Struct async_std::io::BufWriter

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 Write. For example, every call to write on TcpStream results in a system call. 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 written out. However, any errors that happen in the process of flushing the buffer when the writer is dropped will be ignored. Code that wishes to handle such errors must manually call flush before the writer is dropped.

This type is an async version of std::io::BufReader.

Examples

Let's write the numbers one through ten to a TcpStream:

use async_std::net::TcpStream;
use async_std::prelude::*;

let mut stream = TcpStream::connect("127.0.0.1:34254").await?;

for i in 0..10 {
    let arr = [i+1];
    stream.write(&arr).await?;
}

Because we're not buffering, we write each one in turn, incurring the overhead of a system call per byte written. We can fix this with a BufWriter:

use async_std::io::BufWriter;
use async_std::net::TcpStream;
use async_std::prelude::*;

let mut stream = BufWriter::new(TcpStream::connect("127.0.0.1:34254").await?);
for i in 0..10 {
    let arr = [i+1];
    stream.write(&arr).await?;
};

By wrapping the stream with a BufWriter, these ten writes are all grouped together by the buffer, and will all be written out in one system call when the stream is dropped.

Methods

impl<W: AsyncWrite> BufWriter<W>

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

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

Examples

use async_std::io::BufWriter;
use async_std::net::TcpStream;

let mut buffer = BufWriter::new(TcpStream::connect("127.0.0.1:34254").await?);

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

Creates a new BufWriter with the specified buffer capacity.

Examples

Creating a buffer with a buffer of a hundred bytes.

use async_std::io::BufWriter;
use async_std::net::TcpStream;

let stream = TcpStream::connect("127.0.0.1:34254").await?;
let mut buffer = BufWriter::with_capacity(100, stream);

Important traits for &'_ mut F

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

pub fn get_ref(&self) -> &W

Gets a reference to the underlying writer.

Examples

use async_std::io::BufWriter;
use async_std::net::TcpStream;

let mut buffer = BufWriter::new(TcpStream::connect("127.0.0.1:34254").await?);

// We can use reference just like buffer
let reference = buffer.get_ref();

Important traits for &'_ mut F

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

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

Gets a mutable reference to the underlying writer.

It is inadvisable to directly write to the underlying writer.

Examples

use async_std::io::BufWriter;
use async_std::net::TcpStream;

let mut buffer = BufWriter::new(TcpStream::connect("127.0.0.1:34254").await?);

// We can use reference just like buffer
let reference = buffer.get_mut();

pub fn into_inner(self) -> W

Consumes BufWriter, returning the underlying writer

This method will not write leftover data, it will be lost. For method that will attempt to write before returning the writer see poll_into_inner

Important traits for &'_ mut [u8]

impl<'_> Write for &'_ mut [u8]impl<'_> Read for &'_ [u8]

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

Returns a reference to the internally buffered data.

Examples

use async_std::io::BufWriter;
use async_std::net::TcpStream;

let buf_writer = BufWriter::new(TcpStream::connect("127.0.0.1:34251").await?);

// See how many bytes are currently buffered
let bytes_buffered = buf_writer.buffer().len();

Trait Implementations

impl<W: AsyncWrite + Debug> Debug for BufWriter<W>

fn fmt(&self, f: &mut Formatter) -> Result

Formats the value using the given formatter.

impl<W: AsyncWrite> AsyncWrite for BufWriter<W>

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

Attempt to write bytes from buf into the object.

fn poll_flush(self: Pin<&mut Self>, cx: &mut Context) -> Poll<Result<()>>

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

fn poll_close(self: Pin<&mut Self>, cx: &mut Context) -> Poll<Result<()>>

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: AsyncWrite + AsyncSeek> AsyncSeek for BufWriter<W>

fn poll_seek(
    self: Pin<&mut Self>,
    cx: &mut Context,
    pos: SeekFrom
) -> Poll<Result<u64>>

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

Seeking always writes out the internal buffer before seeking.