# Trait futures_core::stream::Stream [−] [src]

    #[must_use = "streams do nothing unless polled"]pub trait Stream {
        type Item;
        fn poll_next(
            self: Pin<&mut Self>,
            cx: &mut Context<'_>
        ) -> Poll<Option<Self::Item>>;

        fn size_hint(&self) -> (usize, Option<usize>) { ... }
    }

A stream of values produced asynchronously.

If `Future<Output = T>` is an asynchronous version of `T`, then `Stream<Item = T>` is an asynchronous version of `Iterator<Item = T>`. A stream represents a sequence of value-producing events that occur asynchronously to the caller.

The trait is modeled after `Future`, but allows `poll_next` to be called even after a value has been produced, yielding `None` once the stream has been fully exhausted.

## Associated Types

### `type Item` [src]

Values yielded by the stream.

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## Required methods

### `fn poll_next(
self: Pin<&mut Self>,
cx: &mut Context<'_>
) -> Poll<Option<Self::Item>>` [src]

Attempt to pull out the next value of this stream, registering the current task for wakeup if the value is not yet available, and returning `None` if the stream is exhausted.

# Return value

There are several possible return values, each indicating a distinct stream state:

- `Poll::Pending` means that this stream’s next value is not ready yet. Implementations will ensure that the current task will be notified when the next value may be ready.

- `Poll::Ready(Some(val))` means that the stream has successfully produced a value, `val`, and may produce further values on subsequent `poll_next` calls.

- `Poll::Ready(None)` means that the stream has terminated, and `poll_next` should not be invoked again.

# Panics

Once a stream has finished (returned `Ready(None)` from `poll_next`), calling its `poll_next` method again may panic, block forever, or cause other kinds of problems; the `Stream` trait places no requirements on the effects of such a call. However, as the `poll_next` 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 stream’s state.

If this is difficult to guard against then the `fuse` adapter can be used to ensure that `poll_next` always returns `Ready(None)` in subsequent calls.

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## Provided methods

### `fn size_hint(&self) -> (usize, Option<usize>)` [src]

Returns the bounds on the remaining length of the stream.

Specifically, `size_hint()` returns a tuple where the first element is the lower bound, and the second element is the upper bound.

The second half of the tuple that is returned is an `Option` `<` `usize` `>`. A `None` here means that either there is no known upper bound, or the upper bound is larger than `usize`.

# Implementation notes

It is not enforced that a stream implementation yields the declared number of elements. A buggy stream may yield less than the lower bound or more than the upper bound of elements.

`size_hint()` is primarily intended to be used for optimizations such as reserving space for the elements of the stream, but must not be trusted to e.g., omit bounds checks in unsafe code. An incorrect implementation of `size_hint()` should not lead to memory safety violations.

That said, the implementation should provide a correct estimation, because otherwise it would be a violation of the trait’s protocol.

The default implementation returns `(0,` `None` `)` which is correct for any stream.

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## Implementations on Foreign Types

### `impl<S: ?Sized + Stream + Unpin> Stream for &mut S` [src]

#### `type Item = S::Item`

#### `fn poll_next(
self: Pin<&mut Self>,
cx: &mut Context<'_>
) -> Poll<Option<Self::Item>>` [src]

#### `fn size_hint(&self) -> (usize, Option<usize>)` [src]

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

#### `type Item = <P::Target as Stream>::Item`

#### `fn poll_next(
self: Pin<&mut Self>,
cx: &mut Context<'_>
) -> Poll<Option<Self::Item>>` [src]

#### `fn size_hint(&self) -> (usize, Option<usize>)` [src]

### `impl<S: ?Sized + Stream + Unpin> Stream for Box<S>` [src]

#### `type Item = S::Item`

#### `fn poll_next(
self: Pin<&mut Self>,
cx: &mut Context<'_>
) -> Poll<Option<Self::Item>>` [src]

#### `fn size_hint(&self) -> (usize, Option<usize>)` [src]

### `impl<S: Stream> Stream for AssertUnwindSafe<S>` [src]

#### `type Item = S::Item`

#### `fn poll_next(
self: Pin<&mut Self>,
cx: &mut Context<'_>
) -> Poll<Option<S::Item>>` [src]

#### `fn size_hint(&self) -> (usize, Option<usize>)` [src]

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## Implementors

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