Struct std::io::Cursor

pub struct Cursor<T> {
    // some fields omitted
}

A Cursor wraps another type and provides it with a Seek implementation.

Cursors are typically used with in-memory buffers 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:

fn main() { 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<()> { try!(writer.seek(SeekFrom::End(-10))); for i in 0..10 { try!(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 = try!(File::create("foo.txt")); try!(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 more slow 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]); } }

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<()> {
    try!(writer.seek(SeekFrom::End(-10)));

    for i in 0..10 {
        try!(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 = try!(File::create("foo.txt"));

try!(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 more slow 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]);
}

Methods

impl<T> Cursor<T>

fn new(inner: T) -> Cursor<T>

Creates a new cursor wrapping the provided underlying I/O object.

Examples

fn main() { use std::io::Cursor; let buff = Cursor::new(Vec::new()); fn force_inference(_: &Cursor<Vec<u8>>) {} force_inference(&buff); }

use std::io::Cursor;

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

fn into_inner(self) -> T

Consumes this cursor, returning the underlying value.

Examples

fn main() { use std::io::Cursor; let buff = Cursor::new(Vec::new()); fn force_inference(_: &Cursor<Vec<u8>>) {} force_inference(&buff); let vec = buff.into_inner(); }

use std::io::Cursor;

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

let vec = buff.into_inner();

fn get_ref(&self) -> &T

Gets a reference to the underlying value in this cursor.

Examples

fn main() { use std::io::Cursor; let buff = Cursor::new(Vec::new()); fn force_inference(_: &Cursor<Vec<u8>>) {} force_inference(&buff); let reference = buff.get_ref(); }

use std::io::Cursor;

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

let reference = buff.get_ref();

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

fn main() { 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(); }

use std::io::Cursor;

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

let reference = buff.get_mut();

fn position(&self) -> u64

Returns the current position of this cursor.

Examples

fn main() { 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); }

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);

fn set_position(&mut self, pos: u64)

Sets the position of this cursor.

Examples

fn main() { 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); }

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);

Trait Implementations

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

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

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

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

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

Read the exact number of bytes required to fill 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 chars(self) -> Chars<Self> where Self: Sized

Transforms this Read instance to an Iterator over chars.

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

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

Fills the internal buffer of this object, returning the buffer contents.

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

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

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.

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<'a> Write for Cursor<&'a mut [u8]>

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

Write a buffer into this object, 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 write.

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 Write for Cursor<Vec<u8>>

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

Write a buffer into this object, 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 write.

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 Write for Cursor<Box<[u8]>>

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

Write a buffer into this object, 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 write.

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.

Derived Implementations

impl<T: Debug> Debug for Cursor<T>

fn fmt(&self, __arg_0: &mut Formatter) -> Result

Formats the value using the given formatter.

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.