Boost spirit qi - parse string into boolean

I need to parse a string into a boolean value; in particular

• if the string is ack I want the parser sets a boolean variable with true;
• if the string is noack I want the parser sets a boolean variable with false;

Note that I do not want to use the return value of the function qi::parse; I want the qi::parse function sets a boolean variable given as argument. Something like the following (not compiling) code:

#include <boost/spirit/include/qi.hpp>
#include <iostream>
#include <string>

using namespace boost::spirit;

int main() {
    bool b;
    std::string s("ack");
    auto it = s.begin();
    if (qi::parse(it, s.end(), < need a rule here... >, b))
        std::cout << "Value: " << std::boolalpha << b << std::endl;
    return 0;
}

The simplest approach that comes to mind is "noack" >> qi::attr(false) and its complement. Add an eoi to ensure full input is parsed:

Live On Coliru

#define BOOST_SPIRIT_DEBUG
#include <boost/spirit/include/qi.hpp>
#include <iostream>
#include <string>

namespace qi = boost::spirit::qi;

int main() {
    for (std::string const s : {"ack","noack","oops","ack 2",""}) {
        bool b;
        if (qi::parse(s.begin(), s.end(), ("ack" >> qi::attr(true) | "noack" >> qi::attr(false)) >> qi::eoi, b))
            std::cout << "'" << s << "' -> " << std::boolalpha << b << std::endl;
        else
            std::cout << "'" << s << "' -> FAIL\n";
    }
}

Prints

'ack' -> true
'noack' -> false
'oops' -> FAIL
'ack 2' -> FAIL
'' -> FAIL

BONUS:

Use symbol for more elegance:

Live On Coliru

#include <boost/spirit/include/qi.hpp>
#include <iostream>
#include <string>

namespace qi = boost::spirit::qi;

struct acknoack_type : qi::symbols<char, bool> {
    acknoack_type() { this->add("ack", true)("noack", false); }
} static const acknoack;

int main() {
    for (std::string const s : {"ack","noack","oops","ack 2",""}) {
        bool b;
        if (qi::parse(s.begin(), s.end(), acknoack >> qi::eoi, b))
            std::cout << "'" << s << "' -> " << std::boolalpha << b << std::endl;
        else
            std::cout << "'" << s << "' -> FAIL\n";
    }
}