clone.rs.html -- source
Description: Source to the Rust file `src/libcore/clone.rs`.
Keywords: rust, rustlang, rust-lang

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    //! The `Clone` trait for types that cannot be 'implicitly copied'.
    //!
    //! In Rust, some simple types are "implicitly copyable" and when you
    //! assign them or pass them as arguments, the receiver will get a copy,
    //! leaving the original value in place. These types do not require
    //! allocation to copy and do not have finalizers (i.e., they do not
    //! contain owned boxes or implement [`Drop`]), so the compiler considers
    //! them cheap and safe to copy. For other types copies must be made
    //! explicitly, by convention implementing the [`Clone`] trait and calling
    //! the [`clone`][clone] method.
    //!
    //! [`Clone`]: trait.Clone.html
    //! [clone]: trait.Clone.html#tymethod.clone
    //! [`Drop`]: ../../std/ops/trait.Drop.html
    //!
    //! Basic usage example:
    //!
    //! ```
    //! let s = String::new(); // String type implements Clone
    //! let copy = s.clone(); // so we can clone it
    //! ```
    //!
    //! To easily implement the Clone trait, you can also use
    //! `#[derive(Clone)]`. Example:
    //!
    //! ```
    //! #[derive(Clone)] // we add the Clone trait to Morpheus struct
    //! struct Morpheus {
    //!    blue_pill: f32,
    //!    red_pill: i64,
    //! }
    //!
    //! fn main() {
    //!    let f = Morpheus { blue_pill: 0.0, red_pill: 0 };
    //!    let copy = f.clone(); // and now we can clone it!
    //! }
    //! ```

    #![stable(feature = "rust1", since = "1.0.0")]

    /// A common trait for the ability to explicitly duplicate an object.
    ///
    /// Differs from [`Copy`] in that [`Copy`] is implicit and extremely inexpensive, while
    /// `Clone` is always explicit and may or may not be expensive. In order to enforce
    /// these characteristics, Rust does not allow you to reimplement [`Copy`], but you
    /// may reimplement `Clone` and run arbitrary code.
    ///
    /// Since `Clone` is more general than [`Copy`], you can automatically make anything
    /// [`Copy`] be `Clone` as well.
    ///
    /// ## Derivable
    ///
    /// This trait can be used with `#[derive]` if all fields are `Clone`. The `derive`d
    /// implementation of [`clone`] calls [`clone`] on each field.
    ///
    /// For a generic struct, `#[derive]` implements `Clone` conditionally by adding bound `Clone` on
    /// generic parameters.
    ///
    /// ```
    /// // `derive` implements Clone for Reading<T> when T is Clone.
    /// #[derive(Clone)]
    /// struct Reading<T> {
    ///     frequency: T,
    /// }
    /// ```
    ///
    /// ## How can I implement `Clone`?
    ///
    /// Types that are [`Copy`] should have a trivial implementation of `Clone`. More formally:
    /// if `T: Copy`, `x: T`, and `y: &T`, then `let x = y.clone();` is equivalent to `let x = *y;`.
    /// Manual implementations should be careful to uphold this invariant; however, unsafe code
    /// must not rely on it to ensure memory safety.
    ///
    /// An example is a generic struct holding a function pointer. In this case, the
    /// implementation of `Clone` cannot be `derive`d, but can be implemented as:
    ///
    /// [`Copy`]: ../../std/marker/trait.Copy.html
    /// [`clone`]: trait.Clone.html#tymethod.clone
    ///
    /// ```
    /// struct Generate<T>(fn() -> T);
    ///
    /// impl<T> Copy for Generate<T> {}
    ///
    /// impl<T> Clone for Generate<T> {
    ///     fn clone(&self) -> Self {
    ///         *self
    ///     }
    /// }
    /// ```
    ///
    /// ## Additional implementors
    ///
    /// In addition to the [implementors listed below][impls],
    /// the following types also implement `Clone`:
    ///
    /// * Function item types (i.e., the distinct types defined for each function)
    /// * Function pointer types (e.g., `fn() -> i32`)
    /// * Array types, for all sizes, if the item type also implements `Clone` (e.g., `[i32; 123456]`)
    /// * Tuple types, if each component also implements `Clone` (e.g., `()`, `(i32, bool)`)
    /// * Closure types, if they capture no value from the environment
    ///   or if all such captured values implement `Clone` themselves.
    ///   Note that variables captured by shared reference always implement `Clone`
    ///   (even if the referent doesn't),
    ///   while variables captured by mutable reference never implement `Clone`.
    ///
    /// [impls]: #implementors
    #[stable(feature = "rust1", since = "1.0.0")]
    #[lang = "clone"]
    pub trait Clone : Sized {
        /// Returns a copy of the value.
        ///
        /// # Examples
        ///
        /// ```
        /// let hello = "Hello"; // &str implements Clone
        ///
        /// assert_eq!("Hello", hello.clone());
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        #[must_use = "cloning is often expensive and is not expected to have side effects"]
        fn clone(&self) -> Self;

        /// Performs copy-assignment from `source`.
        ///
        /// `a.clone_from(&b)` is equivalent to `a = b.clone()` in functionality,
        /// but can be overridden to reuse the resources of `a` to avoid unnecessary
        /// allocations.
        #[inline]
        #[stable(feature = "rust1", since = "1.0.0")]
        fn clone_from(&mut self, source: &Self) {
            *self = source.clone()
        }
    }

    /// Derive macro generating an impl of the trait `Clone`.
    #[cfg(not(bootstrap))]
    #[rustc_builtin_macro]
    #[rustc_macro_transparency = "semitransparent"]
    #[stable(feature = "builtin_macro_prelude", since = "1.38.0")]
    #[allow_internal_unstable(core_intrinsics, derive_clone_copy)]
    pub macro Clone($item:item) { /* compiler built-in */ }

    // FIXME(aburka): these structs are used solely by #[derive] to
    // assert that every component of a type implements Clone or Copy.
    //
    // These structs should never appear in user code.
    #[doc(hidden)]
    #[allow(missing_debug_implementations)]
    #[unstable(feature = "derive_clone_copy",
               reason = "deriving hack, should not be public",
               issue = "0")]
    pub struct AssertParamIsClone<T: Clone + ?Sized> { _field: crate::marker::PhantomData<T> }
    #[doc(hidden)]
    #[allow(missing_debug_implementations)]
    #[unstable(feature = "derive_clone_copy",
               reason = "deriving hack, should not be public",
               issue = "0")]
    pub struct AssertParamIsCopy<T: Copy + ?Sized> { _field: crate::marker::PhantomData<T> }

    /// Implementations of `Clone` for primitive types.
    ///
    /// Implementations that cannot be described in Rust
    /// are implemented in `SelectionContext::copy_clone_conditions()` in librustc.
    mod impls {

        use super::Clone;

        macro_rules! impl_clone {
            ($($t:ty)*) => {
                $(
                    #[stable(feature = "rust1", since = "1.0.0")]
                    impl Clone for $t {
                        #[inline]
                        fn clone(&self) -> Self {
                            *self
                        }
                    }
                )*
            }
        }

        impl_clone! {
            usize u8 u16 u32 u64 u128
            isize i8 i16 i32 i64 i128
            f32 f64
            bool char
        }

        #[unstable(feature = "never_type", issue = "35121")]
        impl Clone for ! {
            #[inline]
            fn clone(&self) -> Self {
                *self
            }
        }

        #[stable(feature = "rust1", since = "1.0.0")]
        impl<T: ?Sized> Clone for *const T {
            #[inline]
            fn clone(&self) -> Self {
                *self
            }
        }

        #[stable(feature = "rust1", since = "1.0.0")]
        impl<T: ?Sized> Clone for *mut T {
            #[inline]
            fn clone(&self) -> Self {
                *self
            }
        }

        // Shared references can be cloned, but mutable references *cannot*!
        #[stable(feature = "rust1", since = "1.0.0")]
        impl<T: ?Sized> Clone for &T {
            #[inline]
            fn clone(&self) -> Self {
                *self
            }
        }

    }

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