1 //! A module for working with borrowed data.
3 #![stable(feature = "rust1", since = "1.0.0")]
5 use core::cmp::Ordering;
6 use core::hash::{Hash, Hasher};
7 use core::ops::{Add, AddAssign, Deref};
9 #[stable(feature = "rust1", since = "1.0.0")]
10 pub use core::borrow::{Borrow, BorrowMut};
13 use crate::string::String;
17 #[stable(feature = "rust1", since = "1.0.0")]
18 impl<'a, B: ?Sized> Borrow<B> for Cow<'a, B>
20 <B as ToOwned>::Owned: 'a
22 fn borrow(&self) -> &B {
27 /// A generalization of `Clone` to borrowed data.
29 /// Some types make it possible to go from borrowed to owned, usually by
30 /// implementing the `Clone` trait. But `Clone` works only for going from `&T`
31 /// to `T`. The `ToOwned` trait generalizes `Clone` to construct owned data
32 /// from any borrow of a given type.
33 #[stable(feature = "rust1", since = "1.0.0")]
35 /// The resulting type after obtaining ownership.
36 #[stable(feature = "rust1", since = "1.0.0")]
37 type Owned: Borrow<Self>;
39 /// Creates owned data from borrowed data, usually by cloning.
46 /// let s: &str = "a";
47 /// let ss: String = s.to_owned();
49 /// let v: &[i32] = &[1, 2];
50 /// let vv: Vec<i32> = v.to_owned();
52 #[stable(feature = "rust1", since = "1.0.0")]
53 #[must_use = "cloning is often expensive and is not expected to have side effects"]
54 fn to_owned(&self) -> Self::Owned;
56 /// Uses borrowed data to replace owned data, usually by cloning.
58 /// This is borrow-generalized version of `Clone::clone_from`.
65 /// # #![feature(toowned_clone_into)]
66 /// let mut s: String = String::new();
67 /// "hello".clone_into(&mut s);
69 /// let mut v: Vec<i32> = Vec::new();
70 /// [1, 2][..].clone_into(&mut v);
72 #[unstable(feature = "toowned_clone_into",
73 reason = "recently added",
75 fn clone_into(&self, target: &mut Self::Owned) {
76 *target = self.to_owned();
80 #[stable(feature = "rust1", since = "1.0.0")]
85 fn to_owned(&self) -> T {
89 fn clone_into(&self, target: &mut T) {
90 target.clone_from(self);
94 /// A clone-on-write smart pointer.
96 /// The type `Cow` is a smart pointer providing clone-on-write functionality: it
97 /// can enclose and provide immutable access to borrowed data, and clone the
98 /// data lazily when mutation or ownership is required. The type is designed to
99 /// work with general borrowed data via the `Borrow` trait.
101 /// `Cow` implements `Deref`, which means that you can call
102 /// non-mutating methods directly on the data it encloses. If mutation
103 /// is desired, `to_mut` will obtain a mutable reference to an owned
104 /// value, cloning if necessary.
109 /// use std::borrow::Cow;
111 /// fn abs_all(input: &mut Cow<[i32]>) {
112 /// for i in 0..input.len() {
113 /// let v = input[i];
115 /// // Clones into a vector if not already owned.
116 /// input.to_mut()[i] = -v;
121 /// // No clone occurs because `input` doesn't need to be mutated.
122 /// let slice = [0, 1, 2];
123 /// let mut input = Cow::from(&slice[..]);
124 /// abs_all(&mut input);
126 /// // Clone occurs because `input` needs to be mutated.
127 /// let slice = [-1, 0, 1];
128 /// let mut input = Cow::from(&slice[..]);
129 /// abs_all(&mut input);
131 /// // No clone occurs because `input` is already owned.
132 /// let mut input = Cow::from(vec![-1, 0, 1]);
133 /// abs_all(&mut input);
136 /// Another example showing how to keep `Cow` in a struct:
139 /// use std::borrow::Cow;
141 /// struct Items<'a, X: 'a> where [X]: ToOwned<Owned = Vec<X>> {
142 /// values: Cow<'a, [X]>,
145 /// impl<'a, X: Clone + 'a> Items<'a, X> where [X]: ToOwned<Owned = Vec<X>> {
146 /// fn new(v: Cow<'a, [X]>) -> Self {
147 /// Items { values: v }
151 /// // Creates a container from borrowed values of a slice
152 /// let readonly = [1, 2];
153 /// let borrowed = Items::new((&readonly[..]).into());
155 /// Items { values: Cow::Borrowed(b) } => println!("borrowed {:?}", b),
156 /// _ => panic!("expect borrowed value"),
159 /// let mut clone_on_write = borrowed;
160 /// // Mutates the data from slice into owned vec and pushes a new value on top
161 /// clone_on_write.values.to_mut().push(3);
162 /// println!("clone_on_write = {:?}", clone_on_write.values);
164 /// // The data was mutated. Let check it out.
165 /// match clone_on_write {
166 /// Items { values: Cow::Owned(_) } => println!("clone_on_write contains owned data"),
167 /// _ => panic!("expect owned data"),
170 #[stable(feature = "rust1", since = "1.0.0")]
171 pub enum Cow<'a, B: ?Sized + 'a>
175 #[stable(feature = "rust1", since = "1.0.0")]
176 Borrowed(#[stable(feature = "rust1", since = "1.0.0")]
180 #[stable(feature = "rust1", since = "1.0.0")]
181 Owned(#[stable(feature = "rust1", since = "1.0.0")]
182 <B as ToOwned>::Owned),
185 #[stable(feature = "rust1", since = "1.0.0")]
186 impl<B: ?Sized + ToOwned> Clone for Cow<'_, B> {
187 fn clone(&self) -> Self {
189 Borrowed(b) => Borrowed(b),
191 let b: &B = o.borrow();
197 fn clone_from(&mut self, source: &Self) {
198 if let Owned(ref mut dest) = *self {
199 if let Owned(ref o) = *source {
200 o.borrow().clone_into(dest);
205 *self = source.clone();
209 impl<B: ?Sized + ToOwned> Cow<'_, B> {
210 /// Returns true if the data is borrowed, i.e. if `to_mut` would require additional work.
215 /// #![feature(cow_is_borrowed)]
216 /// use std::borrow::Cow;
218 /// let cow = Cow::Borrowed("moo");
219 /// assert!(cow.is_borrowed());
221 /// let bull: Cow<'_, str> = Cow::Owned("...moo?".to_string());
222 /// assert!(!bull.is_borrowed());
224 #[unstable(feature = "cow_is_borrowed", issue = "65143")]
225 pub fn is_borrowed(&self) -> bool {
232 /// Returns true if the data is owned, i.e. if `to_mut` would be a no-op.
237 /// #![feature(cow_is_borrowed)]
238 /// use std::borrow::Cow;
240 /// let cow: Cow<'_, str> = Cow::Owned("moo".to_string());
241 /// assert!(cow.is_owned());
243 /// let bull = Cow::Borrowed("...moo?");
244 /// assert!(!bull.is_owned());
246 #[unstable(feature = "cow_is_borrowed", issue = "65143")]
247 pub fn is_owned(&self) -> bool {
251 /// Acquires a mutable reference to the owned form of the data.
253 /// Clones the data if it is not already owned.
258 /// use std::borrow::Cow;
260 /// let mut cow = Cow::Borrowed("foo");
261 /// cow.to_mut().make_ascii_uppercase();
265 /// Cow::Owned(String::from("FOO")) as Cow<str>
268 #[stable(feature = "rust1", since = "1.0.0")]
269 pub fn to_mut(&mut self) -> &mut <B as ToOwned>::Owned {
271 Borrowed(borrowed) => {
272 *self = Owned(borrowed.to_owned());
274 Borrowed(..) => unreachable!(),
275 Owned(ref mut owned) => owned,
278 Owned(ref mut owned) => owned,
282 /// Extracts the owned data.
284 /// Clones the data if it is not already owned.
288 /// Calling `into_owned` on a `Cow::Borrowed` clones the underlying data
289 /// and becomes a `Cow::Owned`:
292 /// use std::borrow::Cow;
294 /// let s = "Hello world!";
295 /// let cow = Cow::Borrowed(s);
298 /// cow.into_owned(),
303 /// Calling `into_owned` on a `Cow::Owned` is a no-op:
306 /// use std::borrow::Cow;
308 /// let s = "Hello world!";
309 /// let cow: Cow<str> = Cow::Owned(String::from(s));
312 /// cow.into_owned(),
316 #[stable(feature = "rust1", since = "1.0.0")]
317 pub fn into_owned(self) -> <B as ToOwned>::Owned {
319 Borrowed(borrowed) => borrowed.to_owned(),
320 Owned(owned) => owned,
325 #[stable(feature = "rust1", since = "1.0.0")]
326 impl<B: ?Sized + ToOwned> Deref for Cow<'_, B> {
329 fn deref(&self) -> &B {
331 Borrowed(borrowed) => borrowed,
332 Owned(ref owned) => owned.borrow(),
337 #[stable(feature = "rust1", since = "1.0.0")]
338 impl<B: ?Sized> Eq for Cow<'_, B> where B: Eq + ToOwned {}
340 #[stable(feature = "rust1", since = "1.0.0")]
341 impl<B: ?Sized> Ord for Cow<'_, B>
342 where B: Ord + ToOwned
345 fn cmp(&self, other: &Self) -> Ordering {
346 Ord::cmp(&**self, &**other)
350 #[stable(feature = "rust1", since = "1.0.0")]
351 impl<'a, 'b, B: ?Sized, C: ?Sized> PartialEq<Cow<'b, C>> for Cow<'a, B>
352 where B: PartialEq<C> + ToOwned,
356 fn eq(&self, other: &Cow<'b, C>) -> bool {
357 PartialEq::eq(&**self, &**other)
361 #[stable(feature = "rust1", since = "1.0.0")]
362 impl<'a, B: ?Sized> PartialOrd for Cow<'a, B>
363 where B: PartialOrd + ToOwned
366 fn partial_cmp(&self, other: &Cow<'a, B>) -> Option<Ordering> {
367 PartialOrd::partial_cmp(&**self, &**other)
371 #[stable(feature = "rust1", since = "1.0.0")]
372 impl<B: ?Sized> fmt::Debug for Cow<'_, B>
374 B: fmt::Debug + ToOwned<Owned: fmt::Debug>,
376 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
378 Borrowed(ref b) => fmt::Debug::fmt(b, f),
379 Owned(ref o) => fmt::Debug::fmt(o, f),
384 #[stable(feature = "rust1", since = "1.0.0")]
385 impl<B: ?Sized> fmt::Display for Cow<'_, B>
387 B: fmt::Display + ToOwned<Owned: fmt::Display>,
389 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
391 Borrowed(ref b) => fmt::Display::fmt(b, f),
392 Owned(ref o) => fmt::Display::fmt(o, f),
397 #[stable(feature = "default", since = "1.11.0")]
398 impl<B: ?Sized> Default for Cow<'_, B>
400 B: ToOwned<Owned: Default>,
402 /// Creates an owned Cow<'a, B> with the default value for the contained owned value.
403 fn default() -> Self {
404 Owned(<B as ToOwned>::Owned::default())
408 #[stable(feature = "rust1", since = "1.0.0")]
409 impl<B: ?Sized> Hash for Cow<'_, B>
410 where B: Hash + ToOwned
413 fn hash<H: Hasher>(&self, state: &mut H) {
414 Hash::hash(&**self, state)
418 #[stable(feature = "rust1", since = "1.0.0")]
419 impl<T: ?Sized + ToOwned> AsRef<T> for Cow<'_, T> {
420 fn as_ref(&self) -> &T {
425 #[stable(feature = "cow_add", since = "1.14.0")]
426 impl<'a> Add<&'a str> for Cow<'a, str> {
427 type Output = Cow<'a, str>;
430 fn add(mut self, rhs: &'a str) -> Self::Output {
436 #[stable(feature = "cow_add", since = "1.14.0")]
437 impl<'a> Add<Cow<'a, str>> for Cow<'a, str> {
438 type Output = Cow<'a, str>;
441 fn add(mut self, rhs: Cow<'a, str>) -> Self::Output {
447 #[stable(feature = "cow_add", since = "1.14.0")]
448 impl<'a> AddAssign<&'a str> for Cow<'a, str> {
449 fn add_assign(&mut self, rhs: &'a str) {
451 *self = Cow::Borrowed(rhs)
452 } else if rhs.is_empty() {
455 if let Cow::Borrowed(lhs) = *self {
456 let mut s = String::with_capacity(lhs.len() + rhs.len());
458 *self = Cow::Owned(s);
460 self.to_mut().push_str(rhs);
465 #[stable(feature = "cow_add", since = "1.14.0")]
466 impl<'a> AddAssign<Cow<'a, str>> for Cow<'a, str> {
467 fn add_assign(&mut self, rhs: Cow<'a, str>) {
470 } else if rhs.is_empty() {
473 if let Cow::Borrowed(lhs) = *self {
474 let mut s = String::with_capacity(lhs.len() + rhs.len());
476 *self = Cow::Owned(s);
478 self.to_mut().push_str(&rhs);