# tokio::io::BufWriter - Rust
API documentation for the Rust `BufWriter` struct in crate `tokio`.
Keywords: rust, rustlang, rust-lang, BufWriter

# Struct tokio::io::BufWriter [−] [src]

    pub struct BufWriter<W> { /* fields omitted */ }

Wraps a writer and buffers its output.

It can be excessively inefficient to work directly with something that implements `AsyncWrite`. A `BufWriter` keeps an in-memory buffer of data and writes it to an underlying writer in large, infrequent batches.

`BufWriter` can improve the speed of programs that make small and repeated write calls to the same file or network socket. It does not help when writing very large amounts at once, or writing just one or a few times. It also provides no advantage when writing to a destination that is in memory, like a `Vec<u8>`.

When the `BufWriter` is dropped, the contents of its buffer will be discarded. Creating multiple instances of a `BufWriter` on the same stream can cause data loss. If you need to write out the contents of its buffer, you must manually call flush before the writer is dropped.

## Implementations

### `impl<W: AsyncWrite> BufWriter<W>` [src]

#### `pub fn new(inner: W) -> Self` [src]

Creates a new `BufWriter` with a default buffer capacity. The default is currently 8 KB, but may change in the future.

#### `pub fn with_capacity(cap: usize, inner: W) -> Self` [src]

Creates a new `BufWriter` with the specified buffer capacity.

#### `pub fn get_ref(&self) -> &Wⓘ

### Notable traits for &'_ mut F

`impl<'_, F> Future for &'_ mut F where
F: Unpin + Future + ?Sized, type Output = <F as Future>::Output;`
` [src]

Gets a reference to the underlying writer.

#### `pub fn get_mut(&mut self) -> &mut Wⓘ

### Notable traits for &'_ mut F

`impl<'_, F> Future for &'_ mut F where
F: Unpin + Future + ?Sized, type Output = <F as Future>::Output;`
` [src]

Gets a mutable reference to the underlying writer.

It is inadvisable to directly write to the underlying writer.

#### `pub fn get_pin_mut(self: Pin<&mut Self>) -> Pin<&mut W>ⓘ

### Notable traits for Pin<P>

`impl<P> Future for Pin<P> where
P: Unpin + DerefMut,
<P as Deref>::Target: Future, type Output = <<P as Deref>::Target as Future>::Output;`
` [src]

Gets a pinned mutable reference to the underlying writer.

It is inadvisable to directly write to the underlying writer.

#### `pub fn into_inner(self) -> W` [src]

Consumes this `BufWriter`, returning the underlying writer.

Note that any leftover data in the internal buffer is lost.

#### `pub fn buffer(&self) -> &[u8]` [src]

Returns a reference to the internally buffered data.

## Trait Implementations

### `impl<W: AsyncWrite + AsyncBufRead> AsyncBufRead for BufWriter<W>` [src]

#### `fn poll_fill_buf(
self: Pin<&mut Self>,
cx: &mut Context<'_>
) -> Poll<Result<&[u8]>>` [src]

Attempts to return the contents of the internal buffer, filling it with more data from the inner reader if it is empty. Read more

#### `fn consume(self: Pin<&mut Self>, amt: usize)` [src]

Tells this buffer that `amt` bytes have been consumed from the buffer, so they should no longer be returned in calls to `poll_read`. Read more

### `impl<W: AsyncWrite + AsyncRead> AsyncRead for BufWriter<W>` [src]

#### `fn poll_read(
self: Pin<&mut Self>,
cx: &mut Context<'_>,
buf: &mut ReadBuf<'_>
) -> Poll<Result<()>>` [src]

Attempts to read from the `AsyncRead` into `buf`. Read more

### `impl<W: AsyncWrite> AsyncWrite for BufWriter<W>` [src]

#### `fn poll_write(
self: Pin<&mut Self>,
cx: &mut Context<'_>,
buf: &[u8]
) -> Poll<Result<usize>>` [src]

Attempt to write bytes from `buf` into the object. Read more

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

Attempts to flush the object, ensuring that any buffered data reach their destination. Read more

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

Initiates or attempts to shut down this writer, returning success when the I/O connection has completely shut down. Read more

#### `fn poll_write_vectored(
self: Pin<&mut Self>,
cx: &mut Context<'_>,
bufs: &[IoSlice<'_>]
) -> Poll<Result<usize, Error>>` [src]

Like `poll_write`, except that it writes from a slice of buffers. Read more

#### `fn is_write_vectored(&self) -> bool` [src]

Determines if this writer has an efficient `poll_write_vectored` implementation. Read more

### `impl<W: Debug> Debug for BufWriter<W>` [src]

#### `fn fmt(&self, f: &mut Formatter<'_>) -> Result` [src]

Formats the value using the given formatter. Read more

### `impl<RW> From<BufWriter<BufReader<RW>>> for BufStream<RW>` [src]

#### `fn from(b: BufWriter<BufReader<RW>>) -> Self` [src]

Performs the conversion.

### `impl<'__pin, W> Unpin for BufWriter<W> where
__Origin<'__pin, W>: Unpin,` [src]

## Auto Trait Implementations

### `impl<W> RefUnwindSafe for BufWriter<W> where
W: RefUnwindSafe,` [src]

### `impl<W> Send for BufWriter<W> where
W: Send,` [src]

### `impl<W> Sync for BufWriter<W> where
W: Sync,` [src]

### `impl<W> UnwindSafe for BufWriter<W> where
W: UnwindSafe,` [src]

## Blanket Implementations

### `impl<T> Any for T where
T: 'static + ?Sized,` [src]

#### `pub fn type_id(&self) -> TypeId` [src]

Gets the `TypeId` of `self`. Read more

### `impl<T> Borrow<T> for T where
T: ?Sized,` [src]

#### `pub fn borrow(&self) -> &Tⓘ

### Notable traits for &'_ mut F

`impl<'_, F> Future for &'_ mut F where
F: Unpin + Future + ?Sized, type Output = <F as Future>::Output;`
` [src]

Immutably borrows from an owned value. Read more

### `impl<T> BorrowMut<T> for T where
T: ?Sized,` [src]

#### `pub fn borrow_mut(&mut self) -> &mut Tⓘ

### Notable traits for &'_ mut F

`impl<'_, F> Future for &'_ mut F where
F: Unpin + Future + ?Sized, type Output = <F as Future>::Output;`
` [src]

Mutably borrows from an owned value. Read more

### `impl<T> From<T> for T` [src]

#### `pub fn from(t: T) -> T` [src]

Performs the conversion.

### `impl<T, U> Into<U> for T where
U: From<T>,` [src]

#### `pub fn into(self) -> U` [src]

Performs the conversion.

### `impl<T, U> TryFrom<U> for T where
U: Into<T>,` [src]

#### `type Error = Infallible`

The type returned in the event of a conversion error.

#### `pub fn try_from(value: U) -> Result<T, <T as TryFrom<U>>::Error>` [src]

Performs the conversion.

### `impl<T, U> TryInto<U> for T where
U: TryFrom<T>,` [src]

#### `type Error = <U as TryFrom<T>>::Error`

The type returned in the event of a conversion error.

#### `pub fn try_into(self) -> Result<U, <U as TryFrom<T>>::Error>` [src]

Performs the conversion.