# Trait std::future::Future

1.36.0 · source ·

    pub trait Future {
        type Output;

        // Required method
        fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output>;
    }

Expand description

A future represents an asynchronous computation obtained by use of `async`.

A future is a value that might 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 `context` passed to the `poll` method can provide a `Waker`, which is a handle for waking up the current task.

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

## Required Associated Types§

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#### type Output

The type of value produced on completion.

## Required Methods§

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#### fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output>

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 `Waker` copied from the current `Context`. This `Waker` is then woken once the future can make progress. For example, a future waiting for a socket to become readable would call `.clone()` on the `Waker` and store it. When a signal arrives elsewhere indicating that the socket is readable, `Waker::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 `Waker` from the `Context` passed to the most recent call 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 should not 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.

##### Panics

Once a future has completed (returned `Ready` from `poll`), calling its `poll` method again may panic, block forever, or cause other kinds of problems; the `Future` trait places no requirements on the effects of such a call. However, as the `poll` method is not marked `unsafe`, Rust’s usual rules apply: calls must never cause undefined behavior (memory corruption, incorrect use of `unsafe` functions, or the like), regardless of the future’s state.

## Implementors§

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### impl<F> Future for &mut F where F: Future + Unpin + ?Sized,

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#### type Output = <F as Future>::Output

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### impl<F> Future for AssertUnwindSafe<F>where F: Future,

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#### type Output = <F as Future>::Output

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### impl<F, A> Future for Box<F, A>where F: Future + Unpin + ?Sized, A: Allocator + 'static,

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#### type Output = <F as Future>::Output

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### impl<P> Future for Pin<P>where P: DerefMut, <P as Deref>::Target: Future,

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

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### impl<T> Future for Exclusive<T>where T: Future + ?Sized,

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#### type Output = <T as Future>::Output

1.48.0 · source§

### impl<T> Future for Pending<T>

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#### type Output = T

1.48.0 · source§

### impl<T> Future for Ready<T>

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#### type Output = T

1.64.0 · source§

### impl<T, F> Future for PollFn<F>where F: FnMut(&mut Context<'_>) -> Poll<T>,

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#### type Output = T