Cursor in alloc::io - Rust
Description: A `Cursor` wraps an in-memory buffer and provides it with a `Seek` implementation.

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## Cursor

# Struct Cursor

Source

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

🔬This is a nightly-only experimental API. (`alloc_io` #154046)
Expand description

A `Cursor` wraps an in-memory buffer and provides it with a `Seek` implementation.

`Cursor`s 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§

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### impl<T> Cursor<T>

1.0.0 (const: 1.79.0) · Source

#### 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());

1.0.0 · Source

#### 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();

1.0.0 (const: 1.79.0) · Source

#### 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();

1.0.0 (const: 1.86.0) · Source

#### pub const 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();

1.0.0 (const: 1.79.0) · Source

#### 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);

1.0.0 (const: 1.86.0) · Source

#### pub const 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);

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

Source

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

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

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()));

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### impl<T> Cursor<T>
where T: AsMut<[u8]>,

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

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

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§

1.0.0 · Source§

### impl<T> Clone for Cursor<T>
where T: Clone,

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

Returns a duplicate of the value. Read more
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#### fn clone_from(&mut self, other: &Cursor<T>)

Performs copy-assignment from `source`. Read more
1.0.0 · Source§

### impl<T> Debug for Cursor<T>
where T: Debug,

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

Formats the value using the given formatter. Read more
1.0.0 · Source§

### impl<T> Default for Cursor<T>
where T: Default,

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#### fn default() -> Cursor<T>

Returns the “default value” for a type. Read more
1.0.0 · Source§

### impl<T> Eq for Cursor<T>
where T: Eq,

1.0.0 · Source§

### impl<T> PartialEq for Cursor<T>
where T: PartialEq,

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

Tests for `self` and `other` values to be equal, and is used by `==`.
1.0.0 (const: unstable) · Source§

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

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

### impl<T> StructuralPartialEq for Cursor<T>
where T: PartialEq,

## Auto Trait Implementations§

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### impl<T> Freeze for Cursor<T>
where T: Freeze,

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### impl<T> RefUnwindSafe for Cursor<T>
where T: RefUnwindSafe,

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### impl<T> Send for Cursor<T>
where T: Send,

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### impl<T> Sync for Cursor<T>
where T: Sync,

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### impl<T> Unpin for Cursor<T>
where T: Unpin,

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### impl<T> UnsafeUnpin for Cursor<T>
where T: UnsafeUnpin,

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### impl<T> UnwindSafe for Cursor<T>
where T: UnwindSafe,

## Blanket Implementations§

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### impl<T> Any for T
where T: 'static + ?Sized,

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#### fn type_id(&self) -> TypeId

Gets the `TypeId` of `self`. Read more
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### impl<T> Borrow<T> for T
where T: ?Sized,

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#### fn borrow(&self) -> &T

Immutably borrows from an owned value. Read more
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### impl<T> BorrowMut<T> for T
where T: ?Sized,

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#### fn borrow_mut(&mut self) -> &mut T

Mutably borrows from an owned value. Read more
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### impl<T> CloneToUninit for T
where T: Clone,

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#### unsafe fn clone_to_uninit(&self, dest: *mut u8)

🔬This is a nightly-only experimental API. (`clone_to_uninit` #126799)
Performs copy-assignment from `self` to `dest`. Read more
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### impl<T> From<T> for T

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#### fn from(t: T) -> T

Returns the argument unchanged.

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### impl<T, U> Into<U> for T
where U: From<T>,

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#### fn into(self) -> U

Calls `U::from(self)`.

That is, this conversion is whatever the implementation of `From<T> for U` chooses to do.

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### impl<T> ToOwned for T
where T: Clone,

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#### type Owned = T

The resulting type after obtaining ownership.
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#### fn to_owned(&self) -> T

Creates owned data from borrowed data, usually by cloning. Read more
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#### fn clone_into(&self, target: &mut T)

Uses borrowed data to replace owned data, usually by cloning. Read more
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### impl<T, U> TryFrom<U> for T
where U: Into<T>,

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#### type Error = Infallible

The type returned in the event of a conversion error.
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#### fn try_from(value: U) -> Result<T, <T as TryFrom<U>>::Error>

Performs the conversion.
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### impl<T, U> TryInto<U> for T
where U: TryFrom<T>,

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#### type Error = <U as TryFrom<T>>::Error

The type returned in the event of a conversion error.
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#### fn try_into(self) -> Result<U, <U as TryFrom<T>>::Error>

Performs the conversion.