# Struct std::io::Cursor [−] [src]

    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>`

### `impl<T> Read for Cursor<T> where T: AsRef<[u8]>`

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

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

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

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

#### `fn by_ref(&mut self) -> &mut Self where Self: Sized`

#### `fn bytes(self) -> Bytes<Self> where Self: Sized`

#### `fn chars(self) -> Chars<Self> where Self: Sized`

#### `fn chain<R: Read>(self, next: R) -> Chain<Self, R> where Self: Sized`

#### `fn take(self, limit: u64) -> Take<Self> where Self: Sized`

#### `fn tee<W: Write>(self, out: W) -> Tee<Self, W> where Self: Sized`

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

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

#### `fn consume(&mut self, amt: usize)`

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

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

#### `fn split(self, byte: u8) -> Split<Self> where Self: Sized`

#### `fn lines(self) -> Lines<Self> where Self: Sized`

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

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

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

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

#### `fn write_fmt(&mut self, fmt: Arguments) -> Result<()>`

#### `fn by_ref(&mut self) -> &mut Self where Self: Sized`

#### `fn broadcast<W: Write>(self, other: W) -> Broadcast<Self, W> where Self: Sized`

### `impl Write for Cursor<Vec<u8>>`

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

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

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

#### `fn write_fmt(&mut self, fmt: Arguments) -> Result<()>`

#### `fn by_ref(&mut self) -> &mut Self where Self: Sized`

#### `fn broadcast<W: Write>(self, other: W) -> Broadcast<Self, W> where Self: Sized`

### `impl Write for Cursor<Box<[u8]>>`

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

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

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

#### `fn write_fmt(&mut self, fmt: Arguments) -> Result<()>`

#### `fn by_ref(&mut self) -> &mut Self where Self: Sized`

#### `fn broadcast<W: Write>(self, other: W) -> Broadcast<Self, W> where Self: Sized`

### Derived Implementations

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

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

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

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

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

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