Trait std::future::Future[][src]

pub trait Future {
    type Output;
    fn poll(self: PinMut<Self>, cx: &mut Context) -> Poll<Self::Output>;
🔬 This is a nightly-only experimental API. (futures_api #50547)

futures in libcore are unstable

A future represents an asychronous computation.

A future is a value that may not have finished computing yet. This kind of "asynchronous value" makes it possible for a thread to continue doing useful work while it waits for the value to become available.

The poll method

The core method of future, poll, attempts to resolve the future into a final value. This method does not block if the value is not ready. Instead, the current task is scheduled to be woken up when it's possible to make further progress by polling again. The wake up is performed using cx.waker(), a handle for waking up the current task.

When using a future, you generally won't call poll directly, but instead await! the value.

Associated Types

🔬 This is a nightly-only experimental API. (futures_api #50547)

futures in libcore are unstable

The result of the Future.

Required Methods

🔬 This is a nightly-only experimental API. (futures_api #50547)

futures in libcore are unstable

Attempt to resolve the future to a final value, registering the current task for wakeup if the value is not yet available.

Return value

This function returns:

  • Poll::Pending if the future is not ready yet
  • Poll::Ready(val) with the result val of this future if it finished successfully.

Once a future has finished, clients should not poll it again.

When a future is not ready yet, poll returns Poll::Pending. The future will also register the interest of the current task in the value being produced. For example, if the future represents the availability of data on a socket, then the task is recorded so that when data arrives, it is woken up (via cx.waker()). Once a task has been woken up, it should attempt to poll the future again, which may or may not produce a final value.

Note that if Pending is returned it only means that the current task (represented by the argument cx) will receive a notification. Tasks from previous calls to poll will not receive notifications.

Runtime characteristics

Futures alone are inert; they must be actively polled to make progress, meaning that each time the current task is woken up, it should actively re-poll pending futures that it still has an interest in.

The poll function is not called repeatedly in a tight loop for futures, but only whenever the future itself is ready, as signaled via the Waker inside task::Context. If you're familiar with the poll(2) or select(2) syscalls on Unix it's worth noting that futures typically do not suffer the same problems of "all wakeups must poll all events"; they are more like epoll(4).

An implementation of poll should strive to return quickly, and must never block. Returning quickly prevents unnecessarily clogging up threads or event loops. If it is known ahead of time that a call to poll may end up taking awhile, the work should be offloaded to a thread pool (or something similar) to ensure that poll can return quickly.


Once a future has completed (returned Ready from poll), then any future calls to poll may panic, block forever, or otherwise cause bad behavior. The Future trait itself provides no guarantees about the behavior of poll after a future has completed.