Trait Copy

1.0.0 · 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.

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.

Copy

Trait Copy Copy 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 CreationStrategy

impl Copy for AcceptedLedgerTransactionKind

impl Copy for AccessControllerPartitionOffset

impl Copy for AccountLockerPartitionOffset

impl Copy for AccountPartitionOffset

impl Copy for AlignerExecutionMode

impl Copy for AlignerFolderMode

impl Copy for AlwaysVisibleGlobalNodesVersion

impl Copy for AttachedModuleId

impl Copy for AuthZoneField

impl Copy for BlueprintPayloadDef

impl Copy for BootLoaderField

impl Copy for ComponentField

impl Copy for ComponentRoyaltyPartitionOffset

impl Copy for ConsensusManagerCollection

impl Copy for ConsensusManagerFeature

impl Copy for ConsensusManagerField

impl Copy for ConsensusManagerPartitionOffset

impl Copy for CostingTaskMode

impl Copy for CurveType

impl Copy for DateTimeError

impl Copy for DecodeError

impl Copy for DefaultDepositRule

impl Copy for scrypto_test::prelude::EntityType

impl Copy for FungibleBucketField

impl Copy for FungibleProofField

impl Copy for FungibleResourceManagerCollection

impl Copy for FungibleResourceManagerFeature

impl Copy for FungibleResourceManagerField

impl Copy for FungibleResourceManagerPartitionOffset

impl Copy for FungibleVaultCollection

impl Copy for FungibleVaultFeature

impl Copy for FungibleVaultField

impl Copy for FungibleVaultPartitionOffset

impl Copy for IdentityV1MinorVersion

impl Copy for IntentHash

impl Copy for scrypto_test::prelude::Level

impl Copy for LocalRef

impl Copy for LocalTypeId

impl Copy for LockStatus

impl Copy for LowerBound

impl Copy for ManifestAddress

impl Copy for ManifestComponentAddress

impl Copy for ManifestCustomExtension

impl Copy for ManifestCustomTraversal

impl Copy for ManifestCustomValueKind

impl Copy for ManifestExpression

impl Copy for ManifestGlobalAddress

impl Copy for ManifestPackageAddress

impl Copy for ManifestResourceAddress

impl Copy for MetadataPartitionOffset

impl Copy for ModuleId

impl Copy for MultiResourcePoolPartitionOffset

impl Copy for NameChangeRule

impl Copy for NoCustomExtension

impl Copy for NoCustomSchema

impl Copy for NoCustomTerminalValueRef

impl Copy for NoCustomTraversal

impl Copy for NoCustomTypeKindLabel

impl Copy for NoCustomValueKind

impl Copy for NonFungibleBucketField

impl Copy for NonFungibleIdType

impl Copy for NonFungibleProofField

impl Copy for NonFungibleResourceManagerCollection

impl Copy for NonFungibleResourceManagerFeature

impl Copy for NonFungibleResourceManagerField

impl Copy for NonFungibleResourceManagerGeneric

impl Copy for NonFungibleResourceManagerPartitionOffset

impl Copy for NonFungibleVaultCollection