C++ what are std::shared_future and std::promise

Question

I understand what's std::future but i don't understand when and HOW to use std::shared_future and std::promise, and i couldn't find a good explanation online.

I will appreciate some help wrapping my head around it.

By the way the example here isn't really clear http://en.cppreference.com/w/cpp/thread/shared_future

Answer 1

std::shared_future is when you need to have multiple valid copies of a std::future, and possibly multiple consumers of said std::future.

You can move a std::future into a std::shared_future.

Consider std::future:std::shared_future to be like std::unique_ptr:std::shared_ptr.

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std::promise is one way to generate a std::future.

template<class T>
std::future<std::decay_t<T>> make_ready_future( T&& t ) {
  std::promise<std::decay_t<T>> pr;
  auto r = pr.get_future();
  pr.set_value(std::forward<T>(t));
  return r;
}

this is a toy example of std::promise, where I take a T and I generate an already-ready std::future<T> from it.

If you have code that could require threading off, or might already be calculated, you can return a std::future<T> from it. If it isn't calculated, you return the result of a std::async( ... ) call. If it is already calculated, you return a make_ready_future( calculated value ).

A slightly more complex situation:

template<class T>
struct promise_pipe {
  std::future<T> out;
  std::function<void(T)> in;
  promise_pipe() {
    auto pr = std::make_shared<std::promise<T>>();
    out = pr->get_future();
    in = [pr](T t)mutable{
      if (pr) pr->set_value(std::forward<T>(t));
      pr = {};
    };
  }
};

Here I have written a promise_pipe.

A promise_pipe<int> has two fields -- a std::future<int> out and a std::function<void(int)> in.

The out future will be ready if and only if you call in with an int.

So you can have a function that returns a std::future<Bitmap>. You have some network code that is expecting the bitmap to be streamed. So you create a promise_pipe<Bitmap>, return the std::future<Bitmap> and store the in in a "todo" list when the bitmap arrives.

When the bitmap has fully arrived, you just do in(Bitmap) and whomever is waiting on that future will be notified.

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promises are objects designed to allow programmers to generate their own futures, not just use ones created by the std library.

Now a packaged_task is often easier to work with than a std::promise, and serves much the same purpose as the promise_pipe above. It is move-only, so you can end up having to wrap it into a std::shared_ptr and a std::function anyhow (or a custom move-only std::function)