Trait Clone

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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

core 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.

impl Clone for Infallible

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impl Clone for FpCategory

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impl Clone for NumError

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impl Clone for devela::num::Sign

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impl Clone for UnitBi

Available on crate feature unit only.

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impl Clone for UnitSi

Available on crate feature unit only.

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impl Clone for devela::text::AsciiChar

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impl Clone for InvalidText

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impl Clone for TextError

Available on crate feature error and text·· only.

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impl Clone for AllErrorKind

Available on crate feature error only.

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impl Clone for AngleDirection

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impl Clone for AngleKind

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impl Clone for devela::all::AtomicOrdering

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impl Clone for AudioError

Available on crate feature audio only.

Available on crate feature draw only.

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impl Clone for EncodingMode

1.28.0 · Source§

impl Clone for devela::all::FmtAlignment

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impl Clone for devela::all::FontError

Available on crate feature font only.

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impl Clone for ImageError

Available on crate feature image only.

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impl Clone for ErrorKind

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impl Clone for IpAddr

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impl Clone for JsEvent

Available on crate feature js only.

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impl Clone for JsPermission

Available on crate feature js only.

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impl Clone for JsPermissionState

Available on crate feature js only.

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impl Clone for devela::all::LayoutError

Available on crate feature layout only.

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impl Clone for devela::all::LogLevel

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impl Clone for devela::all::LogLevelFilter

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impl Clone for MediaError

Available on media·· only.

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impl Clone for MidiError

Available on crate feature midi only.

impl Clone for SixelError

Available on crate feature alloc and crate feature term and (crate features io or std) and (crate features dep_hashbrown or std) and crate feature image only.

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impl Clone for SixelMean

Available on crate feature alloc and crate feature term and (crate features io or std) and (crate features dep_hashbrown or std) and crate feature image only.

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impl Clone for SixelQuality

Available on crate feature alloc and crate feature term and (crate features io or std) and (crate features dep_hashbrown or std) and crate feature image only.

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impl Clone for SixelSplit

Available on crate feature alloc and crate feature term and (crate features io or std) and (crate features dep_hashbrown or std) and crate feature image only.

1.0.0 · Source§