What is the C++ std::promise and how does it work?

Can you explain what std::promise is in C++?

Sure! std::promise is a class in C++ that allows you to set a value or an exception to be retrieved later by a std::future.

How do I use std::promise with std::future?

You create a std::promise object, then get its associated std::future using the get_future() method. You can set a value in the promise, which can be retrieved from the future.

Can you give me a code example?

Absolutely! Here's a simple example:

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

void setPromise(std::promise<int>& p) {
    p.set_value(42);
}

int main() {
    std::promise<int> promise;
    std::future<int> future = promise.get_future();
    std::thread t(setPromise, std::ref(promise));
    t.join();
    std::cout << "Value from promise: " << future.get() << std::endl;
    return 0;
}

What happens if I set an exception in std::promise?

If you set an exception in std::promise, the associated std::future will throw that exception when you call get() on it.