boost::spirit parsing into a fusion adapted struct

Question

If there's a structure:

struct record
{
    std::string     type;
    std::string     delimiter;
    uint32_t        length;
    std::string     name;

    record()
    {
            type = "";
            delimiter = "";
            length = 0;
            name = "";
    }
};

Which is adapted using boost::fusion, and the below grammar:

struct record_parser : qi::grammar<Iterator, record(), ascii::space_type>
{
    record_parser() : record_parser::base_type(start)
    {
        using qi::lit;
        using qi::uint_;
        using qi::lexeme;

        using ascii::char_;
        using ascii::blank;
        using ascii::string;
        using qi::attr;

        using qi::eps;

            type %= lexeme[+(char_ - (blank|char('(')))];
            delimiter_double_quote %= char('(') >> lexeme[char('"')  >> +(char_ - char('"'))  >> char('"') ] >> char(')');
            delimiter_single_quote %= char('(') >> lexeme[char('\'') >> +(char_ - char('\'')) >> char('\'')] >> char(')');
            delimiter %= (delimiter_double_quote | delimiter_single_quote);
            name %= lexeme[+(char_ - (blank|char(';')))] >> char(';');
            length %= (char('(') >> uint_ >> char(')'));

        start %=
            eps >
            lit("record")
            >> char('{')
            >>  type
            >>  (delimiter | attr("")) >> (length | attr(0))
            >>  name
            >>  char('}')
            ;
    }

    qi::rule<Iterator, std::string(), ascii::space_type> type;
    qi::rule<Iterator, std::string(), ascii::space_type> delimiter_double_quote;
    qi::rule<Iterator, std::string(), ascii::space_type> delimiter_single_quote;
    qi::rule<Iterator, std::string(), ascii::space_type> delimiter;
    qi::rule<Iterator, uint32_t(), ascii::space_type> length;
    qi::rule<Iterator, std::string(), ascii::space_type> name;
    qi::rule<Iterator, record(), ascii::space_type> start;
};

I am looking to parse 'delimiter' and 'length' as optional. However, one of them has to be present, and if one is present, the other one should not exist.

For Example:

record { string(5) Alex; }
record { string("|") Alex; }

But Not:

record { string(5)("|") Alex; }
record { string Alex; }

I have attempted to do it this way, but compilation fails:

start %=
            eps >
            lit("record")
            >> char('{')
            >>  type
            >> ((delimiter >> attr(0)) | (attr("") >> length))
            >>  name
            >>  char('}')
            ;

Thank you for your help in advance. Below is the full source code:

#include <boost/config/warning_disable.hpp>
#include <boost/spirit/include/qi.hpp>
#include <boost/spirit/include/phoenix_core.hpp>
#include <boost/spirit/include/phoenix_operator.hpp>
#include <boost/spirit/include/phoenix_object.hpp>
#include <boost/fusion/include/adapt_struct.hpp>
#include <boost/fusion/include/io.hpp>

#include <string>

namespace client
{
    namespace qi = boost::spirit::qi;
    namespace ascii = boost::spirit::ascii;
    namespace phoenix = boost::phoenix;

    struct record
    {
        std::string     type;
        std::string     delimiter;
        uint32_t        length;
        std::string     name;

        record()
        {
                type = "";
                delimiter = "";
                length = 0;
                name = "";
        }
    };
}

BOOST_FUSION_ADAPT_STRUCT(
    client::record,
    (std::string, type)
    (std::string, delimiter)
    (uint32_t, length)
    (std::string, name)
)

namespace client
{
    template <typename Iterator>
    struct record_parser : qi::grammar<Iterator, record(), ascii::space_type>
    {
        record_parser() : record_parser::base_type(start)
        {
            using qi::lit;
            using qi::uint_;
            using qi::lexeme;

            using ascii::char_;
            using ascii::blank;
            using ascii::string;
            using qi::attr;

            using qi::eps;

                type %= lexeme[+(char_ - (blank|char('(')))];
                delimiter_double_quote %= char('(') >> lexeme[char('"')  >> +(char_ - char('"'))  >> char('"') ] >> char(')');
                delimiter_single_quote %= char('(') >> lexeme[char('\'') >> +(char_ - char('\'')) >> char('\'')] >> char(')');
                delimiter %= (delimiter_double_quote | delimiter_single_quote);
                name %= lexeme[+(char_ - (blank|char(';')))] >> char(';');
                length %= (char('(') >> uint_ >> char(')'));

            start %=
                eps >
                lit("record")
                >> char('{')
                >>  type
                >>  (delimiter | attr("")) >> (length | attr(0))
                >>  name
                >>  char('}')
                ;
        }

        qi::rule<Iterator, std::string(), ascii::space_type> type;
        qi::rule<Iterator, std::string(), ascii::space_type> delimiter_double_quote;
        qi::rule<Iterator, std::string(), ascii::space_type> delimiter_single_quote;
        qi::rule<Iterator, std::string(), ascii::space_type> delimiter;
        qi::rule<Iterator, uint32_t(), ascii::space_type> length;
        qi::rule<Iterator, std::string(), ascii::space_type> name;
        qi::rule<Iterator, record(), ascii::space_type> start;
    };
}

////////////////////////////////////////////////////////////////////////////
//  Main program
////////////////////////////////////////////////////////////////////////////
int main()
{
    std::string storage = "record { string(5) Alex; }";

    using boost::spirit::ascii::space;
    typedef std::string::const_iterator iterator_type;
    typedef client::record_parser<iterator_type> record_parser;

    record_parser g; // Our grammar

        client::record rec;
        std::string::const_iterator iter = storage.begin();
        std::string::const_iterator end = storage.end();
        bool r = phrase_parse(iter, end, g, space, rec);

        if (r && iter == end)
        {
            std::cout << boost::fusion::tuple_open('[');
            std::cout << boost::fusion::tuple_close(']');
            std::cout << boost::fusion::tuple_delimiter(", ");

            std::cout << "-------------------------\n";
            std::cout << "Parsing succeeded\n";
            std::cout << "got: " << boost::fusion::as_vector(rec) << std::endl;
            std::cout << "\n-------------------------\n";
        }
        else
        {
                std::string::const_iterator some = iter+30;
                std::string context(iter, (some>end)?end:some);
                std::cout << "-------------------------\n";
                std::cout << "Parsing failed\n";
                std::cout << "stopped at -->" << context << "...\n";
                std::cout << "-------------------------\n";
        }

    return 0;
}

Answer 1

sehe's first answer is perfect (or would be if he corrected what he realized in the comments), but I just wanted to add an explanation of the problem and a possible alternative. The code below is based on that excellent answer.

You have a couple of problems with the attributes of your start rule. The attribute you want to get is record which is basically tuple<string,string,uint32_t,string>. Let's see the attributes of several parsers:

1. Something similar (but simpler) to your original rule:

   Attribute of: "lit("record") >> char_('{') >> type >> delimiter >> length >> name >> char_('}')" tuple<char,string,string,uint32_t,string,char>
   As you can see you have two extra char caused b your use of char_(has an attribute of char) instead of lit(has no attribute). omit[char_] could also work, but would be a little silly.

2. Let's change char_ to lit:

   Attribute of: "lit("record") >> lit('{') >> type >> delimiter >> length >> name >> lit('}')"
   tuple<string,string,uint32_t,string>
   Which is what we want.

3. Your original rule with lit:

   Attribute of: "lit("record") >> lit('{') >> type >> (delimiter | attr("")) >> (length | attr(0)) >> name >> lit('}')"
   tuple<string,variant<string,char const (&)[1]>,variant<uint32_t,int>,string>
   Since the branches of | aren't identical, you get variants instead of the attribute you want. (In this simple case everything works as if there were no variants though)

4. Let's remove the variants (since they cause errors in more complex scenarios):

   Attribute of: "lit("record") >> lit('{') >> type >> (delimiter | attr(string())) >> (length | attr(uint32_t())) >> name >> lit('}')"
   tuple<string,string,uint32_t,string>
   This works in the cases you want but also when both are missing.

5. sehe's approach:

   Attribute of: "lit("record") >> lit('{') >> type >> ((delimiter >> attr(uint32_t())) | (attr(string()) >> length)) >> name >> lit('}')"
   tuple<string,tuple<string,uint32_t>,string>
   Looking at this synthesized attribute you can see the need to create the param_t helper struct to make your record attribute match.

See on Coliru a way to "calculate" the previous attributes.

---

The possible alternative is a custom directive using boost::fusion::flatten_view. Keep in mind that this directive has very little testing so I would recommend the approach shown by sehe, but it seems to work (at least in this case).

The example in this question with this directive on Wandbox

Several other examples where this directive can be useful

flatten_directive.hpp

#pragma once


#include <boost/spirit/home/qi/meta_compiler.hpp>
#include <boost/spirit/home/qi/skip_over.hpp>
#include <boost/spirit/home/qi/parser.hpp>
#include <boost/spirit/home/support/unused.hpp>
#include <boost/spirit/home/support/common_terminals.hpp>
#include <boost/spirit/home/qi/detail/attributes.hpp>
#include <boost/spirit/home/support/info.hpp>
#include <boost/spirit/home/support/handles_container.hpp>
#include <boost/fusion/include/flatten_view.hpp>
#include <boost/fusion/include/for_each.hpp>
#include <boost/fusion/include/zip_view.hpp>


namespace custom
{
    BOOST_SPIRIT_TERMINAL(flatten);
}

namespace boost {
    namespace spirit
    {
        ///////////////////////////////////////////////////////////////////////////
        // Enablers
        ///////////////////////////////////////////////////////////////////////////
        template <>
        struct use_directive<qi::domain, custom::tag::flatten> // enables flatten
            : mpl::true_ {};
    }
}

namespace custom
{


    template <typename Subject>
    struct flatten_directive : boost::spirit::qi::unary_parser<flatten_directive<Subject> >
    {
        typedef Subject subject_type;
        flatten_directive(Subject const& subject)
            : subject(subject) {}

        template <typename Context, typename Iterator>
        struct attribute
        {
            typedef boost::fusion::flatten_view<typename
                boost::spirit::traits::attribute_of<subject_type, Context, Iterator>::type>
                type;//the attribute of the directive is a flatten_view of whatever is the attribute of the subject
        };

        template <typename Iterator, typename Context
            , typename Skipper, typename Attribute>
            bool parse(Iterator& first, Iterator const& last
                , Context& context, Skipper const& skipper
                , Attribute& attr) const
        {
            Iterator temp = first;
            boost::spirit::qi::skip_over(first, last, skipper);
            typename boost::spirit::traits::attribute_of<subject_type, Context, Iterator>::type original_attr;
            if (subject.parse(first, last, context, skipper, original_attr))//parse normally
            {
                typename attribute<Context, Iterator>::type flattened_attr(original_attr);//flatten the attribute
                typedef boost::fusion::vector<Attribute&,typename attribute<Context,Iterator>::type&> sequences;
                boost::fusion::for_each(//assign to each element of Attribute the corresponding element of the flattened sequence
                    boost::fusion::zip_view<sequences>(
                        sequences(attr,flattened_attr)
                    )
                    ,
                    [](const auto& pair)//substitute with a functor with templated operator() to support c++98/03
                    {
                        boost::spirit::traits::assign_to(boost::fusion::at_c<1>(pair),boost::fusion::at_c<0>(pair));
                    }
                    ); 

                return true;
            }
            first = temp;
            return false;
        }

        template <typename Context>
        boost::spirit::info what(Context& context) const
        {
            return info("flatten", subject.what(context));

        }

        Subject subject;
    };
}//custom
 ///////////////////////////////////////////////////////////////////////////
 // Parser generators: make_xxx function (objects)
 ///////////////////////////////////////////////////////////////////////////
namespace boost {
    namespace spirit {
        namespace qi
        {
            template <typename Subject, typename Modifiers>
            struct make_directive<custom::tag::flatten, Subject, Modifiers>
            {
                typedef custom::flatten_directive<Subject> result_type;
                result_type operator()(unused_type, Subject const& subject, unused_type) const
                {
                    return result_type(subject);
                }
            };
        }
    }
}

namespace boost {
    namespace spirit {
        namespace traits
        {
            ///////////////////////////////////////////////////////////////////////////
            template <typename Subject>
            struct has_semantic_action<custom::flatten_directive<Subject> >
                : unary_has_semantic_action<Subject> {};

            ///////////////////////////////////////////////////////////////////////////
            template <typename Subject, typename Attribute, typename Context
                , typename Iterator>
            struct handles_container<custom::flatten_directive<Subject>, Attribute
                , Context, Iterator>
                : unary_handles_container<Subject, Attribute, Context, Iterator> {};
        }
    }
}

main.cpp

#include <boost/spirit/include/qi.hpp>
#include <boost/fusion/include/adapt_struct.hpp>
#include <boost/fusion/include/io.hpp>
#include <boost/fusion/include/flatten_view.hpp>
#include <boost/fusion/include/copy.hpp>
#include "flatten_directive.hpp"

#include <string>

namespace qi = boost::spirit::qi;

namespace client {

    struct record {
        std::string type;
        std::string delimiter;
        uint32_t length = 0;
        std::string name;
    };
}

BOOST_FUSION_ADAPT_STRUCT(client::record, type, delimiter, length, name)

namespace client {
    std::ostream& operator<<(std::ostream& os, record const& v) { return os << boost::fusion::tuple_open('[') << boost::fusion::tuple_close(']') << boost::fusion::tuple_delimiter(", ") << boost::fusion::as_vector(v); }
}

namespace client
{
    template <typename Iterator, typename Skipper = qi::ascii::space_type>
    struct record_parser : qi::grammar<Iterator, record(), Skipper>
    {
        record_parser() : record_parser::base_type(start)
        {
            using namespace qi;

            type = +(graph - '(');
            delimiter_double_quote = '"' >> +~char_('"') >> '"';
            delimiter_single_quote = "'" >> +~char_("'") >> "'";
            delimiter = '(' >> (delimiter_double_quote | delimiter_single_quote) >> ')';
            name = +(graph - ';');
            length = '(' >> uint_ >> ')';

            start =
                custom::flatten[
                    lit("record")
                    >> '{'
                    >> type
                    >> (
                        delimiter >> attr(uint32_t())//the attributes of both branches must be exactly identical
                        | attr(std::string("")) >> length//const char[1]!=std::string int!=uint32_t
                        )
                    >> name
                    >> ';'
                    >> '}'
                ]
                ;
        }
    private:
        qi::rule<Iterator, record(), Skipper> start;
        qi::rule<Iterator, uint32_t(), Skipper> length;
        qi::rule<Iterator, std::string(), Skipper> delimiter;
        // lexemes
        qi::rule<Iterator, std::string()> type, delimiter_double_quote, delimiter_single_quote, name;
    };
}

int main()
{
    for (std::string const storage : {
        "record { string(5) Alex; }",
        "record { string(\"|\") Alex; }",
        "record { string Alex; }",
        "record { string (\"|\")(5) Alex; }"

    })
    {
        typedef std::string::const_iterator iterator_type;
        typedef client::record_parser<iterator_type> record_parser;

        record_parser g; // Our grammar

        client::record rec;
        auto iter = storage.begin(), end = storage.end();
        bool r = phrase_parse(iter, end, g, qi::ascii::space, rec);

        if (r) {
            std::cout << "Parsing succeeded: " << rec << std::endl;
        }
        else {
            std::cout << "Parsing failed\n";
        }

        if (iter != end) {
            std::cout << "Remaining: '" << std::string(iter, end) << "'...\n";
        }
    }
}

Answer 2

You can just write out the combinations:

    >> (
            delimiter >> attr(0)
         |  attr("")  >> length
         |  attr("")  >> attr(0)
    )

The best way to make it work with automatic attribute propagation is to use an AST structure that is similar:

namespace client {

    struct record {
        std::string type;

        struct param_t {
            std::string delimiter;
            uint32_t    length = 0;
        } param;

        std::string name;
    };
}

BOOST_FUSION_ADAPT_STRUCT(client::record::param_t, delimiter, length)
BOOST_FUSION_ADAPT_STRUCT(client::record, type, param, name)

Full Demo Live On Coliru

Note how much simpler the grammar has been made (all those char(' ') things are unnecessary; use lexemes only if you declare a skipper; use ~char_ instead of character set subtraction; use graph instead of char_ - space etc.).

type                   = +(graph - '(');
delimiter_double_quote = '"' >> +~char_('"') >> '"' ;
delimiter_single_quote = "'" >> +~char_("'") >> "'" ;
delimiter              = '(' >> (delimiter_double_quote | delimiter_single_quote) >> ')';
name                   = +(graph - ';');
length                 = '(' >> uint_ >> ')';

start = eps > lit("record") >> '{' 
    >> type
    >> (
            delimiter >> attr(0)
         |  attr("")  >> length
         |  attr("")  >> attr(0)
    )
    >>  name >> ';' >> '}'
    ;

Full code:

Live On Coliru

#include <boost/spirit/include/qi.hpp>
#include <boost/fusion/include/adapt_struct.hpp>
#include <boost/fusion/include/io.hpp>

#include <string>

namespace qi = boost::spirit::qi;

namespace client {

    struct record {
        std::string type;

        struct param_t {
            std::string delimiter;
            uint32_t    length = 0;
        } param;

        std::string name;
    };
}

BOOST_FUSION_ADAPT_STRUCT(client::record::param_t, delimiter, length)
BOOST_FUSION_ADAPT_STRUCT(client::record, type, param, name)

namespace client {
    std::ostream& operator<<(std::ostream& os, record::param_t const& v) { return os << boost::fusion::as_vector(v); }
    std::ostream& operator<<(std::ostream& os, record const& v)          { return os << boost::fusion::as_vector(v); }
}

namespace client
{
    template <typename Iterator, typename Skipper = qi::ascii::space_type>
    struct record_parser : qi::grammar<Iterator, record(), Skipper>
    {
        record_parser() : record_parser::base_type(start)
        {
            using namespace qi;

            type                   = +(graph - '(');
            delimiter_double_quote = '"' >> +~char_('"') >> '"' ;
            delimiter_single_quote = "'" >> +~char_("'") >> "'" ;
            delimiter              = '(' >> (delimiter_double_quote | delimiter_single_quote) >> ')';
            name                   = +(graph - ';');
            length                 = '(' >> uint_ >> ')';

            start = eps > lit("record") >> '{' 
                >> type
                >> (
                        delimiter >> attr(0)
                     |  attr("")  >> length
                     |  attr("")  >> attr(0)
                )
                >>  name >> ';' >> '}'
                ;
        }
      private: 
        qi::rule<Iterator, record(),      Skipper> start;
        qi::rule<Iterator, uint32_t(),    Skipper> length;
        qi::rule<Iterator, std::string(), Skipper> delimiter;
        // lexemes
        qi::rule<Iterator, std::string()> type, delimiter_double_quote, delimiter_single_quote, name;
    };
}

int main()
{
    for (std::string const storage : {
                "record { string(5) Alex; }",
                "record { string(\"|\") Alex; }",
            })
    {
        typedef std::string::const_iterator iterator_type;
        typedef client::record_parser<iterator_type> record_parser;

        record_parser g; // Our grammar

        client::record rec;
        auto iter = storage.begin(), end = storage.end();
        bool r = phrase_parse(iter, end, g, qi::ascii::space, rec);

        if (r) {
            std::cout << "Parsing succeeded: " << rec << std::endl;
        } else {
            std::cout << "Parsing failed\n";
        }

        if (iter != end) {
            std::cout << "Remaining: '" << std::string(iter, end) << "'...\n";
        }
    }
}

Prints:

Parsing succeeded: (string ( 5) Alex)
Parsing succeeded: (string (| 0) Alex)

Answer 3

Because it's 2016, adding a X3 example too. Once again, taking the variant approach, which I find to be typical in Spirit code.

namespace AST {
    struct record {
        std::string type;
        boost::variant<std::string, uint32_t> param;
        std::string name;
    };
}

BOOST_FUSION_ADAPT_STRUCT(AST::record, type, param, name)

namespace parser {
    using namespace x3;
    auto quoted = [](char q) { return q >> +~char_(q) >> q; };

    static auto const type      = +(graph - '(');
    static auto const delimiter = '(' >> (quoted('"') | quoted('\'')) >> ')';
    static auto const name      = +(graph - ';');
    static auto const length    = '(' >> uint_ >> ')';
    static auto const start     = lit("record") >> '{' >> type >> (delimiter | length) >> name >> ';' >> '}';
}

That's all. The calling code is virtually unchanged:

int main()
{
    for (std::string const storage : {
                "record { string(5) Alex; }",
                "record { string(\"|\") Alex; }",
                "record { string Alex; }",
            })
    {
        typedef std::string::const_iterator iterator_type;

        AST::record rec;
        auto iter = storage.begin(), end = storage.end();
        bool r = phrase_parse(iter, end, parser::start, x3::ascii::space, rec);

        if (r) {
            std::cout << "Parsing succeeded: " << boost::fusion::as_vector(rec) << std::endl;
        } else {
            std::cout << "Parsing failed\n";
        }

        if (iter != end) {
            std::cout << "Remaining: '" << std::string(iter, end) << "'\n";
        }
    }
}

Everything compiles a lot quicker and I'd not be surprised if the resultant code was at least twice as fast at runtime too.

Live On Coliru

#include <boost/spirit/home/x3.hpp>
#include <boost/fusion/include/adapt_struct.hpp>

namespace x3 = boost::spirit::x3;

namespace AST {
    struct record {
        std::string type;
        boost::variant<std::string, uint32_t> param;
        std::string name;
    };
}

BOOST_FUSION_ADAPT_STRUCT(AST::record, type, param, name)

namespace parser {
    using namespace x3;
    auto quoted = [](char q) { return q >> +~char_(q) >> q; };

    static auto const type      = +(graph - '(');
    static auto const delimiter = '(' >> (quoted('"') | quoted('\'')) >> ')';
    static auto const name      = +(graph - ';');
    static auto const length    = '(' >> uint_ >> ')';
    static auto const start     = lit("record") >> '{' >> type >> (delimiter | length) >> name >> ';' >> '}';
}

#include <iostream>
#include <boost/fusion/include/io.hpp>
#include <boost/fusion/include/as_vector.hpp>
#include <boost/optional/optional_io.hpp>

int main()
{
    for (std::string const storage : {
                "record { string(5) Alex; }",
                "record { string(\"|\") Alex; }",
                "record { string Alex; }",
            })
    {
        typedef std::string::const_iterator iterator_type;

        AST::record rec;
        auto iter = storage.begin(), end = storage.end();
        bool r = phrase_parse(iter, end, parser::start, x3::ascii::space, rec);

        if (r) {
            std::cout << "Parsing succeeded: " << boost::fusion::as_vector(rec) << std::endl;
        } else {
            std::cout << "Parsing failed\n";
        }

        if (iter != end) {
            std::cout << "Remaining: '" << std::string(iter, end) << "'\n";
        }
    }
}

Prints

Parsing succeeded: (string 5 Alex)
Parsing succeeded: (string | Alex)
Parsing failed
Remaining: 'record { string Alex; }'

Answer 4

Here's the more typical approach that parsed variant<std::string, uint32_t> so the AST reflects that only one can be present:

With Nil-Param

With the same misunderstanding as in my first answer, allowing both params to be optional:

Live On Coliru

#include <boost/spirit/include/qi.hpp>
#include <boost/fusion/include/adapt_struct.hpp>
#include <boost/fusion/include/io.hpp>
#include <boost/optional/optional_io.hpp>

#include <string>

namespace qi = boost::spirit::qi;

namespace client {
    struct nil { friend std::ostream& operator<<(std::ostream& os, nil) { return os << "(nil)"; } };

    struct record {
        std::string type;
        boost::variant<nil, std::string, uint32_t> param;
        std::string name;
    };
}

BOOST_FUSION_ADAPT_STRUCT(client::record, type, param, name)

namespace client
{
    template <typename Iterator, typename Skipper = qi::ascii::space_type>
    struct record_parser : qi::grammar<Iterator, record(), Skipper>
    {
        record_parser() : record_parser::base_type(start)
        {
            using namespace qi;

            type                   = +(graph - '(');
            delimiter_double_quote = '"' >> +~char_('"') >> '"' ;
            delimiter_single_quote = "'" >> +~char_("'") >> "'" ;
            delimiter              = '(' >> (delimiter_double_quote | delimiter_single_quote) >> ')';
            name                   = +(graph - ';');
            length                 = '(' >> uint_ >> ')';

            start = eps > lit("record") >> '{' 
                >> type
                >> (delimiter | length | attr(nil{}))
                >> name >> ';' >> '}'
                ;
        }
      private: 
        qi::rule<Iterator, record(),      Skipper> start;
        qi::rule<Iterator, uint32_t(),    Skipper> length;
        qi::rule<Iterator, std::string(), Skipper> delimiter;
        // lexemes
        qi::rule<Iterator, std::string()> type, delimiter_double_quote, delimiter_single_quote, name;
    };
}

int main()
{
    for (std::string const storage : {
                "record { string(5) Alex; }",
                "record { string(\"|\") Alex; }",
                "record { string Alex; }",
            })
    {
        typedef std::string::const_iterator iterator_type;
        typedef client::record_parser<iterator_type> record_parser;

        record_parser g; // Our grammar

        client::record rec;
        auto iter = storage.begin(), end = storage.end();
        bool r = phrase_parse(iter, end, g, qi::ascii::space, rec);

        if (r) {
            std::cout << "Parsing succeeded: " << boost::fusion::as_vector(rec) << std::endl;
        } else {
            std::cout << "Parsing failed\n";
        }

        if (iter != end) {
            std::cout << "Remaining: '" << std::string(iter, end) << "'...\n";
        }
    }
}

Prints

Parsing succeeded: (string 5 Alex)
Parsing succeeded: (string | Alex)
Parsing succeeded: (string (nil) Alex)

Without Nil-Param

Requiring exactly one:

Live On Coliru

#include <boost/spirit/include/qi.hpp>
#include <boost/fusion/include/adapt_struct.hpp>
#include <boost/fusion/include/io.hpp>
#include <boost/optional/optional_io.hpp>

#include <string>

namespace qi = boost::spirit::qi;

namespace client {
    struct record {
        std::string type;
        boost::variant<std::string, uint32_t> param;
        std::string name;
    };
}

BOOST_FUSION_ADAPT_STRUCT(client::record, type, param, name)

namespace client
{
    template <typename Iterator, typename Skipper = qi::ascii::space_type>
    struct record_parser : qi::grammar<Iterator, record(), Skipper>
    {
        record_parser() : record_parser::base_type(start)
        {
            using namespace qi;

            type                   = +(graph - '(');
            delimiter_double_quote = '"' >> +~char_('"') >> '"' ;
            delimiter_single_quote = "'" >> +~char_("'") >> "'" ;
            delimiter              = '(' >> (delimiter_double_quote | delimiter_single_quote) >> ')';
            name                   = +(graph - ';');
            length                 = '(' >> uint_ >> ')';

            start = eps > lit("record") >> '{' 
                >> type
                >> (delimiter | length)
                >> name >> ';' >> '}'
                ;
        }
      private: 
        qi::rule<Iterator, record(),      Skipper> start;
        qi::rule<Iterator, uint32_t(),    Skipper> length;
        qi::rule<Iterator, std::string(), Skipper> delimiter;
        // lexemes
        qi::rule<Iterator, std::string()> type, delimiter_double_quote, delimiter_single_quote, name;
    };
}

int main()
{
    for (std::string const storage : {
                "record { string(5) Alex; }",
                "record { string(\"|\") Alex; }",
                "record { string Alex; }",
            })
    {
        typedef std::string::const_iterator iterator_type;
        typedef client::record_parser<iterator_type> record_parser;

        record_parser g; // Our grammar

        client::record rec;
        auto iter = storage.begin(), end = storage.end();
        bool r = phrase_parse(iter, end, g, qi::ascii::space, rec);

        if (r) {
            std::cout << "Parsing succeeded: " << boost::fusion::as_vector(rec) << std::endl;
        } else {
            std::cout << "Parsing failed\n";
        }

        if (iter != end) {
            std::cout << "Remaining: '" << std::string(iter, end) << "'...\n";
        }
    }
}

Prints

Parsing succeeded: (string 5 Alex)
Parsing succeeded: (string | Alex)
terminate called after throwing an instance of 'boost::exception_detail::clone_impl<boost::exception_detail::error_info_injector<boost::spirit::qi::expectation_failure<__gnu_cxx::__normal_iterator<char const*, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > > > > >'
  what():  boost::spirit::qi::expectation_failure