4 use crate::io::prelude::*;
7 use crate::io::{self, Error, ErrorKind, Initializer, IoSlice, IoSliceMut, SeekFrom};
9 use core::convert::TryInto;
11 /// A `Cursor` wraps an in-memory buffer and provides it with a
12 /// [`Seek`] implementation.
14 /// `Cursor`s are used with in-memory buffers, anything implementing
15 /// [`AsRef`]`<[u8]>`, to allow them to implement [`Read`] and/or [`Write`],
16 /// allowing these buffers to be used anywhere you might use a reader or writer
17 /// that does actual I/O.
19 /// The standard library implements some I/O traits on various types which
20 /// are commonly used as a buffer, like `Cursor<`[`Vec`]`<u8>>` and
21 /// `Cursor<`[`&[u8]`][bytes]`>`.
25 /// We may want to write bytes to a [`File`] in our production
26 /// code, but use an in-memory buffer in our tests. We can do this with
29 /// [bytes]: crate::slice
30 /// [`File`]: crate::fs::File
33 /// use std::io::prelude::*;
34 /// use std::io::{self, SeekFrom};
35 /// use std::fs::File;
37 /// // a library function we've written
38 /// fn write_ten_bytes_at_end<W: Write + Seek>(writer: &mut W) -> io::Result<()> {
39 /// writer.seek(SeekFrom::End(-10))?;
42 /// writer.write(&[i])?;
49 /// # fn foo() -> io::Result<()> {
50 /// // Here's some code that uses this library function.
52 /// // We might want to use a BufReader here for efficiency, but let's
53 /// // keep this example focused.
54 /// let mut file = File::create("foo.txt")?;
56 /// write_ten_bytes_at_end(&mut file)?;
60 /// // now let's write a test
62 /// fn test_writes_bytes() {
63 /// // setting up a real File is much slower than an in-memory buffer,
64 /// // let's use a cursor instead
65 /// use std::io::Cursor;
66 /// let mut buff = Cursor::new(vec![0; 15]);
68 /// write_ten_bytes_at_end(&mut buff).unwrap();
70 /// assert_eq!(&buff.get_ref()[5..15], &[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]);
73 #[stable(feature = "rust1", since = "1.0.0")]
74 #[derive(Clone, Debug, Default, Eq, PartialEq)]
75 pub struct Cursor<T> {
81 /// Creates a new cursor wrapping the provided underlying in-memory buffer.
83 /// Cursor initial position is `0` even if underlying buffer (e.g., [`Vec`])
84 /// is not empty. So writing to cursor starts with overwriting [`Vec`]
85 /// content, not with appending to it.
90 /// use std::io::Cursor;
92 /// let buff = Cursor::new(Vec::new());
93 /// # fn force_inference(_: &Cursor<Vec<u8>>) {}
94 /// # force_inference(&buff);
96 #[stable(feature = "rust1", since = "1.0.0")]
97 #[rustc_const_unstable(feature = "const_io_structs", issue = "78812")]
98 pub const fn new(inner: T) -> Cursor<T> {
99 Cursor { pos: 0, inner }
102 /// Consumes this cursor, returning the underlying value.
107 /// use std::io::Cursor;
109 /// let buff = Cursor::new(Vec::new());
110 /// # fn force_inference(_: &Cursor<Vec<u8>>) {}
111 /// # force_inference(&buff);
113 /// let vec = buff.into_inner();
115 #[stable(feature = "rust1", since = "1.0.0")]
116 pub fn into_inner(self) -> T {
120 /// Gets a reference to the underlying value in this cursor.
125 /// use std::io::Cursor;
127 /// let buff = Cursor::new(Vec::new());
128 /// # fn force_inference(_: &Cursor<Vec<u8>>) {}
129 /// # force_inference(&buff);
131 /// let reference = buff.get_ref();
133 #[stable(feature = "rust1", since = "1.0.0")]
134 #[rustc_const_unstable(feature = "const_io_structs", issue = "78812")]
135 pub const fn get_ref(&self) -> &T {
139 /// Gets a mutable reference to the underlying value in this cursor.
141 /// Care should be taken to avoid modifying the internal I/O state of the
142 /// underlying value as it may corrupt this cursor's position.
147 /// use std::io::Cursor;
149 /// let mut buff = Cursor::new(Vec::new());
150 /// # fn force_inference(_: &Cursor<Vec<u8>>) {}
151 /// # force_inference(&buff);
153 /// let reference = buff.get_mut();
155 #[stable(feature = "rust1", since = "1.0.0")]
156 pub fn get_mut(&mut self) -> &mut T {
160 /// Returns the current position of this cursor.
165 /// use std::io::Cursor;
166 /// use std::io::prelude::*;
167 /// use std::io::SeekFrom;
169 /// let mut buff = Cursor::new(vec![1, 2, 3, 4, 5]);
171 /// assert_eq!(buff.position(), 0);
173 /// buff.seek(SeekFrom::Current(2)).unwrap();
174 /// assert_eq!(buff.position(), 2);
176 /// buff.seek(SeekFrom::Current(-1)).unwrap();
177 /// assert_eq!(buff.position(), 1);
179 #[stable(feature = "rust1", since = "1.0.0")]
180 #[rustc_const_unstable(feature = "const_io_structs", issue = "78812")]
181 pub const fn position(&self) -> u64 {
185 /// Sets the position of this cursor.
190 /// use std::io::Cursor;
192 /// let mut buff = Cursor::new(vec![1, 2, 3, 4, 5]);
194 /// assert_eq!(buff.position(), 0);
196 /// buff.set_position(2);
197 /// assert_eq!(buff.position(), 2);
199 /// buff.set_position(4);
200 /// assert_eq!(buff.position(), 4);
202 #[stable(feature = "rust1", since = "1.0.0")]
203 pub fn set_position(&mut self, pos: u64) {
208 #[stable(feature = "rust1", since = "1.0.0")]
209 impl<T> io::Seek for Cursor<T>
213 fn seek(&mut self, style: SeekFrom) -> io::Result<u64> {
214 let (base_pos, offset) = match style {
215 SeekFrom::Start(n) => {
219 SeekFrom::End(n) => (self.inner.as_ref().len() as u64, n),
220 SeekFrom::Current(n) => (self.pos, n),
222 let new_pos = if offset >= 0 {
223 base_pos.checked_add(offset as u64)
225 base_pos.checked_sub((offset.wrapping_neg()) as u64)
232 None => Err(Error::new_const(
233 ErrorKind::InvalidInput,
234 &"invalid seek to a negative or overflowing position",
239 fn stream_len(&mut self) -> io::Result<u64> {
240 Ok(self.inner.as_ref().len() as u64)
243 fn stream_position(&mut self) -> io::Result<u64> {
248 #[stable(feature = "rust1", since = "1.0.0")]
249 impl<T> Read for Cursor<T>
253 fn read(&mut self, buf: &mut [u8]) -> io::Result<usize> {
254 let n = Read::read(&mut self.fill_buf()?, buf)?;
255 self.pos += n as u64;
259 fn read_vectored(&mut self, bufs: &mut [IoSliceMut<'_>]) -> io::Result<usize> {
262 let n = self.read(buf)?;
271 fn is_read_vectored(&self) -> bool {
275 fn read_exact(&mut self, buf: &mut [u8]) -> io::Result<()> {
277 Read::read_exact(&mut self.fill_buf()?, buf)?;
278 self.pos += n as u64;
283 unsafe fn initializer(&self) -> Initializer {
288 #[stable(feature = "rust1", since = "1.0.0")]
289 impl<T> BufRead for Cursor<T>
293 fn fill_buf(&mut self) -> io::Result<&[u8]> {
294 let amt = cmp::min(self.pos, self.inner.as_ref().len() as u64);
295 Ok(&self.inner.as_ref()[(amt as usize)..])
297 fn consume(&mut self, amt: usize) {
298 self.pos += amt as u64;
302 // Non-resizing write implementation
304 fn slice_write(pos_mut: &mut u64, slice: &mut [u8], buf: &[u8]) -> io::Result<usize> {
305 let pos = cmp::min(*pos_mut, slice.len() as u64);
306 let amt = (&mut slice[(pos as usize)..]).write(buf)?;
307 *pos_mut += amt as u64;
312 fn slice_write_vectored(
315 bufs: &[IoSlice<'_>],
316 ) -> io::Result<usize> {
317 let mut nwritten = 0;
319 let n = slice_write(pos_mut, slice, buf)?;
328 // Resizing write implementation
329 fn vec_write(pos_mut: &mut u64, vec: &mut Vec<u8>, buf: &[u8]) -> io::Result<usize> {
330 let pos: usize = (*pos_mut).try_into().map_err(|_| {
332 ErrorKind::InvalidInput,
333 &"cursor position exceeds maximum possible vector length",
336 // Make sure the internal buffer is as least as big as where we
340 // use `resize` so that the zero filling is as efficient as possible
343 // Figure out what bytes will be used to overwrite what's currently
344 // there (left), and what will be appended on the end (right)
346 let space = vec.len() - pos;
347 let (left, right) = buf.split_at(cmp::min(space, buf.len()));
348 vec[pos..pos + left.len()].copy_from_slice(left);
349 vec.extend_from_slice(right);
353 *pos_mut = (pos + buf.len()) as u64;
357 fn vec_write_vectored(
360 bufs: &[IoSlice<'_>],
361 ) -> io::Result<usize> {
362 let mut nwritten = 0;
364 nwritten += vec_write(pos_mut, vec, buf)?;
369 #[stable(feature = "rust1", since = "1.0.0")]
370 impl Write for Cursor<&mut [u8]> {
372 fn write(&mut self, buf: &[u8]) -> io::Result<usize> {
373 slice_write(&mut self.pos, self.inner, buf)
377 fn write_vectored(&mut self, bufs: &[IoSlice<'_>]) -> io::Result<usize> {
378 slice_write_vectored(&mut self.pos, self.inner, bufs)
382 fn is_write_vectored(&self) -> bool {
387 fn flush(&mut self) -> io::Result<()> {
392 #[stable(feature = "cursor_mut_vec", since = "1.25.0")]
393 impl Write for Cursor<&mut Vec<u8>> {
394 fn write(&mut self, buf: &[u8]) -> io::Result<usize> {
395 vec_write(&mut self.pos, self.inner, buf)
398 fn write_vectored(&mut self, bufs: &[IoSlice<'_>]) -> io::Result<usize> {
399 vec_write_vectored(&mut self.pos, self.inner, bufs)
403 fn is_write_vectored(&self) -> bool {
408 fn flush(&mut self) -> io::Result<()> {
413 #[stable(feature = "rust1", since = "1.0.0")]
414 impl Write for Cursor<Vec<u8>> {
415 fn write(&mut self, buf: &[u8]) -> io::Result<usize> {
416 vec_write(&mut self.pos, &mut self.inner, buf)
419 fn write_vectored(&mut self, bufs: &[IoSlice<'_>]) -> io::Result<usize> {
420 vec_write_vectored(&mut self.pos, &mut self.inner, bufs)
424 fn is_write_vectored(&self) -> bool {
429 fn flush(&mut self) -> io::Result<()> {
434 #[stable(feature = "cursor_box_slice", since = "1.5.0")]
435 impl Write for Cursor<Box<[u8]>> {
437 fn write(&mut self, buf: &[u8]) -> io::Result<usize> {
438 slice_write(&mut self.pos, &mut self.inner, buf)
442 fn write_vectored(&mut self, bufs: &[IoSlice<'_>]) -> io::Result<usize> {
443 slice_write_vectored(&mut self.pos, &mut self.inner, bufs)
447 fn is_write_vectored(&self) -> bool {
452 fn flush(&mut self) -> io::Result<()> {