Apply_visitor и std::future

#include <iostream>
#include <future>
#include <vector>
#include <boost/variant.hpp>
 
auto f1 = []{ return 42; };
auto f2 = []{ return 10.002; };
auto f3 = []{ return 55555555L; };
auto f4 = []{ return "Hello"; };
 
using ret_type = boost::variant<int, double, long, std::string>;
 
template <typename F>
ret_type wrapper(const F& f) {
   return ret_type(f());
}
 
template <typename T>
struct my_visitor : boost::static_visitor<T>
{
   T operator ()(int value) const { return value; }
   T operator ()(double value) const { return value; }
   T operator ()(long value) const { return value; }
   T operator ()(const std::string& value) const { return value.size(); }
};
 
 
int main()
{
   std::vector<std::future<ret_type>> futures;
 
   futures.emplace_back(std::async(wrapper<decltype(f1)>, f1 ));
   futures.emplace_back(std::async(wrapper<decltype(f2)>, f2 ));
   futures.emplace_back(std::async(wrapper<decltype(f3)>, f3 ));
   futures.emplace_back(std::async(wrapper<decltype(f4)>, f4 ));
 
   for (auto& f : futures) {
      //не компилируется:
      std::cout << (boost::apply_visitor(my_visitor<double>(), f.get() )) << "\n";
   }
 
   return 0;
}

#include <iostream>
#include <future>
#include <vector>
#include <boost/variant.hpp>
 
//...................//
 
int main()
{
   std::vector<std::future<ret_type>> futures;
 
   futures.emplace_back(std::async(wrapper<decltype(f1)>, f1 ));
   futures.emplace_back(std::async(wrapper<decltype(f2)>, f2 ));
   futures.emplace_back(std::async(wrapper<decltype(f3)>, f3 ));
   futures.emplace_back(std::async(wrapper<decltype(f4)>, f4 ));
 
   for (auto& f : futures) {
      auto res = f.get(); // <<<
      std::cout << (boost::apply_visitor(my_visitor<double>(), res )) << "\n";
   }
 
   return 0;
}