future/example2.cpp

// promise constructors
#include <iostream>       // std::cout
#include <functional>     // std::ref
#include <memory>         // std::allocator, std::allocator_arg
#include <thread>         // std::thread
#include <future>         // std::promise, std::future

void print_int (std::future<int>& fut) {
  int x = fut.get();
  std::cout << "value: " << x << '\n';
}

int main ()
{
std::promise<int> foo;
std::promise<int> bar = std::promise<int>(std::allocator_arg,std::allocator<int>());

std::future<int> fut = bar.get_future();

std::thread th (print_int, std::ref(fut));

bar.set_value (20);

th.join();
return 0;
}