Decode char UTF8 escapes in Boost Spirit

Question

question asked: Spirit-general list

_{Hello all,}

I'm not sure if my subject is correct, but the testcode will probably show what I want to achieve.

I'm trying to parse things like:

• '%40' to '@'
• '%3C' to '<'

I have a minimal testcase below. I don't understand why this doesn't work. It's probably me making a mistake but I don't see it.

Using: Compiler: gcc 4.6 Boost: current trunk

I use the following compile line:

g++ -o main -L/usr/src/boost-trunk/stage/lib -I/usr/src/boost-trunk -g -Werror -Wall -std=c++0x -DBOOST_SPIRIT_USE_PHOENIX_V3 main.cpp

#include <iostream>
#include <string>

#define BOOST_SPIRIT_UNICODE

#include <boost/cstdint.hpp>
#include <boost/spirit/include/qi.hpp>
#include <boost/phoenix/phoenix.hpp>

typedef boost::uint32_t uchar; // Unicode codepoint

namespace qi = boost::spirit::qi;

int main(int argc, char **argv) {

    // Input
    std::string input = "%3C";
    std::string::const_iterator begin = input.begin();
    std::string::const_iterator end = input.end();

    using qi::xdigit;
    using qi::_1;
    using qi::_2;
    using qi::_val;

    qi::rule<std::string::const_iterator, uchar()> pchar =
        ('%' > xdigit > xdigit) [_val = (_1 << 4) + _2];

    std::string result;
    bool r = qi::parse(begin, end, pchar, result);
    if (r && begin == end) {
        std::cout << "Output:   " << result << std::endl;
        std::cout << "Expected: < (LESS-THAN SIGN)" << std::endl;
    } else {
        std::cerr << "Error" << std::endl;
        return 1;
    }

    return 0;
}

Regards,

Matthijs Möhlmann

Answer 1

qi::xdigit does not do what you think it does: it returns the raw character (i.e. '0', not 0x00).

You could leverage qi::uint_parser to your advantage, making your parse much simpler as a bonus:

typedef qi::uint_parser<uchar, 16, 2, 2> xuchar;

• no need to rely on phoenix (making it work on older versions of Boost)
• get both characters in one go (otherwise, you might have needed to add copious casting to prevent integer sign extensions)

Here is a fixed up sample:

#include <iostream>
#include <string>

#define BOOST_SPIRIT_UNICODE

#include <boost/cstdint.hpp>
#include <boost/spirit/include/qi.hpp>

typedef boost::uint32_t uchar; // Unicode codepoint

namespace qi = boost::spirit::qi;

typedef qi::uint_parser<uchar, 16, 2, 2> xuchar;
const static xuchar xuchar_ = xuchar();


int main(int argc, char **argv) {

    // Input
    std::string input = "%3C";
    std::string::const_iterator begin = input.begin();
    std::string::const_iterator end = input.end();

    qi::rule<std::string::const_iterator, uchar()> pchar = '%' > xuchar_;

    uchar result;
    bool r = qi::parse(begin, end, pchar, result);

    if (r && begin == end) {
        std::cout << "Output:   " << result << std::endl;
        std::cout << "Expected: < (LESS-THAN SIGN)" << std::endl;
    } else {
        std::cerr << "Error" << std::endl;
        return 1;
    }

    return 0;
}

Output:

Output:   60
Expected: < (LESS-THAN SIGN)

'<' is indeed ASCII 60