core/io/

cursor.rs

use crate::io::{self, ErrorKind, SeekFrom};

/// A `Cursor` wraps an in-memory buffer and provides it with a
/// [`Seek`] implementation.
///
/// `Cursor`s are used with in-memory buffers, anything implementing
/// <code>[AsRef]<\[u8]></code>, 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 <code>Cursor<[Vec]\<u8>></code> and
/// <code>Cursor<[&\[u8\]][bytes]></code>.
///
/// # 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`:
///
// FIXME(#74481): Hard-links required to link from `core` to `std`
/// [bytes]: crate::slice "slice"
/// [`File`]: ../../std/fs/struct.File.html
/// [`Read`]: ../../std/io/trait.Read.html
/// [`Write`]: ../../std/io/trait.Write.html
/// [`Seek`]: crate::io::Seek
/// [Vec]: ../../alloc/vec/struct.Vec.html
///
/// ```no_run
/// 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(())
/// }
///
/// # fn foo() -> io::Result<()> {
/// // 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")?;
/// // First, we need to allocate 10 bytes to be able to write into.
/// file.set_len(10)?;
///
/// write_ten_bytes_at_end(&mut file)?;
/// # Ok(())
/// # }
///
/// // 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]);
/// }
/// ```
#[stable(feature = "rust1", since = "1.0.0")]
#[derive(Debug, Default, Eq, PartialEq)]
pub struct Cursor<T> {
    inner: T,
    pos: u64,
}

impl<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.
    ///
    // FIXME(#74481): Hard-links required to link from `core` to `alloc`
    /// [`Vec`]: ../../alloc/vec/struct.Vec.html
    ///
    /// # Examples
    ///
    /// ```
    /// use std::io::Cursor;
    ///
    /// let buff = Cursor::new(Vec::new());
    /// # fn force_inference(_: &Cursor<Vec<u8>>) {}
    /// # force_inference(&buff);
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    #[rustc_const_stable(feature = "const_io_structs", since = "1.79.0")]
    pub const fn new(inner: T) -> Cursor<T> {
        Cursor { pos: 0, inner }
    }

    /// Consumes this cursor, returning the underlying value.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::io::Cursor;
    ///
    /// let buff = Cursor::new(Vec::new());
    /// # fn force_inference(_: &Cursor<Vec<u8>>) {}
    /// # force_inference(&buff);
    ///
    /// let vec = buff.into_inner();
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn into_inner(self) -> T {
        self.inner
    }

    /// Gets a reference to the underlying value in this cursor.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::io::Cursor;
    ///
    /// let buff = Cursor::new(Vec::new());
    /// # fn force_inference(_: &Cursor<Vec<u8>>) {}
    /// # force_inference(&buff);
    ///
    /// let reference = buff.get_ref();
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    #[rustc_const_stable(feature = "const_io_structs", since = "1.79.0")]
    pub const fn get_ref(&self) -> &T {
        &self.inner
    }

    /// 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());
    /// # fn force_inference(_: &Cursor<Vec<u8>>) {}
    /// # force_inference(&buff);
    ///
    /// let reference = buff.get_mut();
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    #[rustc_const_stable(feature = "const_mut_cursor", since = "1.86.0")]
    pub const fn get_mut(&mut self) -> &mut T {
        &mut self.inner
    }

    /// 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);
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    #[rustc_const_stable(feature = "const_io_structs", since = "1.79.0")]
    pub const fn position(&self) -> u64 {
        self.pos
    }

    /// 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);
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    #[rustc_const_stable(feature = "const_mut_cursor", since = "1.86.0")]
    pub const fn set_position(&mut self, pos: u64) {
        self.pos = pos;
    }

    #[doc(hidden)]
    #[unstable(feature = "core_io_internals", reason = "exposed only for libstd", issue = "none")]
    #[inline]
    pub const fn into_parts_mut(&mut self) -> (&mut u64, &mut T) {
        (&mut self.pos, &mut self.inner)
    }
}

impl<T> Cursor<T>
where
    T: AsRef<[u8]>,
{
    /// Splits the underlying slice at the cursor position and returns them.
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(cursor_split)]
    /// use std::io::Cursor;
    ///
    /// let mut buff = Cursor::new(vec![1, 2, 3, 4, 5]);
    ///
    /// assert_eq!(buff.split(), ([].as_slice(), [1, 2, 3, 4, 5].as_slice()));
    ///
    /// buff.set_position(2);
    /// assert_eq!(buff.split(), ([1, 2].as_slice(), [3, 4, 5].as_slice()));
    ///
    /// buff.set_position(6);
    /// assert_eq!(buff.split(), ([1, 2, 3, 4, 5].as_slice(), [].as_slice()));
    /// ```
    #[unstable(feature = "cursor_split", issue = "86369")]
    pub fn split(&self) -> (&[u8], &[u8]) {
        let slice = self.inner.as_ref();
        let pos = self.pos.min(slice.len() as u64);
        slice.split_at(pos as usize)
    }
}

impl<T> Cursor<T>
where
    T: AsMut<[u8]>,
{
    /// Splits the underlying slice at the cursor position and returns them
    /// mutably.
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(cursor_split)]
    /// use std::io::Cursor;
    ///
    /// let mut buff = Cursor::new(vec![1, 2, 3, 4, 5]);
    ///
    /// assert_eq!(buff.split_mut(), ([].as_mut_slice(), [1, 2, 3, 4, 5].as_mut_slice()));
    ///
    /// buff.set_position(2);
    /// assert_eq!(buff.split_mut(), ([1, 2].as_mut_slice(), [3, 4, 5].as_mut_slice()));
    ///
    /// buff.set_position(6);
    /// assert_eq!(buff.split_mut(), ([1, 2, 3, 4, 5].as_mut_slice(), [].as_mut_slice()));
    /// ```
    #[unstable(feature = "cursor_split", issue = "86369")]
    pub fn split_mut(&mut self) -> (&mut [u8], &mut [u8]) {
        let slice = self.inner.as_mut();
        let pos = self.pos.min(slice.len() as u64);
        slice.split_at_mut(pos as usize)
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<T> Clone for Cursor<T>
where
    T: Clone,
{
    #[inline]
    fn clone(&self) -> Self {
        Cursor { inner: self.inner.clone(), pos: self.pos }
    }

    #[inline]
    fn clone_from(&mut self, other: &Self) {
        self.inner.clone_from(&other.inner);
        self.pos = other.pos;
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<T> io::Seek for Cursor<T>
where
    T: AsRef<[u8]>,
{
    fn seek(&mut self, style: SeekFrom) -> io::Result<u64> {
        let (base_pos, offset) = match style {
            SeekFrom::Start(n) => {
                self.set_position(n);
                return Ok(n);
            }
            SeekFrom::End(n) => (self.get_ref().as_ref().len() as u64, n),
            SeekFrom::Current(n) => (self.position(), n),
        };
        match base_pos.checked_add_signed(offset) {
            Some(n) => {
                self.set_position(n);
                Ok(n)
            }
            None => Err(io::const_error!(
                ErrorKind::InvalidInput,
                "invalid seek to a negative or overflowing position",
            )),
        }
    }

    fn stream_len(&mut self) -> io::Result<u64> {
        Ok(self.get_ref().as_ref().len() as u64)
    }

    fn stream_position(&mut self) -> io::Result<u64> {
        Ok(self.position())
    }
}