std::io::Cursor - Rust
Description: API documentation for the Rust `Cursor` struct in crate `std`.
Keywords: rust, rustlang, rust-lang, Cursor

# Struct std::io::Cursor 1.0.0 [−] [src]

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

A `Cursor` wraps another type and provides it with a `Seek` implementation.

`Cursor`s 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>`
[src]

####
ⓘImportant traits 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 Write for Cursor<Vec<u8>>impl Write for Cursor<Box<[u8]>>`
`pub fn new(inner: T) -> Cursor<T>`
[src]

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

#### `pub fn into_inner(self) -> T`
[src]

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

### 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`
[src]

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

### 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`
[src]

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

#### `pub fn position(&self) -> u64`
[src]

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

#### `pub fn set_position(&mut self, pos: u64)`
[src]

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>`
[src]

####
ⓘImportant traits 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 Write for Cursor<Vec<u8>>impl Write for Cursor<Box<[u8]>>`
`fn clone(&self) -> Cursor<T>`
[src]

Returns a copy of the value. Read more

#### `fn clone_from(&mut self, source: &Self)`
[src]

Performs copy-assignment from `source`. Read more

### `impl<T: Debug> Debug for Cursor<T>`
[src]

#### `fn fmt(&self, __arg_0: &mut Formatter) -> Result`
[src]

Formats the value using the given formatter. Read more

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

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

Seek to an offset, in bytes, in a stream. Read more

### `impl<T> Read for Cursor<T> where
T: AsRef<[u8]>,`
[src]

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

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

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

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

#### `unsafe fn initializer(&self) -> Initializer`
[src]

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

Determines if this `Read`er can work with buffers of uninitialized memory. Read more

#### `fn read_to_end(&mut self, buf: &mut Vec<u8>) -> Result<usize>`
[src]

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

#### `fn read_to_string(&mut self, buf: &mut String) -> Result<usize>`
[src]

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

####
ⓘImportant traits for &'a mut I

### 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,`
[src]

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

####
ⓘImportant traits for Bytes<R>

### Important traits for Bytes<R>

`impl<R: Read> Iterator for Bytes<R> type Item = Result<u8>;`
`fn bytes(self) -> Bytes<Self> where
Self: Sized,`
[src]

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

####
ⓘImportant traits for Chars<R>

### 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,`
[src]

🔬 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>

### 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,`
[src]

Creates an adaptor which will chain this stream with another. Read more

####
ⓘImportant traits for Take<T>

### Important traits for Take<T>

`impl<T: Read> Read for Take<T>`
`fn take(self, limit: u64) -> Take<Self> where
Self: Sized,`
[src]

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

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

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

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

#### `fn consume(&mut self, amt: usize)`
[src]

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

#### `fn read_until(&mut self, byte: u8, buf: &mut Vec<u8>) -> Result<usize>`
[src]

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

#### `fn read_line(&mut self, buf: &mut String) -> Result<usize>`
[src]

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>

### 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,`
[src]

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

####
ⓘImportant traits for Lines<B>

### Important traits for Lines<B>

`impl<B: BufRead> Iterator for Lines<B> type Item = Result<String>;`
`fn lines(self) -> Lines<Self> where
Self: Sized,`
[src]

Returns an iterator over the lines of this reader. Read more

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

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

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

#### `fn flush(&mut self) -> Result<()>`
[src]

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

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

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

#### `fn write_fmt(&mut self, fmt: Arguments) -> Result<()>`
[src]

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

####
ⓘImportant traits for &'a mut I

### 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,`
[src]

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

### `impl Write for Cursor<Vec<u8>>`
[src]

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

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

#### `fn flush(&mut self) -> Result<()>`
[src]

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

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

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

#### `fn write_fmt(&mut self, fmt: Arguments) -> Result<()>`
[src]

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

####
ⓘImportant traits for &'a mut I

### 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,`
[src]

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

### `impl Write for Cursor<Box<[u8]>>`
1.5.0
[src]

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

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

#### `fn flush(&mut self) -> Result<()>`
[src]

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

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

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

#### `fn write_fmt(&mut self, fmt: Arguments) -> Result<()>`
[src]

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

####
ⓘImportant traits for &'a mut I

### 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,`
[src]

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

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