clone.rs

// Copyright 2012-2013 The Rust Project Developers. See the COPYRIGHT
// file at the top-level directory of this distribution and at
// http://rust-lang.org/COPYRIGHT.
//
// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
// option. This file may not be copied, modified, or distributed
// except according to those terms.

//! 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.
///
/// ## Closures
///
/// Closure types automatically implement `Clone` if they capture no value from the environment
/// or if all such captured values implement `Clone` themselves.
///
/// ## 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 an array holding more than 32 elements of a type that is `Clone`; the standard
/// library only implements `Clone` up until arrays of size 32. 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
///
/// ```
/// #[derive(Copy)]
/// struct Stats {
///    frequencies: [i32; 100],
/// }
///
/// impl Clone for Stats {
///     fn clone(&self) -> Stats { *self }
/// }
/// ```
#[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: ::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: ::marker::PhantomData<T> }