future/example21.cpp

// std::future_errc example:
#include <iostream>     // std::cerr
#include <future>       // std::promise, std::future_error, std::future_errc

int main ()
{
 std::promise<int> prom;

 try {
   prom.get_future();
   prom.get_future();   // throws std::future_error with future_already_retrieved
 }
 catch (std::future_error& e) {
   if (e.code() == std::make_error_condition(std::future_errc::future_already_retrieved))
   std::cerr << "[future already retrieved]\n";
   else std::cerr << "[unknown exception]\n";
 }

 return 0;
}