Trait Copy

1.0.0 · Source

pub trait Copy: Clone { }

Expand description

core Types whose values can be duplicated simply by copying bits.

By default, variable bindings have ‘move semantics.’ In other words:

#[derive(Debug)]
struct Foo;

let x = Foo;

let y = x;

// `x` has moved into `y`, and so cannot be used

// println!("{x:?}"); // error: use of moved value

However, if a type implements Copy, it instead has ‘copy semantics’:

// We can derive a `Copy` implementation. `Clone` is also required, as it's
// a supertrait of `Copy`.
#[derive(Debug, Copy, Clone)]
struct Foo;

let x = Foo;

let y = x;

// `y` is a copy of `x`

println!("{x:?}"); // A-OK!

It’s important to note that in these two examples, the only difference is whether you are allowed to access x after the assignment. Under the hood, both a copy and a move can result in bits being copied in memory, although this is sometimes optimized away.

§How can I implement `Copy`?

There are two ways to implement Copy on your type. The simplest is to use derive:

#[derive(Copy, Clone)]
struct MyStruct;

You can also implement Copy and Clone manually:

struct MyStruct;

impl Copy for MyStruct { }

impl Clone for MyStruct {
    fn clone(&self) -> MyStruct {
        *self
    }
}

There is a small difference between the two. The derive strategy will also place a Copy bound on type parameters:

#[derive(Clone)]
struct MyStruct<T>(T);

impl<T: Copy> Copy for MyStruct<T> { }

This isn’t always desired. For example, shared references (&T) can be copied regardless of whether T is Copy. Likewise, a generic struct containing markers such as PhantomData could potentially be duplicated with a bit-wise copy.

§What’s the difference between `Copy` and `Clone`?

Copies happen implicitly, for example as part of an assignment y = x. The behavior of Copy is not overloadable; it is always a simple bit-wise copy.

Cloning is an explicit action, x.clone(). The implementation of Clone can provide any type-specific behavior necessary to duplicate values safely. For example, the implementation of Clone for String needs to copy the pointed-to string buffer in the heap. A simple bitwise copy of String values would merely copy the pointer, leading to a double free down the line. For this reason, String is Clone but not Copy.

Clone is a supertrait of Copy, so everything which is Copy must also implement Clone. If a type is Copy then its Clone implementation only needs to return *self (see the example above).

§When can my type be `Copy`?

A type can implement Copy if all of its components implement Copy. For example, this struct can be Copy:

#[derive(Copy, Clone)]
struct Point {
   x: i32,
   y: i32,
}

A struct can be Copy, and i32 is Copy, therefore Point is eligible to be Copy. By contrast, consider

struct PointList {
    points: Vec<Point>,
}

The struct PointList cannot implement Copy, because Vec<T> is not Copy. If we attempt to derive a Copy implementation, we’ll get an error:

the trait `Copy` cannot be implemented for this type; field `points` does not implement `Copy`

Shared references (&T) are also Copy, so a type can be Copy, even when it holds shared references of types T that are not Copy. Consider the following struct, which can implement Copy, because it only holds a shared reference to our non-Copy type PointList from above:

#[derive(Copy, Clone)]
struct PointListWrapper<'a> {
    point_list_ref: &'a PointList,
}

§When can’t my type be `Copy`?

Some types can’t be copied safely. For example, copying &mut T would create an aliased mutable reference. Copying String would duplicate responsibility for managing the String’s buffer, leading to a double free.

Generalizing the latter case, any type implementing Drop can’t be Copy, because it’s managing some resource besides its own size_of::<T> bytes.

If you try to implement Copy on a struct or enum containing non-Copy data, you will get the error E0204.

§When should my type be `Copy`?

Generally speaking, if your type can implement Copy, it should. Keep in mind, though, that implementing Copy is part of the public API of your type. If the type might become non-Copy in the future, it could be prudent to omit the Copy implementation now, to avoid a breaking API change.

§Additional implementors

In addition to the implementors listed below, the following types also implement Copy:

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 Copy themselves. Note that variables captured by shared reference always implement Copy (even if the referent doesn’t), while variables captured by mutable reference never implement Copy.

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 CopyCopy item path

§How can I implement Copy?

§What’s the difference between Copy and Clone?

§When can my type be Copy?

§When can’t my type be Copy?

§When should my type be Copy?

§Additional implementors

Dyn Compatibility§

Implementors§

impl Copy for devela::_core::ascii::Char

impl Copy for FromBytesWithNulError

impl Copy for DebugAsHex

impl Copy for devela::_core::fmt::Sign

impl Copy for Ipv6MulticastScope

impl Copy for SearchStep

impl Copy for SeekFrom

impl Copy for BacktraceStyle

impl Copy for CheckedCastError

impl Copy for PodCastError

impl Copy for SetCursorStyle

impl Copy for devela::_dep::crossterm::event::KeyCode

impl Copy for KeyEventKind

impl Copy for MediaKeyCode

impl Copy for ModifierKeyCode

impl Copy for devela::_dep::crossterm::event::MouseButton

impl Copy for MouseEventKind

impl Copy for Attribute

impl Copy for devela::_dep::crossterm::style::Color

impl Copy for Colored

impl Copy for ClearType

impl Copy for devela::_dep::ffmpeg_the_third::codec::encoder::Comparison

impl Copy for Decision

impl Copy for MotionEstimation

impl Copy for Compliance

impl Copy for devela::_dep::ffmpeg_the_third::codec::Id

impl Copy for Profile

impl Copy for devela::_dep::ffmpeg_the_third::codec::packet::side_data::Type

impl Copy for AAC

impl Copy for DTS

impl Copy for H264

impl Copy for HEVC

impl Copy for JPEG2000

impl Copy for MPEG2

impl Copy for MPEG4

impl Copy for VC1

impl Copy for VP9

impl Copy for devela::_dep::ffmpeg_the_third::codec::subtitle::Type

impl Copy for devela::_dep::ffmpeg_the_third::codec::threading::Type

impl Copy for AudioService

impl Copy for devela::_dep::ffmpeg_the_third::Channel

impl Copy for ChannelOrder

impl Copy for Discard

impl Copy for devela::_dep::ffmpeg_the_third::Error

impl Copy for FieldOrder

impl Copy for Rounding

impl Copy for Mode

impl Copy for Pixel

impl Copy for Sample

impl Copy for devela::_dep::ffmpeg_the_third::software::resampling::Dither

impl Copy for Engine

impl Copy for Filter

impl Copy for devela::_dep::ffmpeg_the_third::software::scaling::ColorSpace

impl Copy for AVActiveFormatDescription

impl Copy for AVAppToDevMessageType

impl Copy for AVAudioServiceType

impl Copy for AVCRCId

impl Copy for AVChannel

impl Copy for AVChannelOrder

impl Copy for AVChromaLocation

impl Copy for AVClassCategory

impl Copy for AVCodecID

impl Copy for AVColorPrimaries

impl Copy for AVColorRange

impl Copy for AVColorSpace

impl Copy for AVColorTransferCharacteristic

impl Copy for AVDevToAppMessageType

impl Copy for AVDiscard

impl Copy for AVDownmixType

impl Copy for AVDurationEstimationMethod