alloc/
borrow.rs

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) => Borrowed(b),
197            Owned(ref o) => {
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), &Owned(ref o)) => o.borrow().clone_into(dest),
207            (t, s) => *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) => borrowed,
338            Owned(ref owned) => 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) => fmt::Debug::fmt(b, f),
401            Owned(ref o) => fmt::Debug::fmt(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) => fmt::Display::fmt(b, f),
414            Owned(ref o) => fmt::Display::fmt(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) = *self {
480                let mut s = String::with_capacity(lhs.len() + rhs.len());
481                s.push_str(lhs);
482                *self = Cow::Owned(s);
483            }
484            self.to_mut().push_str(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) = *self {
497                let mut s = String::with_capacity(lhs.len() + rhs.len());
498                s.push_str(lhs);
499                *self = Cow::Owned(s);
500            }
501            self.to_mut().push_str(&rhs);
502        }
503    }
504}
alloc/borrow.rs

alloc/
borrow.rs
