What is std::promise in C++?

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 by a std::future. It is used for asynchronous programming.

How does std::promise work with std::future?

When you create a std::promise, you can associate it with a std::future. The future can then be used to retrieve the value set by the promise once it's ready.

Can you give me a simple code example?

Of course! Here's a basic example:

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

void setPromise(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(setPromise, std::ref(promise));
    std::cout << "Waiting for value..." << std::endl;
    std::cout << "Value: " << future.get() << std::endl;
    t.join();
    return 0;
}

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

If you set an exception in std::promise, the std::future associated with it will throw that exception when you call get(). This allows you to handle errors in asynchronous operations.