How do I use futures and promises in C++?

How do I use futures and promises in C++?

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

Can you give me a simple code example?

Sure! Here's a basic example:

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

int calculateSquare(int x) {
    return x * x;
}

int main() {
    std::promise<int> promise;
    std::future<int> future = promise.get_future();

    std::thread t([&promise]() {
        promise.set_value(calculateSquare(5));
    });

    std::cout << "Square: " << future.get() << std::endl;
    t.join();
    return 0;
}

This code calculates the square of a number asynchronously.

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

If you try to get the value of 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 result in an exception.

Are there any limitations I should be aware of?

Yes, one limitation is that a promise can only be fulfilled once. If you try to set a value again, it will throw an exception. Also, futures cannot be copied, only moved.