rustmax::cxx_build::core::prelude::rust_2024

Trait Clone

pub trait Clone: Sized {
    // Required method
    fn clone(&self) -> Self;

    // Provided method
    fn clone_from(&mut self, source: &Self) { ... }
}

🔬This is a nightly-only experimental API. (prelude_2024)

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 AhoCorasickKind

impl Clone for aho_corasick::packed::api::MatchKind

impl Clone for aho_corasick::util::error::MatchErrorKind

impl Clone for aho_corasick::util::prefilter::Candidate

impl Clone for aho_corasick::util::search::Anchored

impl Clone for aho_corasick::util::search::MatchKind

impl Clone for StartKind

impl Clone for Action

impl Clone for anstyle_parse::state::definitions::State

impl Clone for EvalResult

impl Clone for CChar

impl Clone for cexpr::token::Kind

impl Clone for Tz

impl Clone for clang_sys::Version

impl Clone for LabelStyle

impl Clone for Severity

impl Clone for DisplayStyle

impl Clone for colorchoice::ColorChoice

impl Clone for DwarfFileType

impl Clone for gimli::common::Format

impl Clone for SectionId

impl Clone for Vendor

impl Clone for RunTimeEndian

impl Clone for AbbreviationsCacheStrategy

impl Clone for Pointer

impl Clone for gimli::read::Error

impl Clone for IndexSectionId

impl Clone for ColumnType

impl Clone for gimli::read::value::Value

impl Clone for ValueType

impl Clone for globset::ErrorKind

impl Clone for hashbrown::TryReserveError

impl Clone for AdditionError

impl Clone for CreationError

impl Clone for RecordError

impl Clone for SubtractionError

impl Clone for httparse::Error

impl Clone for BaseUnit

impl Clone for FixedAt

impl Clone for Kilo

impl Clone for humantime::date::Error

impl Clone for humantime::duration::Error

impl Clone for TrieResult

impl Clone for TrieType

impl Clone for icu_collections::codepointtrie::error::Error

impl Clone for ExtensionType

impl Clone for ParserError

impl Clone for icu_locid_transform::directionality::Direction

impl Clone for LocaleTransformError

impl Clone for PropertiesError

impl Clone for GeneralCategory

impl Clone for CheckedBidiPairedBracketType

impl Clone for BufferFormat

impl Clone for DataErrorKind

impl Clone for LocaleFallbackPriority

impl Clone for LocaleFallbackSupplement

impl Clone for DnsLength

impl Clone for ErrorPolicy

impl Clone for Hyphens

impl Clone for ProcessingError

impl Clone for ProcessingSuccess

impl Clone for ignore::Error

impl Clone for WalkState

impl Clone for IpAddrRange

impl Clone for IpNet

impl Clone for IpSubnets

impl Clone for itertools::with_position::Position

impl Clone for fsconfig_command

impl Clone for membarrier_cmd

Trait Clone

§How can I implement `Clone`?