Trait Clone

1.6.0 · 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§

1.6.0 · 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§

1.6.0 · 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 wasmtime_environ::component::dfg::CoreDef

impl Clone for Trampoline

impl Clone for ComponentItem

impl Clone for wasmtime_environ::component::CoreDef

impl Clone for wasmtime_environ::component::Export

impl Clone for FixedEncoding

impl Clone for FlatType

impl Clone for InterfaceType

impl Clone for StringEncoding

impl Clone for Transcode

impl Clone for TypeDef

impl Clone for Collector

impl Clone for ConstOp

impl Clone for EngineOrModuleTypeIndex

impl Clone for EntityIndex

impl Clone for wasmtime_environ::EntityType

impl Clone for GcLayout

impl Clone for IndexType

impl Clone for wasmtime_environ::RelocationTarget

impl Clone for SettingKind

impl Clone for TableInitialValue

impl Clone for TableSegmentElements

impl Clone for Trap

impl Clone for VMGcKind

impl Clone for WasmCompositeInnerType

impl Clone for WasmHeapBottomType

impl Clone for WasmHeapTopType

impl Clone for WasmHeapType

impl Clone for WasmStorageType

impl Clone for WasmValType

impl Clone for wasmtime_environ::fact::Import

impl Clone for LibCall

impl Clone for AsciiChar

impl Clone for wasmtime_environ::__core::cmp::Ordering

impl Clone for Infallible

impl Clone for wasmtime_environ::__core::fmt::Alignment

impl Clone for IpAddr

impl Clone for Ipv6MulticastScope

impl Clone for wasmtime_environ::__core::net::SocketAddr

impl Clone for FpCategory

impl Clone for IntErrorKind

impl Clone for SearchStep

impl Clone for wasmtime_environ::__core::sync::atomic::Ordering

impl Clone for TryReserveErrorKind

impl Clone for VarError

impl Clone for std::io::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 _Unwind_Action

impl Clone for _Unwind_Reason_Code

impl Clone for Level

impl Clone for LevelFilter

impl Clone for Op

impl Clone for bool

impl Clone for char

impl Clone for f16

impl Clone for f32

impl Clone for f64

impl Clone for f128

impl Clone for i8

impl Clone for i16

impl Clone for i32

impl Clone for i64

impl Clone for i128

impl Clone for isize

impl Clone for !

Trait Clone

§How can I implement `Clone`?