What are futures and promises in C++?

Can you explain what futures and promises are in C++?

Sure! Futures and promises are used for asynchronous programming in C++. 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 set.

How do I create a promise and a future in C++?

You can create a promise using std::promise<T> and a future using std::future<T>. 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(1));
    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();
}

This code sets a value in a separate thread and retrieves it using the future.

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

If you try to get a future value before it's set, the call to future.get() will block until the value is available. If the promise is set with an exception, calling get() will throw that exception.

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, allowing different threads to set values and retrieve them asynchronously.