linewriter.rs - source
Description: Source of the Rust file `library/std/src/io/buffered/linewriter.rs`.
Keywords: rust, rustlang, rust-lang

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    use crate::fmt;
    use crate::io::{self, buffered::LineWriterShim, BufWriter, IntoInnerError, IoSlice, Write};

    /// 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.
    ///
    /// ```no_run
    /// 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(())
    /// }
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    pub struct LineWriter<W: Write> {
        inner: BufWriter<W>,
    }

    impl<W: Write> LineWriter<W> {
        /// Creates a new `LineWriter`.
        ///
        /// # Examples
        ///
        /// ```no_run
        /// 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(())
        /// }
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn new(inner: W) -> LineWriter<W> {
            // Lines typically aren't that long, don't use a giant buffer
            LineWriter::with_capacity(1024, inner)
        }

        /// Creates a new `LineWriter` with a specified capacity for the internal
        /// buffer.
        ///
        /// # Examples
        ///
        /// ```no_run
        /// 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(())
        /// }
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn with_capacity(capacity: usize, inner: W) -> LineWriter<W> {
            LineWriter { inner: BufWriter::with_capacity(capacity, inner) }
        }

        /// Gets a reference to the underlying writer.
        ///
        /// # Examples
        ///
        /// ```no_run
        /// 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(())
        /// }
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn get_ref(&self) -> &W {
            self.inner.get_ref()
        }

        /// 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
        ///
        /// ```no_run
        /// 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(())
        /// }
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn get_mut(&mut self) -> &mut W {
            self.inner.get_mut()
        }

        /// 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
        ///
        /// ```no_run
        /// 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(())
        /// }
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn into_inner(self) -> Result<W, IntoInnerError<LineWriter<W>>> {
            self.inner.into_inner().map_err(|err| err.new_wrapped(|inner| LineWriter { inner }))
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<W: Write> Write for LineWriter<W> {
        fn write(&mut self, buf: &[u8]) -> io::Result<usize> {
            LineWriterShim::new(&mut self.inner).write(buf)
        }

        fn flush(&mut self) -> io::Result<()> {
            self.inner.flush()
        }

        fn write_vectored(&mut self, bufs: &[IoSlice<'_>]) -> io::Result<usize> {
            LineWriterShim::new(&mut self.inner).write_vectored(bufs)
        }

        fn is_write_vectored(&self) -> bool {
            self.inner.is_write_vectored()
        }

        fn write_all(&mut self, buf: &[u8]) -> io::Result<()> {
            LineWriterShim::new(&mut self.inner).write_all(buf)
        }

        fn write_all_vectored(&mut self, bufs: &mut [IoSlice<'_>]) -> io::Result<()> {
            LineWriterShim::new(&mut self.inner).write_all_vectored(bufs)
        }

        fn write_fmt(&mut self, fmt: fmt::Arguments<'_>) -> io::Result<()> {
            LineWriterShim::new(&mut self.inner).write_fmt(fmt)
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<W: Write> fmt::Debug for LineWriter<W>
    where
        W: fmt::Debug,
    {
        fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
            fmt.debug_struct("LineWriter")
                .field("writer", &self.get_ref())
                .field(
                    "buffer",
                    &format_args!("{}/{}", self.inner.buffer().len(), self.inner.capacity()),
                )
                .finish()
        }
    }