# Struct std::io::BufWriter

1.0.0 · source ·

    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();
    }

Run

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();

Run

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§

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### impl<W: Write> BufWriter<W>

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#### 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

    use std::io::BufWriter;
    use std::net::TcpStream;

    let mut buffer = BufWriter::new(TcpStream::connect("127.0.0.1:34254").unwrap());

Run
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#### 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.

    use std::io::BufWriter;
    use std::net::TcpStream;

    let stream = TcpStream::connect("127.0.0.1:34254").unwrap();
    let mut buffer = BufWriter::with_capacity(100, stream);

Run
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#### 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();

Run
1.56.0 · source

#### 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");

Run
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### impl<W: ?Sized + Write> BufWriter<W>

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#### pub fn get_ref(&self) -> &W

Gets a reference 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_ref();

Run
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#### 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();

Run
1.37.0 · source

#### pub fn buffer(&self) -> &[u8] ⓘ

Returns a reference to the internally buffered data.

##### Examples

    use std::io::BufWriter;
    use std::net::TcpStream;

    let buf_writer = BufWriter::new(TcpStream::connect("127.0.0.1:34254").unwrap());

    // See how many bytes are currently buffered
    let bytes_buffered = buf_writer.buffer().len();

Run
1.46.0 · source

#### 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();

Run

## Trait Implementations§

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### impl<W> Debug for BufWriter<W>where W: Debug + ?Sized + Write,

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

Formats the value using the given formatter. Read more
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### impl<W: ?Sized + Write> Drop for BufWriter<W>

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#### fn drop(&mut self)

Executes the destructor for this type. Read more
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### impl<W: ?Sized + Write + Seek> Seek for BufWriter<W>

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#### 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.

1.55.0 · source§

#### fn rewind(&mut self) -> Result<()>

Rewind to the beginning of a stream. Read more
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#### fn stream_len(&mut self) -> Result<u64>

🔬This is a nightly-only experimental API. (`seek_stream_len` #59359)
Returns the length of this stream (in bytes). Read more
1.51.0 · source§

#### fn stream_position(&mut self) -> Result<u64>

Returns the current seek position from the start of the stream. Read more
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### impl<W: ?Sized + Write> Write for BufWriter<W>

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#### fn write(&mut self, buf: &[u8]) -> Result<usize>

Write a buffer into this writer, returning how many bytes were written. Read more
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#### fn write_all(&mut self, buf: &[u8]) -> Result<()>

Attempts to write an entire buffer into this writer. Read more
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#### fn write_vectored(&mut self, bufs: &[IoSlice<'_>]) -> Result<usize>

Like `write`, except that it writes from a slice of buffers. Read more
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#### fn is_write_vectored(&self) -> bool

🔬This is a nightly-only experimental API. (`can_vector` #69941)
Determines if this `Write`r has an efficient `write_vectored` implementation. Read more
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#### fn flush(&mut self) -> Result<()>

Flush this output stream, ensuring that all intermediately buffered contents reach their destination. Read more
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#### fn write_all_vectored(&mut self, bufs: &mut [IoSlice<'_>]) -> Result<()>

🔬This is a nightly-only experimental API. (`write_all_vectored` #70436)
Attempts to write multiple buffers into this writer. Read more
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#### fn write_fmt(&mut self, fmt: Arguments<'_>) -> Result<()>

Writes a formatted string into this writer, returning any error encountered. Read more

## Auto Trait Implementations§

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### impl<W: ?Sized> RefUnwindSafe for BufWriter<W>where W: RefUnwindSafe,

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### impl<W: ?Sized> Send for BufWriter<W>where W: Send,

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### impl<W: ?Sized> Sync for BufWriter<W>where W: Sync,

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### impl<W: ?Sized> Unpin for BufWriter<W>where W: Unpin,

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### impl<W: ?Sized> UnwindSafe for BufWriter<W>where W: UnwindSafe,

## Blanket Implementations§

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### impl<T> Any for Twhere T: 'static + ?Sized,

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#### fn type_id(&self) -> TypeId

Gets the `TypeId` of `self`. Read more
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### impl<T> Borrow<T> for Twhere T: ?Sized,

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#### fn borrow(&self) -> &T

Immutably borrows from an owned value. Read more
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### impl<T> BorrowMut<T> for Twhere T: ?Sized,

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#### fn borrow_mut(&mut self) -> &mut T

Mutably borrows from an owned value. Read more
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### impl<T> From<T> for T

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#### fn from(t: T) -> T

Returns the argument unchanged.

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### impl<T, U> Into<U> for Twhere U: From<T>,

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#### fn into(self) -> U

Calls `U::from(self)`.

That is, this conversion is whatever the implementation of `From<T> for U` chooses to do.

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### impl<T, U> TryFrom<U> for Twhere U: Into<T>,

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#### type Error = Infallible

The type returned in the event of a conversion error.
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#### fn try_from(value: U) -> Result<T, <T as TryFrom<U>>::Error>

Performs the conversion.
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### impl<T, U> TryInto<U> for Twhere U: TryFrom<T>,

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#### type Error = <U as TryFrom<T>>::Error

The type returned in the event of a conversion error.
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#### fn try_into(self) -> Result<U, <U as TryFrom<T>>::Error>

Performs the conversion.