how to change the parser of a rule

问题

Is it possible to modify the parser of a rule at runtime? I am able to dynamically create parsers (classes that are derived from parser_base), but I dont know how I can assign the new parser to an existing rule.

Basically my problem is that I want to define a parser for a line that consists of elements like numbers (lets call them constants in the sense that they are invariant over all my parser input) and symbols (the variants that I want to capture with a dynamic parser approach). Since the symbols are based on the current environment at runtime I think I need a dynamic parser.

minimalistic issue, I want symbols and a line-end:

namespace parser
{
    x3::rule<class line, ast::line> line = "line";
    auto const line_def = variants_def >> lineend_def;
    BOOST_SPIRIT_DEFINE(line);
}

using line_type = boost::spirit::x3::rule<class client::parser::line, ast::line>;
line_type line(boost::spirit::x3::plus<boost::spirit::x3::symbols_parser<boost::spirit::char_encoding::standard, client::ast::command, boost::spirit::x3::tst<boost::spirit::char_encoding::standard::char_type, client::ast::command>>> symbols_parser)
{
    auto line_end = lineend();
    auto const line_def2 = symbols_parser >> line_end;
    return parser::line; // <-- how can I change the line_type to use line_def2?
}

This code might be far from minimal, but unfortunately Im not so familiar with C++ yet.

回答1:

You cannot.

Actually, I lied, you can have different parsers with the same id in dynamic libraries and change the rule's parser by switching dynamic libraries back and forth, but please, do not do this.

来源：https://stackoverflow.com/questions/56103411/how-to-change-the-parser-of-a-rule