Struct BufWriter

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

Adds buffering to a writer.

It can be excessively inefficient to work directly with something that implements AsyncWrite. For example, every call to write() on a TCP stream results in a system call. A BufWriter keeps an in-memory buffer of data and writes it to the underlying writer in large, infrequent batches.

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

Unlike std::io::BufWriter, this type does not write out the contents of its buffer when it is dropped. Therefore, it is important that users explicitly flush the buffer before dropping the BufWriter.

Examples

use futures_lite::io::{AsyncWriteExt, BufWriter};

let mut output = Vec::new();
let mut writer = BufWriter::new(&mut output);

writer.write_all(b"hello").await?;
writer.flush().await?;

Implementations

impl<W> BufWriter<W>

where W: AsyncWrite,

pub fn new(inner: W) -> BufWriter<W>

Creates a buffered writer with the default buffer capacity.

The default capacity is currently 8 KB, but that may change in the future.

Examples

use futures_lite::io::BufWriter;

let mut output = Vec::new();
let writer = BufWriter::new(&mut output);

pub fn with_capacity(capacity: usize, inner: W) -> BufWriter<W>

Creates a buffered writer with the specified buffer capacity.

Examples

use futures_lite::io::BufWriter;

let mut output = Vec::new();
let writer = BufWriter::with_capacity(100, &mut output);

pub fn get_ref(&self) -> &W

Gets a reference to the underlying writer.

Examples

use futures_lite::io::BufWriter;

let mut output = Vec::new();
let writer = BufWriter::new(&mut output);

let r = writer.get_ref();

pub fn get_mut(&mut self) -> &mut W

Gets a mutable reference to the underlying writer.

It is not advisable to directly write to the underlying writer.

Examples

use futures_lite::io::BufWriter;

let mut output = Vec::new();
let mut writer = BufWriter::new(&mut output);

let r = writer.get_mut();

pub fn into_inner(self) -> W

Unwraps the buffered writer, returning the underlying writer.

Note that any leftover data in the internal buffer will be lost. If you don’t want to lose that data, flush the buffered writer before unwrapping it.

Examples

use futures_lite::io::{AsyncWriteExt, BufWriter};

let mut output = vec![1, 2, 3];
let mut writer = BufWriter::new(&mut output);

writer.write_all(&[4]).await?;
writer.flush().await?;
assert_eq!(writer.into_inner(), &[1, 2, 3, 4]);

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

Returns a reference to the internal buffer.

Examples

use futures_lite::io::BufWriter;

let mut output = Vec::new();
let writer = BufWriter::new(&mut output);

// The internal buffer is empty until the first write request.
assert_eq!(writer.buffer(), &[]);

Trait Implementations

impl<W> AsyncSeek for BufWriter<W>

where W: AsyncWrite + AsyncSeek,

fn poll_seek( self: Pin<&mut BufWriter<W>>, cx: &mut Context<'_>, pos: SeekFrom, ) -> Poll<Result<u64, Error>>

Seek to the offset, in bytes, in the underlying writer.

Seeking always writes out the internal buffer before seeking.

impl<W> AsyncWrite for BufWriter<W>

where W: AsyncWrite,

fn poll_write( self: Pin<&mut BufWriter<W>>, cx: &mut Context<'_>, buf: &[u8], ) -> Poll<Result<usize, Error>>

Attempt to write bytes from buf into the object.

fn poll_flush( self: Pin<&mut BufWriter<W>>, cx: &mut Context<'_>, ) -> Poll<Result<(), Error>>

Attempt to flush the object, ensuring that any buffered data reach their destination.

fn poll_close( self: Pin<&mut BufWriter<W>>, cx: &mut Context<'_>, ) -> Poll<Result<(), Error>>

Attempt to close the object.

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

Attempt to write bytes from bufs into the object using vectored IO operations.

impl<W> Debug for BufWriter<W>

where W: Debug,

fn fmt(&self, f: &mut Formatter<'_>) -> Result<(), Error>

Formats the value using the given formatter.

impl<'__pin, W> Unpin for BufWriter<W>

where <PinnedFieldsOfHelperStruct<__Origin<'__pin, W>> as PinnedFieldsOfHelperTrait>::Actual: Unpin,