std::io

Struct Cursor

1.0.0 · Source

pub struct Cursor<T> { /* private fields */ }

Expand description

A Cursor wraps an in-memory buffer and provides it with a Seek implementation.

Cursors are used with in-memory buffers, anything implementing AsRef<[u8]>, to allow them to implement Read and/or Write, allowing these buffers to be used anywhere you might use a reader or writer that does actual I/O.

The standard library implements some I/O traits on various types which are commonly used as a buffer, like Cursor<Vec<u8>> and Cursor<&[u8]>.

§Examples

We may want to write bytes to a File in our production code, but use an in-memory buffer in our tests. We can do this with Cursor:

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

// a library function we've written
fn write_ten_bytes_at_end<W: Write + Seek>(mut writer: W) -> io::Result<()> {
    writer.seek(SeekFrom::End(-10))?;

    for i in 0..10 {
        writer.write(&[i])?;
    }

    // all went well
    Ok(())
}

// Here's some code that uses this library function.
//
// We might want to use a BufReader here for efficiency, but let's
// keep this example focused.
let mut file = File::create("foo.txt")?;
// First, we need to allocate 10 bytes to be able to write into.
file.set_len(10)?;

write_ten_bytes_at_end(&mut file)?;

// now let's write a test
#[test]
fn test_writes_bytes() {
    // setting up a real File is much slower than an in-memory buffer,
    // let's use a cursor instead
    use std::io::Cursor;
    let mut buff = Cursor::new(vec![0; 15]);

    write_ten_bytes_at_end(&mut buff).unwrap();

    assert_eq!(&buff.get_ref()[5..15], &[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]);
}

Implementations§

Source §

impl<T> Cursor<T>

1.0.0 (const: 1.79.0) · Source

pub const fn new(inner: T) -> Cursor<T> ⓘ

Creates a new cursor wrapping the provided underlying in-memory buffer.

Cursor initial position is 0 even if underlying buffer (e.g., Vec) is not empty. So writing to cursor starts with overwriting Vec content, not with appending to it.

§Examples

use std::io::Cursor;

let buff = Cursor::new(Vec::new());

1.0.0 · Source

pub fn into_inner(self) -> T

Consumes this cursor, returning the underlying value.

§Examples

use std::io::Cursor;

let buff = Cursor::new(Vec::new());

let vec = buff.into_inner();

1.0.0 (const: 1.79.0) · Source

pub const fn get_ref(&self) -> &T

Gets a reference to the underlying value in this cursor.

§Examples

use std::io::Cursor;

let buff = Cursor::new(Vec::new());

let reference = buff.get_ref();

1.0.0 (const: unstable) · Source

pub fn get_mut(&mut self) -> &mut T

Gets a mutable reference to the underlying value in this cursor.

Care should be taken to avoid modifying the internal I/O state of the underlying value as it may corrupt this cursor’s position.

§Examples

use std::io::Cursor;

let mut buff = Cursor::new(Vec::new());

let reference = buff.get_mut();

1.0.0 (const: 1.79.0) · Source

pub const fn position(&self) -> u64

Returns the current position of this cursor.

§Examples

use std::io::Cursor;
use std::io::prelude::*;
use std::io::SeekFrom;

let mut buff = Cursor::new(vec![1, 2, 3, 4, 5]);

assert_eq!(buff.position(), 0);

buff.seek(SeekFrom::Current(2)).unwrap();
assert_eq!(buff.position(), 2);

buff.seek(SeekFrom::Current(-1)).unwrap();
assert_eq!(buff.position(), 1);

1.0.0 (const: unstable) · Source

pub fn set_position(&mut self, pos: u64)

Sets the position of this cursor.

§Examples

use std::io::Cursor;

let mut buff = Cursor::new(vec![1, 2, 3, 4, 5]);

assert_eq!(buff.position(), 0);

buff.set_position(2);
assert_eq!(buff.position(), 2);

buff.set_position(4);
assert_eq!(buff.position(), 4);

Source §

impl<T> Cursor<T>
where T: AsRef<[u8]>,

Source

pub fn split(&self) -> (&[u8], &[u8])

🔬This is a nightly-only experimental API. (cursor_split #86369)

Splits the underlying slice at the cursor position and returns them.

§Examples

#![feature(cursor_split)]
use std::io::Cursor;

let mut buff = Cursor::new(vec![1, 2, 3, 4, 5]);

assert_eq!(buff.split(), ([].as_slice(), [1, 2, 3, 4, 5].as_slice()));

buff.set_position(2);
assert_eq!(buff.split(), ([1, 2].as_slice(), [3, 4, 5].as_slice()));

buff.set_position(6);
assert_eq!(buff.split(), ([1, 2, 3, 4, 5].as_slice(), [].as_slice()));

Source §

impl<T> Cursor<T>
where T: AsMut<[u8]>,

Source

pub fn split_mut(&mut self) -> (&mut [u8], &mut [u8])

🔬This is a nightly-only experimental API. (cursor_split #86369)

Splits the underlying slice at the cursor position and returns them mutably.

§Examples

#![feature(cursor_split)]
use std::io::Cursor;

let mut buff = Cursor::new(vec![1, 2, 3, 4, 5]);

assert_eq!(buff.split_mut(), ([].as_mut_slice(), [1, 2, 3, 4, 5].as_mut_slice()));

buff.set_position(2);
assert_eq!(buff.split_mut(), ([1, 2].as_mut_slice(), [3, 4, 5].as_mut_slice()));

buff.set_position(6);
assert_eq!(buff.split_mut(), ([1, 2, 3, 4, 5].as_mut_slice(), [].as_mut_slice()));