What is the difference between futures and promises in C++?

What is the difference between futures and promises in C++?

In C++, a promise is a way to set a value that will be retrieved later, while a future is a way to retrieve that value. A promise is used to fulfill a future.

Can you give me an example of how to use them?

Sure! You can create a promise and a future like this:

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

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

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

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

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

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

Are there any performance considerations I should be aware of?

Yes, using futures and promises can introduce overhead due to thread management and synchronization. It's important to use them judiciously, especially in performance-critical applications.