src/libcore/clone.rs

   1 // Copyright 2012-2013 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 //! The `Clone` trait for types that cannot be 'implicitly copied'.
  12 //!
  13 //! In Rust, some simple types are "implicitly copyable" and when you
  14 //! assign them or pass them as arguments, the receiver will get a copy,
  15 //! leaving the original value in place. These types do not require
  16 //! allocation to copy and do not have finalizers (i.e. they do not
  17 //! contain owned boxes or implement [`Drop`]), so the compiler considers
  18 //! them cheap and safe to copy. For other types copies must be made
  19 //! explicitly, by convention implementing the [`Clone`] trait and calling
  20 //! the [`clone`][clone] method.
  21 //!
  22 //! [`Clone`]: trait.Clone.html
  23 //! [clone]: trait.Clone.html#tymethod.clone
  24 //! [`Drop`]: ../../std/ops/trait.Drop.html
  25 //!
  26 //! Basic usage example:
  27 //!
  28 //! ```
  29 //! let s = String::new(); // String type implements Clone
  30 //! let copy = s.clone(); // so we can clone it
  31 //! ```
  32 //!
  33 //! To easily implement the Clone trait, you can also use
  34 //! `#[derive(Clone)]`. Example:
  35 //!
  36 //! ```
  37 //! #[derive(Clone)] // we add the Clone trait to Morpheus struct
  38 //! struct Morpheus {
  39 //!    blue_pill: f32,
  40 //!    red_pill: i64,
  41 //! }
  42 //!
  43 //! fn main() {
  44 //!    let f = Morpheus { blue_pill: 0.0, red_pill: 0 };
  45 //!    let copy = f.clone(); // and now we can clone it!
  46 //! }
  47 //! ```
  48
  49 #![stable(feature = "rust1", since = "1.0.0")]
  50
  51 /// A common trait for the ability to explicitly duplicate an object.
  52 ///
  53 /// Differs from [`Copy`] in that [`Copy`] is implicit and extremely inexpensive, while
  54 /// `Clone` is always explicit and may or may not be expensive. In order to enforce
  55 /// these characteristics, Rust does not allow you to reimplement [`Copy`], but you
  56 /// may reimplement `Clone` and run arbitrary code.
  57 ///
  58 /// Since `Clone` is more general than [`Copy`], you can automatically make anything
  59 /// [`Copy`] be `Clone` as well.
  60 ///
  61 /// ## Derivable
  62 ///
  63 /// This trait can be used with `#[derive]` if all fields are `Clone`. The `derive`d
  64 /// implementation of [`clone()`] calls [`clone()`] on each field.
  65 ///
  66 /// ## How can I implement `Clone`?
  67 ///
  68 /// Types that are [`Copy`] should have a trivial implementation of `Clone`. More formally:
  69 /// if `T: Copy`, `x: T`, and `y: &T`, then `let x = y.clone();` is equivalent to `let x = *y;`.
  70 /// Manual implementations should be careful to uphold this invariant; however, unsafe code
  71 /// must not rely on it to ensure memory safety.
  72 ///
  73 /// An example is an array holding more than 32 elements of a type that is `Clone`; the standard
  74 /// library only implements `Clone` up until arrays of size 32. In this case, the implementation of
  75 /// `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 /// #[derive(Copy)]
  82 /// struct Stats {
  83 ///    frequencies: [i32; 100],
  84 /// }
  85 ///
  86 /// impl Clone for Stats {
  87 ///     fn clone(&self) -> Stats { *self }
  88 /// }
  89 /// ```
  90 #[stable(feature = "rust1", since = "1.0.0")]
  91 pub trait Clone : Sized {
  92     /// Returns a copy of the value.
  93     ///
  94     /// # Examples
  95     ///
  96     /// ```
  97     /// let hello = "Hello"; // &str implements Clone
  98     ///
  99     /// assert_eq!("Hello", hello.clone());
 100     /// ```
 101     #[stable(feature = "rust1", since = "1.0.0")]
 102     fn clone(&self) -> Self;
 103
 104     /// Performs copy-assignment from `source`.
 105     ///
 106     /// `a.clone_from(&b)` is equivalent to `a = b.clone()` in functionality,
 107     /// but can be overridden to reuse the resources of `a` to avoid unnecessary
 108     /// allocations.
 109     #[inline(always)]
 110     #[stable(feature = "rust1", since = "1.0.0")]
 111     fn clone_from(&mut self, source: &Self) {
 112         *self = source.clone()
 113     }
 114 }
 115
 116 // FIXME(aburka): these structs are used solely by #[derive] to
 117 // assert that every component of a type implements Clone or Copy.
 118 //
 119 // These structs should never appear in user code.
 120 #[doc(hidden)]
 121 #[allow(missing_debug_implementations)]
 122 #[unstable(feature = "derive_clone_copy",
 123            reason = "deriving hack, should not be public",
 124            issue = "0")]
 125 pub struct AssertParamIsClone<T: Clone + ?Sized> { _field: ::marker::PhantomData<T> }
 126 #[doc(hidden)]
 127 #[allow(missing_debug_implementations)]
 128 #[unstable(feature = "derive_clone_copy",
 129            reason = "deriving hack, should not be public",
 130            issue = "0")]
 131 pub struct AssertParamIsCopy<T: Copy + ?Sized> { _field: ::marker::PhantomData<T> }
 132
 133 #[stable(feature = "rust1", since = "1.0.0")]
 134 impl<'a, T: ?Sized> Clone for &'a T {
 135     /// Returns a shallow copy of the reference.
 136     #[inline]
 137     fn clone(&self) -> &'a T { *self }
 138 }
 139
 140 macro_rules! clone_impl {
 141     ($t:ty) => {
 142         #[stable(feature = "rust1", since = "1.0.0")]
 143         impl Clone for $t {
 144             /// Returns a deep copy of the value.
 145             #[inline]
 146             fn clone(&self) -> $t { *self }
 147         }
 148     }
 149 }
 150
 151 clone_impl! { isize }
 152 clone_impl! { i8 }
 153 clone_impl! { i16 }
 154 clone_impl! { i32 }
 155 clone_impl! { i64 }
 156 #[cfg(not(stage0))]
 157 clone_impl! { i128 }
 158
 159 clone_impl! { usize }
 160 clone_impl! { u8 }
 161 clone_impl! { u16 }
 162 clone_impl! { u32 }
 163 clone_impl! { u64 }
 164 #[cfg(not(stage0))]
 165 clone_impl! { u128 }
 166
 167 clone_impl! { f32 }
 168 clone_impl! { f64 }
 169
 170 clone_impl! { () }
 171 clone_impl! { bool }
 172 clone_impl! { char }