# Struct std::io::LineWriter

1.0.0 · source ·

    pub struct LineWriter<W: ?Sized + Write> { /* private fields */ }

Expand description

Wraps a writer and buffers output to it, flushing whenever a newline (`0x0a`, `'\n'`) is detected.

The `BufWriter` struct wraps a writer and buffers its output. But it only does this batched write when it goes out of scope, or when the internal buffer is full. Sometimes, you’d prefer to write each line as it’s completed, rather than the entire buffer at once. Enter `LineWriter`. It does exactly that.

Like `BufWriter`, a `LineWriter`’s buffer will also be flushed when the `LineWriter` goes out of scope or when its internal buffer is full.

If there’s still a partial line in the buffer when the `LineWriter` is dropped, it will flush those contents.

## Examples

We can use `LineWriter` to write one line at a time, significantly reducing the number of actual writes to the file.

    use std::fs::{self, File};
    use std::io::prelude::*;
    use std::io::LineWriter;

    fn main() -> std::io::Result<()> {
        let road_not_taken = b"I shall be telling this with a sigh
    Somewhere ages and ages hence:
    Two roads diverged in a wood, and I -
    I took the one less traveled by,
    And that has made all the difference.";

        let file = File::create("poem.txt")?;
        let mut file = LineWriter::new(file);

        file.write_all(b"I shall be telling this with a sigh")?;

        // No bytes are written until a newline is encountered (or
        // the internal buffer is filled).
        assert_eq!(fs::read_to_string("poem.txt")?, "");
        file.write_all(b"\n")?;
        assert_eq!(
            fs::read_to_string("poem.txt")?,
            "I shall be telling this with a sigh\n",
        );

        // Write the rest of the poem.
        file.write_all(b"Somewhere ages and ages hence:
    Two roads diverged in a wood, and I -
    I took the one less traveled by,
    And that has made all the difference.")?;

        // The last line of the poem doesn't end in a newline, so
        // we have to flush or drop the `LineWriter` to finish
        // writing.
        file.flush()?;

        // Confirm the whole poem was written.
        assert_eq!(fs::read("poem.txt")?, &road_not_taken[..]);
        Ok(())
    }

Run

## Implementations§

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### impl<W: Write> LineWriter<W>

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#### pub fn new(inner: W) -> LineWriter<W> ⓘ

Creates a new `LineWriter`.

##### Examples

    use std::fs::File;
    use std::io::LineWriter;

    fn main() -> std::io::Result<()> {
        let file = File::create("poem.txt")?;
        let file = LineWriter::new(file);
        Ok(())
    }

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#### pub fn with_capacity(capacity: usize, inner: W) -> LineWriter<W> ⓘ

Creates a new `LineWriter` with at least the specified capacity for the internal buffer.

##### Examples

    use std::fs::File;
    use std::io::LineWriter;

    fn main() -> std::io::Result<()> {
        let file = File::create("poem.txt")?;
        let file = LineWriter::with_capacity(100, file);
        Ok(())
    }

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#### pub fn get_mut(&mut self) -> &mut W

Gets a mutable reference to the underlying writer.

Caution must be taken when calling methods on the mutable reference returned as extra writes could corrupt the output stream.

##### Examples

    use std::fs::File;
    use std::io::LineWriter;

    fn main() -> std::io::Result<()> {
        let file = File::create("poem.txt")?;
        let mut file = LineWriter::new(file);

        // we can use reference just like file
        let reference = file.get_mut();
        Ok(())
    }

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#### pub fn into_inner(self) -> Result<W, IntoInnerError<LineWriter<W>>>

Unwraps this `LineWriter`, returning the underlying writer.

The internal 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::fs::File;
    use std::io::LineWriter;

    fn main() -> std::io::Result<()> {
        let file = File::create("poem.txt")?;

        let writer: LineWriter<File> = LineWriter::new(file);

        let file: File = writer.into_inner()?;
        Ok(())
    }

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### impl<W: ?Sized + Write> LineWriter<W>

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

Gets a reference to the underlying writer.

##### Examples

    use std::fs::File;
    use std::io::LineWriter;

    fn main() -> std::io::Result<()> {
        let file = File::create("poem.txt")?;
        let file = LineWriter::new(file);

        let reference = file.get_ref();
        Ok(())
    }

Run

## Trait Implementations§

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### impl<W> Debug for LineWriter<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> Write for LineWriter<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 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_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 write_all(&mut self, buf: &[u8]) -> Result<()>

Attempts to write an entire buffer into this writer. 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 LineWriter<W>where W: RefUnwindSafe,

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

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

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

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### impl<W: ?Sized> UnwindSafe for LineWriter<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.