Struct core2::io::Cursor

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

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 core2::io::{self, Seek, SeekFrom, Write};
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").map_err(|e| io::Error::from(e))?;

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 core2::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 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 core2::io::Cursor;

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

pub fn into_inner(self) -> T

Consumes this cursor, returning the underlying value.

Examples

use core2::io::Cursor;

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

let vec = buff.into_inner();

pub fn get_ref(&self) -> &T

Gets a reference to the underlying value in this cursor.

Examples

use core2::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 core2::io::Cursor;

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

let reference = buff.get_mut();

pub fn position(&self) -> u64

Returns the current position of this cursor.

Examples

use core2::io::{Cursor, Seek, SeekFrom};
use std::io::prelude::*;

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 core2::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);

Trait Implementations

impl<T> BufRead for Cursor<T>

where T: AsRef<[u8]>,

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.

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.

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

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

Returns a copy of the value.

fn clone_from(&mut self, source: &Self)

Performs copy-assignment from source.

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

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

Formats the value using the given formatter.

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

fn default() -> Cursor<T>

Returns the “default value” for a type.

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

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

This method tests for self and other values to be equal, and is used by ==.

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

This method 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>

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

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

Read the exact number of bytes required to fill buf.

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

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

unsafe fn initializer(&self) -> Initializer

Determines if this Reader can work with buffers of uninitialized memory.

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

where Self: Sized,

Creates an adaptor which will chain this stream with another.

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

where Self: Sized,

Creates an adaptor 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>

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

impl Write for Cursor<&mut [u8]>

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

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

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

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

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

Attempts to write an entire buffer into this writer.

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

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” adaptor for this instance of Write.

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

impl<T> StructuralPartialEq for Cursor<T>