src/libcore/clone.rs

   1 //! The `Clone` trait for types that cannot be 'implicitly copied'.
   2 //!
   3 //! In Rust, some simple types are "implicitly copyable" and when you
   4 //! assign them or pass them as arguments, the receiver will get a copy,
   5 //! leaving the original value in place. These types do not require
   6 //! allocation to copy and do not have finalizers (i.e., they do not
   7 //! contain owned boxes or implement [`Drop`]), so the compiler considers
   8 //! them cheap and safe to copy. For other types copies must be made
   9 //! explicitly, by convention implementing the [`Clone`] trait and calling
  10 //! the [`clone`][clone] method.
  11 //!
  12 //! [`Clone`]: trait.Clone.html
  13 //! [clone]: trait.Clone.html#tymethod.clone
  14 //! [`Drop`]: ../../std/ops/trait.Drop.html
  15 //!
  16 //! Basic usage example:
  17 //!
  18 //! ```
  19 //! let s = String::new(); // String type implements Clone
  20 //! let copy = s.clone(); // so we can clone it
  21 //! ```
  22 //!
  23 //! To easily implement the Clone trait, you can also use
  24 //! `#[derive(Clone)]`. Example:
  25 //!
  26 //! ```
  27 //! #[derive(Clone)] // we add the Clone trait to Morpheus struct
  28 //! struct Morpheus {
  29 //!    blue_pill: f32,
  30 //!    red_pill: i64,
  31 //! }
  32 //!
  33 //! fn main() {
  34 //!    let f = Morpheus { blue_pill: 0.0, red_pill: 0 };
  35 //!    let copy = f.clone(); // and now we can clone it!
  36 //! }
  37 //! ```
  38
  39 #![stable(feature = "rust1", since = "1.0.0")]
  40
  41 /// A common trait for the ability to explicitly duplicate an object.
  42 ///
  43 /// Differs from [`Copy`] in that [`Copy`] is implicit and extremely inexpensive, while
  44 /// `Clone` is always explicit and may or may not be expensive. In order to enforce
  45 /// these characteristics, Rust does not allow you to reimplement [`Copy`], but you
  46 /// may reimplement `Clone` and run arbitrary code.
  47 ///
  48 /// Since `Clone` is more general than [`Copy`], you can automatically make anything
  49 /// [`Copy`] be `Clone` as well.
  50 ///
  51 /// ## Derivable
  52 ///
  53 /// This trait can be used with `#[derive]` if all fields are `Clone`. The `derive`d
  54 /// implementation of [`clone`] calls [`clone`] on each field.
  55 ///
  56 /// For a generic struct, `#[derive]` implements `Clone` conditionally by adding bound `Clone` on
  57 /// generic parameters.
  58 ///
  59 /// ```
  60 /// // `derive` implements Clone for Reading<T> when T is Clone.
  61 /// #[derive(Clone)]
  62 /// struct Reading<T> {
  63 ///     frequency: T,
  64 /// }
  65 /// ```
  66 ///
  67 /// ## How can I implement `Clone`?
  68 ///
  69 /// Types that are [`Copy`] should have a trivial implementation of `Clone`. More formally:
  70 /// if `T: Copy`, `x: T`, and `y: &T`, then `let x = y.clone();` is equivalent to `let x = *y;`.
  71 /// Manual implementations should be careful to uphold this invariant; however, unsafe code
  72 /// must not rely on it to ensure memory safety.
  73 ///
  74 /// An example is a generic struct holding a function pointer. In this case, the
  75 /// implementation of `Clone` cannot be `derive`d, but can be implemented as:
  76 ///
  77 /// [`Copy`]: ../../std/marker/trait.Copy.html
  78 /// [`clone`]: trait.Clone.html#tymethod.clone
  79 ///
  80 /// ```
  81 /// struct Generate<T>(fn() -> T);
  82 ///
  83 /// impl<T> Copy for Generate<T> {}
  84 ///
  85 /// impl<T> Clone for Generate<T> {
  86 ///     fn clone(&self) -> Self {
  87 ///         *self
  88 ///     }
  89 /// }
  90 /// ```
  91 ///
  92 /// ## Additional implementors
  93 ///
  94 /// In addition to the [implementors listed below][impls],
  95 /// the following types also implement `Clone`:
  96 ///
  97 /// * Function item types (i.e., the distinct types defined for each function)
  98 /// * Function pointer types (e.g., `fn() -> i32`)
  99 /// * Array types, for all sizes, if the item type also implements `Clone` (e.g., `[i32; 123456]`)
 100 /// * Tuple types, if each component also implements `Clone` (e.g., `()`, `(i32, bool)`)
 101 /// * Closure types, if they capture no value from the environment
 102 ///   or if all such captured values implement `Clone` themselves.
 103 ///   Note that variables captured by shared reference always implement `Clone`
 104 ///   (even if the referent doesn't),
 105 ///   while variables captured by mutable reference never implement `Clone`.
 106 ///
 107 /// [impls]: #implementors
 108 #[stable(feature = "rust1", since = "1.0.0")]
 109 #[lang = "clone"]
 110 pub trait Clone: Sized {
 111     /// Returns a copy of the value.
 112     ///
 113     /// # Examples
 114     ///
 115     /// ```
 116     /// let hello = "Hello"; // &str implements Clone
 117     ///
 118     /// assert_eq!("Hello", hello.clone());
 119     /// ```
 120     #[stable(feature = "rust1", since = "1.0.0")]
 121     #[must_use = "cloning is often expensive and is not expected to have side effects"]
 122     fn clone(&self) -> Self;
 123
 124     /// Performs copy-assignment from `source`.
 125     ///
 126     /// `a.clone_from(&b)` is equivalent to `a = b.clone()` in functionality,
 127     /// but can be overridden to reuse the resources of `a` to avoid unnecessary
 128     /// allocations.
 129     #[inline]
 130     #[stable(feature = "rust1", since = "1.0.0")]
 131     fn clone_from(&mut self, source: &Self) {
 132         *self = source.clone()
 133     }
 134 }
 135
 136 /// Derive macro generating an impl of the trait `Clone`.
 137 #[rustc_builtin_macro]
 138 #[stable(feature = "builtin_macro_prelude", since = "1.38.0")]
 139 #[allow_internal_unstable(core_intrinsics, derive_clone_copy)]
 140 pub macro Clone($item:item) {
 141     /* compiler built-in */
 142 }
 143
 144 // FIXME(aburka): these structs are used solely by #[derive] to
 145 // assert that every component of a type implements Clone or Copy.
 146 //
 147 // These structs should never appear in user code.
 148 #[doc(hidden)]
 149 #[allow(missing_debug_implementations)]
 150 #[unstable(
 151     feature = "derive_clone_copy",
 152     reason = "deriving hack, should not be public",
 153     issue = "none"
 154 )]
 155 pub struct AssertParamIsClone<T: Clone + ?Sized> {
 156     _field: crate::marker::PhantomData<T>,
 157 }
 158 #[doc(hidden)]
 159 #[allow(missing_debug_implementations)]
 160 #[unstable(
 161     feature = "derive_clone_copy",
 162     reason = "deriving hack, should not be public",
 163     issue = "none"
 164 )]
 165 pub struct AssertParamIsCopy<T: Copy + ?Sized> {
 166     _field: crate::marker::PhantomData<T>,
 167 }
 168
 169 /// Implementations of `Clone` for primitive types.
 170 ///
 171 /// Implementations that cannot be described in Rust
 172 /// are implemented in `SelectionContext::copy_clone_conditions()` in librustc.
 173 mod impls {
 174
 175     use super::Clone;
 176
 177     macro_rules! impl_clone {
 178         ($($t:ty)*) => {
 179             $(
 180                 #[stable(feature = "rust1", since = "1.0.0")]
 181                 impl Clone for $t {
 182                     #[inline]
 183                     fn clone(&self) -> Self {
 184                         *self
 185                     }
 186                 }
 187             )*
 188         }
 189     }
 190
 191     impl_clone! {
 192         usize u8 u16 u32 u64 u128
 193         isize i8 i16 i32 i64 i128
 194         f32 f64
 195         bool char
 196     }
 197
 198     #[unstable(feature = "never_type", issue = "35121")]
 199     impl Clone for ! {
 200         #[inline]
 201         fn clone(&self) -> Self {
 202             *self
 203         }
 204     }
 205
 206     #[stable(feature = "rust1", since = "1.0.0")]
 207     impl<T: ?Sized> Clone for *const T {
 208         #[inline]
 209         fn clone(&self) -> Self {
 210             *self
 211         }
 212     }
 213
 214     #[stable(feature = "rust1", since = "1.0.0")]
 215     impl<T: ?Sized> Clone for *mut T {
 216         #[inline]
 217         fn clone(&self) -> Self {
 218             *self
 219         }
 220     }
 221
 222     // Shared references can be cloned, but mutable references *cannot*!
 223     #[stable(feature = "rust1", since = "1.0.0")]
 224     impl<T: ?Sized> Clone for &T {
 225         #[inline]
 226         fn clone(&self) -> Self {
 227             *self
 228         }
 229     }
 230 }