src/liballoc/borrow.rs

   1 //! A module for working with borrowed data.
   2
   3 #![stable(feature = "rust1", since = "1.0.0")]
   4
   5 use core::cmp::Ordering;
   6 use core::hash::{Hash, Hasher};
   7 use core::ops::{Add, AddAssign, Deref};
   8
   9 #[stable(feature = "rust1", since = "1.0.0")]
  10 pub use core::borrow::{Borrow, BorrowMut};
  11
  12 use crate::fmt;
  13 use crate::string::String;
  14
  15 use Cow::*;
  16
  17 #[stable(feature = "rust1", since = "1.0.0")]
  18 impl<'a, B: ?Sized> Borrow<B> for Cow<'a, B>
  19     where B: ToOwned,
  20           <B as ToOwned>::Owned: 'a
  21 {
  22     fn borrow(&self) -> &B {
  23         &**self
  24     }
  25 }
  26
  27 /// A generalization of `Clone` to borrowed data.
  28 ///
  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")]
  34 pub trait ToOwned {
  35     /// The resulting type after obtaining ownership.
  36     #[stable(feature = "rust1", since = "1.0.0")]
  37     type Owned: Borrow<Self>;
  38
  39     /// Creates owned data from borrowed data, usually by cloning.
  40     ///
  41     /// # Examples
  42     ///
  43     /// Basic usage:
  44     ///
  45     /// ```
  46     /// let s: &str = "a";
  47     /// let ss: String = s.to_owned();
  48     ///
  49     /// let v: &[i32] = &[1, 2];
  50     /// let vv: Vec<i32> = v.to_owned();
  51     /// ```
  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;
  55
  56     /// Uses borrowed data to replace owned data, usually by cloning.
  57     ///
  58     /// This is borrow-generalized version of `Clone::clone_from`.
  59     ///
  60     /// # Examples
  61     ///
  62     /// Basic usage:
  63     ///
  64     /// ```
  65     /// # #![feature(toowned_clone_into)]
  66     /// let mut s: String = String::new();
  67     /// "hello".clone_into(&mut s);
  68     ///
  69     /// let mut v: Vec<i32> = Vec::new();
  70     /// [1, 2][..].clone_into(&mut v);
  71     /// ```
  72     #[unstable(feature = "toowned_clone_into",
  73                reason = "recently added",
  74                issue = "41263")]
  75     fn clone_into(&self, target: &mut Self::Owned) {
  76         *target = self.to_owned();
  77     }
  78 }
  79
  80 #[stable(feature = "rust1", since = "1.0.0")]
  81 impl<T> ToOwned for T
  82     where T: Clone
  83 {
  84     type Owned = T;
  85     fn to_owned(&self) -> T {
  86         self.clone()
  87     }
  88
  89     fn clone_into(&self, target: &mut T) {
  90         target.clone_from(self);
  91     }
  92 }
  93
  94 /// A clone-on-write smart pointer.
  95 ///
  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.
 100 ///
 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.
 105 ///
 106 /// # Examples
 107 ///
 108 /// ```
 109 /// use std::borrow::Cow;
 110 ///
 111 /// fn abs_all(input: &mut Cow<[i32]>) {
 112 ///     for i in 0..input.len() {
 113 ///         let v = input[i];
 114 ///         if v < 0 {
 115 ///             // Clones into a vector if not already owned.
 116 ///             input.to_mut()[i] = -v;
 117 ///         }
 118 ///     }
 119 /// }
 120 ///
 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);
 125 ///
 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);
 130 ///
 131 /// // No clone occurs because `input` is already owned.
 132 /// let mut input = Cow::from(vec![-1, 0, 1]);
 133 /// abs_all(&mut input);
 134 /// ```
 135 ///
 136 /// Another example showing how to keep `Cow` in a struct:
 137 ///
 138 /// ```
 139 /// use std::borrow::Cow;
 140 ///
 141 /// struct Items<'a, X: 'a> where [X]: ToOwned<Owned = Vec<X>> {
 142 ///     values: Cow<'a, [X]>,
 143 /// }
 144 ///
 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 }
 148 ///     }
 149 /// }
 150 ///
 151 /// // Creates a container from borrowed values of a slice
 152 /// let readonly = [1, 2];
 153 /// let borrowed = Items::new((&readonly[..]).into());
 154 /// match borrowed {
 155 ///     Items { values: Cow::Borrowed(b) } => println!("borrowed {:?}", b),
 156 ///     _ => panic!("expect borrowed value"),
 157 /// }
 158 ///
 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);
 163 ///
 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"),
 168 /// }
 169 /// ```
 170 #[stable(feature = "rust1", since = "1.0.0")]
 171 pub enum Cow<'a, B: ?Sized + 'a>
 172     where B: ToOwned
 173 {
 174     /// Borrowed data.
 175     #[stable(feature = "rust1", since = "1.0.0")]
 176     Borrowed(#[stable(feature = "rust1", since = "1.0.0")]
 177              &'a B),
 178
 179     /// Owned data.
 180     #[stable(feature = "rust1", since = "1.0.0")]
 181     Owned(#[stable(feature = "rust1", since = "1.0.0")]
 182           <B as ToOwned>::Owned),
 183 }
 184
 185 #[stable(feature = "rust1", since = "1.0.0")]
 186 impl<B: ?Sized + ToOwned> Clone for Cow<'_, B> {
 187     fn clone(&self) -> Self {
 188         match *self {
 189             Borrowed(b) => Borrowed(b),
 190             Owned(ref o) => {
 191                 let b: &B = o.borrow();
 192                 Owned(b.to_owned())
 193             }
 194         }
 195     }
 196
 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);
 201                 return;
 202             }
 203         }
 204
 205         *self = source.clone();
 206     }
 207 }
 208
 209 impl<B: ?Sized + ToOwned> Cow<'_, B> {
 210     /// Returns true if the data is borrowed, i.e. if `to_mut` would require additional work.
 211     ///
 212     /// # Examples
 213     ///
 214     /// ```
 215     /// #![feature(cow_is_borrowed)]
 216     /// use std::borrow::Cow;
 217     ///
 218     /// let cow = Cow::Borrowed("moo");
 219     /// assert!(cow.is_borrowed());
 220     ///
 221     /// let bull: Cow<'_, str> = Cow::Owned("...moo?".to_string());
 222     /// assert!(!bull.is_borrowed());
 223     /// ```
 224     #[unstable(feature = "cow_is_borrowed", issue = "65143")]
 225     pub fn is_borrowed(&self) -> bool {
 226         match *self {
 227             Borrowed(_) => true,
 228             Owned(_) => false,
 229         }
 230     }
 231
 232     /// Returns true if the data is owned, i.e. if `to_mut` would be a no-op.
 233     ///
 234     /// # Examples
 235     ///
 236     /// ```
 237     /// #![feature(cow_is_borrowed)]
 238     /// use std::borrow::Cow;
 239     ///
 240     /// let cow: Cow<'_, str> = Cow::Owned("moo".to_string());
 241     /// assert!(cow.is_owned());
 242     ///
 243     /// let bull = Cow::Borrowed("...moo?");
 244     /// assert!(!bull.is_owned());
 245     /// ```
 246     #[unstable(feature = "cow_is_borrowed", issue = "65143")]
 247     pub fn is_owned(&self) -> bool {
 248         !self.is_borrowed()
 249     }
 250
 251     /// Acquires a mutable reference to the owned form of the data.
 252     ///
 253     /// Clones the data if it is not already owned.
 254     ///
 255     /// # Examples
 256     ///
 257     /// ```
 258     /// use std::borrow::Cow;
 259     ///
 260     /// let mut cow = Cow::Borrowed("foo");
 261     /// cow.to_mut().make_ascii_uppercase();
 262     ///
 263     /// assert_eq!(
 264     ///   cow,
 265     ///   Cow::Owned(String::from("FOO")) as Cow<str>
 266     /// );
 267     /// ```
 268     #[stable(feature = "rust1", since = "1.0.0")]
 269     pub fn to_mut(&mut self) -> &mut <B as ToOwned>::Owned {
 270         match *self {
 271             Borrowed(borrowed) => {
 272                 *self = Owned(borrowed.to_owned());
 273                 match *self {
 274                     Borrowed(..) => unreachable!(),
 275                     Owned(ref mut owned) => owned,
 276                 }
 277             }
 278             Owned(ref mut owned) => owned,
 279         }
 280     }
 281
 282     /// Extracts the owned data.
 283     ///
 284     /// Clones the data if it is not already owned.
 285     ///
 286     /// # Examples
 287     ///
 288     /// Calling `into_owned` on a `Cow::Borrowed` clones the underlying data
 289     /// and becomes a `Cow::Owned`:
 290     ///
 291     /// ```
 292     /// use std::borrow::Cow;
 293     ///
 294     /// let s = "Hello world!";
 295     /// let cow = Cow::Borrowed(s);
 296     ///
 297     /// assert_eq!(
 298     ///   cow.into_owned(),
 299     ///   String::from(s)
 300     /// );
 301     /// ```
 302     ///
 303     /// Calling `into_owned` on a `Cow::Owned` is a no-op:
 304     ///
 305     /// ```
 306     /// use std::borrow::Cow;
 307     ///
 308     /// let s = "Hello world!";
 309     /// let cow: Cow<str> = Cow::Owned(String::from(s));
 310     ///
 311     /// assert_eq!(
 312     ///   cow.into_owned(),
 313     ///   String::from(s)
 314     /// );
 315     /// ```
 316     #[stable(feature = "rust1", since = "1.0.0")]
 317     pub fn into_owned(self) -> <B as ToOwned>::Owned {
 318         match self {
 319             Borrowed(borrowed) => borrowed.to_owned(),
 320             Owned(owned) => owned,
 321         }
 322     }
 323 }
 324
 325 #[stable(feature = "rust1", since = "1.0.0")]
 326 impl<B: ?Sized + ToOwned> Deref for Cow<'_, B> {
 327     type Target = B;
 328
 329     fn deref(&self) -> &B {
 330         match *self {
 331             Borrowed(borrowed) => borrowed,
 332             Owned(ref owned) => owned.borrow(),
 333         }
 334     }
 335 }
 336
 337 #[stable(feature = "rust1", since = "1.0.0")]
 338 impl<B: ?Sized> Eq for Cow<'_, B> where B: Eq + ToOwned {}
 339
 340 #[stable(feature = "rust1", since = "1.0.0")]
 341 impl<B: ?Sized> Ord for Cow<'_, B>
 342     where B: Ord + ToOwned
 343 {
 344     #[inline]
 345     fn cmp(&self, other: &Self) -> Ordering {
 346         Ord::cmp(&**self, &**other)
 347     }
 348 }
 349
 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,
 353           C: ToOwned
 354 {
 355     #[inline]
 356     fn eq(&self, other: &Cow<'b, C>) -> bool {
 357         PartialEq::eq(&**self, &**other)
 358     }
 359 }
 360
 361 #[stable(feature = "rust1", since = "1.0.0")]
 362 impl<'a, B: ?Sized> PartialOrd for Cow<'a, B>
 363     where B: PartialOrd + ToOwned
 364 {
 365     #[inline]
 366     fn partial_cmp(&self, other: &Cow<'a, B>) -> Option<Ordering> {
 367         PartialOrd::partial_cmp(&**self, &**other)
 368     }
 369 }
 370
 371 #[stable(feature = "rust1", since = "1.0.0")]
 372 impl<B: ?Sized> fmt::Debug for Cow<'_, B>
 373 where
 374     B: fmt::Debug + ToOwned<Owned: fmt::Debug>,
 375 {
 376     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
 377         match *self {
 378             Borrowed(ref b) => fmt::Debug::fmt(b, f),
 379             Owned(ref o) => fmt::Debug::fmt(o, f),
 380         }
 381     }
 382 }
 383
 384 #[stable(feature = "rust1", since = "1.0.0")]
 385 impl<B: ?Sized> fmt::Display for Cow<'_, B>
 386 where
 387     B: fmt::Display + ToOwned<Owned: fmt::Display>,
 388 {
 389     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
 390         match *self {
 391             Borrowed(ref b) => fmt::Display::fmt(b, f),
 392             Owned(ref o) => fmt::Display::fmt(o, f),
 393         }
 394     }
 395 }
 396
 397 #[stable(feature = "default", since = "1.11.0")]
 398 impl<B: ?Sized> Default for Cow<'_, B>
 399 where
 400     B: ToOwned<Owned: Default>,
 401 {
 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())
 405     }
 406 }
 407
 408 #[stable(feature = "rust1", since = "1.0.0")]
 409 impl<B: ?Sized> Hash for Cow<'_, B>
 410     where B: Hash + ToOwned
 411 {
 412     #[inline]
 413     fn hash<H: Hasher>(&self, state: &mut H) {
 414         Hash::hash(&**self, state)
 415     }
 416 }
 417
 418 #[stable(feature = "rust1", since = "1.0.0")]
 419 impl<T: ?Sized + ToOwned> AsRef<T> for Cow<'_, T> {
 420     fn as_ref(&self) -> &T {
 421         self
 422     }
 423 }
 424
 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>;
 428
 429     #[inline]
 430     fn add(mut self, rhs: &'a str) -> Self::Output {
 431         self += rhs;
 432         self
 433     }
 434 }
 435
 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>;
 439
 440     #[inline]
 441     fn add(mut self, rhs: Cow<'a, str>) -> Self::Output {
 442         self += rhs;
 443         self
 444     }
 445 }
 446
 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) {
 450         if self.is_empty() {
 451             *self = Cow::Borrowed(rhs)
 452         } else if rhs.is_empty() {
 453             return;
 454         } else {
 455             if let Cow::Borrowed(lhs) = *self {
 456                 let mut s = String::with_capacity(lhs.len() + rhs.len());
 457                 s.push_str(lhs);
 458                 *self = Cow::Owned(s);
 459             }
 460             self.to_mut().push_str(rhs);
 461         }
 462     }
 463 }
 464
 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>) {
 468         if self.is_empty() {
 469             *self = rhs
 470         } else if rhs.is_empty() {
 471             return;
 472         } else {
 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 }