Boost.x3: attribute accumulates between alternatives

问题

I have a parser for parsing an Identifier like foo, bar, baz and one for parsing also nested identifiers like foo::bar, foo::bar.baz, foo::bar.baz.baham They both parse into the same ast struct, which looks like this:

struct identifier : x3::position_tagged{
    std::vector <std::string> namespaces;
    std::vector <std::string> classes;
    std::string identifier;

};

The parser for an identifier looks like this:

#define VEC_ATR x3::attr(std::vector<std::string>({})) //ugly hack

auto const identifier_def =
                VEC_ATR
                >> VEC_ATR
                >> id_string;

and for the nested_identifier like this:

auto const nested_identifier_def =
        x3::lexeme[
                (+(id_string >> "::") >> +(id_string >> ".") > id_string)
                | (+(id_string >> "::") >> VEC_ATR > id_string)
                | (VEC_ATR >> +(id_string >> ".") > id_string)
                | identifier

        ];

I know shame on me for the macro. The identifier parser works fine, but the nested_identifier has a strange behaviour if I try to parse something like foo::bar::baz the ast objects which falls out of the parser, has all the namespaces, in this case foo and bar twice in the namespaces vector. I have a small example of this strange behaviour here. Can anybody explain me why this happens, and how I can avoid this?

回答1:

The reason why you get that behaviour is that the alternative parser does not automatically rollback the changes made to the external attribute when one of its branches fails.

In your case this is what happens:

• Initially the attribute is [{},{},""].
• The first alternative branch is tried.
• id_string >> "::" matches twice and adds foo and bar to the first vector ->[{foo,bar},{},""].
• id_string >> "." fails to match -> the sequence fails -> the alternative branch fails (leaving the attribute unchanged).
• The second alternative branch is tried.
• id_string >> "::" matches twice and adds foo and bar to the first vector ->[{foo,bar,foo,bar},{},""].
• attr(vector<string>({})) succeeds (attr always succeeds) and substitutes the empty second vector with a vector with an empty string -> [{foo,bar,foo,bar},{""},""].
• id_string matches and baz is added to the attribute ->[{foo,bar,foo,bar},{""},baz].
• The second alternative branch succeeds.

In Spirit.Qi the solution in this case is quite easy, simply use the hold directive. Unfortunately this directive is not yet implemented in Spirit.X3. A possible alternative could be putting each of the alternative branches in its own x3::rule either explicitly or with as<ast::identifier>(alternative_branch) as used here by sehe. Here is a simplified example that shows the as approach.

Another possibility could be implementing the hold directive, here is my attempt(running on WandBox):

#include <boost/spirit/home/x3/support/context.hpp>
#include <boost/spirit/home/x3/core/skip_over.hpp>
#include <boost/spirit/home/x3/core/parser.hpp>

namespace boost { namespace spirit { namespace x3
{
    template <typename Subject>
    struct hold_directive : unary_parser<Subject, hold_directive<Subject>>
    {
        typedef unary_parser<Subject, hold_directive<Subject> > base_type;
        static bool const is_pass_through_unary = true;
        static bool const handles_container = Subject::handles_container;

        hold_directive(Subject const& subject)
          : base_type(subject) {}

        template <typename Iterator, typename Context
          , typename RContext, typename Attribute>
        bool parse(Iterator& first, Iterator const& last
          , Context const& context, RContext& rcontext, Attribute& attr) const
        {
            Attribute copy(attr);
            if (this->subject.parse(first, last, context, rcontext, copy))
            {
                traits::move_to(copy, attr);
                return true;
            }
            return false;
        }

    };

    struct hold_gen
    {
        template <typename Subject>
        hold_directive<typename extension::as_parser<Subject>::value_type>
        operator[](Subject const& subject) const
        {
            return { as_parser(subject) };
        }
    };

    auto const hold = hold_gen{};
}}}

回答2:

Please note that as of Boost1.70, the solution proposed by @sehe does not work anymore (see this discussion).

The only workaround now is to refactor the grammar, so that the rollback wouldn't be needed.

来源：https://stackoverflow.com/questions/39537675/boost-x3-attribute-accumulates-between-alternatives