Trait core2::io::Write

source · [−]
pub trait Write {
    fn write(&mut self, buf: &[u8]) -> Result<usize>;

    fn flush(&mut self) -> Result<()>;

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

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

    fn by_ref(&mut self) -> &mut Self
    where
        Self: Sized,
    { ... }
}
Expand description

A trait for objects which are byte-oriented sinks.

Implementors of the Write trait are sometimes called ‘writers’.

Writers are defined by two required methods, write and flush:

  • The write method will attempt to write some data into the object, returning how many bytes were successfully written.

  • The flush method is useful for adaptors and explicit buffers themselves for ensuring that all buffered data has been pushed out to the ‘true sink’.

Writers are intended to be composable with one another. Many implementors throughout std::io take and provide types which implement the Write trait.

Examples

use std::io::prelude::*;
use std::fs::File;

fn main() -> std::io::Result<()> {
    let data = b"some bytes";

    let mut pos = 0;
    let mut buffer = File::create("foo.txt")?;

    while pos < data.len() {
        let bytes_written = buffer.write(&data[pos..])?;
        pos += bytes_written;
    }
    Ok(())
}

The trait also provides convenience methods like write_all, which calls write in a loop until its entire input has been written.

Required methods

Write a buffer into this writer, returning how many bytes were written.

This function will attempt to write the entire contents of buf, but the entire write may not succeed, or the write may also generate an error. A call to write represents at most one attempt to write to any wrapped object.

Calls to write are not guaranteed to block waiting for data to be written, and a write which would otherwise block can be indicated through an Err variant.

If the return value is Ok(n) then it must be guaranteed that n <= buf.len(). A return value of 0 typically means that the underlying object is no longer able to accept bytes and will likely not be able to in the future as well, or that the buffer provided is empty.

Errors

Each call to write may generate an I/O error indicating that the operation could not be completed. If an error is returned then no bytes in the buffer were written to this writer.

It is not considered an error if the entire buffer could not be written to this writer.

An error of the ErrorKind::Interrupted kind is non-fatal and the write operation should be retried if there is nothing else to do.

Examples
use std::io::prelude::*;
use std::fs::File;

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

    // Writes some prefix of the byte string, not necessarily all of it.
    buffer.write(b"some bytes")?;
    Ok(())
}

Flush this output stream, ensuring that all intermediately buffered contents reach their destination.

Errors

It is considered an error if not all bytes could be written due to I/O errors or EOF being reached.

Examples
use std::io::prelude::*;
use std::io::BufWriter;
use std::fs::File;

fn main() -> std::io::Result<()> {
    let mut buffer = BufWriter::new(File::create("foo.txt")?);

    buffer.write_all(b"some bytes")?;
    buffer.flush()?;
    Ok(())
}

Provided methods

Attempts to write an entire buffer into this writer.

This method will continuously call write until there is no more data to be written or an error of non-ErrorKind::Interrupted kind is returned. This method will not return until the entire buffer has been successfully written or such an error occurs. The first error that is not of ErrorKind::Interrupted kind generated from this method will be returned.

If the buffer contains no data, this will never call write.

Errors

This function will return the first error of non-ErrorKind::Interrupted kind that write returns.

Examples
use std::io::prelude::*;
use std::fs::File;

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

    buffer.write_all(b"some bytes")?;
    Ok(())
}

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

This method is primarily used to interface with the format_args!() macro, but it is rare that this should explicitly be called. The write!() macro should be favored to invoke this method instead.

This function internally uses the write_all method on this trait and hence will continuously write data so long as no errors are received. This also means that partial writes are not indicated in this signature.

Errors

This function will return any I/O error reported while formatting.

Examples
use std::io::prelude::*;
use std::fs::File;

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

    // this call
    write!(buffer, "{:.*}", 2, 1.234567)?;
    // turns into this:
    buffer.write_fmt(format_args!("{:.*}", 2, 1.234567))?;
    Ok(())
}

Creates a “by reference” adaptor for this instance of Write.

The returned adaptor also implements Write and will simply borrow this current writer.

Examples
use std::io::Write;
use std::fs::File;

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

    let reference = buffer.by_ref();

    // we can use reference just like our original buffer
    reference.write_all(b"some bytes")?;
    Ok(())
}

Implementations on Foreign Types

Write is implemented for &mut [u8] by copying into the slice, overwriting its data.

Note that writing updates the slice to point to the yet unwritten part. The slice will be empty when it has been completely overwritten.

Write is implemented for Vec<u8> by appending to the vector. The vector will grow as needed.

Implementors