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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()
}
}
// 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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