What are futures and promises in C++?

What are futures and promises in C++?

Futures and promises are part of the C++ standard library that help manage asynchronous operations. A promise is a way to set a value that will be available in the future, while a future is a way to retrieve that value once it's ready.

How do I use them in my code?

You can use std::promise to set a value and std::future to get that value. Here's a simple example:

#include <iostream>
#include <future>
#include <thread>

void asyncTask(std::promise<int> &&p) {
    std::this_thread::sleep_for(std::chrono::seconds(2));
    p.set_value(42);
}

int main() {
    std::promise<int> promise;
    std::future<int> future = promise.get_future();
    std::thread t(asyncTask, std::move(promise));
    std::cout << "Waiting for the result..." << std::endl;
    std::cout << "Result: " << future.get() << std::endl;
    t.join();
    return 0;
}

What happens if I try to get a future value before it's set?

If you try to get a value from a future before it's set, the call to future.get() will block until the value is available. If the promise is never fulfilled, it will throw a std::future_error.

Can I use futures and promises with multiple threads?

Yes, futures and promises are designed to work with multiple threads. You can create multiple promises and futures, and they can be set and retrieved from different threads safely.