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.

§`Clone` and `PartialEq`/`Eq`

Clone is intended for the duplication of objects. Consequently, when implementing both Clone and PartialEq, the following property is expected to hold:

x == x -> x.clone() == x

In other words, if an object compares equal to itself, its clone must also compare equal to the original.

For types that also implement Eq – for which x == x always holds – this implies that x.clone() == x must always be true. Standard library collections such as HashMap, HashSet, BTreeMap, BTreeSet and BinaryHeap rely on their keys respecting this property for correct behavior. Furthermore, these collections require that cloning a key preserves the outcome of the Hash and Ord methods. Thankfully, this follows automatically from x.clone() == x if Hash and Ord are correctly implemented according to their own requirements.

When deriving both Clone and PartialEq using #[derive(Clone, PartialEq)] or when additionally deriving Eq using #[derive(Clone, PartialEq, Eq)], then this property is automatically upheld – provided that it is satisfied by the underlying types.

Violating this property is a logic error. The behavior resulting from a logic error is not specified, but users of the trait must ensure that such logic errors do not result in undefined behavior. This means that unsafe code must not rely on this property being satisfied.

§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 Clone Copy item path

§Derivable

§How can I implement Clone?

§Clone and PartialEq/Eq

§Additional implementors

Required Methods§

fn clone(&self) -> Self

§Examples

Provided Methods§

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

Dyn Compatibility§

Implementors§

impl Clone for AddressBech32DecodeError

impl Clone for AddressBech32EncodeError

impl Clone for AlwaysVisibleGlobalNodesVersion

impl Clone for SystemExecution

impl Clone for ParseBlsPublicKeyError

impl Clone for ParseBlsSignatureError

impl Clone for ParseEd25519PublicKeyError

impl Clone for ParseEd25519SignatureError

impl Clone for ParseHashError

impl Clone for ParseSecp256k1PublicKeyError

impl Clone for ParseSecp256k1SignatureError

impl Clone for radix_common::crypto::PublicKey

impl Clone for PublicKeyHash

impl Clone for ManifestCustomExtension

impl Clone for ManifestCustomTraversal

impl Clone for ManifestCustomValue

impl Clone for ManifestCustomValueKind

impl Clone for ManifestToRustValueError

impl Clone for RustToManifestValueError

impl Clone for AllowedIds

impl Clone for LowerBound

impl Clone for ManifestAddress

impl Clone for ManifestBucketBatch

impl Clone for ManifestComponentAddress

impl Clone for ManifestExpression

impl Clone for ManifestGlobalAddress

impl Clone for ManifestNonFungibleLocalId

impl Clone for ManifestNonFungibleLocalIdValidationError

impl Clone for ManifestPackageAddress

impl Clone for ManifestProofBatch

impl Clone for ManifestResourceAddress

impl Clone for ManifestResourceConstraint

impl Clone for ParseManifestAddressReservationError

impl Clone for ParseManifestBlobRefError

impl Clone for ParseManifestBucketError

impl Clone for ParseManifestDecimalError

impl Clone for ParseManifestExpressionError

impl Clone for ParseManifestPreciseDecimalError

impl Clone for ParseManifestProofError

impl Clone for ResourceConstraintError

impl Clone for ResourceConstraintsError

impl Clone for UpperBound

impl Clone for OwnValidation

impl Clone for ReferenceValidation

impl Clone for ScryptoCustomTraversal

impl Clone for ScryptoCustomTypeKind

impl Clone for ScryptoCustomTypeKindLabel

impl Clone for ScryptoCustomTypeValidation

impl Clone for ScryptoCustomValue

impl Clone for ScryptoCustomValueKind

impl Clone for ContentValidationError

impl Clone for NonFungibleIdType

impl Clone for NonFungibleLocalId

impl Clone for ParseNonFungibleLocalIdError

impl Clone for ParseOwnError

impl Clone for ParseReferenceError

impl Clone for ParseI192Error

impl Clone for ParseI256Error

impl Clone for ParseI320Error

impl Clone for ParseI384Error

impl Clone for ParseI448Error

impl Clone for ParseI512Error

impl Clone for ParseI768Error

impl Clone for ParseU192Error

impl Clone for ParseU256Error

impl Clone for ParseU320Error

impl Clone for ParseU384Error

impl Clone for ParseU448Error

Trait Clone

§How can I implement `Clone`?

§`Clone` and `PartialEq`/`Eq`