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 PrivateInput

impl Clone for Dictionary

impl Clone for OffsetValue

impl Clone for BuiltinName

impl Clone for ApUpdate

impl Clone for FpUpdate

impl Clone for Op1Addr

impl Clone for Opcode

impl Clone for PcUpdate

impl Clone for Register

impl Clone for Res

impl Clone for LayoutName

impl Clone for MaybeRelocatable

impl Clone for BuiltinRunner

impl Clone for BuiltinAdditionalData

impl Clone for CairoArg

impl Clone for RunnerMode

impl Clone for cairo_vm::with_std::cmp::Ordering

impl Clone for Infallible

impl Clone for cairo_vm::with_std::fmt::Alignment

impl Clone for FpCategory

impl Clone for IntErrorKind

impl Clone for SearchStep

impl Clone for cairo_vm::with_std::sync::atomic::Ordering

impl Clone for RecvTimeoutError

impl Clone for TryRecvError

impl Clone for alloc::collections::TryReserveErrorKind

impl Clone for AsciiChar

impl Clone for IpAddr

impl Clone for Ipv6MulticastScope

impl Clone for SocketAddr

impl Clone for VarError

impl Clone for SeekFrom

impl Clone for std::io::error::ErrorKind

impl Clone for Shutdown

impl Clone for BacktraceStyle

impl Clone for allocator_api2::stable::raw_vec::TryReserveErrorKind

impl Clone for Compress

impl Clone for Validate

impl Clone for ark_std::io::error::ErrorKind

impl Clone for byteorder::BigEndian

impl Clone for byteorder::LittleEndian

impl Clone for TruncSide

impl Clone for FlushCompress

impl Clone for FlushDecompress

impl Clone for Status

impl Clone for hashbrown::TryReserveError

impl Clone for FromHexError

impl Clone for RootsConfig

impl Clone for PrefilterConfig

impl Clone for CompressionStrategy

impl Clone for TDEFLFlush

impl Clone for TDEFLStatus

impl Clone for CompressionLevel

impl Clone for DataFormat

impl Clone for MZError

impl Clone for MZFlush

impl Clone for MZStatus

impl Clone for TINFLStatus

impl Clone for nom::error::ErrorKind

impl Clone for Needed

impl Clone for Endianness

impl Clone for Sign

impl Clone for Primality

impl Clone for BernoulliError

impl Clone for WeightedError

impl Clone for IndexVec

impl Clone for IndexVecIntoIter

impl Clone for RoundingStrategy

Trait Clone

§How can I implement `Clone`?