src/libcollections/borrow.rs

   1 // Copyright 2014 The Rust Project Developers. See the COPYRIGHT
   2 // file at the top-level directory of this distribution and at
   3 // http://rust-lang.org/COPYRIGHT.
   4 //
   5 // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
   6 // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
   7 // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
   8 // option. This file may not be copied, modified, or distributed
   9 // except according to those terms.
  10
  11 //! A module for working with borrowed data.
  12
  13 #![stable(feature = "rust1", since = "1.0.0")]
  14
  15 use core::clone::Clone;
  16 use core::cmp::{Eq, Ord, Ordering, PartialEq, PartialOrd};
  17 use core::convert::AsRef;
  18 use core::hash::{Hash, Hasher};
  19 use core::marker::Sized;
  20 use core::ops::Deref;
  21 use core::option::Option;
  22
  23 use fmt;
  24
  25 use self::Cow::*;
  26
  27 #[stable(feature = "rust1", since = "1.0.0")]
  28 pub use core::borrow::{Borrow, BorrowMut};
  29
  30 #[stable(feature = "rust1", since = "1.0.0")]
  31 impl<'a, B: ?Sized> Borrow<B> for Cow<'a, B>
  32     where B: ToOwned,
  33           <B as ToOwned>::Owned: 'a
  34 {
  35     fn borrow(&self) -> &B {
  36         &**self
  37     }
  38 }
  39
  40 /// A generalization of `Clone` to borrowed data.
  41 ///
  42 /// Some types make it possible to go from borrowed to owned, usually by
  43 /// implementing the `Clone` trait. But `Clone` works only for going from `&T`
  44 /// to `T`. The `ToOwned` trait generalizes `Clone` to construct owned data
  45 /// from any borrow of a given type.
  46 #[stable(feature = "rust1", since = "1.0.0")]
  47 pub trait ToOwned {
  48     #[stable(feature = "rust1", since = "1.0.0")]
  49     type Owned: Borrow<Self>;
  50
  51     /// Creates owned data from borrowed data, usually by cloning.
  52     #[stable(feature = "rust1", since = "1.0.0")]
  53     fn to_owned(&self) -> Self::Owned;
  54 }
  55
  56 #[stable(feature = "rust1", since = "1.0.0")]
  57 impl<T> ToOwned for T where T: Clone {
  58     type Owned = T;
  59     fn to_owned(&self) -> T {
  60         self.clone()
  61     }
  62 }
  63
  64 /// A clone-on-write smart pointer.
  65 ///
  66 /// The type `Cow` is a smart pointer providing clone-on-write functionality: it
  67 /// can enclose and provide immutable access to borrowed data, and clone the
  68 /// data lazily when mutation or ownership is required. The type is designed to
  69 /// work with general borrowed data via the `Borrow` trait.
  70 ///
  71 /// `Cow` implements `Deref`, which means that you can call
  72 /// non-mutating methods directly on the data it encloses. If mutation
  73 /// is desired, `to_mut` will obtain a mutable reference to an owned
  74 /// value, cloning if necessary.
  75 ///
  76 /// # Examples
  77 ///
  78 /// ```
  79 /// use std::borrow::Cow;
  80 ///
  81 /// # #[allow(dead_code)]
  82 /// fn abs_all(input: &mut Cow<[i32]>) {
  83 ///     for i in 0..input.len() {
  84 ///         let v = input[i];
  85 ///         if v < 0 {
  86 ///             // clones into a vector the first time (if not already owned)
  87 ///             input.to_mut()[i] = -v;
  88 ///         }
  89 ///     }
  90 /// }
  91 /// ```
  92 #[stable(feature = "rust1", since = "1.0.0")]
  93 pub enum Cow<'a, B: ?Sized + 'a>
  94     where B: ToOwned
  95 {
  96     /// Borrowed data.
  97     #[stable(feature = "rust1", since = "1.0.0")]
  98     Borrowed(&'a B),
  99
 100     /// Owned data.
 101     #[stable(feature = "rust1", since = "1.0.0")]
 102     Owned(<B as ToOwned>::Owned),
 103 }
 104
 105 #[stable(feature = "rust1", since = "1.0.0")]
 106 impl<'a, B: ?Sized> Clone for Cow<'a, B> where B: ToOwned {
 107     fn clone(&self) -> Cow<'a, B> {
 108         match *self {
 109             Borrowed(b) => Borrowed(b),
 110             Owned(ref o) => {
 111                 let b: &B = o.borrow();
 112                 Owned(b.to_owned())
 113             }
 114         }
 115     }
 116 }
 117
 118 impl<'a, B: ?Sized> Cow<'a, B> where B: ToOwned {
 119     /// Acquires a mutable reference to the owned form of the data.
 120     ///
 121     /// Clones the data if it is not already owned.
 122     ///
 123     /// # Examples
 124     ///
 125     /// ```
 126     /// use std::borrow::Cow;
 127     ///
 128     /// let mut cow: Cow<[_]> = Cow::Owned(vec![1, 2, 3]);
 129     ///
 130     /// let hello = cow.to_mut();
 131     ///
 132     /// assert_eq!(hello, &[1, 2, 3]);
 133     /// ```
 134     #[stable(feature = "rust1", since = "1.0.0")]
 135     pub fn to_mut(&mut self) -> &mut <B as ToOwned>::Owned {
 136         match *self {
 137             Borrowed(borrowed) => {
 138                 *self = Owned(borrowed.to_owned());
 139                 self.to_mut()
 140             }
 141             Owned(ref mut owned) => owned,
 142         }
 143     }
 144
 145     /// Extracts the owned data.
 146     ///
 147     /// Clones the data if it is not already owned.
 148     ///
 149     /// # Examples
 150     ///
 151     /// ```
 152     /// use std::borrow::Cow;
 153     ///
 154     /// let cow: Cow<[_]> = Cow::Owned(vec![1, 2, 3]);
 155     ///
 156     /// let hello = cow.into_owned();
 157     ///
 158     /// assert_eq!(vec![1, 2, 3], hello);
 159     /// ```
 160     #[stable(feature = "rust1", since = "1.0.0")]
 161     pub fn into_owned(self) -> <B as ToOwned>::Owned {
 162         match self {
 163             Borrowed(borrowed) => borrowed.to_owned(),
 164             Owned(owned) => owned,
 165         }
 166     }
 167 }
 168
 169 #[stable(feature = "rust1", since = "1.0.0")]
 170 impl<'a, B: ?Sized> Deref for Cow<'a, B> where B: ToOwned {
 171     type Target = B;
 172
 173     fn deref(&self) -> &B {
 174         match *self {
 175             Borrowed(borrowed) => borrowed,
 176             Owned(ref owned) => owned.borrow(),
 177         }
 178     }
 179 }
 180
 181 #[stable(feature = "rust1", since = "1.0.0")]
 182 impl<'a, B: ?Sized> Eq for Cow<'a, B> where B: Eq + ToOwned {}
 183
 184 #[stable(feature = "rust1", since = "1.0.0")]
 185 impl<'a, B: ?Sized> Ord for Cow<'a, B> where B: Ord + ToOwned {
 186     #[inline]
 187     fn cmp(&self, other: &Cow<'a, B>) -> Ordering {
 188         Ord::cmp(&**self, &**other)
 189     }
 190 }
 191
 192 #[stable(feature = "rust1", since = "1.0.0")]
 193 impl<'a, 'b, B: ?Sized, C: ?Sized> PartialEq<Cow<'b, C>> for Cow<'a, B>
 194     where B: PartialEq<C> + ToOwned,
 195           C: ToOwned
 196 {
 197     #[inline]
 198     fn eq(&self, other: &Cow<'b, C>) -> bool {
 199         PartialEq::eq(&**self, &**other)
 200     }
 201 }
 202
 203 #[stable(feature = "rust1", since = "1.0.0")]
 204 impl<'a, B: ?Sized> PartialOrd for Cow<'a, B> where B: PartialOrd + ToOwned {
 205     #[inline]
 206     fn partial_cmp(&self, other: &Cow<'a, B>) -> Option<Ordering> {
 207         PartialOrd::partial_cmp(&**self, &**other)
 208     }
 209 }
 210
 211 #[stable(feature = "rust1", since = "1.0.0")]
 212 impl<'a, B: ?Sized> fmt::Debug for Cow<'a, B>
 213     where B: fmt::Debug + ToOwned,
 214           <B as ToOwned>::Owned: fmt::Debug
 215 {
 216     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
 217         match *self {
 218             Borrowed(ref b) => fmt::Debug::fmt(b, f),
 219             Owned(ref o) => fmt::Debug::fmt(o, f),
 220         }
 221     }
 222 }
 223
 224 #[stable(feature = "rust1", since = "1.0.0")]
 225 impl<'a, B: ?Sized> fmt::Display for Cow<'a, B>
 226     where B: fmt::Display + ToOwned,
 227           <B as ToOwned>::Owned: fmt::Display
 228 {
 229     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
 230         match *self {
 231             Borrowed(ref b) => fmt::Display::fmt(b, f),
 232             Owned(ref o) => fmt::Display::fmt(o, f),
 233         }
 234     }
 235 }
 236
 237 #[stable(feature = "rust1", since = "1.0.0")]
 238 impl<'a, B: ?Sized> Hash for Cow<'a, B> where B: Hash + ToOwned {
 239     #[inline]
 240     fn hash<H: Hasher>(&self, state: &mut H) {
 241         Hash::hash(&**self, state)
 242     }
 243 }
 244
 245 /// Trait for moving into a `Cow`.
 246 #[unstable(feature = "into_cow", reason = "may be replaced by `convert::Into`",
 247            issue = "27735")]
 248 pub trait IntoCow<'a, B: ?Sized> where B: ToOwned {
 249     /// Moves `self` into `Cow`
 250     fn into_cow(self) -> Cow<'a, B>;
 251 }
 252
 253 #[stable(feature = "rust1", since = "1.0.0")]
 254 impl<'a, B: ?Sized> IntoCow<'a, B> for Cow<'a, B> where B: ToOwned {
 255     fn into_cow(self) -> Cow<'a, B> {
 256         self
 257     }
 258 }
 259
 260 #[stable(feature = "rust1", since = "1.0.0")]
 261 impl<'a, T: ?Sized + ToOwned> AsRef<T> for Cow<'a, T> {
 262     fn as_ref(&self) -> &T {
 263         self
 264     }
 265 }