pub struct BufWriter<W: ?Sized + Write> { /* private fields */ }
Expand description
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<W>
keeps an in-memory buffer of data and writes it to an underlying
writer in large, infrequent batches.
BufWriter<W>
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>
.
It is critical to call flush
before BufWriter<W>
is dropped. Though
dropping will attempt to flush the contents of the buffer, any errors
that happen in the process of dropping will be ignored. Calling flush
ensures that the buffer is empty and thus dropping will not even attempt
file operations.
§Examples
Let’s write the numbers one through ten to a TcpStream
:
use std::io::prelude::*;
use std::net::TcpStream;
let mut stream = TcpStream::connect("127.0.0.1:34254").unwrap();
for i in 0..10 {
stream.write(&[i+1]).unwrap();
}
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<W>
:
use std::io::prelude::*;
use std::io::BufWriter;
use std::net::TcpStream;
let mut stream = BufWriter::new(TcpStream::connect("127.0.0.1:34254").unwrap());
for i in 0..10 {
stream.write(&[i+1]).unwrap();
}
stream.flush().unwrap();
By wrapping the stream with a BufWriter<W>
, these ten writes are all grouped
together by the buffer and will all be written out in one system call when
the stream
is flushed.
Implementations§
source§impl<W: Write> BufWriter<W>
impl<W: Write> BufWriter<W>
1.0.0 · sourcepub fn new(inner: W) -> BufWriter<W> ⓘ
pub fn new(inner: W) -> BufWriter<W> ⓘ
Creates a new BufWriter<W>
with a default buffer capacity. The default is currently 8 KiB,
but may change in the future.
§Examples
1.0.0 · sourcepub fn with_capacity(capacity: usize, inner: W) -> BufWriter<W> ⓘ
pub fn with_capacity(capacity: usize, inner: W) -> BufWriter<W> ⓘ
Creates a new BufWriter<W>
with at least the specified buffer capacity.
§Examples
Creating a buffer with a buffer of at least a hundred bytes.
1.0.0 · sourcepub fn into_inner(self) -> Result<W, IntoInnerError<BufWriter<W>>>
pub fn into_inner(self) -> Result<W, IntoInnerError<BufWriter<W>>>
1.56.0 · sourcepub fn into_parts(self) -> (W, Result<Vec<u8>, WriterPanicked>)
pub fn into_parts(self) -> (W, Result<Vec<u8>, WriterPanicked>)
Disassembles this BufWriter<W>
, returning the underlying writer, and any buffered but
unwritten data.
If the underlying writer panicked, it is not known what portion of the data was written.
In this case, we return WriterPanicked
for the buffered data (from which the buffer
contents can still be recovered).
into_parts
makes no attempt to flush data and cannot fail.
§Examples
use std::io::{BufWriter, Write};
let mut buffer = [0u8; 10];
let mut stream = BufWriter::new(buffer.as_mut());
write!(stream, "too much data").unwrap();
stream.flush().expect_err("it doesn't fit");
let (recovered_writer, buffered_data) = stream.into_parts();
assert_eq!(recovered_writer.len(), 0);
assert_eq!(&buffered_data.unwrap(), b"ata");
source§impl<W: ?Sized + Write> BufWriter<W>
impl<W: ?Sized + Write> BufWriter<W>
1.0.0 · sourcepub fn get_mut(&mut self) -> &mut W
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
1.37.0 · sourcepub fn buffer(&self) -> &[u8] ⓘ
pub fn buffer(&self) -> &[u8] ⓘ
Returns a reference to the internally buffered data.
§Examples
1.46.0 · sourcepub fn capacity(&self) -> usize
pub fn capacity(&self) -> usize
Returns the number of bytes the internal buffer can hold without flushing.
§Examples
use std::io::BufWriter;
use std::net::TcpStream;
let buf_writer = BufWriter::new(TcpStream::connect("127.0.0.1:34254").unwrap());
// Check the capacity of the inner buffer
let capacity = buf_writer.capacity();
// Calculate how many bytes can be written without flushing
let without_flush = capacity - buf_writer.buffer().len();
Trait Implementations§
1.0.0 · source§impl<W: ?Sized + Write + Seek> Seek for BufWriter<W>
impl<W: ?Sized + Write + Seek> Seek for BufWriter<W>
source§fn seek(&mut self, pos: SeekFrom) -> Result<u64>
fn seek(&mut self, pos: SeekFrom) -> Result<u64>
Seek to the offset, in bytes, in the underlying writer.
Seeking always writes out the internal buffer before seeking.
source§fn stream_len(&mut self) -> Result<u64>
fn stream_len(&mut self) -> Result<u64>
seek_stream_len
#59359)1.0.0 · source§impl<W: ?Sized + Write> Write for BufWriter<W>
impl<W: ?Sized + Write> Write for BufWriter<W>
source§fn write(&mut self, buf: &[u8]) -> Result<usize>
fn write(&mut self, buf: &[u8]) -> Result<usize>
source§fn write_all(&mut self, buf: &[u8]) -> Result<()>
fn write_all(&mut self, buf: &[u8]) -> Result<()>
source§fn is_write_vectored(&self) -> bool
fn is_write_vectored(&self) -> bool
can_vector
#69941)