Struct std::io::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>(writer: &mut 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")?;

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]);
}

Run

Implementations

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impl<T> Cursor<T>

const: unstable · source

pub 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());

Run

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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();

Run

const: unstable · source

pub 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();

Run

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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();

Run

const: unstable · source

pub 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);

Run

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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);

Run

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

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pub fn remaining_slice(&self) -> &[u8]ⓘ

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

Returns the remaining slice.

Examples

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

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

assert_eq!(buff.remaining_slice(), &[1, 2, 3, 4, 5]);

buff.set_position(2);
assert_eq!(buff.remaining_slice(), &[3, 4, 5]);

buff.set_position(4);
assert_eq!(buff.remaining_slice(), &[5]);

buff.set_position(6);
assert_eq!(buff.remaining_slice(), &[]);

Run

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pub fn is_empty(&self) -> bool

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

Returns true if the remaining slice is empty.

Examples

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

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

buff.set_position(2);
assert!(!buff.is_empty());

buff.set_position(5);
assert!(buff.is_empty());

buff.set_position(10);
assert!(buff.is_empty());

Run

Trait Implementations

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

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fn fill_buf(&mut self) -> Result<&[u8]>

Returns the contents of the internal buffer, filling it with more data from the inner reader if it is empty. Read more

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fn consume(&mut self, amt: usize)

Tells this buffer that amt bytes have been consumed from the buffer, so they should no longer be returned in calls to read. Read more

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fn has_data_left(&mut self) -> Result<bool>

🔬This is a nightly-only experimental API. (buf_read_has_data_left #86423)

Check if the underlying Read has any data left to be read. Read more

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fn read_until(&mut self, byte: u8, buf: &mut Vec<u8>) -> Result<usize>

Read all bytes into buf until the delimiter byte or EOF is reached. Read more

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fn read_line(&mut self, buf: &mut String) -> Result<usize>

Read all bytes until a newline (the 0xA byte) is reached, and append them to the provided buffer. You do not need to clear the buffer before appending. Read more

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