# Struct std::io::Cursor

1.0.0 · source ·

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

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")?;

    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]);
    }

Run

## Implementations§

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### impl<T> Cursor<T>

const: unstable · source

#### 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 std::io::Cursor;

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

Run
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#### 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();

Run
const: unstable · source

#### pub 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();

Run
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#### 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();

Run
const: unstable · source

#### pub 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);

Run
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#### 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);

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

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#### pub fn remaining_slice(&self) -> &[u8] ⓘ

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

Returns the remaining slice.

##### Examples

    #![feature(cursor_remaining)]
    use std::io::Cursor;

    let mut buff = Cursor::new(vec![1, 2, 3, 4, 5]);

    assert_eq!(buff.remaining_slice(), &[1, 2, 3, 4, 5]);

    buff.set_position(2);
    assert_eq!(buff.remaining_slice(), &[3, 4, 5]);

    buff.set_position(4);
    assert_eq!(buff.remaining_slice(), &[5]);

    buff.set_position(6);
    assert_eq!(buff.remaining_slice(), &[]);

Run
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#### pub fn is_empty(&self) -> bool

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

Returns `true` if the remaining slice is empty.

##### Examples

    #![feature(cursor_remaining)]
    use std::io::Cursor;

    let mut buff = Cursor::new(vec![1, 2, 3, 4, 5]);

    buff.set_position(2);
    assert!(!buff.is_empty());

    buff.set_position(5);
    assert!(buff.is_empty());

    buff.set_position(10);
    assert!(buff.is_empty());

Run

## Trait Implementations§

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

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#### 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. Read more
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#### 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`. Read more
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#### fn has_data_left(&mut self) -> Result<bool>

🔬This is a nightly-only experimental API. (`buf_read_has_data_left` #86423)
Check if the underlying `Read` has any data left to be read. Read more
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#### fn read_until(&mut self, byte: u8, buf: &mut Vec<u8>) -> Result<usize>

Read all bytes into `buf` until the delimiter `byte` or EOF is reached. Read more
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#### 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 `String` buffer. Read more
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#### fn split(self, byte: u8) -> Split<Self> ⓘwhere Self: Sized,

Returns an iterator over the contents of this reader split on the byte `byte`. Read more
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#### fn lines(self) -> Lines<Self> ⓘwhere Self: Sized,

Returns an iterator over the lines of this reader. Read more
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### impl<T> Clone for Cursor<T>where T: Clone,

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#### fn clone(&self) -> Self

Returns a copy of the value. Read more
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#### fn clone_from(&mut self, other: &Self)

Performs copy-assignment from `source`. Read more
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### impl<T: Debug> Debug for Cursor<T>

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

Formats the value using the given formatter. Read more
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### impl<T: Default> Default for Cursor<T>

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#### fn default() -> Cursor<T> ⓘ

Returns the “default value” for a type. Read more
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### impl<T: PartialEq> PartialEq for Cursor<T>

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

This method tests for `self` and `other` values to be equal, and is used by `==`.
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#### 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.
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### impl<T> Read for Cursor<T>where T: AsRef<[u8]>,

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#### 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. Read more
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#### fn read_buf(&mut self, cursor: BorrowedCursor<'_>) -> Result<()>

🔬This is a nightly-only experimental API. (`read_buf` #78485)
Pull some bytes from this source into the specified buffer. Read more
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#### fn read_vectored(&mut self, bufs: &mut [IoSliceMut<'_>]) -> Result<usize>

Like `read`, except that it reads into a slice of buffers. Read more
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#### fn is_read_vectored(&self) -> bool

🔬This is a nightly-only experimental API. (`can_vector` #69941)
Determines if this `Read`er has an efficient `read_vectored` implementation. Read more
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#### fn read_exact(&mut self, buf: &mut [u8]) -> Result<()>

Read the exact number of bytes required to fill `buf`. Read more
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#### fn read_to_end(&mut self, buf: &mut Vec<u8>) -> Result<usize>

Read all bytes until EOF in this source, placing them into `buf`. Read more
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#### fn read_to_string(&mut self, buf: &mut String) -> Result<usize>

Read all bytes until EOF in this source, appending them to `buf`. Read more
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#### fn read_buf_exact(&mut self, cursor: BorrowedCursor<'_>) -> Result<()>

🔬This is a nightly-only experimental API. (`read_buf` #78485)
Read the exact number of bytes required to fill `cursor`. Read more
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#### fn by_ref(&mut self) -> &mut Self where Self: Sized,

Creates a “by reference” adaptor for this instance of `Read`. Read more
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#### fn bytes(self) -> Bytes<Self> ⓘwhere Self: Sized,

Transforms this `Read` instance to an `Iterator` over its bytes. Read more
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#### fn chain<R: Read>(self, next: R) -> Chain<Self, R> ⓘwhere Self: Sized,

Creates an adapter which will chain this stream with another. Read more
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#### fn take(self, limit: u64) -> Take<Self> ⓘwhere Self: Sized,

Creates an adapter which will read at most `limit` bytes from it. Read more
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### impl<T> Seek for Cursor<T>where T: AsRef<[u8]>,

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

Seek to an offset, in bytes, in a stream. Read more
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#### fn stream_len(&mut self) -> Result<u64>

🔬This is a nightly-only experimental API. (`seek_stream_len` #59359)
Returns the length of this stream (in bytes). Read more
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#### fn stream_position(&mut self) -> Result<u64>

Returns the current seek position from the start of the stream. Read more
1.55.0 · source§

#### fn rewind(&mut self) -> Result<()>

Rewind to the beginning of a stream. Read more
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### impl Write for Cursor<&mut [u8]>

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

Write a buffer into this writer, returning how many bytes were written. Read more
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#### fn write_vectored(&mut self, bufs: &[IoSlice<'_>]) -> Result<usize>

Like `write`, except that it writes from a slice of buffers. Read more
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#### fn is_write_vectored(&self) -> bool

🔬This is a nightly-only experimental API. (`can_vector` #69941)
Determines if this `Write`r has an efficient `write_vectored` implementation. Read more
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#### fn flush(&mut self) -> Result<()>

Flush this output stream, ensuring that all intermediately buffered contents reach their destination. Read more
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#### fn write_all(&mut self, buf: &[u8]) -> Result<()>

Attempts to write an entire buffer into this writer. Read more
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#### fn write_all_vectored(&mut self, bufs: &mut [IoSlice<'_>]) -> Result<()>

🔬This is a nightly-only experimental API. (`write_all_vectored` #70436)
Attempts to write multiple buffers into this writer. Read more
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#### fn write_fmt(&mut self, fmt: Arguments<'_>) -> Result<()>

Writes a formatted string into this writer, returning any error encountered. Read more
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#### fn by_ref(&mut self) -> &mut Self where Self: Sized,

Creates a “by reference” adapter for this instance of `Write`. Read more
1.25.0 · source§

### impl<A> Write for Cursor<&mut Vec<u8, A>>where A: Allocator,

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

Write a buffer into this writer, returning how many bytes were written. Read more
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#### fn write_vectored(&mut self, bufs: &[IoSlice<'_>]) -> Result<usize>

Like `write`, except that it writes from a slice of buffers. Read more
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#### fn is_write_vectored(&self) -> bool

🔬This is a nightly-only experimental API. (`can_vector` #69941)
Determines if this `Write`r has an efficient `write_vectored` implementation. Read more
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#### fn flush(&mut self) -> Result<()>

Flush this output stream, ensuring that all intermediately buffered contents reach their destination. Read more
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#### fn write_all(&mut self, buf: &[u8]) -> Result<()>

Attempts to write an entire buffer into this writer. Read more
source§

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

🔬This is a nightly-only experimental API. (`write_all_vectored` #70436)
Attempts to write multiple buffers into this writer. Read more
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#### fn write_fmt(&mut self, fmt: Arguments<'_>) -> Result<()>

Writes a formatted string into this writer, returning any error encountered. Read more
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#### fn by_ref(&mut self) -> &mut Self where Self: Sized,

Creates a “by reference” adapter for this instance of `Write`. Read more
1.61.0 · source§

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

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

Write a buffer into this writer, returning how many bytes were written. Read more
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#### fn write_vectored(&mut self, bufs: &[IoSlice<'_>]) -> Result<usize>

Like `write`, except that it writes from a slice of buffers. Read more
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#### fn is_write_vectored(&self) -> bool

🔬This is a nightly-only experimental API. (`can_vector` #69941)
Determines if this `Write`r has an efficient `write_vectored` implementation. Read more
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#### fn flush(&mut self) -> Result<()>

Flush this output stream, ensuring that all intermediately buffered contents reach their destination. Read more
source§

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

Attempts to write an entire buffer into this writer. Read more
source§

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

🔬This is a nightly-only experimental API. (`write_all_vectored` #70436)
Attempts to write multiple buffers into this writer. Read more
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#### fn write_fmt(&mut self, fmt: Arguments<'_>) -> Result<()>

Writes a formatted string into this writer, returning any error encountered. Read more
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#### fn by_ref(&mut self) -> &mut Self where Self: Sized,

Creates a “by reference” adapter for this instance of `Write`. Read more
1.5.0 · source§

### impl<A> Write for Cursor<Box<[u8], A>>where A: Allocator,

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

Write a buffer into this writer, returning how many bytes were written. Read more
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#### fn write_vectored(&mut self, bufs: &[IoSlice<'_>]) -> Result<usize>

Like `write`, except that it writes from a slice of buffers. Read more
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#### fn is_write_vectored(&self) -> bool

🔬This is a nightly-only experimental API. (`can_vector` #69941)
Determines if this `Write`r has an efficient `write_vectored` implementation. Read more
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#### fn flush(&mut self) -> Result<()>

Flush this output stream, ensuring that all intermediately buffered contents reach their destination. Read more
source§

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

Attempts to write an entire buffer into this writer. Read more
source§

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

🔬This is a nightly-only experimental API. (`write_all_vectored` #70436)
Attempts to write multiple buffers into this writer. Read more
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#### fn write_fmt(&mut self, fmt: Arguments<'_>) -> Result<()>

Writes a formatted string into this writer, returning any error encountered. Read more
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#### fn by_ref(&mut self) -> &mut Self where Self: Sized,

Creates a “by reference” adapter for this instance of `Write`. Read more
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### impl<A> Write for Cursor<Vec<u8, A>>where A: Allocator,

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

Write a buffer into this writer, returning how many bytes were written. Read more
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#### fn write_vectored(&mut self, bufs: &[IoSlice<'_>]) -> Result<usize>

Like `write`, except that it writes from a slice of buffers. Read more
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#### fn is_write_vectored(&self) -> bool

🔬This is a nightly-only experimental API. (`can_vector` #69941)
Determines if this `Write`r has an efficient `write_vectored` implementation. Read more
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#### fn flush(&mut self) -> Result<()>

Flush this output stream, ensuring that all intermediately buffered contents reach their destination. Read more
source§

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

Attempts to write an entire buffer into this writer. Read more
source§

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

🔬This is a nightly-only experimental API. (`write_all_vectored` #70436)
Attempts to write multiple buffers into this writer. Read more
source§

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

Writes a formatted string into this writer, returning any error encountered. Read more
source§

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

Creates a “by reference” adapter for this instance of `Write`. Read more
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### impl<T: Eq> Eq for Cursor<T>

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### impl<T> StructuralEq for Cursor<T>

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### impl<T> StructuralPartialEq for Cursor<T>

## Auto Trait Implementations§

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

## Blanket Implementations§

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### impl<T> Any for Twhere 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 Twhere 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 Twhere 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> 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 Twhere 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 Twhere 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 Twhere 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 Twhere 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.