lib::core::clone

Trait Clone

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. Copy has no methods, so you cannot change its behavior, but when implementing Clone, the clone method you provide may run arbitrary code.

Since Clone is a supertrait of Copy, any type that implements Copy must also implement Clone.

§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 (const: unstable) · 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 (const: unstable) · 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".

Implementors§

1.100.0 (const: unstable) · Source§

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 !

impl Clone for ATerm

impl Clone for futures_util::abortable::AbortHandle

impl Clone for tokio::runtime::task::abort::AbortHandle

impl Clone for Aborted

impl Clone for Access

impl Clone for AccessError

impl Clone for AccessKind

impl Clone for AccessMode

impl Clone for anstyle_parse::state::definitions::Action

impl Clone for tower_http::follow_redirect::policy::Action

impl Clone for lib::net::AddrParseError

impl Clone for rustls_pki_types::server_name::AddrParseError

impl Clone for ipnet::parser::AddrParseError

impl Clone for Adler32

impl Clone for Advice

impl Clone for AesMode

impl Clone for AhoCorasick

impl Clone for AhoCorasickBuilder

impl Clone for AhoCorasickKind

impl Clone for AlertDescription

impl Clone for ring::hkdf::Algorithm

impl Clone for ring::hmac::Algorithm

impl Clone for ring::pbkdf2::Algorithm

impl Clone for AlgorithmIdentifier

impl Clone for lib::core::mem::Alignment

impl Clone for lib::core::fmt::Alignment

impl Clone for icu_datetime::options::Alignment

impl Clone for AllocErr

impl Clone for core::alloc::AllocError

impl Clone for allocator_api2::stable::alloc::AllocError

impl Clone for Alphabet

impl Clone for Alphabetic

impl Clone for Alphanumeric

impl Clone for AlternateHandling

impl Clone for Alternation

impl Clone for AlwaysResolvesClientRawPublicKeys

impl Clone for AlwaysResolvesServerRawPublicKeys

impl Clone for regex_automata::util::search::Anchored

impl Clone for aho_corasick::util::search::Anchored

impl Clone for AndCondition

impl Clone for Ansi256Color

impl Clone for AnsiColor

impl Clone for AnyCalendar

impl Clone for AnyCalendarKind

impl Clone for AnyColumn

impl Clone for AnyConnectOptions

impl Clone for AnyKind

impl Clone for AnyRow

impl Clone for AnyStatement

impl Clone for AnyTypeInfo

impl Clone for AnyTypeInfoKind

impl Clone for AnyValue

impl Clone for AnyValueKind

impl Clone for AppliedMigration

impl Clone for ArchiveOffset

impl Clone for Arg

impl Clone for ArgAction

impl Clone for ArgCursor

impl Clone for ArgGroup

impl Clone for ArgMatches

impl Clone for ArgPredicate

impl Clone for AsciiChar

impl Clone for AsciiDenyList

impl Clone for AsciiParser

impl Clone for AsciiProbeResult

impl Clone for Assertion

impl Clone for AssertionKind

Trait Clone

§How can I implement `Clone`?

§`Clone` and `PartialEq`/`Eq`