filter

#include <iostream>
#include <functional>
#include <cstdlib>

struct test
    //: boost::static_visitor<>
{

    using result_type = void;

    template< typename... P >
    result_type apply_filter(P &&... _p) const
    {
        return operator () (filter(_p)...);
    }

    template< typename... T >
    result_type operator () (std::string const & _s, T &&... _tail) const
    {
        std::cout << _s << std::endl;
        return operator () (std::forward< T >(_tail)...);
    }

    template< typename... T >
    result_type operator () (double const & _x, T &&... _tail) const
    {
        std::cout << _x << std::endl;
        return operator () (std::forward< T >(_tail)...);
    }

private :

    result_type operator () () const { return; }

    std::string filter(std::string const & _s) const
    {
        return "\"" + _s + "\"";
    }

    double filter(double const & _x) const
    {
        return _x + 1.0;
    }

};

int main()
{
    test const test_;
    test_.apply_filter(1.0, "x", 0.0, "y");
    return EXIT_SUCCESS;
}