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

# 1.0.0[−][src]Struct std::io::Cursor

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

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>(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

## Implementations

### `impl<T> Cursor<T>`[src]

#### `pub fn new(inner: T) -> Cursor<T>`[src]

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

#### `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

#### `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

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

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

Returns the contents of the internal buffer, filling it with more data from the inner reader if it is empty. 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

#### `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

#### `fn lines(self) -> Lines<Self> where
Self: Sized,` [src]

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

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

#### `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, f: &mut Formatter) -> Result`[src]

Formats the value using the given formatter. Read more

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

#### `fn default() -> Cursor<T>`[src]

Returns the "default value" for a type. Read more

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

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

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

This method tests for `self` and `other` values to be equal, and is used by `==`. Read more

#### `fn ne(&self, other: &Cursor<T>) -> bool`[src]

This method tests for `!=`.

### `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_vectored(&mut self, bufs: &mut [IoSliceMut]) -> Result<usize>`[src]

Like `read`, except that it reads into a slice of buffers. Read more

#### `fn is_read_vectored(&self) -> bool`[src]

🔬 This is a nightly-only experimental API. (`can_vector` #69941)

Determines if this `Read`er has an efficient `read_vectored` implementation. 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, appending them to `buf`. Read more

#### `fn by_ref(&mut self) -> &mut Self where
Self: Sized,` [src]

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

#### `fn bytes(self) -> Bytes<Self> where
Self: Sized,` [src]

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

#### `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

#### `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> 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

#### `fn stream_len(&mut self) -> Result<u64>`[src]

🔬 This is a nightly-only experimental API. (`seek_convenience` #59359)

Returns the length of this stream (in bytes). Read more

#### `fn stream_position(&mut self) -> Result<u64>`[src]

🔬 This is a nightly-only experimental API. (`seek_convenience` #59359)

Returns the current seek position from the start of the stream. Read more

### `impl<T> StructuralEq for Cursor<T>`[src]

### `impl<T> StructuralPartialEq for Cursor<T>`[src]

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

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

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

#### `fn write_vectored(&mut self, bufs: &[IoSlice]) -> Result<usize>`[src]

Like `write`, except that it writes from a slice of buffers. Read more

#### `fn is_write_vectored(&self) -> bool`[src]

🔬 This is a nightly-only experimental API. (`can_vector` #69941)

Determines if this `Write`er has an efficient `write_vectored` implementation. 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 writer. Read more

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

🔬 This is a nightly-only experimental API. (`write_all_vectored` #70436)

Attempts to write multiple buffers into this writer. Read more

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

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

#### `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<&'_ mut Vec<u8>>`1.25.0[src]

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

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

#### `fn write_vectored(&mut self, bufs: &[IoSlice]) -> Result<usize>`[src]

Like `write`, except that it writes from a slice of buffers. Read more

#### `fn is_write_vectored(&self) -> bool`[src]

🔬 This is a nightly-only experimental API. (`can_vector` #69941)

Determines if this `Write`er has an efficient `write_vectored` implementation. 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 writer. Read more

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

🔬 This is a nightly-only experimental API. (`write_all_vectored` #70436)

Attempts to write multiple buffers into this writer. Read more

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

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

#### `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 writer, returning how many bytes were written. Read more

#### `fn write_vectored(&mut self, bufs: &[IoSlice]) -> Result<usize>`[src]

Like `write`, except that it writes from a slice of buffers. Read more

#### `fn is_write_vectored(&self) -> bool`[src]

🔬 This is a nightly-only experimental API. (`can_vector` #69941)

Determines if this `Write`er has an efficient `write_vectored` implementation. 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 writer. Read more

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

🔬 This is a nightly-only experimental API. (`write_all_vectored` #70436)

Attempts to write multiple buffers into this writer. Read more

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

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

#### `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 writer, returning how many bytes were written. Read more

#### `fn write_vectored(&mut self, bufs: &[IoSlice]) -> Result<usize>`[src]

Like `write`, except that it writes from a slice of buffers. Read more

#### `fn is_write_vectored(&self) -> bool`[src]

🔬 This is a nightly-only experimental API. (`can_vector` #69941)

Determines if this `Write`er has an efficient `write_vectored` implementation. 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 writer. Read more

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

🔬 This is a nightly-only experimental API. (`write_all_vectored` #70436)

Attempts to write multiple buffers into this writer. Read more

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

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

#### `fn by_ref(&mut self) -> &mut Self where
Self: Sized,` [src]

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

## 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

### `impl<T> Any for T where
T: 'static + ?Sized,` [src]

#### `fn type_id(&self) -> TypeId`[src]

Gets the `TypeId` of `self`. Read more

### `impl<T> Borrow<T> for T where
T: ?Sized,` [src]

#### `fn borrow(&self) -> &T`[src]

Immutably borrows from an owned value. Read more

### `impl<T> BorrowMut<T> for T where
T: ?Sized,` [src]

#### `fn borrow_mut(&mut self) -> &mut T`[src]

Mutably borrows from an owned value. Read more

### `impl<T> From<T> for T`[src]

#### `fn from(t: T) -> T`[src]

Performs the conversion.

### `impl<T, U> Into<U> for T where
U: From<T>,` [src]

#### `fn into(self) -> U`[src]

Performs the conversion.

### `impl<T> ToOwned for T where
T: Clone,` [src]

#### `type Owned = T`

The resulting type after obtaining ownership.

#### `fn to_owned(&self) -> T`[src]

Creates owned data from borrowed data, usually by cloning. Read more

#### `fn clone_into(&self, target: &mut T)`[src]

🔬 This is a nightly-only experimental API. (`toowned_clone_into` #41263)

recently added

Uses borrowed data to replace owned data, usually by cloning. Read more

### `impl<T, U> TryFrom<U> for T where
U: Into<T>,` [src]

#### `type Error = Infallible`

The type returned in the event of a conversion error.

#### `fn try_from(value: U) -> Result<T, <T as TryFrom<U>>::Error>`[src]

Performs the conversion.

### `impl<T, U> TryInto<U> for T where
U: TryFrom<T>,` [src]

#### `type Error = <U as TryFrom<T>>::Error`

The type returned in the event of a conversion error.

#### `fn try_into(self) -> Result<U, <U as TryFrom<T>>::Error>`[src]

Performs the conversion.