4 use crate::io::prelude::*;
6 use crate::alloc::Allocator;
8 use crate::io::{self, BorrowedCursor, ErrorKind, IoSlice, IoSliceMut, SeekFrom};
10 /// A `Cursor` wraps an in-memory buffer and provides it with a
11 /// [`Seek`] implementation.
13 /// `Cursor`s are used with in-memory buffers, anything implementing
14 /// <code>[AsRef]<\[u8]></code>, to allow them to implement [`Read`] and/or [`Write`],
15 /// allowing these buffers to be used anywhere you might use a reader or writer
16 /// that does actual I/O.
18 /// The standard library implements some I/O traits on various types which
19 /// are commonly used as a buffer, like <code>Cursor<[Vec]\<u8>></code> and
20 /// <code>Cursor<[&\[u8\]][bytes]></code>.
24 /// We may want to write bytes to a [`File`] in our production
25 /// code, but use an in-memory buffer in our tests. We can do this with
28 /// [bytes]: crate::slice "slice"
29 /// [`File`]: crate::fs::File
32 /// use std::io::prelude::*;
33 /// use std::io::{self, SeekFrom};
34 /// use std::fs::File;
36 /// // a library function we've written
37 /// fn write_ten_bytes_at_end<W: Write + Seek>(writer: &mut W) -> io::Result<()> {
38 /// writer.seek(SeekFrom::End(-10))?;
41 /// writer.write(&[i])?;
48 /// # fn foo() -> io::Result<()> {
49 /// // Here's some code that uses this library function.
51 /// // We might want to use a BufReader here for efficiency, but let's
52 /// // keep this example focused.
53 /// let mut file = File::create("foo.txt")?;
55 /// write_ten_bytes_at_end(&mut file)?;
59 /// // now let's write a test
61 /// fn test_writes_bytes() {
62 /// // setting up a real File is much slower than an in-memory buffer,
63 /// // let's use a cursor instead
64 /// use std::io::Cursor;
65 /// let mut buff = Cursor::new(vec![0; 15]);
67 /// write_ten_bytes_at_end(&mut buff).unwrap();
69 /// assert_eq!(&buff.get_ref()[5..15], &[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]);
72 #[stable(feature = "rust1", since = "1.0.0")]
73 #[derive(Debug, Default, Eq, PartialEq)]
74 pub struct Cursor<T> {
80 /// Creates a new cursor wrapping the provided underlying in-memory buffer.
82 /// Cursor initial position is `0` even if underlying buffer (e.g., [`Vec`])
83 /// is not empty. So writing to cursor starts with overwriting [`Vec`]
84 /// content, not with appending to it.
89 /// use std::io::Cursor;
91 /// let buff = Cursor::new(Vec::new());
92 /// # fn force_inference(_: &Cursor<Vec<u8>>) {}
93 /// # force_inference(&buff);
95 #[stable(feature = "rust1", since = "1.0.0")]
96 #[rustc_const_unstable(feature = "const_io_structs", issue = "78812")]
97 pub const fn new(inner: T) -> Cursor<T> {
98 Cursor { pos: 0, inner }
101 /// Consumes this cursor, returning the underlying value.
106 /// use std::io::Cursor;
108 /// let buff = Cursor::new(Vec::new());
109 /// # fn force_inference(_: &Cursor<Vec<u8>>) {}
110 /// # force_inference(&buff);
112 /// let vec = buff.into_inner();
114 #[stable(feature = "rust1", since = "1.0.0")]
115 pub fn into_inner(self) -> T {
119 /// Gets a reference to the underlying value in this cursor.
124 /// use std::io::Cursor;
126 /// let buff = Cursor::new(Vec::new());
127 /// # fn force_inference(_: &Cursor<Vec<u8>>) {}
128 /// # force_inference(&buff);
130 /// let reference = buff.get_ref();
132 #[stable(feature = "rust1", since = "1.0.0")]
133 #[rustc_const_unstable(feature = "const_io_structs", issue = "78812")]
134 pub const fn get_ref(&self) -> &T {
138 /// Gets a mutable reference to the underlying value in this cursor.
140 /// Care should be taken to avoid modifying the internal I/O state of the
141 /// underlying value as it may corrupt this cursor's position.
146 /// use std::io::Cursor;
148 /// let mut buff = Cursor::new(Vec::new());
149 /// # fn force_inference(_: &Cursor<Vec<u8>>) {}
150 /// # force_inference(&buff);
152 /// let reference = buff.get_mut();
154 #[stable(feature = "rust1", since = "1.0.0")]
155 pub fn get_mut(&mut self) -> &mut T {
159 /// Returns the current position of this cursor.
164 /// use std::io::Cursor;
165 /// use std::io::prelude::*;
166 /// use std::io::SeekFrom;
168 /// let mut buff = Cursor::new(vec![1, 2, 3, 4, 5]);
170 /// assert_eq!(buff.position(), 0);
172 /// buff.seek(SeekFrom::Current(2)).unwrap();
173 /// assert_eq!(buff.position(), 2);
175 /// buff.seek(SeekFrom::Current(-1)).unwrap();
176 /// assert_eq!(buff.position(), 1);
178 #[stable(feature = "rust1", since = "1.0.0")]
179 #[rustc_const_unstable(feature = "const_io_structs", issue = "78812")]
180 pub const fn position(&self) -> u64 {
184 /// Sets the position of this cursor.
189 /// use std::io::Cursor;
191 /// let mut buff = Cursor::new(vec![1, 2, 3, 4, 5]);
193 /// assert_eq!(buff.position(), 0);
195 /// buff.set_position(2);
196 /// assert_eq!(buff.position(), 2);
198 /// buff.set_position(4);
199 /// assert_eq!(buff.position(), 4);
201 #[stable(feature = "rust1", since = "1.0.0")]
202 pub fn set_position(&mut self, pos: u64) {
211 /// Returns the remaining slice.
216 /// #![feature(cursor_remaining)]
217 /// use std::io::Cursor;
219 /// let mut buff = Cursor::new(vec![1, 2, 3, 4, 5]);
221 /// assert_eq!(buff.remaining_slice(), &[1, 2, 3, 4, 5]);
223 /// buff.set_position(2);
224 /// assert_eq!(buff.remaining_slice(), &[3, 4, 5]);
226 /// buff.set_position(4);
227 /// assert_eq!(buff.remaining_slice(), &[5]);
229 /// buff.set_position(6);
230 /// assert_eq!(buff.remaining_slice(), &[]);
232 #[unstable(feature = "cursor_remaining", issue = "86369")]
233 pub fn remaining_slice(&self) -> &[u8] {
234 let len = self.pos.min(self.inner.as_ref().len() as u64);
235 &self.inner.as_ref()[(len as usize)..]
238 /// Returns `true` if the remaining slice is empty.
243 /// #![feature(cursor_remaining)]
244 /// use std::io::Cursor;
246 /// let mut buff = Cursor::new(vec![1, 2, 3, 4, 5]);
248 /// buff.set_position(2);
249 /// assert!(!buff.is_empty());
251 /// buff.set_position(5);
252 /// assert!(buff.is_empty());
254 /// buff.set_position(10);
255 /// assert!(buff.is_empty());
257 #[unstable(feature = "cursor_remaining", issue = "86369")]
258 pub fn is_empty(&self) -> bool {
259 self.pos >= self.inner.as_ref().len() as u64
263 #[stable(feature = "rust1", since = "1.0.0")]
264 impl<T> Clone for Cursor<T>
269 fn clone(&self) -> Self {
270 Cursor { inner: self.inner.clone(), pos: self.pos }
274 fn clone_from(&mut self, other: &Self) {
275 self.inner.clone_from(&other.inner);
276 self.pos = other.pos;
280 #[stable(feature = "rust1", since = "1.0.0")]
281 impl<T> io::Seek for Cursor<T>
285 fn seek(&mut self, style: SeekFrom) -> io::Result<u64> {
286 let (base_pos, offset) = match style {
287 SeekFrom::Start(n) => {
291 SeekFrom::End(n) => (self.inner.as_ref().len() as u64, n),
292 SeekFrom::Current(n) => (self.pos, n),
294 match base_pos.checked_add_signed(offset) {
299 None => Err(io::const_io_error!(
300 ErrorKind::InvalidInput,
301 "invalid seek to a negative or overflowing position",
306 fn stream_len(&mut self) -> io::Result<u64> {
307 Ok(self.inner.as_ref().len() as u64)
310 fn stream_position(&mut self) -> io::Result<u64> {
315 #[stable(feature = "rust1", since = "1.0.0")]
316 impl<T> Read for Cursor<T>
320 fn read(&mut self, buf: &mut [u8]) -> io::Result<usize> {
321 let n = Read::read(&mut self.remaining_slice(), buf)?;
322 self.pos += n as u64;
326 fn read_buf(&mut self, mut cursor: BorrowedCursor<'_>) -> io::Result<()> {
327 let prev_written = cursor.written();
329 Read::read_buf(&mut self.fill_buf()?, cursor.reborrow())?;
331 self.pos += (cursor.written() - prev_written) as u64;
336 fn read_vectored(&mut self, bufs: &mut [IoSliceMut<'_>]) -> io::Result<usize> {
339 let n = self.read(buf)?;
348 fn is_read_vectored(&self) -> bool {
352 fn read_exact(&mut self, buf: &mut [u8]) -> io::Result<()> {
354 Read::read_exact(&mut self.remaining_slice(), buf)?;
355 self.pos += n as u64;
360 #[stable(feature = "rust1", since = "1.0.0")]
361 impl<T> BufRead for Cursor<T>
365 fn fill_buf(&mut self) -> io::Result<&[u8]> {
366 Ok(self.remaining_slice())
368 fn consume(&mut self, amt: usize) {
369 self.pos += amt as u64;
373 // Non-resizing write implementation
375 fn slice_write(pos_mut: &mut u64, slice: &mut [u8], buf: &[u8]) -> io::Result<usize> {
376 let pos = cmp::min(*pos_mut, slice.len() as u64);
377 let amt = (&mut slice[(pos as usize)..]).write(buf)?;
378 *pos_mut += amt as u64;
383 fn slice_write_vectored(
386 bufs: &[IoSlice<'_>],
387 ) -> io::Result<usize> {
388 let mut nwritten = 0;
390 let n = slice_write(pos_mut, slice, buf)?;
399 /// Reserves the required space, and pads the vec with 0s if necessary.
400 fn reserve_and_pad<A: Allocator>(
402 vec: &mut Vec<u8, A>,
404 ) -> io::Result<usize> {
405 let pos: usize = (*pos_mut).try_into().map_err(|_| {
407 ErrorKind::InvalidInput,
408 "cursor position exceeds maximum possible vector length",
412 // For safety reasons, we don't want these numbers to overflow
413 // otherwise our allocation won't be enough
414 let desired_cap = pos.saturating_add(buf_len);
415 if desired_cap > vec.capacity() {
416 // We want our vec's total capacity
417 // to have room for (pos+buf_len) bytes. Reserve allocates
418 // based on additional elements from the length, so we need to
419 // reserve the difference
420 vec.reserve(desired_cap - vec.len());
422 // Pad if pos is above the current len.
424 let diff = pos - vec.len();
425 // Unfortunately, `resize()` would suffice but the optimiser does not
426 // realise the `reserve` it does can be eliminated. So we do it manually
427 // to eliminate that extra branch
428 let spare = vec.spare_capacity_mut();
429 debug_assert!(spare.len() >= diff);
430 // Safety: we have allocated enough capacity for this.
431 // And we are only writing, not reading
433 spare.get_unchecked_mut(..diff).fill(core::mem::MaybeUninit::new(0));
441 /// Writes the slice to the vec without allocating
442 /// # Safety: vec must have buf.len() spare capacity
443 unsafe fn vec_write_unchecked<A>(pos: usize, vec: &mut Vec<u8, A>, buf: &[u8]) -> usize
447 debug_assert!(vec.capacity() >= pos + buf.len());
448 vec.as_mut_ptr().add(pos).copy_from(buf.as_ptr(), buf.len());
452 /// Resizing write implementation for [`Cursor`]
454 /// Cursor is allowed to have a pre-allocated and initialised
455 /// vector body, but with a position of 0. This means the [`Write`]
456 /// will overwrite the contents of the vec.
458 /// This also allows for the vec body to be empty, but with a position of N.
459 /// This means that [`Write`] will pad the vec with 0 initially,
460 /// before writing anything from that point
461 fn vec_write<A>(pos_mut: &mut u64, vec: &mut Vec<u8, A>, buf: &[u8]) -> io::Result<usize>
465 let buf_len = buf.len();
466 let mut pos = reserve_and_pad(pos_mut, vec, buf_len)?;
468 // Write the buf then progress the vec forward if necessary
469 // Safety: we have ensured that the capacity is available
470 // and that all bytes get written up to pos
472 pos = vec_write_unchecked(pos, vec, buf);
479 *pos_mut += buf_len as u64;
483 /// Resizing write_vectored implementation for [`Cursor`]
485 /// Cursor is allowed to have a pre-allocated and initialised
486 /// vector body, but with a position of 0. This means the [`Write`]
487 /// will overwrite the contents of the vec.
489 /// This also allows for the vec body to be empty, but with a position of N.
490 /// This means that [`Write`] will pad the vec with 0 initially,
491 /// before writing anything from that point
492 fn vec_write_vectored<A>(
494 vec: &mut Vec<u8, A>,
495 bufs: &[IoSlice<'_>],
496 ) -> io::Result<usize>
500 // For safety reasons, we don't want this sum to overflow ever.
501 // If this saturates, the reserve should panic to avoid any unsound writing.
502 let buf_len = bufs.iter().fold(0usize, |a, b| a.saturating_add(b.len()));
503 let mut pos = reserve_and_pad(pos_mut, vec, buf_len)?;
505 // Write the buf then progress the vec forward if necessary
506 // Safety: we have ensured that the capacity is available
507 // and that all bytes get written up to the last pos
510 pos = vec_write_unchecked(pos, vec, buf);
518 *pos_mut += buf_len as u64;
522 #[stable(feature = "rust1", since = "1.0.0")]
523 impl Write for Cursor<&mut [u8]> {
525 fn write(&mut self, buf: &[u8]) -> io::Result<usize> {
526 slice_write(&mut self.pos, self.inner, buf)
530 fn write_vectored(&mut self, bufs: &[IoSlice<'_>]) -> io::Result<usize> {
531 slice_write_vectored(&mut self.pos, self.inner, bufs)
535 fn is_write_vectored(&self) -> bool {
540 fn flush(&mut self) -> io::Result<()> {
545 #[stable(feature = "cursor_mut_vec", since = "1.25.0")]
546 impl<A> Write for Cursor<&mut Vec<u8, A>>
550 fn write(&mut self, buf: &[u8]) -> io::Result<usize> {
551 vec_write(&mut self.pos, self.inner, buf)
554 fn write_vectored(&mut self, bufs: &[IoSlice<'_>]) -> io::Result<usize> {
555 vec_write_vectored(&mut self.pos, self.inner, bufs)
559 fn is_write_vectored(&self) -> bool {
564 fn flush(&mut self) -> io::Result<()> {
569 #[stable(feature = "rust1", since = "1.0.0")]
570 impl<A> Write for Cursor<Vec<u8, A>>
574 fn write(&mut self, buf: &[u8]) -> io::Result<usize> {
575 vec_write(&mut self.pos, &mut self.inner, buf)
578 fn write_vectored(&mut self, bufs: &[IoSlice<'_>]) -> io::Result<usize> {
579 vec_write_vectored(&mut self.pos, &mut self.inner, bufs)
583 fn is_write_vectored(&self) -> bool {
588 fn flush(&mut self) -> io::Result<()> {
593 #[stable(feature = "cursor_box_slice", since = "1.5.0")]
594 impl<A> Write for Cursor<Box<[u8], A>>
599 fn write(&mut self, buf: &[u8]) -> io::Result<usize> {
600 slice_write(&mut self.pos, &mut self.inner, buf)
604 fn write_vectored(&mut self, bufs: &[IoSlice<'_>]) -> io::Result<usize> {
605 slice_write_vectored(&mut self.pos, &mut self.inner, bufs)
609 fn is_write_vectored(&self) -> bool {
614 fn flush(&mut self) -> io::Result<()> {
619 #[stable(feature = "cursor_array", since = "1.61.0")]
620 impl<const N: usize> Write for Cursor<[u8; N]> {
622 fn write(&mut self, buf: &[u8]) -> io::Result<usize> {
623 slice_write(&mut self.pos, &mut self.inner, buf)
627 fn write_vectored(&mut self, bufs: &[IoSlice<'_>]) -> io::Result<usize> {
628 slice_write_vectored(&mut self.pos, &mut self.inner, bufs)
632 fn is_write_vectored(&self) -> bool {
637 fn flush(&mut self) -> io::Result<()> {