Struct async_std::io::BufWriter

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

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>.

Unlike the BufWriter type in std, this type does not write out the contents of its buffer when it is dropped. Therefore, it is absolutely critical that users explicitly flush the buffer before dropping a BufWriter.

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

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?;
};

stream.flush().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 stream.flush() completes. (As mentioned above, dropping a BufWriter does not flush its buffers, so a flush call is essential.)

Implementations

impl<W: Write> 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);

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();

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 async fn into_inner(self) -> Result<W, IntoInnerError<BufWriter<W>>>

where Self: Unpin,

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

Examples

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

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

// unwrap the TcpStream and flush the buffer
let stream = buf_writer.into_inner().await.unwrap();

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: Write + Seek> 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.

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

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

Formats the value using the given formatter.

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

where __Origin<'__pin, W>: Unpin,