Struct std::io::Cursor1.0.0 [] [src]

pub struct Cursor<T> { /* 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:

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]);
}Run

Methods

impl<T> Cursor<T>
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Important traits for 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());Run

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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();Run

Important traits for &'a mut I
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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();Run

Important traits for &'a mut I
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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();Run

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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);Run

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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);Run

Trait Implementations

impl<T: Clone> Clone for Cursor<T>
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Important traits for Cursor<T>
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Returns a copy of the value. Read more

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Performs copy-assignment from source. Read more

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

impl<T> Seek for Cursor<T> where
    T: AsRef<[u8]>, 
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Seek to an offset, in bytes, in a stream. Read more

impl<T> Read for Cursor<T> where
    T: AsRef<[u8]>, 
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Pull some bytes from this source into the specified buffer, returning how many bytes were read. Read more

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Read the exact number of bytes required to fill buf. Read more

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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. Read more

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

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

Important traits for &'a mut I
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Creates a "by reference" adaptor for this instance of Read. Read more

Important traits for Bytes<R>
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Transforms this Read instance to an [Iterator] over its bytes. Read more

Important traits for Chars<R>
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🔬 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. Read more

Important traits for Chain<T, U>
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Creates an adaptor which will chain this stream with another. Read more

Important traits for Take<T>
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Creates an adaptor which will read at most limit bytes from it. Read more

impl<T> BufRead for Cursor<T> where
    T: AsRef<[u8]>, 
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Fills the internal buffer of this object, returning the buffer contents. Read more

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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. Read more

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

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

Important traits for Split<B>
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Returns an iterator over the contents of this reader split on the byte byte. Read more

Important traits for Lines<B>
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Returns an iterator over the lines of this reader. Read more

impl<'a> Write for Cursor<&'a mut [u8]>
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Write a buffer into this object, returning how many bytes were written. Read more

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

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Attempts to write an entire buffer into this write. Read more

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

Important traits for &'a mut I
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Creates a "by reference" adaptor for this instance of Write. Read more

impl<'a> Write for Cursor<&'a mut Vec<u8>>
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Write a buffer into this object, returning how many bytes were written. Read more

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

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Attempts to write an entire buffer into this write. Read more

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

Important traits for &'a mut I
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Creates a "by reference" adaptor for this instance of Write. Read more

impl Write for Cursor<Vec<u8>>
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Write a buffer into this object, returning how many bytes were written. Read more

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

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Attempts to write an entire buffer into this write. Read more

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

Important traits for &'a mut I
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Creates a "by reference" adaptor for this instance of Write. Read more

impl Write for Cursor<Box<[u8]>>
1.5.0
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Write a buffer into this object, returning how many bytes were written. Read more

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

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Attempts to write an entire buffer into this write. Read more

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

Important traits for &'a mut I
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Creates a "by reference" adaptor for this instance of Write. Read more