# [−][src]Trait core::future::Future

    #[must_use]
    pub trait Future {
        type Output;
        fn poll(self: Pin<&mut Self>, lw: &LocalWaker) -> Poll<Self::Output>;
    }

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

futures in libcore are unstable

A future represents an asynchronous 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 `poll`ing 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

### `type Output`

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

futures in libcore are unstable

The result of the `Future`.

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## Required methods

### `fn poll(self: Pin<&mut Self>, lw: &LocalWaker) -> Poll<Self::Output>`

🔬 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` and stores a clone of the `LocalWaker` to be woken once the future can make progress. For example, a future waiting for a socket to become readable would call `.clone()` on the `LocalWaker` and store it. When a signal arrives elsewhere indicating that the socket is readable, `[LocalWaker::wake]` is called and the socket future's task is awoken. 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 on multiple calls to `poll`, only the most recent `LocalWaker` passed to `poll` should be scheduled to receive a wakeup.

# Runtime characteristics

Futures alone are inert; they must be actively `poll`ed 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-- instead, it should only be called when the future indicates that it is ready to make progress (by calling `wake()`). 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.

# `LocalWaker`, `Waker` and thread-safety

The `poll` function takes a `LocalWaker`, an object which knows how to awaken the current task. `LocalWaker` is not `Send` nor `Sync`, so in order to make thread-safe futures the `LocalWaker::into_waker` method should be used to convert the `LocalWaker` into a thread-safe version. `LocalWaker::wake` implementations have the ability to be more efficient, however, so when thread safety is not necessary, `LocalWaker` should be preferred.

# Panics

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.

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## Implementors

###

`impl<'a, F: ?Sized + Future + Unpin> Future for &'a mut F` | [src]

#### `type Output = F::Output`

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

futures in libcore are unstable

####

`fn poll(self: Pin<&mut Self>, lw: &LocalWaker) -> Poll<Self::Output>` | [src]

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

futures in libcore are unstable

###

`impl<P> Future for Pin<P> where P: Unpin + DerefMut, P::Target: Future,` | [src]

#### `type Output = <<P as Deref>::Target as Future>::Output`

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

futures in libcore are unstable

####

`fn poll(self: Pin<&mut Self>, lw: &LocalWaker) -> Poll<Self::Output>` | [src]

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

futures in libcore are unstable

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