Struct async_std::io::BufReader

pub struct BufReader<R> { /* fields omitted */ }

Adds buffering to any reader.

It can be excessively inefficient to work directly with a Read instance. A BufReader performs large, infrequent reads on the underlying Read and maintains an in-memory buffer of the incoming byte stream.

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

When the BufReader is dropped, the contents of its buffer will be discarded. Creating multiple instances of a BufReader on the same stream can cause data loss.

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

Examples

use async_std::fs::File;
use async_std::io::BufReader;
use async_std::prelude::*;

let mut f = BufReader::new(File::open("a.txt").await?);

let mut line = String::new();
f.read_line(&mut line).await?;

Methods

impl<R: AsyncRead> BufReader<R>

pub fn new(inner: R) -> BufReader<R>

Creates a buffered reader with default buffer capacity.

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

Examples

use async_std::fs::File;
use async_std::io::BufReader;

let f = BufReader::new(File::open("a.txt").await?);

pub fn with_capacity(capacity: usize, inner: R) -> BufReader<R>

Creates a new buffered reader with the specified capacity.

Examples

use async_std::fs::File;
use async_std::io::BufReader;

let f = BufReader::with_capacity(1024, File::open("a.txt").await?);

impl<R> BufReader<R>

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) -> &R

Gets a reference to the underlying reader.

It is inadvisable to directly read from the underlying reader.

Examples

use async_std::fs::File;
use async_std::io::BufReader;

let f = BufReader::new(File::open("a.txt").await?);
let inner = f.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 R

Gets a mutable reference to the underlying reader.

It is inadvisable to directly read from the underlying reader.

Examples

use async_std::fs::File;
use async_std::io::BufReader;

let mut f = BufReader::new(File::open("a.txt").await?);
let inner = f.get_mut();

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

Returns a reference to the internal buffer.

This function will not attempt to fill the buffer if it is empty.

Examples

use async_std::fs::File;
use async_std::io::BufReader;

let f = BufReader::new(File::open("a.txt").await?);
let buffer = f.buffer();

pub fn into_inner(self) -> R

Unwraps the buffered reader, returning the underlying reader.

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

Examples

use async_std::fs::File;
use async_std::io::BufReader;

let f = BufReader::new(File::open("a.txt").await?);
let inner = f.into_inner();

Trait Implementations

impl<R: AsyncRead + Debug> Debug for BufReader<R>

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

Formats the value using the given formatter.

impl<R: AsyncRead> AsyncRead for BufReader<R>

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

Attempt to read from the AsyncRead into buf.

fn poll_read_vectored(
    self: Pin<&mut Self>,
    cx: &mut Context,
    bufs: &mut [IoSliceMut]
) -> Poll<Result<usize>>

Attempt to read from the AsyncRead into bufs using vectored IO operations.

unsafe fn initializer(&self) -> Initializer

Determines if this AsyncReader can work with buffers of uninitialized memory.

impl<R: AsyncRead> AsyncBufRead for BufReader<R>

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

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

fn consume(self: Pin<&mut Self>, amt: usize)

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

impl<R: AsyncSeek> AsyncSeek for BufReader<R>

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

Seeks to an offset, in bytes, in the underlying reader.

The position used for seeking with SeekFrom::Current(_) is the position the underlying reader would be at if the BufReader had no internal buffer.

Seeking always discards the internal buffer, even if the seek position would otherwise fall within it. This guarantees that calling .into_inner() immediately after a seek yields the underlying reader at the same position.

See Seek for more details.

Note: In the edge case where you're seeking with SeekFrom::Current(n) where n minus the internal buffer length overflows an i64, two seeks will be performed instead of one. If the second seek returns Err, the underlying reader will be left at the same position it would have if you called seek with SeekFrom::Current(0).