Struct std::io::Cursor

[+] Show type declaration

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

[−] Expand description

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:

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<()> {
    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]);
}

Methods

impl<T> Cursor<T>

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Important traits for Cursor<T>

impl<T> Read for Cursor<T> where
    T: AsRef<[u8]>, impl<'a> Write for Cursor<&'a mut [u8]>impl<'a> Write for Cursor<&'a mut Vec<u8>>impl Write for Cursor<Vec<u8>>impl Write for Cursor<Box<[u8]>>

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

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

Cursor initial position is 0 even if underlying object (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());

pub fn into_inner(self) -> T

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Consumes this cursor, returning the underlying value.

Examples

use std::io::Cursor;

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

let vec = buff.into_inner();

Important traits for &'a mut I

impl<'a, I> Iterator for &'a mut I where
    I: Iterator + ?Sized,  type Item = <I as Iterator>::Item;impl<'a, R: Read + ?Sized> Read for &'a mut Rimpl<'a, W: Write + ?Sized> Write for &'a mut W

pub fn get_ref(&self) -> &T

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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();

Important traits for &'a mut I

impl<'a, I> Iterator for &'a mut I where
    I: Iterator + ?Sized,  type Item = <I as Iterator>::Item;impl<'a, R: Read + ?Sized> Read for &'a mut Rimpl<'a, W: Write + ?Sized> Write for &'a mut W

pub fn get_mut(&mut self) -> &mut T

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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();

pub fn position(&self) -> u64

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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);

pub fn set_position(&mut self, pos: u64)

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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);

Trait Implementations

impl<T: Clone> Clone for Cursor<T>

[+]

Important traits for Cursor<T>

impl<T> Read for Cursor<T> where
    T: AsRef<[u8]>, impl<'a> Write for Cursor<&'a mut [u8]>impl<'a> Write for Cursor<&'a mut Vec<u8>>impl Write for Cursor<Vec<u8>>impl Write for Cursor<Box<[u8]>>

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

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Returns a copy of the value.

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

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Performs copy-assignment from source.

impl<T: Debug> Debug for Cursor<T>

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

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Formats the value using the given formatter.

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

[+]

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

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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>

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Pull some bytes from this source into the specified buffer, returning how many bytes were read.

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

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Read the exact number of bytes required to fill buf.

unsafe fn initializer(&self) -> Initializer

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🔬 This is a nightly-only experimental API. (read_initializer #42788)

Determines if this Reader can work with buffers of uninitialized memory.

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

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

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

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Read all bytes until EOF in this source, appending them to buf.

Important traits for &'a mut I

impl<'a, I> Iterator for &'a mut I where
    I: Iterator + ?Sized,  type Item = <I as Iterator>::Item;impl<'a, R: Read + ?Sized> Read for &'a mut Rimpl<'a, W: Write + ?Sized> Write for &'a mut W

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

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Creates a "by reference" adaptor for this instance of Read.

Important traits for Bytes<R>

impl<R: Read> Iterator for Bytes<R> type Item = Result<u8>;

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

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Transforms this Read instance to an [Iterator] over its bytes.

Important traits for Chars<R>

impl<R: Read> Iterator for Chars<R> type Item = Result<char, CharsError>;

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

[−]

🔬 This is a nightly-only experimental API. (io #27802)

the semantics of a partial read/write of where errors happen is currently unclear and may change

Transforms this Read instance to an [Iterator] over [char]s.

Important traits for Chain<T, U>

impl<T: Read, U: Read> Read for Chain<T, U>

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

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Creates an adaptor which will chain this stream with another.

Important traits for Take<T>

impl<T: Read> Read for Take<T>

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

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Creates an adaptor which will read at most limit bytes from it.

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

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

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

fn consume(&mut self, amt: usize)

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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>

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

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

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Read all bytes until a newline (the 0xA byte) is reached, and append them to the provided buffer.

Important traits for Split<B>

impl<B: BufRead> Iterator for Split<B> type Item = Result<Vec<u8>>;

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

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Returns an iterator over the contents of this reader split on the byte byte.

Important traits for Lines<B>

impl<B: BufRead> Iterator for Lines<B> type Item = Result<String>;

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

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Returns an iterator over the lines of this reader.

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

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

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Write a buffer into this object, returning how many bytes were written.

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

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Flush this output stream, ensuring that all intermediately buffered contents reach their destination.

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

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Attempts to write an entire buffer into this write.

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

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Writes a formatted string into this writer, returning any error encountered.

Important traits for &'a mut I

impl<'a, I> Iterator for &'a mut I where
    I: Iterator + ?Sized,  type Item = <I as Iterator>::Item;impl<'a, R: Read + ?Sized> Read for &'a mut Rimpl<'a, W: Write + ?Sized> Write for &'a mut W

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

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Creates a "by reference" adaptor for this instance of Write.

impl<'a> Write for Cursor<&'a mut Vec<u8>>

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

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Write a buffer into this object, returning how many bytes were written.

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

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Flush this output stream, ensuring that all intermediately buffered contents reach their destination.

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

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Attempts to write an entire buffer into this write.

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

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Writes a formatted string into this writer, returning any error encountered.

Important traits for &'a mut I

impl<'a, I> Iterator for &'a mut I where
    I: Iterator + ?Sized,  type Item = <I as Iterator>::Item;impl<'a, R: Read + ?Sized> Read for &'a mut Rimpl<'a, W: Write + ?Sized> Write for &'a mut W

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

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Creates a "by reference" adaptor for this instance of Write.

impl Write for Cursor<Vec<u8>>

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

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Write a buffer into this object, returning how many bytes were written.

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

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Flush this output stream, ensuring that all intermediately buffered contents reach their destination.

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

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Attempts to write an entire buffer into this write.

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

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Writes a formatted string into this writer, returning any error encountered.

Important traits for &'a mut I

impl<'a, I> Iterator for &'a mut I where
    I: Iterator + ?Sized,  type Item = <I as Iterator>::Item;impl<'a, R: Read + ?Sized> Read for &'a mut Rimpl<'a, W: Write + ?Sized> Write for &'a mut W

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

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Creates a "by reference" adaptor for this instance of Write.

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

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

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Write a buffer into this object, returning how many bytes were written.

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

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Flush this output stream, ensuring that all intermediately buffered contents reach their destination.

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

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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.

Important traits for &'a mut I

impl<'a, I> Iterator for &'a mut I where
    I: Iterator + ?Sized,  type Item = <I as Iterator>::Item;impl<'a, R: Read + ?Sized> Read for &'a mut Rimpl<'a, W: Write + ?Sized> Write for &'a mut W

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

[−]

Creates a "by reference" adaptor for this instance of Write.