Trait Clone

1.0.0 · Source

pub trait Clone: Sized {
    // Required method
    fn clone(&self) -> Self;

    // Provided method
    fn clone_from(&mut self, source: &Self) { ... }
}

Expand description

A common trait for the ability to explicitly duplicate an object.

Differs from Copy in that Copy is implicit and an inexpensive bit-wise copy, 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 derived 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 derived, but can be implemented as:

struct Generate<T>(fn() -> T);

impl<T> Copy for Generate<T> {}

impl<T> Clone for Generate<T> {
    fn clone(&self) -> Self {
        *self
    }
}

If we derive:

#[derive(Copy, Clone)]
struct Generate<T>(fn() -> T);

the auto-derived implementations will have unnecessary T: Copy and T: Clone bounds:


// Automatically derived
impl<T: Copy> Copy for Generate<T> { }

// Automatically derived
impl<T: Clone> Clone for Generate<T> {
    fn clone(&self) -> Generate<T> {
        Generate(Clone::clone(&self.0))
    }
}

The bounds are unnecessary because clearly the function itself should be copy- and cloneable even if its return type is not:

#[derive(Copy, Clone)]
struct Generate<T>(fn() -> T);

struct NotCloneable;

fn generate_not_cloneable() -> NotCloneable {
    NotCloneable
}

Generate(generate_not_cloneable).clone(); // error: trait bounds were not satisfied
// Note: With the manual implementations the above line will compile.

§Additional implementors

In addition to the implementors listed below, 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)
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.

Required Methods§

1.0.0 · Source

fn clone(&self) -> Self

Returns a copy of the value.

§Examples

let hello = "Hello"; // &str implements Clone

assert_eq!("Hello", hello.clone());

Provided Methods§

1.0.0 · Source

fn clone_from(&mut self, source: &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.

Dyn Compatibility§

This trait is not dyn compatible.

In older versions of Rust, dyn compatibility was called "object safety", so this trait is not object safe.

Trait CloneCopy item path

§Derivable

§How can I implement Clone?

§Additional implementors

Required Methods§

fn clone(&self) -> Self

§Examples

Provided Methods§

fn clone_from(&mut self, source: &Self)

Dyn Compatibility§

Implementors§

impl Clone for BuildPattern

impl Clone for Case

impl Clone for shadow_rs::fmt::Alignment

impl Clone for DebugAsHex

impl Clone for Sign

impl Clone for TryReserveErrorKind

impl Clone for AsciiChar

impl Clone for core::cmp::Ordering

impl Clone for Infallible

impl Clone for FromBytesWithNulError

impl Clone for IpAddr

impl Clone for Ipv6MulticastScope

impl Clone for core::net::socket_addr::SocketAddr

impl Clone for FpCategory

impl Clone for IntErrorKind

impl Clone for GetDisjointMutError

impl Clone for SearchStep

impl Clone for core::sync::atomic::Ordering

impl Clone for VarError

impl Clone for SeekFrom

impl Clone for ErrorKind

impl Clone for Shutdown

impl Clone for BacktraceStyle

impl Clone for RecvTimeoutError

impl Clone for TryRecvError

impl Clone for Cfg

impl Clone for CfgExpr

impl Clone for Platform

impl Clone for DependencyKind

impl Clone for Applicability

impl Clone for DiagnosticLevel

impl Clone for CargoOpt

impl Clone for CrateType

impl Clone for Edition

impl Clone for TargetKind

impl Clone for ArtifactDebuginfo

impl Clone for Message

impl Clone for ApplyLocation

impl Clone for CloneLocal

impl Clone for SshHostKeyType

impl Clone for DiffBinaryKind

impl Clone for DiffLineType

impl Clone for AutotagOption

impl Clone for BranchType

impl Clone for ConfigLevel

impl Clone for Delta

impl Clone for DiffFormat

impl Clone for git2::Direction

impl Clone for ErrorClass

impl Clone for ErrorCode

impl Clone for FetchPrune

impl Clone for FileFavor

impl Clone for FileMode

impl Clone for ObjectType

impl Clone for ReferenceType

impl Clone for RepositoryState

impl Clone for ResetType

impl Clone for StashApplyProgress

impl Clone for SubmoduleIgnore

impl Clone for SubmoduleUpdate

impl Clone for PackBuilderStage

impl Clone for StatusShow

impl Clone for TraceLevel

impl Clone for Service

impl Clone for TreeWalkMode

impl Clone for TrieResult

impl Clone for TrieType

impl Clone for icu_collections::codepointtrie::error::Error

impl Clone for ExtensionType

Trait Clone

§How can I implement `Clone`?