library/core/src/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`] method.
  11 //!
  12 //! [`clone`]: Clone::clone
  13 //!
  14 //! Basic usage example:
  15 //!
  16 //! ```
  17 //! let s = String::new(); // String type implements Clone
  18 //! let copy = s.clone(); // so we can clone it
  19 //! ```
  20 //!
  21 //! To easily implement the Clone trait, you can also use
  22 //! `#[derive(Clone)]`. Example:
  23 //!
  24 //! ```
  25 //! #[derive(Clone)] // we add the Clone trait to Morpheus struct
  26 //! struct Morpheus {
  27 //!    blue_pill: f32,
  28 //!    red_pill: i64,
  29 //! }
  30 //!
  31 //! fn main() {
  32 //!    let f = Morpheus { blue_pill: 0.0, red_pill: 0 };
  33 //!    let copy = f.clone(); // and now we can clone it!
  34 //! }
  35 //! ```
  36
  37 #![stable(feature = "rust1", since = "1.0.0")]
  38
  39 /// A common trait for the ability to explicitly duplicate an object.
  40 ///
  41 /// Differs from [`Copy`] in that [`Copy`] is implicit and extremely inexpensive, while
  42 /// `Clone` is always explicit and may or may not be expensive. In order to enforce
  43 /// these characteristics, Rust does not allow you to reimplement [`Copy`], but you
  44 /// may reimplement `Clone` and run arbitrary code.
  45 ///
  46 /// Since `Clone` is more general than [`Copy`], you can automatically make anything
  47 /// [`Copy`] be `Clone` as well.
  48 ///
  49 /// ## Derivable
  50 ///
  51 /// This trait can be used with `#[derive]` if all fields are `Clone`. The `derive`d
  52 /// implementation of [`Clone`] calls [`clone`] on each field.
  53 ///
  54 /// [`clone`]: Clone::clone
  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 /// ```
  78 /// struct Generate<T>(fn() -> T);
  79 ///
  80 /// impl<T> Copy for Generate<T> {}
  81 ///
  82 /// impl<T> Clone for Generate<T> {
  83 ///     fn clone(&self) -> Self {
  84 ///         *self
  85 ///     }
  86 /// }
  87 /// ```
  88 ///
  89 /// ## Additional implementors
  90 ///
  91 /// In addition to the [implementors listed below][impls],
  92 /// the following types also implement `Clone`:
  93 ///
  94 /// * Function item types (i.e., the distinct types defined for each function)
  95 /// * Function pointer types (e.g., `fn() -> i32`)
  96 /// * Array types, for all sizes, if the item type also implements `Clone` (e.g., `[i32; 123456]`)
  97 /// * Tuple types, if each component also implements `Clone` (e.g., `()`, `(i32, bool)`)
  98 /// * Closure types, if they capture no value from the environment
  99 ///   or if all such captured values implement `Clone` themselves.
 100 ///   Note that variables captured by shared reference always implement `Clone`
 101 ///   (even if the referent doesn't),
 102 ///   while variables captured by mutable reference never implement `Clone`.
 103 ///
 104 /// [impls]: #implementors
 105 #[stable(feature = "rust1", since = "1.0.0")]
 106 #[lang = "clone"]
 107 pub trait Clone: Sized {
 108     /// Returns a copy of the value.
 109     ///
 110     /// # Examples
 111     ///
 112     /// ```
 113     /// let hello = "Hello"; // &str implements Clone
 114     ///
 115     /// assert_eq!("Hello", hello.clone());
 116     /// ```
 117     #[stable(feature = "rust1", since = "1.0.0")]
 118     #[must_use = "cloning is often expensive and is not expected to have side effects"]
 119     fn clone(&self) -> Self;
 120
 121     /// Performs copy-assignment from `source`.
 122     ///
 123     /// `a.clone_from(&b)` is equivalent to `a = b.clone()` in functionality,
 124     /// but can be overridden to reuse the resources of `a` to avoid unnecessary
 125     /// allocations.
 126     #[inline]
 127     #[stable(feature = "rust1", since = "1.0.0")]
 128     fn clone_from(&mut self, source: &Self) {
 129         *self = source.clone()
 130     }
 131 }
 132
 133 /// Derive macro generating an impl of the trait `Clone`.
 134 #[rustc_builtin_macro]
 135 #[stable(feature = "builtin_macro_prelude", since = "1.38.0")]
 136 #[allow_internal_unstable(core_intrinsics, derive_clone_copy)]
 137 pub macro Clone($item:item) {
 138     /* compiler built-in */
 139 }
 140
 141 // FIXME(aburka): these structs are used solely by #[derive] to
 142 // assert that every component of a type implements Clone or Copy.
 143 //
 144 // These structs should never appear in user code.
 145 #[doc(hidden)]
 146 #[allow(missing_debug_implementations)]
 147 #[unstable(
 148     feature = "derive_clone_copy",
 149     reason = "deriving hack, should not be public",
 150     issue = "none"
 151 )]
 152 pub struct AssertParamIsClone<T: Clone + ?Sized> {
 153     _field: crate::marker::PhantomData<T>,
 154 }
 155 #[doc(hidden)]
 156 #[allow(missing_debug_implementations)]
 157 #[unstable(
 158     feature = "derive_clone_copy",
 159     reason = "deriving hack, should not be public",
 160     issue = "none"
 161 )]
 162 pub struct AssertParamIsCopy<T: Copy + ?Sized> {
 163     _field: crate::marker::PhantomData<T>,
 164 }
 165
 166 /// Implementations of `Clone` for primitive types.
 167 ///
 168 /// Implementations that cannot be described in Rust
 169 /// are implemented in `traits::SelectionContext::copy_clone_conditions()`
 170 /// in `rustc_trait_selection`.
 171 mod impls {
 172
 173     use super::Clone;
 174
 175     macro_rules! impl_clone {
 176         ($($t:ty)*) => {
 177             $(
 178                 #[stable(feature = "rust1", since = "1.0.0")]
 179                 impl Clone for $t {
 180                     #[inline]
 181                     fn clone(&self) -> Self {
 182                         *self
 183                     }
 184                 }
 185             )*
 186         }
 187     }
 188
 189     impl_clone! {
 190         usize u8 u16 u32 u64 u128
 191         isize i8 i16 i32 i64 i128
 192         f32 f64
 193         bool char
 194     }
 195
 196     #[unstable(feature = "never_type", issue = "35121")]
 197     impl Clone for ! {
 198         #[inline]
 199         fn clone(&self) -> Self {
 200             *self
 201         }
 202     }
 203
 204     #[stable(feature = "rust1", since = "1.0.0")]
 205     impl<T: ?Sized> Clone for *const T {
 206         #[inline]
 207         fn clone(&self) -> Self {
 208             *self
 209         }
 210     }
 211
 212     #[stable(feature = "rust1", since = "1.0.0")]
 213     impl<T: ?Sized> Clone for *mut T {
 214         #[inline]
 215         fn clone(&self) -> Self {
 216             *self
 217         }
 218     }
 219
 220     /// Shared references can be cloned, but mutable references *cannot*!
 221     #[stable(feature = "rust1", since = "1.0.0")]
 222     impl<T: ?Sized> Clone for &T {
 223         #[inline]
 224         fn clone(&self) -> Self {
 225             *self
 226         }
 227     }
 228
 229     /// Shared references can be cloned, but mutable references *cannot*!
 230     #[stable(feature = "rust1", since = "1.0.0")]
 231     impl<T: ?Sized> !Clone for &mut T {}
 232 }