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 later by a std::future.

How does it work with std::future?

std::future is used to retrieve the value set by std::promise. When you create a std::promise, you can get a std::future from it, which will eventually hold the value.

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;
}

This code sets a promise in a separate thread and retrieves the value in the main thread.

What happens if I set an exception instead of a value?

If you set an exception using std::promise::set_exception, the std::future will throw that exception when you call future.get(). This allows for error handling in asynchronous operations.