Improve use of alternative parser

Question

I extended the Mini XML example from the spirit manual.
The grammar describes a xml tag that can be closed with '/>' and has no child nodes or which is closed like in the example with a closing tag '' and can optionally have children.

#include <boost/spirit/include/qi.hpp>
#include <boost/spirit/include/phoenix_core.hpp>
#include <boost/spirit/include/phoenix_operator.hpp>
#include <boost/fusion/include/adapt_struct.hpp>
#include <boost/variant.hpp>
#include <boost/variant/recursive_variant.hpp>

struct XmlTree;

typedef boost::variant<boost::recursive_wrapper<XmlTree>, std::string>
    mini_xml_node;

typedef std::vector<mini_xml_node> Children;

struct XmlTree
{
    std::string name;
    Children childs;
};

BOOST_FUSION_ADAPT_STRUCT(
XmlTree,
(std::string, name)
(Children, childs)
)

typedef std::string::const_iterator Iterator;

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

class XmlParserGrammar : public qi::grammar<Iterator, XmlTree(), qi::locals<std::string*>, ascii::space_type>
{
public:
XmlParserGrammar() : XmlParserGrammar::base_type(xml, "xml")
{
    using qi::lit;
    using qi::lexeme;
    using qi::attr;
    using ascii::space;
    using ascii::char_;
    using ascii::alnum;
    using phoenix::val;

    xml %=
        startTag[qi::_a = &qi::_1]  >>
        (
        (
            lit("/>") > attr(Children()) //can i remove this somehow?
        )
        |
        (
            lit(">")
            >> *node_
            > endTag(*qi::_a)
        )
        );

    startTag %= '<' >> !lit('/') >> lexeme[ +(alnum - (space | '>' | "/>")) ] ;

    node_ %= xml | text;

    endTag = "</" > lit(qi::_r1) > '>';

    text %= lexeme[+(char_ - '<')];
}

private:
    qi::rule<Iterator, XmlTree(), qi::locals<std::string*>, ascii::space_type> xml;
    qi::rule<Iterator, std::string(), ascii::space_type> startTag;
    qi::rule<Iterator, mini_xml_node(), ascii::space_type> node_;
    qi::rule<Iterator, void(std::string&), ascii::space_type> endTag;
    qi::rule<Iterator, std::string(), ascii::space_type> text;
};

Is it possible to write this rule without the attr(Children()) tag? I think it is more or less a performance lag. I need it to avoid the optional attribute of the alternative parser. If there are no child tags the attribute should only be an empty vector.

Answer 1

You should be able to write:

xml %= startTag[_a = &_1] 
       >> attributes 
       >> (  "/>" >> eps
          |  ">" >> *node > endTag(*_a) 
          )
    ;

That leaves the vector attribute unchanged (and empty).