Struct Cursor

pub struct Cursor<T> { pub(in ::io::cursor) inner: T, pub(in ::io::cursor) pos: u64 }

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(())
}

# fn foo() -> io::Result<()> {
// 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)?;
# Ok(())
# }

// 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]);
}

Fields

inner: T
pos: u64

Implementations

impl<T> Cursor<T>

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());
# fn force_inference(_: &Cursor<Vec<u8>>) {}
# force_inference(&buff);

fn into_inner(self) -> T

Consumes this cursor, returning the underlying value.

Examples

use std::io::Cursor;

let buff = Cursor::new(Vec::new());
# fn force_inference(_: &Cursor<Vec<u8>>) {}
# force_inference(&buff);

let vec = buff.into_inner();

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());
# fn force_inference(_: &Cursor<Vec<u8>>) {}
# force_inference(&buff);

let reference = buff.get_ref();

const 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());
# fn force_inference(_: &Cursor<Vec<u8>>) {}
# force_inference(&buff);

let reference = buff.get_mut();

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);

const 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);

const fn into_parts_mut(&mut self) -> (&mut u64, &mut T)

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

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

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()));

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

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

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()));

Trait Implementations

impl Write for Cursor<&mut [u8]>

fn write(&mut self, buf: &[u8]) -> Result<usize>
fn write_vectored(&mut self, bufs: &[IoSlice<'_>]) -> Result<usize>
fn is_write_vectored(&self) -> bool
fn write_all(&mut self, buf: &[u8]) -> Result<()>
fn write_all_vectored(&mut self, bufs: &mut [IoSlice<'_>]) -> Result<()>
fn flush(&mut self) -> Result<()>

impl<T> Clone for Cursor<T> where T: Clone,

fn clone(&self) -> Self
fn clone_from(&mut self, other: &Self)

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

fn seek(&mut self, style: SeekFrom) -> Result<u64>
fn stream_len(&mut self) -> Result<u64>
fn stream_position(&mut self) -> Result<u64>

impl<T: Debug> Debug for Cursor<T>

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

impl<T: Default> Default for Cursor<T>

fn default() -> Cursor<T>

impl<T: Eq> Eq for Cursor<T>

fn assert_fields_are_eq(&self)

impl<T: PartialEq> PartialEq for Cursor<T>

fn eq(&self, other: &Cursor<T>) -> bool

impl<T: PartialEq> StructuralPartialEq for Cursor<T>

impl<W: WriteThroughCursor> Write for Cursor<W>

fn write(&mut self, buf: &[u8]) -> Result<usize>
fn write_vectored(&mut self, bufs: &[IoSlice<'_>]) -> Result<usize>
fn is_write_vectored(&self) -> bool
fn write_all(&mut self, buf: &[u8]) -> Result<()>
fn write_all_vectored(&mut self, bufs: &mut [IoSlice<'_>]) -> Result<()>
fn flush(&mut self) -> Result<()>

impl<const N: usize> Write for Cursor<[u8; N]>

fn write(&mut self, buf: &[u8]) -> Result<usize>
fn write_vectored(&mut self, bufs: &[IoSlice<'_>]) -> Result<usize>
fn is_write_vectored(&self) -> bool
fn write_all(&mut self, buf: &[u8]) -> Result<()>
fn write_all_vectored(&mut self, bufs: &mut [IoSlice<'_>]) -> Result<()>
fn flush(&mut self) -> Result<()>

Auto Trait Implementations

impl<T> Freeze for Cursor<T> where T: Freeze,

impl<T> RefUnwindSafe for Cursor<T> where T: RefUnwindSafe,

impl<T> Send for Cursor<T> where T: Send,

impl<T> Sync for Cursor<T> where T: Sync,

impl<T> Unpin for Cursor<T> where T: Unpin,

impl<T> UnsafeUnpin for Cursor<T> where T: UnsafeUnpin,

impl<T> UnwindSafe for Cursor<T> where T: UnwindSafe,

Blanket Implementations

impl<T> Any for Cursor<T> where T: 'static + ?Sized,

fn type_id(&self) -> TypeId

impl<T> Borrow<T> for Cursor<T> where T: ?Sized,

fn borrow(&self) -> &T

impl<T> BorrowMut<T> for Cursor<T> where T: ?Sized,

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

impl<T> CloneToUninit for Cursor<T> where T: Clone,

unsafe fn clone_to_uninit(&self, dest: *mut u8)

impl<T> From<T> for Cursor<T>

fn from(t: T) -> T

Returns the argument unchanged.

impl<T> SizeHint for Cursor<T> where T: ?Sized,

fn lower_bound(&self) -> usize
fn upper_bound(&self) -> Option<usize>

impl<T> SizedTypeProperties for Cursor<T>

impl<T, U> Into<U> for Cursor<T> where U: From<T>,

fn into(self) -> U

Calls U::from(self).

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

impl<T, U> TryFrom<U> for Cursor<T> where U: Into<T>,

type Error = Infallible;
fn try_from(value: U) -> Result<T, <T as TryFrom<U>>::Error>

impl<T, U> TryInto<U> for Cursor<T> where U: TryFrom<T>,

type Error = <U as TryFrom<T>>::Error;
fn try_into(self) -> Result<U, <U as TryFrom<T>>::Error>