Struct Cursor

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

Available on crate feature std only.

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

impl<T> Cursor<T>

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());

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();

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();

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();

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);

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);

impl<T> Cursor<T>

where T: AsRef<[u8]>,

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

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

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()));

impl<T> Cursor<T>

where T: AsMut<[u8]>,

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

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

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()));

Trait Implementations

impl<T> BufRead for Cursor<T>

where T: AsRef<[u8]>,

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

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

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.

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

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

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

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

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

fn skip_until(&mut self, byte: u8) -> Result<usize, Error>

Skips all bytes until the delimiter byte or EOF is reached.

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

Reads all bytes until a newline (the 0xA byte) is reached, and append them to the provided String buffer.

fn split(self, byte: u8) -> Split<Self>

where Self: Sized,

Returns an iterator over the contents of this reader split on the byte byte.

fn lines(self) -> Lines<Self>

where Self: Sized,

Returns an iterator over the lines of this reader.

impl<T> Clone for Cursor<T>

where T: Clone,

fn clone(&self) -> Cursor<T>

Returns a copy of the value.

fn clone_from(&mut self, other: &Cursor<T>)

Performs copy-assignment from source.

impl<T> Debug for Cursor<T>

where T: Debug,

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

Formats the value using the given formatter.

impl<T> Default for Cursor<T>

where T: Default,

fn default() -> Cursor<T>

Returns the “default value” for a type.

impl<T> PartialEq for Cursor<T>

where T: PartialEq,

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

Tests for self and other values to be equal, and is used by ==.

fn ne(&self, other: &Rhs) -> bool

Tests for !=. The default implementation is almost always sufficient, and should not be overridden without very good reason.

impl<T> Read for Cursor<T>

where T: AsRef<[u8]>,

fn read(&mut self, buf: &mut [u8]) -> Result<usize, Error>

Pull some bytes from this source into the specified buffer, returning how many bytes were read.

fn read_buf(&mut self, cursor: BorrowedCursor<'_>) -> Result<(), Error>

🔬This is a nightly-only experimental API. (read_buf)

Pull some bytes from this source into the specified buffer.

fn read_vectored(&mut self, bufs: &mut [IoSliceMut<'_>]) -> Result<usize, Error>

Like read, except that it reads into a slice of buffers.

fn is_read_vectored(&self) -> bool

🔬This is a nightly-only experimental API. (can_vector)

Determines if this Reader has an efficient read_vectored implementation.

fn read_exact(&mut self, buf: &mut [u8]) -> Result<(), Error>

Reads the exact number of bytes required to fill buf.

fn read_buf_exact(&mut self, cursor: BorrowedCursor<'_>) -> Result<(), Error>

🔬This is a nightly-only experimental API. (read_buf)

Reads the exact number of bytes required to fill cursor.

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

Reads all bytes until EOF in this source, placing them into buf.

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

Reads all bytes until EOF in this source, appending them to buf.

fn by_ref(&mut self) -> &mut Self

where Self: Sized,

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

fn bytes(self) -> Bytes<Self>

where Self: Sized,

Transforms this Read instance to an Iterator over its bytes.

fn chain<R>(self, next: R) -> Chain<Self, R>

where R: Read, Self: Sized,

Creates an adapter which will chain this stream with another.

fn take(self, limit: u64) -> Take<Self>

where Self: Sized,

Creates an adapter which will read at most limit bytes from it.

impl<T> Seek for Cursor<T>

where T: AsRef<[u8]>,

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

Seek to an offset, in bytes, in a stream.

fn stream_len(&mut self) -> Result<u64, Error>

🔬This is a nightly-only experimental API. (seek_stream_len)

Returns the length of this stream (in bytes).

fn stream_position(&mut self) -> Result<u64, Error>

Returns the current seek position from the start of the stream.

fn rewind(&mut self) -> Result<(), Error>

Rewind to the beginning of a stream.

fn seek_relative(&mut self, offset: i64) -> Result<(), Error>

Seeks relative to the current position.

impl Write for Cursor<&mut [u8]>

fn write(&mut self, buf: &[u8]) -> Result<usize, Error>

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

fn write_vectored(&mut self, bufs: &[IoSlice<'_>]) -> Result<usize, Error>

Like write, except that it writes from a slice of buffers.

fn is_write_vectored(&self) -> bool

🔬This is a nightly-only experimental API. (can_vector)

Determines if this Writer has an efficient write_vectored implementation.

fn flush(&mut self) -> Result<(), Error>

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

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

Attempts to write an entire buffer into this writer.

fn write_all_vectored(&mut self, bufs: &mut [IoSlice<'_>]) -> Result<(), Error>

🔬This is a nightly-only experimental API. (write_all_vectored)

Attempts to write multiple buffers into this writer.

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

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

fn by_ref(&mut self) -> &mut Self

where Self: Sized,

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

impl<A> Write for Cursor<&mut Vec<u8, A>>

where A: Allocator,

fn write(&mut self, buf: &[u8]) -> Result<usize, Error>

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

fn write_vectored(&mut self, bufs: &[IoSlice<'_>]) -> Result<usize, Error>

Like write, except that it writes from a slice of buffers.

fn is_write_vectored(&self) -> bool

🔬This is a nightly-only experimental API. (can_vector)

Determines if this Writer has an efficient write_vectored implementation.

fn flush(&mut self) -> Result<(), Error>

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

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

Attempts to write an entire buffer into this writer.

fn write_all_vectored(&mut self, bufs: &mut [IoSlice<'_>]) -> Result<(), Error>

🔬This is a nightly-only experimental API. (write_all_vectored)

Attempts to write multiple buffers into this writer.

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

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

fn by_ref(&mut self) -> &mut Self

where Self: Sized,

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

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

fn write(&mut self, buf: &[u8]) -> Result<usize, Error>

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

fn write_vectored(&mut self, bufs: &[IoSlice<'_>]) -> Result<usize, Error>

Like write, except that it writes from a slice of buffers.

fn is_write_vectored(&self) -> bool

🔬This is a nightly-only experimental API. (can_vector)

Determines if this Writer has an efficient write_vectored implementation.

fn flush(&mut self) -> Result<(), Error>

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

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

Attempts to write an entire buffer into this writer.

fn write_all_vectored(&mut self, bufs: &mut [IoSlice<'_>]) -> Result<(), Error>

🔬This is a nightly-only experimental API. (write_all_vectored)

Attempts to write multiple buffers into this writer.

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

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

fn by_ref(&mut self) -> &mut Self

where Self: Sized,

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

impl<A> Write for Cursor<Box<[u8], A>>

where A: Allocator,

fn write(&mut self, buf: &[u8]) -> Result<usize, Error>

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

fn write_vectored(&mut self, bufs: &[IoSlice<'_>]) -> Result<usize, Error>

Like write, except that it writes from a slice of buffers.

fn is_write_vectored(&self) -> bool

🔬This is a nightly-only experimental API. (can_vector)

Determines if this Writer has an efficient write_vectored implementation.

fn flush(&mut self) -> Result<(), Error>

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

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

Attempts to write an entire buffer into this writer.

fn write_all_vectored(&mut self, bufs: &mut [IoSlice<'_>]) -> Result<(), Error>

🔬This is a nightly-only experimental API. (write_all_vectored)

Attempts to write multiple buffers into this writer.

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

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

fn by_ref(&mut self) -> &mut Self

where Self: Sized,

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

impl<A> Write for Cursor<Vec<u8, A>>

where A: Allocator,

fn write(&mut self, buf: &[u8]) -> Result<usize, Error>

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

fn write_vectored(&mut self, bufs: &[IoSlice<'_>]) -> Result<usize, Error>

Like write, except that it writes from a slice of buffers.

fn is_write_vectored(&self) -> bool

🔬This is a nightly-only experimental API. (can_vector)

Determines if this Writer has an efficient write_vectored implementation.

fn flush(&mut self) -> Result<(), Error>

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

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

Attempts to write an entire buffer into this writer.

fn write_all_vectored(&mut self, bufs: &mut [IoSlice<'_>]) -> Result<(), Error>

🔬This is a nightly-only experimental API. (write_all_vectored)

Attempts to write multiple buffers into this writer.

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

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

fn by_ref(&mut self) -> &mut Self

where Self: Sized,

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

impl<T> Eq for Cursor<T>

where T: Eq,

impl<T> StructuralPartialEq for Cursor<T>

Layout

Note: Unable to compute type layout, possibly due to this type having generic parameters. Layout can only be computed for concrete, fully-instantiated types.