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Struct std::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 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]);
}

Implementations

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impl<T> Cursor<T>
const: unstablesource 
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());
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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();
const: unstablesource 
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();
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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();
const: unstablesource 
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);
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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);
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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(), &[]);
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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());

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 Reader 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]>,
source 
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
1.55.0source 
fn rewind(&mut self) -> Result<()>
Rewind to the beginning of a stream. Read more
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impl Write for Cursor<&mut [u8]>
source 
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
source 
fn is_write_vectored(&self) -> bool
🔬This is a nightly-only experimental API. (can_vector #69941)
Determines if this Writer 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
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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
source 
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
1.25.0source 
impl<A> Write for Cursor<&mut Vec<u8, A>>
where
    A: Allocator,
source 
fn write(&mut self, buf: &[u8]) -> Result<usize>
Write a buffer into this writer, returning how many bytes were written. Read more
source 
fn write_vectored(&mut self, bufs: &[IoSlice<'_>]) -> Result<usize>
Like write, except that it writes from a slice of buffers. Read more
source 
fn is_write_vectored(&self) -> bool
🔬This is a nightly-only experimental API. (can_vector #69941)
Determines if this Writer has an efficient write_vectored implementation. Read more
source 
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
1.61.0source 
impl<const N: usize> Write for Cursor<[u8; N]>
source 
fn write(&mut self, buf: &[u8]) -> Result<usize>
Write a buffer into this writer, returning how many bytes were written. Read more
source 
fn write_vectored(&mut self, bufs: &[IoSlice<'_>]) -> Result<usize>
Like write, except that it writes from a slice of buffers. Read more
source 
fn is_write_vectored(&self) -> bool
🔬This is a nightly-only experimental API. (can_vector #69941)
Determines if this Writer has an efficient write_vectored implementation. Read more
source 
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
1.5.0source 
impl<A> Write for Cursor<Box<[u8], A>>
where
    A: Allocator,
source 
fn write(&mut self, buf: &[u8]) -> Result<usize>
Write a buffer into this writer, returning how many bytes were written. Read more
source 
fn write_vectored(&mut self, bufs: &[IoSlice<'_>]) -> Result<usize>
Like write, except that it writes from a slice of buffers. Read more
source 
fn is_write_vectored(&self) -> bool
🔬This is a nightly-only experimental API. (can_vector #69941)
Determines if this Writer has an efficient write_vectored implementation. Read more
source 
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
source 
impl<A> Write for Cursor<Vec<u8, A>>
where
    A: Allocator,
source 
fn write(&mut self, buf: &[u8]) -> Result<usize>
Write a buffer into this writer, returning how many bytes were written. Read more
source 
fn write_vectored(&mut self, bufs: &[IoSlice<'_>]) -> Result<usize>
Like write, except that it writes from a slice of buffers. Read more
source 
fn is_write_vectored(&self) -> bool
🔬This is a nightly-only experimental API. (can_vector #69941)
Determines if this Writer has an efficient write_vectored implementation. Read more
source 
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
source 
impl<T: Eq> Eq for Cursor<T>
source 
impl<T> StructuralEq for Cursor<T>
source 
impl<T> StructuralPartialEq for Cursor<T>

Auto Trait Implementations

impl<T> RefUnwindSafe for Cursor<T>
where
    T: RefUnwindSafe,
impl<T> Send for Cursor<T>
where
    T: Send,
impl<T> Sync for Cursor<T>
where
    T: Sync,
impl<T> Unpin for Cursor<T>
where
    T: Unpin,
impl<T> UnwindSafe for Cursor<T>
where
    T: UnwindSafe,

Blanket Implementations

source 
impl<T> Any for T
where
    T: 'static + ?Sized,
source 
fn type_id(&self) -> TypeId
Gets the TypeId of self. Read more
source 
impl<T> Borrow<T> for T
where
    T: ?Sized,
source 
fn borrow(&self) -> &T
Immutably borrows from an owned value. Read more
source 
impl<T> BorrowMut<T> for T
where
    T: ?Sized,
source 
fn borrow_mut(&mut self) -> &mut T
Mutably borrows from an owned value. Read more
source 
impl<T> From<T> for T
source 
fn from(t: T) -> T

Returns the argument unchanged.

source 
impl<T, U> Into<U> for T
where
    U: From<T>,
source 
fn into(self) -> U

Calls U::from(self).

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

source 
impl<T> ToOwned for T
where
    T: Clone,
 
type Owned = T
The resulting type after obtaining ownership.
source 
fn to_owned(&self) -> T
Creates owned data from borrowed data, usually by cloning. Read more
source 
fn clone_into(&self, target: &mut T)
Uses borrowed data to replace owned data, usually by cloning. Read more
source 
impl<T, U> TryFrom<U> for T
where
    U: Into<T>,
 
type Error = Infallible
The type returned in the event of a conversion error.
source 
fn try_from(value: U) -> Result<T, <T as TryFrom<U>>::Error>
Performs the conversion.
source 
impl<T, U> TryInto<U> for T
where
    U: TryFrom<T>,
 
type Error = <U as TryFrom<T>>::Error
The type returned in the event of a conversion error.
source 
fn try_into(self) -> Result<U, <U as TryFrom<T>>::Error>
Performs the conversion.

© 2010 The Rust Project Developers
Licensed under the Apache License, Version 2.0 or the MIT license, at your option.
https://doc.rust-lang.org/std/io/struct.Cursor.html