</attributes>
    </value>
    <success></success>
    <attributes>[[[B, r, i, l, e, y], -40.9054]]</attributes>
    </pair>
    <pair>
    <try></try>
    <key>
        <try></try>
        <fail/>
    </key>
    <fail/>
    </pair>
    <success></success>
    <attributes>[[[[B, r, i, l, e, y], -40.9054], [[C, l, a, r, a], -92.4086], [[J, a, m, i, y, a], [J, e, l, l, y,  , D, r, o, p]], [[J, a, n, e, l, l, e], 132349223], [[S, a, r, a, i], 52.7312]]]</attributes>
</query>
<success></success>
<attributes>[[[[B, r, i, l, e, y], -40.9054], [[C, l, a, r, a], -92.4086], [[J, a, m, i, y, a], [J, e, l, l, y,  , D, r, o, p]], [[J, a, n, e, l, l, e], 132349223], [[S, a, r, a, i], 52.7312]]]</attributes>
</start>