Trait frame_support::dispatch::marker::Copy1.0.0[−][src]

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, which isn’t always desired.

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` may not 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)
Array types, for all sizes, if the item type also implements Copy (e.g., [i32; 123456])
Tuple types, if each component also implements Copy (e.g., (), (i32, bool))
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.

Trait frame_support::dispatch::marker::Copy1.0.0[−][src]

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

Implementations on Foreign Types

Implementors

impl Copy for BenchmarkSelector

impl Copy for BenchmarkParameter

impl<T: Copy + Config> Copy for Module<T>

impl Copy for SizeType

impl Copy for Mode

impl Copy for DatabaseType

impl Copy for DatabaseSize

impl Copy for ProxyType

impl Copy for Runtime

impl Copy for Runtime

impl Copy for BlockType

impl Copy for DatabaseType

impl Copy for Profile

impl Copy for DestroyWitness

impl Copy for ConversionError

impl Copy for Reasons

impl<T: Copy + Config> Copy for Module<T>

impl<T: Copy + Config<I>, I: Copy + Instance> Copy for Module<T, I>

impl Copy for ReturnFlags

impl Copy for Conviction

impl<Balance: Copy> Copy for Delegations<Balance>

impl Copy for Vote

impl<Balance: Copy> Copy for AccountVote<Balance>

impl Copy for VoteThreshold

impl<Bn: Copy> Copy for Phase<Bn>

impl Copy for ElectionCompute

impl Copy for SolutionOrSnapshotSize

impl<Balance: Copy> Copy for VoterInfo<Balance>

impl Copy for GrandpaTimeSlot

impl<Balance: Copy + Encode + Decode + MaxEncodedLen + Clone + Debug + Eq + PartialEq> Copy for Judgement<Balance>

impl Copy for IdentityField

impl Copy for IdentityFields

impl<T: Copy + Config<I>, I: Copy + Instance> Copy for Module<T, I>

impl<BlockNumber: Copy> Copy for Timepoint<BlockNumber>

impl<AccountId: Copy, ProxyType: Copy, BlockNumber: Copy> Copy for ProxyDefinition<AccountId, ProxyType, BlockNumber>

impl<AccountId: Copy, Hash: Copy, BlockNumber: Copy> Copy for Announcement<AccountId, Hash, BlockNumber>

impl<T: Copy + Config> Copy for Module<T>

impl<T: Copy + Config> Copy for Module<T>

impl Copy for Vote

impl Copy for Judgement

impl<Balance: Copy, BlockNumber: Copy> Copy for Payout<Balance, BlockNumber>

impl Copy for VouchingStatus

impl<AccountId: Copy, Balance: Copy> Copy for Bid<AccountId, Balance>

impl<AccountId: Copy, Balance: Copy> Copy for BidKind<AccountId, Balance>

impl<T: Copy + Config<I>, I: Copy + Instance> Copy for Module<T, I>

impl<AccountId: Copy> Copy for RewardDestination<AccountId>

impl Copy for Forcing

impl<T: Copy + Config> Copy for Module<T>

impl Copy for DestroyWitness

impl<Balance: Copy, BlockNumber: Copy> Copy for VestingInfo<Balance, BlockNumber>

impl Copy for RecordProof

impl Copy for WasmExecutionMethod

impl Copy for TracingReceiver

impl Copy for NodeKeyType

impl Copy for CryptoScheme

impl Copy for OutputType

impl Copy for ExecutionStrategy

impl Copy for RpcMethods

impl Copy for Database

impl Copy for SyncMode

impl Copy for NewBlockState

impl Copy for MemorySize

impl Copy for KeepBlocks

impl Copy for TransactionStorageMode

impl Copy for ForkChoiceStrategy

impl Copy for CheckForEquivocation

impl Copy for EpochIdentifierPosition

impl<T: Copy> Copy for SlotDuration<T>

impl Copy for WasmExecutionMethod

impl Copy for SupervisorFuncIndex

impl Copy for IfDisconnected

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`?

`impl Copy for BenchmarkSelector`

`impl Copy for BenchmarkParameter`

`impl<T: Copy + Config> Copy for Module<T>`

`impl Copy for SizeType`

`impl Copy for Mode`

`impl Copy for DatabaseType`

`impl Copy for DatabaseSize`

`impl Copy for ProxyType`

`impl Copy for Runtime`

`impl Copy for Runtime`

`impl Copy for BlockType`

`impl Copy for DatabaseType`

`impl Copy for Profile`

`impl Copy for DestroyWitness`

`impl Copy for ConversionError`

`impl Copy for Reasons`

`impl<T: Copy + Config> Copy for Module<T>`

`impl<T: Copy + Config<I>, I: Copy + Instance> Copy for Module<T, I>`

`impl Copy for ReturnFlags`

`impl Copy for Conviction`

`impl<Balance: Copy> Copy for Delegations<Balance>`

`impl Copy for Vote`

`impl<Balance: Copy> Copy for AccountVote<Balance>`

`impl Copy for VoteThreshold`

`impl<Bn: Copy> Copy for Phase<Bn>`

`impl Copy for ElectionCompute`

`impl Copy for SolutionOrSnapshotSize`

`impl<Balance: Copy> Copy for VoterInfo<Balance>`

`impl Copy for GrandpaTimeSlot`

`impl<Balance: Copy + Encode + Decode + MaxEncodedLen + Clone + Debug + Eq + PartialEq> Copy for Judgement<Balance>`

`impl Copy for IdentityField`

`impl Copy for IdentityFields`

`impl<T: Copy + Config<I>, I: Copy + Instance> Copy for Module<T, I>`

`impl<BlockNumber: Copy> Copy for Timepoint<BlockNumber>`

`impl<AccountId: Copy, ProxyType: Copy, BlockNumber: Copy> Copy for ProxyDefinition<AccountId, ProxyType, BlockNumber>`

`impl<AccountId: Copy, Hash: Copy, BlockNumber: Copy> Copy for Announcement<AccountId, Hash, BlockNumber>`

`impl<T: Copy + Config> Copy for Module<T>`

`impl<T: Copy + Config> Copy for Module<T>`

`impl Copy for Vote`

`impl Copy for Judgement`

`impl<Balance: Copy, BlockNumber: Copy> Copy for Payout<Balance, BlockNumber>`

`impl Copy for VouchingStatus`

`impl<AccountId: Copy, Balance: Copy> Copy for Bid<AccountId, Balance>`

`impl<AccountId: Copy, Balance: Copy> Copy for BidKind<AccountId, Balance>`

`impl<T: Copy + Config<I>, I: Copy + Instance> Copy for Module<T, I>`

`impl<AccountId: Copy> Copy for RewardDestination<AccountId>`

`impl Copy for Forcing`

`impl<T: Copy + Config> Copy for Module<T>`

`impl Copy for DestroyWitness`

`impl<Balance: Copy, BlockNumber: Copy> Copy for VestingInfo<Balance, BlockNumber>`

`impl Copy for RecordProof`

`impl Copy for WasmExecutionMethod`

`impl Copy for TracingReceiver`

`impl Copy for NodeKeyType`

`impl Copy for CryptoScheme`

`impl Copy for OutputType`

`impl Copy for ExecutionStrategy`

`impl Copy for RpcMethods`

`impl Copy for Database`

`impl Copy for SyncMode`

`impl Copy for NewBlockState`

`impl Copy for MemorySize`

`impl Copy for KeepBlocks`

`impl Copy for TransactionStorageMode`

`impl Copy for ForkChoiceStrategy`

`impl Copy for CheckForEquivocation`

`impl Copy for EpochIdentifierPosition`

`impl<T: Copy> Copy for SlotDuration<T>`

`impl Copy for WasmExecutionMethod`

`impl Copy for SupervisorFuncIndex`

`impl Copy for IfDisconnected`

`impl Copy for BitswapConfig`

`impl Copy for TransactionImport`

`impl Copy for MessageIntent`

`impl Copy for SetId`