Boost Spirit Karma equivalent to Qi matches

Question

What is the equivalent to boost::spirit::qi::matches in boost::spirit::karma? For example I want to generate a literal "array" only if a boolean flag is set true.

Answer 1

Use the specified value form of bool_ to consume the attribute only if it matches the supplied value:

Live On Coliru

#include <boost/spirit/include/karma.hpp>

namespace karma = boost::spirit::karma;

int main() {
    using namespace karma;

    for (int i = 0; i < 10; ++i)
    {
        bool b = (0 == i%3);
        std::cout << format_delimited(
                (omit[bool_(true)] << "array" | omit[bool_(false) << "vector"]),
                ';',
                b
            ) << "\n";
    }

}

Prints

array;


array;


array;


array;