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§

1.0.0 · 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 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 ConstOp

impl Clone for EngineOrModuleTypeIndex

impl Clone for EntityIndex

impl Clone for wasmtime_environ::EntityType

impl Clone for MemoryStyle

impl Clone for wasmtime_environ::RelocationTarget

impl Clone for SettingKind

impl Clone for TableInitialValue

impl Clone for TableSegmentElements

impl Clone for TableStyle

impl Clone for Trap

impl Clone for VMGcKind

impl Clone for WasmCompositeType

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 wasmtime_environ::wasmparser::AbstractHeapType

impl Clone for wasmtime_environ::wasmparser::BlockType

impl Clone for CanonicalFunction

impl Clone for wasmtime_environ::wasmparser::CanonicalOption

impl Clone for wasmtime_environ::wasmparser::Catch

impl Clone for ComdatSymbolKind

impl Clone for ComponentExternalKind

impl Clone for wasmtime_environ::wasmparser::ComponentOuterAliasKind

impl Clone for wasmtime_environ::wasmparser::ComponentTypeRef

impl Clone for wasmtime_environ::wasmparser::ComponentValType

impl Clone for wasmtime_environ::wasmparser::CompositeInnerType

impl Clone for wasmtime_environ::wasmparser::CoreDumpValue

impl Clone for wasmtime_environ::wasmparser::Encoding

impl Clone for ExternalKind

impl Clone for FrameKind

impl Clone for wasmtime_environ::wasmparser::HeapType

impl Clone for InstantiationArgKind

impl Clone for wasmtime_environ::wasmparser::Ordering

impl Clone for OuterAliasKind

impl Clone for wasmtime_environ::wasmparser::PrimitiveValType

impl Clone for RelocAddendKind

impl Clone for RelocationType

impl Clone for wasmtime_environ::wasmparser::StorageType

impl Clone for wasmtime_environ::wasmparser::TagKind

impl Clone for wasmtime_environ::wasmparser::TypeBounds

impl Clone for TypeRef

impl Clone for UnpackedIndex

impl Clone for wasmtime_environ::wasmparser::ValType

impl Clone for AnyTypeId

impl Clone for ComponentAnyTypeId

impl Clone for ComponentCoreTypeId

impl Clone for wasmtime_environ::wasmparser::types::ComponentDefinedType

impl Clone for ComponentEntityType

impl Clone for wasmtime_environ::wasmparser::types::ComponentValType

impl Clone for CoreInstanceTypeKind

impl Clone for wasmtime_environ::wasmparser::types::EntityType

impl Clone for AsciiChar

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

impl Clone for Infallible

Trait Clone

§How can I implement `Clone`?