How to capture character without consuming it in boost::spirit::qi

Question

I'm using boost::spirit::qi to parse a "template" format that looks something like this:

/path/to/:somewhere:/nifty.json

where :somewhere: represents any string identified by the name somewhere (the name can be any series of characters between two : characters). I have a working parser for this, but I want to make one additional improvement.

I would like to know what character follows the :somewhere: placeholder (in this case a /). But the rest of my parser still needs to know about this / and consume it as part of the next section.

How can I "read" the / after :somewhere: without actually consuming it so that the rest of the parser will see it and consume it.

Answer 1

As sehe mentioned this can be done using the lookahead parser operator &, but if you want to emit the character as well you'll also need boost.phoenix, qi::locals and qi::attr.

For example:

#include <boost/fusion/include/std_pair.hpp>
#include <boost/spirit/include/phoenix.hpp>
#include <boost/spirit/include/qi.hpp>

#include <iostream>
#include <string>

namespace qi = boost::spirit::qi;

int main(int argc, char** argv)
{
    std::string input("foo:/bar");
    std::pair<char, std::string> output;

    std::string::const_iterator begin = input.begin(),
                                end = input.end();

    qi::rule<std::string::const_iterator, qi::locals<char>, std::pair<char, std::string>()> duplicate =
          "foo"
       >> qi::omit[
             &(":" >> qi::char_[qi::_a = qi::_1])
          ]
       >> qi::attr(qi::_a)
       >> ":"
       >> *qi::char_;

    bool r = qi::parse(begin,
                       end,
                       duplicate,
                       output);

    std::cout << std::boolalpha
              << r << " "
              << (begin == end) << " '"
              << output.first << "' \""
              << output.second << "\""
              << std::endl;

    return 0;
}

This outputs:

true true '/' "/bar"