Loading...
Note: File does not exist in v5.9.
1// SPDX-License-Identifier: Apache-2.0 OR MIT
2
3//! A module for working with borrowed data.
4
5#![stable(feature = "rust1", since = "1.0.0")]
6
7use core::cmp::Ordering;
8use core::hash::{Hash, Hasher};
9use core::ops::Deref;
10#[cfg(not(no_global_oom_handling))]
11use core::ops::{Add, AddAssign};
12
13#[stable(feature = "rust1", since = "1.0.0")]
14pub use core::borrow::{Borrow, BorrowMut};
15
16use core::fmt;
17#[cfg(not(no_global_oom_handling))]
18use crate::string::String;
19
20use Cow::*;
21
22#[stable(feature = "rust1", since = "1.0.0")]
23impl<'a, B: ?Sized> Borrow<B> for Cow<'a, B>
24where
25 B: ToOwned,
26 <B as ToOwned>::Owned: 'a,
27{
28 fn borrow(&self) -> &B {
29 &**self
30 }
31}
32
33/// A generalization of `Clone` to borrowed data.
34///
35/// Some types make it possible to go from borrowed to owned, usually by
36/// implementing the `Clone` trait. But `Clone` works only for going from `&T`
37/// to `T`. The `ToOwned` trait generalizes `Clone` to construct owned data
38/// from any borrow of a given type.
39#[cfg_attr(not(test), rustc_diagnostic_item = "ToOwned")]
40#[stable(feature = "rust1", since = "1.0.0")]
41pub trait ToOwned {
42 /// The resulting type after obtaining ownership.
43 #[stable(feature = "rust1", since = "1.0.0")]
44 type Owned: Borrow<Self>;
45
46 /// Creates owned data from borrowed data, usually by cloning.
47 ///
48 /// # Examples
49 ///
50 /// Basic usage:
51 ///
52 /// ```
53 /// let s: &str = "a";
54 /// let ss: String = s.to_owned();
55 ///
56 /// let v: &[i32] = &[1, 2];
57 /// let vv: Vec<i32> = v.to_owned();
58 /// ```
59 #[stable(feature = "rust1", since = "1.0.0")]
60 #[must_use = "cloning is often expensive and is not expected to have side effects"]
61 fn to_owned(&self) -> Self::Owned;
62
63 /// Uses borrowed data to replace owned data, usually by cloning.
64 ///
65 /// This is borrow-generalized version of `Clone::clone_from`.
66 ///
67 /// # Examples
68 ///
69 /// Basic usage:
70 ///
71 /// ```
72 /// # #![feature(toowned_clone_into)]
73 /// let mut s: String = String::new();
74 /// "hello".clone_into(&mut s);
75 ///
76 /// let mut v: Vec<i32> = Vec::new();
77 /// [1, 2][..].clone_into(&mut v);
78 /// ```
79 #[unstable(feature = "toowned_clone_into", reason = "recently added", issue = "41263")]
80 fn clone_into(&self, target: &mut Self::Owned) {
81 *target = self.to_owned();
82 }
83}
84
85#[stable(feature = "rust1", since = "1.0.0")]
86impl<T> ToOwned for T
87where
88 T: Clone,
89{
90 type Owned = T;
91 fn to_owned(&self) -> T {
92 self.clone()
93 }
94
95 fn clone_into(&self, target: &mut T) {
96 target.clone_from(self);
97 }
98}
99
100/// A clone-on-write smart pointer.
101///
102/// The type `Cow` is a smart pointer providing clone-on-write functionality: it
103/// can enclose and provide immutable access to borrowed data, and clone the
104/// data lazily when mutation or ownership is required. The type is designed to
105/// work with general borrowed data via the `Borrow` trait.
106///
107/// `Cow` implements `Deref`, which means that you can call
108/// non-mutating methods directly on the data it encloses. If mutation
109/// is desired, `to_mut` will obtain a mutable reference to an owned
110/// value, cloning if necessary.
111///
112/// If you need reference-counting pointers, note that
113/// [`Rc::make_mut`][crate::rc::Rc::make_mut] and
114/// [`Arc::make_mut`][crate::sync::Arc::make_mut] can provide clone-on-write
115/// functionality as well.
116///
117/// # Examples
118///
119/// ```
120/// use std::borrow::Cow;
121///
122/// fn abs_all(input: &mut Cow<[i32]>) {
123/// for i in 0..input.len() {
124/// let v = input[i];
125/// if v < 0 {
126/// // Clones into a vector if not already owned.
127/// input.to_mut()[i] = -v;
128/// }
129/// }
130/// }
131///
132/// // No clone occurs because `input` doesn't need to be mutated.
133/// let slice = [0, 1, 2];
134/// let mut input = Cow::from(&slice[..]);
135/// abs_all(&mut input);
136///
137/// // Clone occurs because `input` needs to be mutated.
138/// let slice = [-1, 0, 1];
139/// let mut input = Cow::from(&slice[..]);
140/// abs_all(&mut input);
141///
142/// // No clone occurs because `input` is already owned.
143/// let mut input = Cow::from(vec![-1, 0, 1]);
144/// abs_all(&mut input);
145/// ```
146///
147/// Another example showing how to keep `Cow` in a struct:
148///
149/// ```
150/// use std::borrow::Cow;
151///
152/// struct Items<'a, X: 'a> where [X]: ToOwned<Owned = Vec<X>> {
153/// values: Cow<'a, [X]>,
154/// }
155///
156/// impl<'a, X: Clone + 'a> Items<'a, X> where [X]: ToOwned<Owned = Vec<X>> {
157/// fn new(v: Cow<'a, [X]>) -> Self {
158/// Items { values: v }
159/// }
160/// }
161///
162/// // Creates a container from borrowed values of a slice
163/// let readonly = [1, 2];
164/// let borrowed = Items::new((&readonly[..]).into());
165/// match borrowed {
166/// Items { values: Cow::Borrowed(b) } => println!("borrowed {b:?}"),
167/// _ => panic!("expect borrowed value"),
168/// }
169///
170/// let mut clone_on_write = borrowed;
171/// // Mutates the data from slice into owned vec and pushes a new value on top
172/// clone_on_write.values.to_mut().push(3);
173/// println!("clone_on_write = {:?}", clone_on_write.values);
174///
175/// // The data was mutated. Let's check it out.
176/// match clone_on_write {
177/// Items { values: Cow::Owned(_) } => println!("clone_on_write contains owned data"),
178/// _ => panic!("expect owned data"),
179/// }
180/// ```
181#[stable(feature = "rust1", since = "1.0.0")]
182#[cfg_attr(not(test), rustc_diagnostic_item = "Cow")]
183pub enum Cow<'a, B: ?Sized + 'a>
184where
185 B: ToOwned,
186{
187 /// Borrowed data.
188 #[stable(feature = "rust1", since = "1.0.0")]
189 Borrowed(#[stable(feature = "rust1", since = "1.0.0")] &'a B),
190
191 /// Owned data.
192 #[stable(feature = "rust1", since = "1.0.0")]
193 Owned(#[stable(feature = "rust1", since = "1.0.0")] <B as ToOwned>::Owned),
194}
195
196#[stable(feature = "rust1", since = "1.0.0")]
197impl<B: ?Sized + ToOwned> Clone for Cow<'_, B> {
198 fn clone(&self) -> Self {
199 match *self {
200 Borrowed(b) => Borrowed(b),
201 Owned(ref o) => {
202 let b: &B = o.borrow();
203 Owned(b.to_owned())
204 }
205 }
206 }
207
208 fn clone_from(&mut self, source: &Self) {
209 match (self, source) {
210 (&mut Owned(ref mut dest), &Owned(ref o)) => o.borrow().clone_into(dest),
211 (t, s) => *t = s.clone(),
212 }
213 }
214}
215
216impl<B: ?Sized + ToOwned> Cow<'_, B> {
217 /// Returns true if the data is borrowed, i.e. if `to_mut` would require additional work.
218 ///
219 /// # Examples
220 ///
221 /// ```
222 /// #![feature(cow_is_borrowed)]
223 /// use std::borrow::Cow;
224 ///
225 /// let cow = Cow::Borrowed("moo");
226 /// assert!(cow.is_borrowed());
227 ///
228 /// let bull: Cow<'_, str> = Cow::Owned("...moo?".to_string());
229 /// assert!(!bull.is_borrowed());
230 /// ```
231 #[unstable(feature = "cow_is_borrowed", issue = "65143")]
232 #[rustc_const_unstable(feature = "const_cow_is_borrowed", issue = "65143")]
233 pub const fn is_borrowed(&self) -> bool {
234 match *self {
235 Borrowed(_) => true,
236 Owned(_) => false,
237 }
238 }
239
240 /// Returns true if the data is owned, i.e. if `to_mut` would be a no-op.
241 ///
242 /// # Examples
243 ///
244 /// ```
245 /// #![feature(cow_is_borrowed)]
246 /// use std::borrow::Cow;
247 ///
248 /// let cow: Cow<'_, str> = Cow::Owned("moo".to_string());
249 /// assert!(cow.is_owned());
250 ///
251 /// let bull = Cow::Borrowed("...moo?");
252 /// assert!(!bull.is_owned());
253 /// ```
254 #[unstable(feature = "cow_is_borrowed", issue = "65143")]
255 #[rustc_const_unstable(feature = "const_cow_is_borrowed", issue = "65143")]
256 pub const fn is_owned(&self) -> bool {
257 !self.is_borrowed()
258 }
259
260 /// Acquires a mutable reference to the owned form of the data.
261 ///
262 /// Clones the data if it is not already owned.
263 ///
264 /// # Examples
265 ///
266 /// ```
267 /// use std::borrow::Cow;
268 ///
269 /// let mut cow = Cow::Borrowed("foo");
270 /// cow.to_mut().make_ascii_uppercase();
271 ///
272 /// assert_eq!(
273 /// cow,
274 /// Cow::Owned(String::from("FOO")) as Cow<str>
275 /// );
276 /// ```
277 #[stable(feature = "rust1", since = "1.0.0")]
278 pub fn to_mut(&mut self) -> &mut <B as ToOwned>::Owned {
279 match *self {
280 Borrowed(borrowed) => {
281 *self = Owned(borrowed.to_owned());
282 match *self {
283 Borrowed(..) => unreachable!(),
284 Owned(ref mut owned) => owned,
285 }
286 }
287 Owned(ref mut owned) => owned,
288 }
289 }
290
291 /// Extracts the owned data.
292 ///
293 /// Clones the data if it is not already owned.
294 ///
295 /// # Examples
296 ///
297 /// Calling `into_owned` on a `Cow::Borrowed` returns a clone of the borrowed data:
298 ///
299 /// ```
300 /// use std::borrow::Cow;
301 ///
302 /// let s = "Hello world!";
303 /// let cow = Cow::Borrowed(s);
304 ///
305 /// assert_eq!(
306 /// cow.into_owned(),
307 /// String::from(s)
308 /// );
309 /// ```
310 ///
311 /// Calling `into_owned` on a `Cow::Owned` returns the owned data. The data is moved out of the
312 /// `Cow` without being cloned.
313 ///
314 /// ```
315 /// use std::borrow::Cow;
316 ///
317 /// let s = "Hello world!";
318 /// let cow: Cow<str> = Cow::Owned(String::from(s));
319 ///
320 /// assert_eq!(
321 /// cow.into_owned(),
322 /// String::from(s)
323 /// );
324 /// ```
325 #[stable(feature = "rust1", since = "1.0.0")]
326 pub fn into_owned(self) -> <B as ToOwned>::Owned {
327 match self {
328 Borrowed(borrowed) => borrowed.to_owned(),
329 Owned(owned) => owned,
330 }
331 }
332}
333
334#[stable(feature = "rust1", since = "1.0.0")]
335#[rustc_const_unstable(feature = "const_deref", issue = "88955")]
336impl<B: ?Sized + ToOwned> const Deref for Cow<'_, B>
337where
338 B::Owned: ~const Borrow<B>,
339{
340 type Target = B;
341
342 fn deref(&self) -> &B {
343 match *self {
344 Borrowed(borrowed) => borrowed,
345 Owned(ref owned) => owned.borrow(),
346 }
347 }
348}
349
350#[stable(feature = "rust1", since = "1.0.0")]
351impl<B: ?Sized> Eq for Cow<'_, B> where B: Eq + ToOwned {}
352
353#[stable(feature = "rust1", since = "1.0.0")]
354impl<B: ?Sized> Ord for Cow<'_, B>
355where
356 B: Ord + ToOwned,
357{
358 #[inline]
359 fn cmp(&self, other: &Self) -> Ordering {
360 Ord::cmp(&**self, &**other)
361 }
362}
363
364#[stable(feature = "rust1", since = "1.0.0")]
365impl<'a, 'b, B: ?Sized, C: ?Sized> PartialEq<Cow<'b, C>> for Cow<'a, B>
366where
367 B: PartialEq<C> + ToOwned,
368 C: ToOwned,
369{
370 #[inline]
371 fn eq(&self, other: &Cow<'b, C>) -> bool {
372 PartialEq::eq(&**self, &**other)
373 }
374}
375
376#[stable(feature = "rust1", since = "1.0.0")]
377impl<'a, B: ?Sized> PartialOrd for Cow<'a, B>
378where
379 B: PartialOrd + ToOwned,
380{
381 #[inline]
382 fn partial_cmp(&self, other: &Cow<'a, B>) -> Option<Ordering> {
383 PartialOrd::partial_cmp(&**self, &**other)
384 }
385}
386
387#[stable(feature = "rust1", since = "1.0.0")]
388impl<B: ?Sized> fmt::Debug for Cow<'_, B>
389where
390 B: fmt::Debug + ToOwned<Owned: fmt::Debug>,
391{
392 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
393 match *self {
394 Borrowed(ref b) => fmt::Debug::fmt(b, f),
395 Owned(ref o) => fmt::Debug::fmt(o, f),
396 }
397 }
398}
399
400#[stable(feature = "rust1", since = "1.0.0")]
401impl<B: ?Sized> fmt::Display for Cow<'_, B>
402where
403 B: fmt::Display + ToOwned<Owned: fmt::Display>,
404{
405 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
406 match *self {
407 Borrowed(ref b) => fmt::Display::fmt(b, f),
408 Owned(ref o) => fmt::Display::fmt(o, f),
409 }
410 }
411}
412
413#[stable(feature = "default", since = "1.11.0")]
414impl<B: ?Sized> Default for Cow<'_, B>
415where
416 B: ToOwned<Owned: Default>,
417{
418 /// Creates an owned Cow<'a, B> with the default value for the contained owned value.
419 fn default() -> Self {
420 Owned(<B as ToOwned>::Owned::default())
421 }
422}
423
424#[stable(feature = "rust1", since = "1.0.0")]
425impl<B: ?Sized> Hash for Cow<'_, B>
426where
427 B: Hash + ToOwned,
428{
429 #[inline]
430 fn hash<H: Hasher>(&self, state: &mut H) {
431 Hash::hash(&**self, state)
432 }
433}
434
435#[stable(feature = "rust1", since = "1.0.0")]
436impl<T: ?Sized + ToOwned> AsRef<T> for Cow<'_, T> {
437 fn as_ref(&self) -> &T {
438 self
439 }
440}
441
442#[cfg(not(no_global_oom_handling))]
443#[stable(feature = "cow_add", since = "1.14.0")]
444impl<'a> Add<&'a str> for Cow<'a, str> {
445 type Output = Cow<'a, str>;
446
447 #[inline]
448 fn add(mut self, rhs: &'a str) -> Self::Output {
449 self += rhs;
450 self
451 }
452}
453
454#[cfg(not(no_global_oom_handling))]
455#[stable(feature = "cow_add", since = "1.14.0")]
456impl<'a> Add<Cow<'a, str>> for Cow<'a, str> {
457 type Output = Cow<'a, str>;
458
459 #[inline]
460 fn add(mut self, rhs: Cow<'a, str>) -> Self::Output {
461 self += rhs;
462 self
463 }
464}
465
466#[cfg(not(no_global_oom_handling))]
467#[stable(feature = "cow_add", since = "1.14.0")]
468impl<'a> AddAssign<&'a str> for Cow<'a, str> {
469 fn add_assign(&mut self, rhs: &'a str) {
470 if self.is_empty() {
471 *self = Cow::Borrowed(rhs)
472 } else if !rhs.is_empty() {
473 if let Cow::Borrowed(lhs) = *self {
474 let mut s = String::with_capacity(lhs.len() + rhs.len());
475 s.push_str(lhs);
476 *self = Cow::Owned(s);
477 }
478 self.to_mut().push_str(rhs);
479 }
480 }
481}
482
483#[cfg(not(no_global_oom_handling))]
484#[stable(feature = "cow_add", since = "1.14.0")]
485impl<'a> AddAssign<Cow<'a, str>> for Cow<'a, str> {
486 fn add_assign(&mut self, rhs: Cow<'a, str>) {
487 if self.is_empty() {
488 *self = rhs
489 } else if !rhs.is_empty() {
490 if let Cow::Borrowed(lhs) = *self {
491 let mut s = String::with_capacity(lhs.len() + rhs.len());
492 s.push_str(lhs);
493 *self = Cow::Owned(s);
494 }
495 self.to_mut().push_str(&rhs);
496 }
497 }
498}