Trait Clone

1.0.0 (const: unstable) · 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 that allows explicit creation of a duplicate value.

Calling clone always produces a new value. However, for types that are references to other data (such as smart pointers or references), the new value may still point to the same underlying data, rather than duplicating it. See Clone::clone for more details.

This distinction is especially important when using #[derive(Clone)] on structs containing smart pointers like Arc<Mutex<T>> - the cloned struct will share mutable state with the original.

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 duplicate of the value.

Note that what “duplicate” means varies by type:

For most types, this creates a deep, independent copy
For reference types like &T, this creates another reference to the same value
For smart pointers like Arc or Rc, this increments the reference count but still points to the same underlying data

§Examples

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

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

Example with a reference-counted type:

use std::sync::{Arc, Mutex};

let data = Arc::new(Mutex::new(vec![1, 2, 3]));
let data_clone = data.clone(); // Creates another Arc pointing to the same Mutex

{
    let mut lock = data.lock().unwrap();
    lock.push(4);
}

// Changes are visible through the clone because they share the same underlying data
assert_eq!(*data_clone.lock().unwrap(), vec![1, 2, 3, 4]);

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 AcknowledgementStatus

impl Clone for ibc_core::channel::types::channel::Order

impl Clone for ibc_core::channel::types::channel::State

impl Clone for ChannelMsg

impl Clone for ibc_core::channel::types::msgs::PacketMsg

impl Clone for PacketMsgType

impl Clone for Receipt

impl Clone for ibc_core::channel::types::proto::v1::acknowledgement::Response

impl Clone for ibc_core::channel::types::proto::v1::Order

impl Clone for ResponseResultType

impl Clone for ibc_core::channel::types::proto::v1::State

impl Clone for TimeoutHeight

impl Clone for TimeoutTimestamp

impl Clone for ClientMsg

impl Clone for Status

impl Clone for UpdateKind

impl Clone for ibc_core::commitment_types::proto::ics23::batch_entry::Proof

impl Clone for ibc_core::commitment_types::proto::ics23::commitment_proof::Proof

impl Clone for ibc_core::commitment_types::proto::ics23::compressed_batch_entry::Proof

impl Clone for HashOp

impl Clone for LengthOp

impl Clone for ibc_core::connection::types::State

impl Clone for ConnectionMsg

impl Clone for ibc_core::connection::types::proto::v1::State

impl Clone for IbcEvent

impl Clone for MessageEvent

impl Clone for MsgEnvelope

impl Clone for ibc_core::host::types::path::Path

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 core::fmt::Alignment

impl Clone for DebugAsHex

impl Clone for Sign

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 core::sync::atomic::Ordering

impl Clone for VarError

impl Clone for SeekFrom

impl Clone for std::io::error::ErrorKind

impl Clone for Shutdown

impl Clone for BacktraceStyle

impl Clone for RecvTimeoutError

impl Clone for TryRecvError

impl Clone for arbitrary::error::Error

impl Clone for base64::decode::DecodeError

impl Clone for base64::decode::DecodeError

impl Clone for base64::decode::DecodeSliceError

impl Clone for base64::decode::DecodeSliceError

impl Clone for base64::encode::EncodeSliceError

impl Clone for base64::encode::EncodeSliceError

impl Clone for base64::engine::DecodePaddingMode

impl Clone for base64::engine::DecodePaddingMode

impl Clone for borsh::nostd_io::ErrorKind

impl Clone for byte_slice_cast::Error

impl Clone for Case

impl Clone for cosmos_sdk_proto::cosmos::gov::v1::ProposalStatus

impl Clone for cosmos_sdk_proto::cosmos::gov::v1::VoteOption

impl Clone for cosmos_sdk_proto::cosmos::gov::v1beta1::ProposalStatus

impl Clone for cosmos_sdk_proto::cosmos::gov::v1beta1::VoteOption

impl Clone for AuthorizationType

impl Clone for BondStatus

impl Clone for Infraction

Trait Clone

§How can I implement `Clone`?