Trait Copy

1.6.0 · source

pub trait Copy: Clone { }

Expand description

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.

Object Safety§

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

Object Safety§

Implementors§

impl Copy for ComponentItem

impl Copy for FixedEncoding

impl Copy for FlatType

impl Copy for InterfaceType

impl Copy for StringEncoding

impl Copy for Transcode

impl Copy for TypeDef

impl Copy for ConstOp

impl Copy for EngineOrModuleTypeIndex

impl Copy for EntityIndex

impl Copy for wasmtime_environ::RelocationTarget

impl Copy for SettingKind

impl Copy for Trap

impl Copy for VMGcKind

impl Copy for WasmHeapTopType

impl Copy for WasmHeapType

impl Copy for WasmValType

impl Copy for LibCall

impl Copy for wasmtime_environ::wasmparser::AbstractHeapType

impl Copy for wasmtime_environ::wasmparser::BlockType

impl Copy for wasmtime_environ::wasmparser::CanonicalOption

impl Copy for Catch

impl Copy for ComdatSymbolKind

impl Copy for ComponentExternalKind

impl Copy for wasmtime_environ::wasmparser::ComponentOuterAliasKind

impl Copy for wasmtime_environ::wasmparser::ComponentTypeRef

impl Copy for wasmtime_environ::wasmparser::ComponentValType

impl Copy for wasmtime_environ::wasmparser::Encoding

impl Copy for ExternalKind

impl Copy for FrameKind

impl Copy for wasmtime_environ::wasmparser::HeapType

impl Copy for InstantiationArgKind

impl Copy for wasmtime_environ::wasmparser::Ordering

impl Copy for OuterAliasKind

impl Copy for wasmtime_environ::wasmparser::PrimitiveValType

impl Copy for RelocAddendKind

impl Copy for RelocationType

impl Copy for wasmtime_environ::wasmparser::StorageType

impl Copy for wasmtime_environ::wasmparser::TagKind

impl Copy for wasmtime_environ::wasmparser::TypeBounds

impl Copy for TypeRef

impl Copy for UnpackedIndex

impl Copy for wasmtime_environ::wasmparser::ValType

impl Copy for AnyTypeId

impl Copy for ComponentAnyTypeId

impl Copy for ComponentCoreTypeId

impl Copy for ComponentEntityType

impl Copy for wasmtime_environ::wasmparser::types::ComponentValType

impl Copy for wasmtime_environ::wasmparser::types::EntityType

impl Copy for AsciiChar

impl Copy for wasmtime_environ::__core::cmp::Ordering

impl Copy for Infallible

impl Copy for wasmtime_environ::__core::fmt::Alignment

impl Copy for IpAddr

impl Copy for Ipv6MulticastScope

impl Copy for SocketAddr

impl Copy for FpCategory

impl Copy for SearchStep

impl Copy for wasmtime_environ::__core::sync::atomic::Ordering

impl Copy for std::io::SeekFrom

impl Copy for std::io::error::ErrorKind

impl Copy for Shutdown

impl Copy for BacktraceStyle

impl Copy for RecvTimeoutError

impl Copy for TryRecvError

impl Copy for _Unwind_Action

impl Copy for _Unwind_Reason_Code

impl Copy for DemangleNodeType

impl Copy for cpp_demangle::error::Error

impl Copy for embedded_io::ErrorKind