1//! A module for working with borrowed data.
2
3#![stable(feature = "rust1", since = "1.0.0")]
4
5#[stable(feature = "rust1", since = "1.0.0")]
6pub use core::borrow::{Borrow, BorrowMut};
7use core::cmp::Ordering;
8use core::hash::{Hash, Hasher};
9#[cfg(not(no_global_oom_handling))]
10use core::ops::{Add, AddAssign};
11use core::ops::{Deref, DerefPure};
12
13use Cow::*;
14
15use crate::fmt;
16#[cfg(not(no_global_oom_handling))]
17use crate::string::String;
18
19#[stable(feature = "rust1", since = "1.0.0")]
20impl<'a, B: ?Sized> Borrow<B> for Cow<'a, B>
21where
22 B: ToOwned,
23{
24 fn borrow(&self) -> &B {
25 &**self
26 }
27}
28
29/// A generalization of `Clone` to borrowed data.
30///
31/// Some types make it possible to go from borrowed to owned, usually by
32/// implementing the `Clone` trait. But `Clone` works only for going from `&T`
33/// to `T`. The `ToOwned` trait generalizes `Clone` to construct owned data
34/// from any borrow of a given type.
35#[rustc_diagnostic_item = "ToOwned"]
36#[stable(feature = "rust1", since = "1.0.0")]
37pub trait ToOwned {
38 /// The resulting type after obtaining ownership.
39 #[stable(feature = "rust1", since = "1.0.0")]
40 type Owned: Borrow<Self>;
41
42 /// Creates owned data from borrowed data, usually by cloning.
43 ///
44 /// # Examples
45 ///
46 /// Basic usage:
47 ///
48 /// ```
49 /// let s: &str = "a";
50 /// let ss: String = s.to_owned();
51 ///
52 /// let v: &[i32] = &[1, 2];
53 /// let vv: Vec<i32> = v.to_owned();
54 /// ```
55 #[stable(feature = "rust1", since = "1.0.0")]
56 #[must_use = "cloning is often expensive and is not expected to have side effects"]
57 #[rustc_diagnostic_item = "to_owned_method"]
58 fn to_owned(&self) -> Self::Owned;
59
60 /// Uses borrowed data to replace owned data, usually by cloning.
61 ///
62 /// This is borrow-generalized version of [`Clone::clone_from`].
63 ///
64 /// # Examples
65 ///
66 /// Basic usage:
67 ///
68 /// ```
69 /// let mut s: String = String::new();
70 /// "hello".clone_into(&mut s);
71 ///
72 /// let mut v: Vec<i32> = Vec::new();
73 /// [1, 2][..].clone_into(&mut v);
74 /// ```
75 #[stable(feature = "toowned_clone_into", since = "1.63.0")]
76 fn clone_into(&self, target: &mut Self::Owned) {
77 *target = self.to_owned();
78 }
79}
80
81#[stable(feature = "rust1", since = "1.0.0")]
82impl<T> ToOwned for T
83where
84 T: Clone,
85{
86 type Owned = T;
87 fn to_owned(&self) -> T {
88 self.clone()
89 }
90
91 fn clone_into(&self, target: &mut T) {
92 target.clone_from(self);
93 }
94}
95
96/// A clone-on-write smart pointer.
97///
98/// The type `Cow` is a smart pointer providing clone-on-write functionality: it
99/// can enclose and provide immutable access to borrowed data, and clone the
100/// data lazily when mutation or ownership is required. The type is designed to
101/// work with general borrowed data via the `Borrow` trait.
102///
103/// `Cow` implements `Deref`, which means that you can call
104/// non-mutating methods directly on the data it encloses. If mutation
105/// is desired, `to_mut` will obtain a mutable reference to an owned
106/// value, cloning if necessary.
107///
108/// If you need reference-counting pointers, note that
109/// [`Rc::make_mut`][crate::rc::Rc::make_mut] and
110/// [`Arc::make_mut`][crate::sync::Arc::make_mut] can provide clone-on-write
111/// functionality as well.
112///
113/// # Examples
114///
115/// ```
116/// use std::borrow::Cow;
117///
118/// fn abs_all(input: &mut Cow<'_, [i32]>) {
119/// for i in 0..input.len() {
120/// let v = input[i];
121/// if v < 0 {
122/// // Clones into a vector if not already owned.
123/// input.to_mut()[i] = -v;
124/// }
125/// }
126/// }
127///
128/// // No clone occurs because `input` doesn't need to be mutated.
129/// let slice = [0, 1, 2];
130/// let mut input = Cow::from(&slice[..]);
131/// abs_all(&mut input);
132///
133/// // Clone occurs because `input` needs to be mutated.
134/// let slice = [-1, 0, 1];
135/// let mut input = Cow::from(&slice[..]);
136/// abs_all(&mut input);
137///
138/// // No clone occurs because `input` is already owned.
139/// let mut input = Cow::from(vec![-1, 0, 1]);
140/// abs_all(&mut input);
141/// ```
142///
143/// Another example showing how to keep `Cow` in a struct:
144///
145/// ```
146/// use std::borrow::Cow;
147///
148/// struct Items<'a, X> where [X]: ToOwned<Owned = Vec<X>> {
149/// values: Cow<'a, [X]>,
150/// }
151///
152/// impl<'a, X: Clone + 'a> Items<'a, X> where [X]: ToOwned<Owned = Vec<X>> {
153/// fn new(v: Cow<'a, [X]>) -> Self {
154/// Items { values: v }
155/// }
156/// }
157///
158/// // Creates a container from borrowed values of a slice
159/// let readonly = [1, 2];
160/// let borrowed = Items::new((&readonly[..]).into());
161/// match borrowed {
162/// Items { values: Cow::Borrowed(b) } => println!("borrowed {b:?}"),
163/// _ => panic!("expect borrowed value"),
164/// }
165///
166/// let mut clone_on_write = borrowed;
167/// // Mutates the data from slice into owned vec and pushes a new value on top
168/// clone_on_write.values.to_mut().push(3);
169/// println!("clone_on_write = {:?}", clone_on_write.values);
170///
171/// // The data was mutated. Let's check it out.
172/// match clone_on_write {
173/// Items { values: Cow::Owned(_) } => println!("clone_on_write contains owned data"),
174/// _ => panic!("expect owned data"),
175/// }
176/// ```
177#[stable(feature = "rust1", since = "1.0.0")]
178#[rustc_diagnostic_item = "Cow"]
179pub enum Cow<'a, B: ?Sized + 'a>
180where
181 B: ToOwned,
182{
183 /// Borrowed data.
184 #[stable(feature = "rust1", since = "1.0.0")]
185 Borrowed(#[stable(feature = "rust1", since = "1.0.0")] &'a B),
186
187 /// Owned data.
188 #[stable(feature = "rust1", since = "1.0.0")]
189 Owned(#[stable(feature = "rust1", since = "1.0.0")] <B as ToOwned>::Owned),
190}
191
192#[stable(feature = "rust1", since = "1.0.0")]
193impl<B: ?Sized + ToOwned> Clone for Cow<'_, B> {
194 fn clone(&self) -> Self {
195 match *self {
196 Borrowed(b: &B) => Borrowed(b),
197 Owned(ref o: &::Owned) => {
198 let b: &B = o.borrow();
199 Owned(b.to_owned())
200 }
201 }
202 }
203
204 fn clone_from(&mut self, source: &Self) {
205 match (self, source) {
206 (&mut Owned(ref mut dest: &mut ::Owned), &Owned(ref o: &::Owned)) => o.borrow().clone_into(target:dest),
207 (t: &mut Cow<'_, B>, s: &Cow<'_, B>) => *t = s.clone(),
208 }
209 }
210}
211
212impl<B: ?Sized + ToOwned> Cow<'_, B> {
213 /// Returns true if the data is borrowed, i.e. if `to_mut` would require additional work.
214 ///
215 /// # Examples
216 ///
217 /// ```
218 /// #![feature(cow_is_borrowed)]
219 /// use std::borrow::Cow;
220 ///
221 /// let cow = Cow::Borrowed("moo");
222 /// assert!(cow.is_borrowed());
223 ///
224 /// let bull: Cow<'_, str> = Cow::Owned("...moo?".to_string());
225 /// assert!(!bull.is_borrowed());
226 /// ```
227 #[unstable(feature = "cow_is_borrowed", issue = "65143")]
228 pub const fn is_borrowed(&self) -> bool {
229 match *self {
230 Borrowed(_) => true,
231 Owned(_) => false,
232 }
233 }
234
235 /// Returns true if the data is owned, i.e. if `to_mut` would be a no-op.
236 ///
237 /// # Examples
238 ///
239 /// ```
240 /// #![feature(cow_is_borrowed)]
241 /// use std::borrow::Cow;
242 ///
243 /// let cow: Cow<'_, str> = Cow::Owned("moo".to_string());
244 /// assert!(cow.is_owned());
245 ///
246 /// let bull = Cow::Borrowed("...moo?");
247 /// assert!(!bull.is_owned());
248 /// ```
249 #[unstable(feature = "cow_is_borrowed", issue = "65143")]
250 pub const fn is_owned(&self) -> bool {
251 !self.is_borrowed()
252 }
253
254 /// Acquires a mutable reference to the owned form of the data.
255 ///
256 /// Clones the data if it is not already owned.
257 ///
258 /// # Examples
259 ///
260 /// ```
261 /// use std::borrow::Cow;
262 ///
263 /// let mut cow = Cow::Borrowed("foo");
264 /// cow.to_mut().make_ascii_uppercase();
265 ///
266 /// assert_eq!(
267 /// cow,
268 /// Cow::Owned(String::from("FOO")) as Cow<'_, str>
269 /// );
270 /// ```
271 #[stable(feature = "rust1", since = "1.0.0")]
272 pub fn to_mut(&mut self) -> &mut <B as ToOwned>::Owned {
273 match *self {
274 Borrowed(borrowed) => {
275 *self = Owned(borrowed.to_owned());
276 match *self {
277 Borrowed(..) => unreachable!(),
278 Owned(ref mut owned) => owned,
279 }
280 }
281 Owned(ref mut owned) => owned,
282 }
283 }
284
285 /// Extracts the owned data.
286 ///
287 /// Clones the data if it is not already owned.
288 ///
289 /// # Examples
290 ///
291 /// Calling `into_owned` on a `Cow::Borrowed` returns a clone of the borrowed data:
292 ///
293 /// ```
294 /// use std::borrow::Cow;
295 ///
296 /// let s = "Hello world!";
297 /// let cow = Cow::Borrowed(s);
298 ///
299 /// assert_eq!(
300 /// cow.into_owned(),
301 /// String::from(s)
302 /// );
303 /// ```
304 ///
305 /// Calling `into_owned` on a `Cow::Owned` returns the owned data. The data is moved out of the
306 /// `Cow` without being cloned.
307 ///
308 /// ```
309 /// use std::borrow::Cow;
310 ///
311 /// let s = "Hello world!";
312 /// let cow: Cow<'_, str> = Cow::Owned(String::from(s));
313 ///
314 /// assert_eq!(
315 /// cow.into_owned(),
316 /// String::from(s)
317 /// );
318 /// ```
319 #[stable(feature = "rust1", since = "1.0.0")]
320 pub fn into_owned(self) -> <B as ToOwned>::Owned {
321 match self {
322 Borrowed(borrowed) => borrowed.to_owned(),
323 Owned(owned) => owned,
324 }
325 }
326}
327
328#[stable(feature = "rust1", since = "1.0.0")]
329impl<B: ?Sized + ToOwned> Deref for Cow<'_, B>
330where
331 B::Owned: Borrow<B>,
332{
333 type Target = B;
334
335 fn deref(&self) -> &B {
336 match *self {
337 Borrowed(borrowed: &B) => borrowed,
338 Owned(ref owned: &impl Borrow) => owned.borrow(),
339 }
340 }
341}
342
343// `Cow<'_, T>` can only implement `DerefPure` if `<T::Owned as Borrow<T>>` (and `BorrowMut<T>`) is trusted.
344// For now, we restrict `DerefPure for Cow<T>` to `T: Sized` (`T as Borrow<T>` is trusted),
345// `str` (`String as Borrow<str>` is trusted) and `[T]` (`Vec<T> as Borrow<[T]>` is trusted).
346// In the future, a `BorrowPure<T>` trait analogous to `DerefPure` might generalize this.
347#[unstable(feature = "deref_pure_trait", issue = "87121")]
348unsafe impl<T: Clone> DerefPure for Cow<'_, T> {}
349#[cfg(not(no_global_oom_handling))]
350#[unstable(feature = "deref_pure_trait", issue = "87121")]
351unsafe impl DerefPure for Cow<'_, str> {}
352#[cfg(not(no_global_oom_handling))]
353#[unstable(feature = "deref_pure_trait", issue = "87121")]
354unsafe impl<T: Clone> DerefPure for Cow<'_, [T]> {}
355
356#[stable(feature = "rust1", since = "1.0.0")]
357impl<B: ?Sized> Eq for Cow<'_, B> where B: Eq + ToOwned {}
358
359#[stable(feature = "rust1", since = "1.0.0")]
360impl<B: ?Sized> Ord for Cow<'_, B>
361where
362 B: Ord + ToOwned,
363{
364 #[inline]
365 fn cmp(&self, other: &Self) -> Ordering {
366 Ord::cmp(&**self, &**other)
367 }
368}
369
370#[stable(feature = "rust1", since = "1.0.0")]
371impl<'a, 'b, B: ?Sized, C: ?Sized> PartialEq<Cow<'b, C>> for Cow<'a, B>
372where
373 B: PartialEq<C> + ToOwned,
374 C: ToOwned,
375{
376 #[inline]
377 fn eq(&self, other: &Cow<'b, C>) -> bool {
378 PartialEq::eq(&**self, &**other)
379 }
380}
381
382#[stable(feature = "rust1", since = "1.0.0")]
383impl<'a, B: ?Sized> PartialOrd for Cow<'a, B>
384where
385 B: PartialOrd + ToOwned,
386{
387 #[inline]
388 fn partial_cmp(&self, other: &Cow<'a, B>) -> Option<Ordering> {
389 PartialOrd::partial_cmp(&**self, &**other)
390 }
391}
392
393#[stable(feature = "rust1", since = "1.0.0")]
394impl<B: ?Sized> fmt::Debug for Cow<'_, B>
395where
396 B: fmt::Debug + ToOwned<Owned: fmt::Debug>,
397{
398 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
399 match *self {
400 Borrowed(ref b: &&B) => fmt::Debug::fmt(self:b, f),
401 Owned(ref o: &impl Debug) => fmt::Debug::fmt(self:o, f),
402 }
403 }
404}
405
406#[stable(feature = "rust1", since = "1.0.0")]
407impl<B: ?Sized> fmt::Display for Cow<'_, B>
408where
409 B: fmt::Display + ToOwned<Owned: fmt::Display>,
410{
411 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
412 match *self {
413 Borrowed(ref b: &&B) => fmt::Display::fmt(self:b, f),
414 Owned(ref o: &impl Display) => fmt::Display::fmt(self:o, f),
415 }
416 }
417}
418
419#[stable(feature = "default", since = "1.11.0")]
420impl<B: ?Sized> Default for Cow<'_, B>
421where
422 B: ToOwned<Owned: Default>,
423{
424 /// Creates an owned Cow<'a, B> with the default value for the contained owned value.
425 fn default() -> Self {
426 Owned(<B as ToOwned>::Owned::default())
427 }
428}
429
430#[stable(feature = "rust1", since = "1.0.0")]
431impl<B: ?Sized> Hash for Cow<'_, B>
432where
433 B: Hash + ToOwned,
434{
435 #[inline]
436 fn hash<H: Hasher>(&self, state: &mut H) {
437 Hash::hash(&**self, state)
438 }
439}
440
441#[stable(feature = "rust1", since = "1.0.0")]
442impl<T: ?Sized + ToOwned> AsRef<T> for Cow<'_, T> {
443 fn as_ref(&self) -> &T {
444 self
445 }
446}
447
448#[cfg(not(no_global_oom_handling))]
449#[stable(feature = "cow_add", since = "1.14.0")]
450impl<'a> Add<&'a str> for Cow<'a, str> {
451 type Output = Cow<'a, str>;
452
453 #[inline]
454 fn add(mut self, rhs: &'a str) -> Self::Output {
455 self += rhs;
456 self
457 }
458}
459
460#[cfg(not(no_global_oom_handling))]
461#[stable(feature = "cow_add", since = "1.14.0")]
462impl<'a> Add<Cow<'a, str>> for Cow<'a, str> {
463 type Output = Cow<'a, str>;
464
465 #[inline]
466 fn add(mut self, rhs: Cow<'a, str>) -> Self::Output {
467 self += rhs;
468 self
469 }
470}
471
472#[cfg(not(no_global_oom_handling))]
473#[stable(feature = "cow_add", since = "1.14.0")]
474impl<'a> AddAssign<&'a str> for Cow<'a, str> {
475 fn add_assign(&mut self, rhs: &'a str) {
476 if self.is_empty() {
477 *self = Cow::Borrowed(rhs)
478 } else if !rhs.is_empty() {
479 if let Cow::Borrowed(lhs: &str) = *self {
480 let mut s: String = String::with_capacity(lhs.len() + rhs.len());
481 s.push_str(string:lhs);
482 *self = Cow::Owned(s);
483 }
484 self.to_mut().push_str(string:rhs);
485 }
486 }
487}
488
489#[cfg(not(no_global_oom_handling))]
490#[stable(feature = "cow_add", since = "1.14.0")]
491impl<'a> AddAssign<Cow<'a, str>> for Cow<'a, str> {
492 fn add_assign(&mut self, rhs: Cow<'a, str>) {
493 if self.is_empty() {
494 *self = rhs
495 } else if !rhs.is_empty() {
496 if let Cow::Borrowed(lhs: &str) = *self {
497 let mut s: String = String::with_capacity(lhs.len() + rhs.len());
498 s.push_str(string:lhs);
499 *self = Cow::Owned(s);
500 }
501 self.to_mut().push_str(&rhs);
502 }
503 }
504}
505