pub struct BufWriter<W: 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();
}
RunBecause 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();
RunBy 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
sourceimpl<W: Write> BufWriter<W>
impl<W: Write> BufWriter<W>
sourcepub fn new(inner: W) -> BufWriter<W>ⓘNotable traits for BufWriter<W>impl<W: Write> Write for BufWriter<W>
pub fn new(inner: W) -> BufWriter<W>ⓘNotable traits for BufWriter<W>impl<W: Write> Write for BufWriter<W>
sourcepub fn with_capacity(capacity: usize, inner: W) -> BufWriter<W>ⓘNotable traits for BufWriter<W>impl<W: Write> Write for BufWriter<W>
pub fn with_capacity(capacity: usize, inner: W) -> BufWriter<W>ⓘNotable traits for BufWriter<W>impl<W: Write> Write for BufWriter<W>
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
use std::io::BufWriter;
use std::net::TcpStream;
let mut buffer = BufWriter::new(TcpStream::connect("127.0.0.1:34254").unwrap());
// we can use reference just like buffer
let reference = buffer.get_mut();
Run1.37.0 · sourcepub fn buffer(&self) -> &[u8]ⓘNotable traits for &[u8]impl Read for &[u8]impl Write for &mut [u8]
pub fn buffer(&self) -> &[u8]ⓘNotable traits for &[u8]impl Read for &[u8]impl Write for &mut [u8]
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();
Runsourcepub fn into_inner(self) -> Result<W, IntoInnerError<BufWriter<W>>>
pub fn into_inner(self) -> Result<W, IntoInnerError<BufWriter<W>>>
Unwraps this BufWriter<W>
, returning the underlying writer.
The buffer is written out before returning the writer.
Errors
An Err
will be returned if an error occurs while flushing the buffer.
Examples
use std::io::BufWriter;
use std::net::TcpStream;
let mut buffer = BufWriter::new(TcpStream::connect("127.0.0.1:34254").unwrap());
// unwrap the TcpStream and flush the buffer
let stream = buffer.into_inner().unwrap();
Run1.56.0 · 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");
RunTrait Implementations
sourceimpl<W: Write + Seek> Seek for BufWriter<W>
impl<W: Write + Seek> Seek for BufWriter<W>
sourcefn 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.
sourcefn stream_len(&mut self) -> Result<u64>
fn stream_len(&mut self) -> Result<u64>
Returns the length of this stream (in bytes). Read more
1.51.0 · sourcefn stream_position(&mut self) -> Result<u64>
fn stream_position(&mut self) -> Result<u64>
Returns the current seek position from the start of the stream. Read more
sourceimpl<W: Write> Write for BufWriter<W>
impl<W: Write> Write for BufWriter<W>
sourcefn write(&mut self, buf: &[u8]) -> Result<usize>
fn write(&mut self, buf: &[u8]) -> Result<usize>
Write a buffer into this writer, returning how many bytes were written. Read more
sourcefn write_all(&mut self, buf: &[u8]) -> Result<()>
fn write_all(&mut self, buf: &[u8]) -> Result<()>
Attempts to write an entire buffer into this writer. Read more
sourcefn write_vectored(&mut self, bufs: &[IoSlice<'_>]) -> Result<usize>
fn write_vectored(&mut self, bufs: &[IoSlice<'_>]) -> Result<usize>
sourcefn is_write_vectored(&self) -> bool
fn is_write_vectored(&self) -> bool
Determines if this Write
r has an efficient write_vectored
implementation. Read more
sourcefn flush(&mut self) -> Result<()>
fn flush(&mut self) -> Result<()>
Flush this output stream, ensuring that all intermediately buffered contents reach their destination. Read more
sourcefn write_all_vectored(&mut self, bufs: &mut [IoSlice<'_>]) -> Result<()>
fn write_all_vectored(&mut self, bufs: &mut [IoSlice<'_>]) -> Result<()>
Attempts to write multiple buffers into this writer. Read more
Auto Trait Implementations
impl<W> RefUnwindSafe for BufWriter<W> where
W: RefUnwindSafe,
impl<W> Send for BufWriter<W> where
W: Send,
impl<W> Sync for BufWriter<W> where
W: Sync,
impl<W> Unpin for BufWriter<W> where
W: Unpin,
impl<W> UnwindSafe for BufWriter<W> where
W: UnwindSafe,
Blanket Implementations
sourceimpl<T> BorrowMut<T> for T where
T: ?Sized,
impl<T> BorrowMut<T> for T where
T: ?Sized,
const: unstable · sourcefn borrow_mut(&mut self) -> &mut T
fn borrow_mut(&mut self) -> &mut T
Mutably borrows from an owned value. Read more