C++: What is std::future and std::promise?

1#include <future>
2
3// Promise/Future pair
4std::promise<int> prom;
5std::future<int> fut = prom.get_future();
6
7std::thread t([&prom] {
8    prom.set_value(42);  // Set result
9});
10
11std::cout << fut.get();  // 42
12t.join();
13
14// std::packaged_task
15std::packaged_task<int(int)> task([](int x) {
16    return x * 2;
17});
18std::future<int> result = task.get_future();
19task(21);  // Execute
20std::cout << result.get();  // 42
21
22// Error propagation
23std::promise<void> p;
24std::future<void> f = p.get_future();
25p.set_exception(
26    std::make_exception_ptr(std::runtime_error("oops"))
27);
28f.get();  // Throws!