karma legacy "variant" with attr_cast, C++ (vc++)

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

namespace ka = boost::spirit::karma;

struct U /* a kind of union (legacy code)*/
    {
        bool kind;
        double foo; /* if kind = true */
        size_t bar; /* if kind = false */
    };

typedef boost::variant<double, size_t> UVariant;

namespace boost { namespace spirit { namespace traits {
    template<>
    struct transform_attribute<const U,UVariant,ka::domain>
    {
        typedef UVariant type;
        static type pre(const U & u) { 
            if(u.kind)
            { 
                return type(u.foo); 
            }
            else
            {
                return type(u.bar);
            }
        }
    };
}}}

typedef std::back_insert_iterator<std::string> iterator;

class grm: public ka::grammar<iterator, U()>
{
public:
    grm():grm::base_type(start)
    {
        start = ka::attr_cast< UVariant >(foo | bar);
        foo = ka::double_;
        bar = ka::uint_;
    }
private:
    ka::rule<iterator,U()> start;
    ka::rule<iterator,double()> foo;
    ka::rule<iterator,size_t()> bar;
};

int main(int argc, char * argv[])
{
    grm g;
    U u;
    u.kind = false;
    u.foo = 1.0;
    u.bar = 34;

std::string generated;
    std::back_insert_iterator<std::string> sink(generated);
    ka::generate(sink,g,u);
    
    std::cout << generated << std::endl;

return 0;
}

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Compilation time: 12,27 sec, absolute running time: 0,22 sec, absolute service time: 12,52 sec