Packaging up tasks for later

Another thing to notice about the preceding diagram is that the shared state doesn't just contain an optional<T>; it actually contains a variant<T, exception_ptr> (for variant and optional, see Chapter 5, Vocabulary Types). This implies that not only can you shove data of type T through the wormhole; you can also shove exceptions through. This is particularly convenient and symmetrical because it allows std::future<T> to represent all the possible outcomes of calling a function with the signature T(). Maybe it returns a T; maybe it throws an exception; and of course maybe it never returns at all. Similarly, a call to f.get() may return a T; or throw an exception; or (if the promise-holding thread loops forever) might never return at all. In order to shove an exception through the wormhole, you'd use the method p.set_exception...