4 use crate::io::prelude::*;
6 use crate::alloc::Allocator;
8 use crate::io::{self, ErrorKind, IoSlice, IoSliceMut, ReadBuf, SeekFrom};
10 use core::convert::TryInto;
12 /// A `Cursor` wraps an in-memory buffer and provides it with a
13 /// [`Seek`] implementation.
15 /// `Cursor`s are used with in-memory buffers, anything implementing
16 /// <code>[AsRef]<\[u8]></code>, to allow them to implement [`Read`] and/or [`Write`],
17 /// allowing these buffers to be used anywhere you might use a reader or writer
18 /// that does actual I/O.
20 /// The standard library implements some I/O traits on various types which
21 /// are commonly used as a buffer, like <code>Cursor<[Vec]\<u8>></code> and
22 /// <code>Cursor<[&\[u8\]][bytes]></code>.
26 /// We may want to write bytes to a [`File`] in our production
27 /// code, but use an in-memory buffer in our tests. We can do this with
30 /// [bytes]: crate::slice "slice"
31 /// [`File`]: crate::fs::File
34 /// use std::io::prelude::*;
35 /// use std::io::{self, SeekFrom};
36 /// use std::fs::File;
38 /// // a library function we've written
39 /// fn write_ten_bytes_at_end<W: Write + Seek>(writer: &mut W) -> io::Result<()> {
40 /// writer.seek(SeekFrom::End(-10))?;
43 /// writer.write(&[i])?;
50 /// # fn foo() -> io::Result<()> {
51 /// // Here's some code that uses this library function.
53 /// // We might want to use a BufReader here for efficiency, but let's
54 /// // keep this example focused.
55 /// let mut file = File::create("foo.txt")?;
57 /// write_ten_bytes_at_end(&mut file)?;
61 /// // now let's write a test
63 /// fn test_writes_bytes() {
64 /// // setting up a real File is much slower than an in-memory buffer,
65 /// // let's use a cursor instead
66 /// use std::io::Cursor;
67 /// let mut buff = Cursor::new(vec![0; 15]);
69 /// write_ten_bytes_at_end(&mut buff).unwrap();
71 /// assert_eq!(&buff.get_ref()[5..15], &[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]);
74 #[stable(feature = "rust1", since = "1.0.0")]
75 #[derive(Debug, Default, Eq, PartialEq)]
76 pub struct Cursor<T> {
82 /// Creates a new cursor wrapping the provided underlying in-memory buffer.
84 /// Cursor initial position is `0` even if underlying buffer (e.g., [`Vec`])
85 /// is not empty. So writing to cursor starts with overwriting [`Vec`]
86 /// content, not with appending to it.
91 /// use std::io::Cursor;
93 /// let buff = Cursor::new(Vec::new());
94 /// # fn force_inference(_: &Cursor<Vec<u8>>) {}
95 /// # force_inference(&buff);
97 #[stable(feature = "rust1", since = "1.0.0")]
98 #[rustc_const_unstable(feature = "const_io_structs", issue = "78812")]
99 pub const fn new(inner: T) -> Cursor<T> {
100 Cursor { pos: 0, inner }
103 /// Consumes this cursor, returning the underlying value.
108 /// use std::io::Cursor;
110 /// let buff = Cursor::new(Vec::new());
111 /// # fn force_inference(_: &Cursor<Vec<u8>>) {}
112 /// # force_inference(&buff);
114 /// let vec = buff.into_inner();
116 #[stable(feature = "rust1", since = "1.0.0")]
117 pub fn into_inner(self) -> T {
121 /// Gets a reference to the underlying value in this cursor.
126 /// use std::io::Cursor;
128 /// let buff = Cursor::new(Vec::new());
129 /// # fn force_inference(_: &Cursor<Vec<u8>>) {}
130 /// # force_inference(&buff);
132 /// let reference = buff.get_ref();
134 #[stable(feature = "rust1", since = "1.0.0")]
135 #[rustc_const_unstable(feature = "const_io_structs", issue = "78812")]
136 pub const fn get_ref(&self) -> &T {
140 /// Gets a mutable reference to the underlying value in this cursor.
142 /// Care should be taken to avoid modifying the internal I/O state of the
143 /// underlying value as it may corrupt this cursor's position.
148 /// use std::io::Cursor;
150 /// let mut buff = Cursor::new(Vec::new());
151 /// # fn force_inference(_: &Cursor<Vec<u8>>) {}
152 /// # force_inference(&buff);
154 /// let reference = buff.get_mut();
156 #[stable(feature = "rust1", since = "1.0.0")]
157 pub fn get_mut(&mut self) -> &mut T {
161 /// Returns the current position of this cursor.
166 /// use std::io::Cursor;
167 /// use std::io::prelude::*;
168 /// use std::io::SeekFrom;
170 /// let mut buff = Cursor::new(vec![1, 2, 3, 4, 5]);
172 /// assert_eq!(buff.position(), 0);
174 /// buff.seek(SeekFrom::Current(2)).unwrap();
175 /// assert_eq!(buff.position(), 2);
177 /// buff.seek(SeekFrom::Current(-1)).unwrap();
178 /// assert_eq!(buff.position(), 1);
180 #[stable(feature = "rust1", since = "1.0.0")]
181 #[rustc_const_unstable(feature = "const_io_structs", issue = "78812")]
182 pub const fn position(&self) -> u64 {
186 /// Sets the position of this cursor.
191 /// use std::io::Cursor;
193 /// let mut buff = Cursor::new(vec![1, 2, 3, 4, 5]);
195 /// assert_eq!(buff.position(), 0);
197 /// buff.set_position(2);
198 /// assert_eq!(buff.position(), 2);
200 /// buff.set_position(4);
201 /// assert_eq!(buff.position(), 4);
203 #[stable(feature = "rust1", since = "1.0.0")]
204 pub fn set_position(&mut self, pos: u64) {
213 /// Returns the remaining slice.
218 /// #![feature(cursor_remaining)]
219 /// use std::io::Cursor;
221 /// let mut buff = Cursor::new(vec![1, 2, 3, 4, 5]);
223 /// assert_eq!(buff.remaining_slice(), &[1, 2, 3, 4, 5]);
225 /// buff.set_position(2);
226 /// assert_eq!(buff.remaining_slice(), &[3, 4, 5]);
228 /// buff.set_position(4);
229 /// assert_eq!(buff.remaining_slice(), &[5]);
231 /// buff.set_position(6);
232 /// assert_eq!(buff.remaining_slice(), &[]);
234 #[unstable(feature = "cursor_remaining", issue = "86369")]
235 pub fn remaining_slice(&self) -> &[u8] {
236 let len = self.pos.min(self.inner.as_ref().len() as u64);
237 &self.inner.as_ref()[(len as usize)..]
240 /// Returns `true` if the remaining slice is empty.
245 /// #![feature(cursor_remaining)]
246 /// use std::io::Cursor;
248 /// let mut buff = Cursor::new(vec![1, 2, 3, 4, 5]);
250 /// buff.set_position(2);
251 /// assert!(!buff.is_empty());
253 /// buff.set_position(5);
254 /// assert!(buff.is_empty());
256 /// buff.set_position(10);
257 /// assert!(buff.is_empty());
259 #[unstable(feature = "cursor_remaining", issue = "86369")]
260 pub fn is_empty(&self) -> bool {
261 self.pos >= self.inner.as_ref().len() as u64
265 #[stable(feature = "rust1", since = "1.0.0")]
266 impl<T> Clone for Cursor<T>
271 fn clone(&self) -> Self {
272 Cursor { inner: self.inner.clone(), pos: self.pos }
276 fn clone_from(&mut self, other: &Self) {
277 self.inner.clone_from(&other.inner);
278 self.pos = other.pos;
282 #[stable(feature = "rust1", since = "1.0.0")]
283 impl<T> io::Seek for Cursor<T>
287 fn seek(&mut self, style: SeekFrom) -> io::Result<u64> {
288 let (base_pos, offset) = match style {
289 SeekFrom::Start(n) => {
293 SeekFrom::End(n) => (self.inner.as_ref().len() as u64, n),
294 SeekFrom::Current(n) => (self.pos, n),
296 match base_pos.checked_add_signed(offset) {
301 None => Err(io::const_io_error!(
302 ErrorKind::InvalidInput,
303 "invalid seek to a negative or overflowing position",
308 fn stream_len(&mut self) -> io::Result<u64> {
309 Ok(self.inner.as_ref().len() as u64)
312 fn stream_position(&mut self) -> io::Result<u64> {
317 #[stable(feature = "rust1", since = "1.0.0")]
318 impl<T> Read for Cursor<T>
322 fn read(&mut self, buf: &mut [u8]) -> io::Result<usize> {
323 let n = Read::read(&mut self.remaining_slice(), buf)?;
324 self.pos += n as u64;
328 fn read_buf(&mut self, buf: &mut ReadBuf<'_>) -> io::Result<()> {
329 let prev_filled = buf.filled_len();
331 Read::read_buf(&mut self.fill_buf()?, buf)?;
333 self.pos += (buf.filled_len() - prev_filled) as u64;
338 fn read_vectored(&mut self, bufs: &mut [IoSliceMut<'_>]) -> io::Result<usize> {
341 let n = self.read(buf)?;
350 fn is_read_vectored(&self) -> bool {
354 fn read_exact(&mut self, buf: &mut [u8]) -> io::Result<()> {
356 Read::read_exact(&mut self.remaining_slice(), buf)?;
357 self.pos += n as u64;
362 #[stable(feature = "rust1", since = "1.0.0")]
363 impl<T> BufRead for Cursor<T>
367 fn fill_buf(&mut self) -> io::Result<&[u8]> {
368 Ok(self.remaining_slice())
370 fn consume(&mut self, amt: usize) {
371 self.pos += amt as u64;
375 // Non-resizing write implementation
377 fn slice_write(pos_mut: &mut u64, slice: &mut [u8], buf: &[u8]) -> io::Result<usize> {
378 let pos = cmp::min(*pos_mut, slice.len() as u64);
379 let amt = (&mut slice[(pos as usize)..]).write(buf)?;
380 *pos_mut += amt as u64;
385 fn slice_write_vectored(
388 bufs: &[IoSlice<'_>],
389 ) -> io::Result<usize> {
390 let mut nwritten = 0;
392 let n = slice_write(pos_mut, slice, buf)?;
401 // Resizing write implementation
402 fn vec_write<A>(pos_mut: &mut u64, vec: &mut Vec<u8, A>, buf: &[u8]) -> io::Result<usize>
406 let pos: usize = (*pos_mut).try_into().map_err(|_| {
408 ErrorKind::InvalidInput,
409 "cursor position exceeds maximum possible vector length",
412 // Make sure the internal buffer is as least as big as where we
416 // use `resize` so that the zero filling is as efficient as possible
419 // Figure out what bytes will be used to overwrite what's currently
420 // there (left), and what will be appended on the end (right)
422 let space = vec.len() - pos;
423 let (left, right) = buf.split_at(cmp::min(space, buf.len()));
424 vec[pos..pos + left.len()].copy_from_slice(left);
425 vec.extend_from_slice(right);
429 *pos_mut = (pos + buf.len()) as u64;
433 fn vec_write_vectored<A>(
435 vec: &mut Vec<u8, A>,
436 bufs: &[IoSlice<'_>],
437 ) -> io::Result<usize>
441 let mut nwritten = 0;
443 nwritten += vec_write(pos_mut, vec, buf)?;
448 #[stable(feature = "rust1", since = "1.0.0")]
449 impl Write for Cursor<&mut [u8]> {
451 fn write(&mut self, buf: &[u8]) -> io::Result<usize> {
452 slice_write(&mut self.pos, self.inner, buf)
456 fn write_vectored(&mut self, bufs: &[IoSlice<'_>]) -> io::Result<usize> {
457 slice_write_vectored(&mut self.pos, self.inner, bufs)
461 fn is_write_vectored(&self) -> bool {
466 fn flush(&mut self) -> io::Result<()> {
471 #[stable(feature = "cursor_mut_vec", since = "1.25.0")]
472 impl<A> Write for Cursor<&mut Vec<u8, A>>
476 fn write(&mut self, buf: &[u8]) -> io::Result<usize> {
477 vec_write(&mut self.pos, self.inner, buf)
480 fn write_vectored(&mut self, bufs: &[IoSlice<'_>]) -> io::Result<usize> {
481 vec_write_vectored(&mut self.pos, self.inner, bufs)
485 fn is_write_vectored(&self) -> bool {
490 fn flush(&mut self) -> io::Result<()> {
495 #[stable(feature = "rust1", since = "1.0.0")]
496 impl<A> Write for Cursor<Vec<u8, A>>
500 fn write(&mut self, buf: &[u8]) -> io::Result<usize> {
501 vec_write(&mut self.pos, &mut self.inner, buf)
504 fn write_vectored(&mut self, bufs: &[IoSlice<'_>]) -> io::Result<usize> {
505 vec_write_vectored(&mut self.pos, &mut self.inner, bufs)
509 fn is_write_vectored(&self) -> bool {
514 fn flush(&mut self) -> io::Result<()> {
519 #[stable(feature = "cursor_box_slice", since = "1.5.0")]
520 impl<A> Write for Cursor<Box<[u8], A>>
525 fn write(&mut self, buf: &[u8]) -> io::Result<usize> {
526 slice_write(&mut self.pos, &mut self.inner, buf)
530 fn write_vectored(&mut self, bufs: &[IoSlice<'_>]) -> io::Result<usize> {
531 slice_write_vectored(&mut self.pos, &mut self.inner, bufs)
535 fn is_write_vectored(&self) -> bool {
540 fn flush(&mut self) -> io::Result<()> {
545 #[stable(feature = "cursor_array", since = "1.61.0")]
546 impl<const N: usize> Write for Cursor<[u8; N]> {
548 fn write(&mut self, buf: &[u8]) -> io::Result<usize> {
549 slice_write(&mut self.pos, &mut self.inner, buf)
553 fn write_vectored(&mut self, bufs: &[IoSlice<'_>]) -> io::Result<usize> {
554 slice_write_vectored(&mut self.pos, &mut self.inner, bufs)
558 fn is_write_vectored(&self) -> bool {
563 fn flush(&mut self) -> io::Result<()> {