1 //! The `Clone` trait for types that cannot be 'implicitly copied'.
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.
12 //! [`clone`]: Clone::clone
14 //! Basic usage example:
17 //! let s = String::new(); // String type implements Clone
18 //! let copy = s.clone(); // so we can clone it
21 //! To easily implement the Clone trait, you can also use
22 //! `#[derive(Clone)]`. Example:
25 //! #[derive(Clone)] // we add the Clone trait to Morpheus struct
32 //! let f = Morpheus { blue_pill: 0.0, red_pill: 0 };
33 //! let copy = f.clone(); // and now we can clone it!
37 #![stable(feature = "rust1", since = "1.0.0")]
39 use crate::marker::Destruct;
41 /// A common trait for the ability to explicitly duplicate an object.
43 /// Differs from [`Copy`] in that [`Copy`] is implicit and an inexpensive bit-wise copy, 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.
48 /// Since `Clone` is more general than [`Copy`], you can automatically make anything
49 /// [`Copy`] be `Clone` as well.
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.
56 /// [`clone`]: Clone::clone
58 /// For a generic struct, `#[derive]` implements `Clone` conditionally by adding bound `Clone` on
59 /// generic parameters.
62 /// // `derive` implements Clone for Reading<T> when T is Clone.
64 /// struct Reading<T> {
69 /// ## How can I implement `Clone`?
71 /// Types that are [`Copy`] should have a trivial implementation of `Clone`. More formally:
72 /// if `T: Copy`, `x: T`, and `y: &T`, then `let x = y.clone();` is equivalent to `let x = *y;`.
73 /// Manual implementations should be careful to uphold this invariant; however, unsafe code
74 /// must not rely on it to ensure memory safety.
76 /// An example is a generic struct holding a function pointer. In this case, the
77 /// implementation of `Clone` cannot be `derive`d, but can be implemented as:
80 /// struct Generate<T>(fn() -> T);
82 /// impl<T> Copy for Generate<T> {}
84 /// impl<T> Clone for Generate<T> {
85 /// fn clone(&self) -> Self {
91 /// ## Additional implementors
93 /// In addition to the [implementors listed below][impls],
94 /// the following types also implement `Clone`:
96 /// * Function item types (i.e., the distinct types defined for each function)
97 /// * Function pointer types (e.g., `fn() -> i32`)
98 /// * Tuple types, if each component also implements `Clone` (e.g., `()`, `(i32, bool)`)
99 /// * Closure types, if they capture no value from the environment
100 /// or if all such captured values implement `Clone` themselves.
101 /// Note that variables captured by shared reference always implement `Clone`
102 /// (even if the referent doesn't),
103 /// while variables captured by mutable reference never implement `Clone`.
105 /// [impls]: #implementors
106 #[stable(feature = "rust1", since = "1.0.0")]
108 #[rustc_diagnostic_item = "Clone"]
109 #[rustc_trivial_field_reads]
110 pub trait Clone: Sized {
111 /// Returns a copy of the value.
116 /// # #![allow(noop_method_call)]
117 /// let hello = "Hello"; // &str implements Clone
119 /// assert_eq!("Hello", hello.clone());
121 #[stable(feature = "rust1", since = "1.0.0")]
122 #[must_use = "cloning is often expensive and is not expected to have side effects"]
123 fn clone(&self) -> Self;
125 /// Performs copy-assignment from `source`.
127 /// `a.clone_from(&b)` is equivalent to `a = b.clone()` in functionality,
128 /// but can be overridden to reuse the resources of `a` to avoid unnecessary
131 #[stable(feature = "rust1", since = "1.0.0")]
132 #[default_method_body_is_const]
133 fn clone_from(&mut self, source: &Self)
135 Self: ~const Destruct,
137 *self = source.clone()
141 /// Derive macro generating an impl of the trait `Clone`.
142 #[rustc_builtin_macro]
143 #[stable(feature = "builtin_macro_prelude", since = "1.38.0")]
144 #[allow_internal_unstable(core_intrinsics, derive_clone_copy)]
145 pub macro Clone($item:item) {
146 /* compiler built-in */
149 // FIXME(aburka): these structs are used solely by #[derive] to
150 // assert that every component of a type implements Clone or Copy.
152 // These structs should never appear in user code.
154 #[allow(missing_debug_implementations)]
156 feature = "derive_clone_copy",
157 reason = "deriving hack, should not be public",
160 pub struct AssertParamIsClone<T: Clone + ?Sized> {
161 _field: crate::marker::PhantomData<T>,
164 #[allow(missing_debug_implementations)]
166 feature = "derive_clone_copy",
167 reason = "deriving hack, should not be public",
170 pub struct AssertParamIsCopy<T: Copy + ?Sized> {
171 _field: crate::marker::PhantomData<T>,
174 /// Implementations of `Clone` for primitive types.
176 /// Implementations that cannot be described in Rust
177 /// are implemented in `traits::SelectionContext::copy_clone_conditions()`
178 /// in `rustc_trait_selection`.
183 macro_rules! impl_clone {
186 #[stable(feature = "rust1", since = "1.0.0")]
187 #[rustc_const_unstable(feature = "const_clone", issue = "91805")]
188 impl const Clone for $t {
190 fn clone(&self) -> Self {
199 usize u8 u16 u32 u64 u128
200 isize i8 i16 i32 i64 i128
205 #[unstable(feature = "never_type", issue = "35121")]
206 #[rustc_const_unstable(feature = "const_clone", issue = "91805")]
207 impl const Clone for ! {
209 fn clone(&self) -> Self {
214 #[stable(feature = "rust1", since = "1.0.0")]
215 #[rustc_const_unstable(feature = "const_clone", issue = "91805")]
216 impl<T: ?Sized> const Clone for *const T {
218 fn clone(&self) -> Self {
223 #[stable(feature = "rust1", since = "1.0.0")]
224 #[rustc_const_unstable(feature = "const_clone", issue = "91805")]
225 impl<T: ?Sized> const Clone for *mut T {
227 fn clone(&self) -> Self {
232 /// Shared references can be cloned, but mutable references *cannot*!
233 #[stable(feature = "rust1", since = "1.0.0")]
234 #[rustc_const_unstable(feature = "const_clone", issue = "91805")]
235 impl<T: ?Sized> const Clone for &T {
237 #[rustc_diagnostic_item = "noop_method_clone"]
238 fn clone(&self) -> Self {
243 /// Shared references can be cloned, but mutable references *cannot*!
244 #[stable(feature = "rust1", since = "1.0.0")]
245 impl<T: ?Sized> !Clone for &mut T {}