Trait Clone

1.0.0 · 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 AsciiChar

impl Clone for origin_studio::cmp::Ordering

impl Clone for TryReserveErrorKind

impl Clone for Infallible

impl Clone for VarError

impl Clone for origin_studio::fmt::Alignment

impl Clone for DebugAsHex

impl Clone for Sign

impl Clone for origin_studio::io::SeekFrom

impl Clone for IpAddr

impl Clone for Ipv6MulticastScope

impl Clone for SocketAddr

impl Clone for FpCategory

impl Clone for IntErrorKind

impl Clone for GetDisjointMutError

impl Clone for SearchStep

impl Clone for origin_studio::sync::atomic::Ordering

impl Clone for FromBytesWithNulError

impl Clone for DwarfFileType

impl Clone for Format

impl Clone for SectionId

impl Clone for Vendor

impl Clone for RunTimeEndian

impl Clone for Pointer

impl Clone for gimli::read::Error

impl Clone for IndexSectionId

impl Clone for Value

impl Clone for ValueType

impl Clone for tpacket_versions

impl Clone for fsconfig_command

impl Clone for membarrier_cmd

impl Clone for membarrier_cmd_flag

impl Clone for procmap_query_flags

impl Clone for rustix::backend::fs::types::Advice

impl Clone for FileType

impl Clone for FlockOperation

impl Clone for rustix::backend::mm::types::Advice

impl Clone for Resource

impl Clone for MembarrierCommand

impl Clone for TimerfdClockId

impl Clone for ClockId

impl Clone for rustix::fs::seek_from::SeekFrom

impl Clone for Direction

impl Clone for DumpableBehavior

impl Clone for EndianMode

impl Clone for FloatingPointMode

impl Clone for MachineCheckMemoryCorruptionKillPolicy

impl Clone for PTracer

impl Clone for SpeculationFeature

impl Clone for TimeStampCounterReadability

impl Clone for TimingMethod

impl Clone for VirtualMemoryMapAddress

impl Clone for FlockOffsetType

impl Clone for FlockType

impl Clone for NanosleepRelativeResult

impl Clone for WakeOp

impl Clone for WakeOpCmp

impl Clone for Capability

impl Clone for CoreSchedulingScope

impl Clone for SecureComputingMode

impl Clone for SysCallUserDispatchFastSwitch

impl Clone for LinkNameSpaceType

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

Trait Clone

§How can I implement `Clone`?