multithreading · advanced

std::future, promise & async

How std::future/promise (and std::async) pass a result across threads: get() blocks until ready, then returns the value or rethrows.

The code

std::promise<int> p;
std::future<int> f = p.get_future();

std::thread worker([&] { p.set_value(42); }); // produce
int r = f.get();                              // blocks until ready -> 42
worker.join();

// one-shot: launch + future together
std::future<int> g = std::async(compute);
int s = g.get(); // result, or rethrows the task's exception

What this lesson walks through

1. 01promise and future share one state
2. 02f.get() blocks until ready
3. 03Worker calls set_value(42)
4. 04get() unblocks and returns 42
5. 05std::async = task + future in one
6. 06Exceptions travel through the future
7. 07The model

std::promise is the write end; the std::future it hands out is the read end. They share a single state object, initially empty.