Trait Clone

Help

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 allocator_api2::stable::raw_vec::TryReserveErrorKind

impl Clone for DwarfFileType

impl Clone for Format

impl Clone for gimli::common::SectionId

impl Clone for gimli::common::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 Value

impl Clone for ValueType

impl Clone for gimli::write::cfi::CallFrameInstruction

impl Clone for ConvertError

impl Clone for Address

impl Clone for gimli::write::Error

impl Clone for Reference

impl Clone for LineString

impl Clone for gimli::write::loc::Location

impl Clone for gimli::write::range::Range

impl Clone for gimli::write::relocate::RelocationTarget

impl Clone for gimli::write::unit::AttributeValue

impl Clone for hashbrown::TryReserveError

impl Clone for GetDisjointMutError

impl Clone for Level

impl Clone for LevelFilter

impl Clone for PrefilterConfig

impl Clone for AddressSize

impl Clone for object::common::Architecture

impl Clone for object::common::BinaryFormat

impl Clone for ComdatKind

impl Clone for FileFlags

impl Clone for RelocationEncoding

impl Clone for RelocationFlags

impl Clone for RelocationKind

impl Clone for SectionFlags

impl Clone for SectionKind

impl Clone for object::common::SegmentFlags

impl Clone for SubArchitecture

impl Clone for SymbolKind

impl Clone for SymbolScope

impl Clone for object::endian::Endianness

impl Clone for ImportType

impl Clone for CompressionFormat

impl Clone for FileKind

impl Clone for ObjectKind

impl Clone for object::read::RelocationTarget

impl Clone for object::read::SymbolSection

impl Clone for CoffExportStyle

impl Clone for object::write::coff::writer::Name

impl Clone for Mangling

impl Clone for StandardSection

impl Clone for StandardSegment

impl Clone for object::write::SymbolSection

impl Clone for postcard::error::Error

impl Clone for Op

impl Clone for CDataModel

impl Clone for Size

impl Clone for ParseError

impl Clone for Aarch64Architecture

impl Clone for target_lexicon::targets::Architecture

impl Clone for ArmArchitecture

impl Clone for target_lexicon::targets::BinaryFormat

impl Clone for CleverArchitecture

impl Clone for CustomVendor

impl Clone for Environment

impl Clone for Mips32Architecture

impl Clone for Mips64Architecture

Trait Clone

§How can I implement `Clone`?