Question

I’m confusing myself with difference between a future and a promise.

Obviously, they have different methods and stuff, but what is the actual use case?

Is it?:

• when I’m managing some async task, I use future to get the value “in future”
• when I’m the async task, I use promise as the return type to allow the user get a future from my promise

Answer 1

Future and Promise are the two separate sides of an asynchronous operation.

std::promise is used by the “producer/writer” of the asynchronous operation.

std::future is used by the “consumer/reader” of the asynchronous operation.

The reason it is separated into these two separate “interfaces” is to hide the “write/set” functionality from the “consumer/reader”.

auto promise = std::promise<std::string>();

auto producer = std::thread([&]
{
    promise.set_value("Hello World");
});

auto future = promise.get_future();

auto consumer = std::thread([&]
{
    std::cout << future.get();
});

producer.join();
consumer.join();

One (incomplete) way to implement std::async using std::promise could be:

template<typename F>
auto async(F&& func) -> std::future<decltype(func())>
{
    typedef decltype(func()) result_type;

    auto promise = std::promise<result_type>();
    auto future  = promise.get_future();

    std::thread(std::bind([=](std::promise<result_type>& promise)
    {
        try
        {
            promise.set_value(func()); // Note: Will not work with std::promise<void>. Needs some meta-template programming which is out of scope for this question.
        }
        catch(...)
        {
            promise.set_exception(std::current_exception());
        }
    }, std::move(promise))).detach();

    return std::move(future);
}

Using std::packaged_task which is a helper (i.e. it basically does what we were doing above) around std::promise you could do the following which is more complete and possibly faster:

template<typename F>
auto async(F&& func) -> std::future<decltype(func())>
{
    auto task   = std::packaged_task<decltype(func())()>(std::forward<F>(func));
    auto future = task.get_future();

    std::thread(std::move(task)).detach();

    return std::move(future);
}

Note that this is slightly different from std::async where the returned std::future will when destructed actually block until the thread is finished.