pub struct BufWriter<W: Write> { /* fields omitted */ }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
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();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();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();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");Trait Implementations
Write a buffer into this writer, returning how many bytes were written. Read more
Attempts to write an entire buffer into this writer. Read more
Flush this output stream, ensuring that all intermediately buffered contents reach their destination. Read more
Writes a formatted string into this writer, returning any error encountered. Read more