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 /// Acquires a mutable reference to the owned form of the data.
212 /// Clones the data if it is not already owned.
217 /// use std::borrow::Cow;
219 /// let mut cow = Cow::Borrowed("foo");
220 /// cow.to_mut().make_ascii_uppercase();
224 /// Cow::Owned(String::from("FOO")) as Cow<str>
227 #[stable(feature = "rust1", since = "1.0.0")]
228 pub fn to_mut(&mut self) -> &mut <B as ToOwned>::Owned {
230 Borrowed(borrowed) => {
231 *self = Owned(borrowed.to_owned());
233 Borrowed(..) => unreachable!(),
234 Owned(ref mut owned) => owned,
237 Owned(ref mut owned) => owned,
241 /// Extracts the owned data.
243 /// Clones the data if it is not already owned.
247 /// Calling `into_owned` on a `Cow::Borrowed` clones the underlying data
248 /// and becomes a `Cow::Owned`:
251 /// use std::borrow::Cow;
253 /// let s = "Hello world!";
254 /// let cow = Cow::Borrowed(s);
257 /// cow.into_owned(),
262 /// Calling `into_owned` on a `Cow::Owned` is a no-op:
265 /// use std::borrow::Cow;
267 /// let s = "Hello world!";
268 /// let cow: Cow<str> = Cow::Owned(String::from(s));
271 /// cow.into_owned(),
275 #[stable(feature = "rust1", since = "1.0.0")]
276 pub fn into_owned(self) -> <B as ToOwned>::Owned {
278 Borrowed(borrowed) => borrowed.to_owned(),
279 Owned(owned) => owned,
284 #[stable(feature = "rust1", since = "1.0.0")]
285 impl<B: ?Sized + ToOwned> Deref for Cow<'_, B> {
288 fn deref(&self) -> &B {
290 Borrowed(borrowed) => borrowed,
291 Owned(ref owned) => owned.borrow(),
296 #[stable(feature = "rust1", since = "1.0.0")]
297 impl<B: ?Sized> Eq for Cow<'_, B> where B: Eq + ToOwned {}
299 #[stable(feature = "rust1", since = "1.0.0")]
300 impl<B: ?Sized> Ord for Cow<'_, B>
301 where B: Ord + ToOwned
304 fn cmp(&self, other: &Self) -> Ordering {
305 Ord::cmp(&**self, &**other)
309 #[stable(feature = "rust1", since = "1.0.0")]
310 impl<'a, 'b, B: ?Sized, C: ?Sized> PartialEq<Cow<'b, C>> for Cow<'a, B>
311 where B: PartialEq<C> + ToOwned,
315 fn eq(&self, other: &Cow<'b, C>) -> bool {
316 PartialEq::eq(&**self, &**other)
320 #[stable(feature = "rust1", since = "1.0.0")]
321 impl<'a, B: ?Sized> PartialOrd for Cow<'a, B>
322 where B: PartialOrd + ToOwned
325 fn partial_cmp(&self, other: &Cow<'a, B>) -> Option<Ordering> {
326 PartialOrd::partial_cmp(&**self, &**other)
330 #[stable(feature = "rust1", since = "1.0.0")]
331 impl<B: ?Sized> fmt::Debug for Cow<'_, B>
333 B: fmt::Debug + ToOwned<Owned: fmt::Debug>,
335 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
337 Borrowed(ref b) => fmt::Debug::fmt(b, f),
338 Owned(ref o) => fmt::Debug::fmt(o, f),
343 #[stable(feature = "rust1", since = "1.0.0")]
344 impl<B: ?Sized> fmt::Display for Cow<'_, B>
346 B: fmt::Display + ToOwned<Owned: fmt::Display>,
348 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
350 Borrowed(ref b) => fmt::Display::fmt(b, f),
351 Owned(ref o) => fmt::Display::fmt(o, f),
356 #[stable(feature = "default", since = "1.11.0")]
357 impl<B: ?Sized> Default for Cow<'_, B>
359 B: ToOwned<Owned: Default>,
361 /// Creates an owned Cow<'a, B> with the default value for the contained owned value.
362 fn default() -> Self {
363 Owned(<B as ToOwned>::Owned::default())
367 #[stable(feature = "rust1", since = "1.0.0")]
368 impl<B: ?Sized> Hash for Cow<'_, B>
369 where B: Hash + ToOwned
372 fn hash<H: Hasher>(&self, state: &mut H) {
373 Hash::hash(&**self, state)
377 #[stable(feature = "rust1", since = "1.0.0")]
378 impl<T: ?Sized + ToOwned> AsRef<T> for Cow<'_, T> {
379 fn as_ref(&self) -> &T {
384 #[stable(feature = "cow_add", since = "1.14.0")]
385 impl<'a> Add<&'a str> for Cow<'a, str> {
386 type Output = Cow<'a, str>;
389 fn add(mut self, rhs: &'a str) -> Self::Output {
395 #[stable(feature = "cow_add", since = "1.14.0")]
396 impl<'a> Add<Cow<'a, str>> for Cow<'a, str> {
397 type Output = Cow<'a, str>;
400 fn add(mut self, rhs: Cow<'a, str>) -> Self::Output {
406 #[stable(feature = "cow_add", since = "1.14.0")]
407 impl<'a> AddAssign<&'a str> for Cow<'a, str> {
408 fn add_assign(&mut self, rhs: &'a str) {
410 *self = Cow::Borrowed(rhs)
411 } else if rhs.is_empty() {
414 if let Cow::Borrowed(lhs) = *self {
415 let mut s = String::with_capacity(lhs.len() + rhs.len());
417 *self = Cow::Owned(s);
419 self.to_mut().push_str(rhs);
424 #[stable(feature = "cow_add", since = "1.14.0")]
425 impl<'a> AddAssign<Cow<'a, str>> for Cow<'a, str> {
426 fn add_assign(&mut self, rhs: Cow<'a, str>) {
429 } else if rhs.is_empty() {
432 if let Cow::Borrowed(lhs) = *self {
433 let mut s = String::with_capacity(lhs.len() + rhs.len());
435 *self = Cow::Owned(s);
437 self.to_mut().push_str(&rhs);