Trait frame_support::dispatch::marker::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, 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` 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.

Trait frame_support::dispatch::marker::Copy

Implementors§

impl Copy for ArithmeticError

impl Copy for Rounding

impl Copy for SignedRounding

impl Copy for DeriveJunction

impl Copy for PublicError

impl Copy for LogLevel

impl Copy for LogLevelFilter

impl Copy for HttpError

impl Copy for HttpRequestStatus

impl Copy for StorageKind

impl Copy for CallContext

impl Copy for TokenError

impl Copy for TransactionalError

impl Copy for Era

impl Copy for sp_runtime::legacy::byte_sized_error::DispatchError

impl Copy for DisableStrategy

impl Copy for sp_version::embed::Error

impl Copy for ReturnValue

impl Copy for sp_wasm_interface::Value

impl Copy for sp_wasm_interface::ValueType

impl Copy for frame_support::pallet_prelude::DispatchError

impl Copy for InvalidTransaction

impl Copy for TransactionSource

impl Copy for TransactionValidityError

impl Copy for UnknownTransaction

impl Copy for ChildType

impl Copy for StateVersion

impl Copy for ProcessMessageError

impl Copy for UpgradeCheckSelect

impl Copy for BalanceStatus

impl Copy for DepositConsequence

impl Copy for ExistenceRequirement

impl Copy for Fortitude

impl Copy for Precision

impl Copy for Preservation

impl Copy for Provenance

impl Copy for Restriction

impl Copy for DispatchClass

impl Copy for Pays

impl Copy for frame_support::dispatch::fmt::Alignment

impl Copy for core::cmp::Ordering

impl Copy for Infallible

impl Copy for Which

impl Copy for IpAddr

impl Copy for Ipv6MulticastScope

impl Copy for SocketAddr

impl Copy for FpCategory

impl Copy for SearchStep

impl Copy for core::sync::atomic::Ordering

impl Copy for proc_macro::diagnostic::Level

impl Copy for proc_macro::Delimiter

impl Copy for proc_macro::Spacing

impl Copy for std::io::SeekFrom

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

impl Copy for Shutdown

impl Copy for BacktraceStyle

impl Copy for std::sync::mpsc::RecvTimeoutError

impl Copy for std::sync::mpsc::TryRecvError

impl Copy for _Unwind_Action

impl Copy for _Unwind_Reason_Code

impl Copy for time::ParseError

impl Copy for PrintFmt

impl Copy for SecondsFormat

impl Copy for Pad

impl Copy for ParseErrorKind

impl Copy for Month

impl Copy for RoundingError

impl Copy for Weekday

impl Copy for hex::error::FromHexError

impl Copy for log::Level

impl Copy for log::LevelFilter

impl Copy for Sign

impl Copy for Grouping

impl Copy for Locale

impl Copy for proc_macro2::Delimiter

impl Copy for proc_macro2::Spacing

impl Copy for Category

impl Copy for AttrStyle