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 aho_corasick::ahocorasick::MatchKind

impl Copy for aho_corasick::packed::api::MatchKind

impl Copy for Colour

impl Copy for PrintFmt

impl Copy for CharacterSet

impl Copy for Language

impl Copy for MnemonicType

impl Copy for byteorder::BigEndian

impl Copy for byteorder::LittleEndian

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 const_oid::error::Error

impl Copy for CFComparisonResult

impl Copy for DemangleNodeType

impl Copy for cpp_demangle::error::Error

impl Copy for cranelift_codegen::binemit::Reloc

impl Copy for CursorPosition

impl Copy for AtomicRmwOp

impl Copy for FloatCC

impl Copy for IntCC

impl Copy for ValueDef

impl Copy for AnyEntity

impl Copy for ArgumentExtension

impl Copy for ArgumentPurpose

impl Copy for InstructionFormat

impl Copy for Opcode

impl Copy for ResolvedConstraint

impl Copy for KnownSymbol

impl Copy for LibCall

impl Copy for cranelift_codegen::ir::memflags::Endianness

impl Copy for ExpandedProgramPoint

impl Copy for StackSlotKind

impl Copy for cranelift_codegen::ir::trapcode::TrapCode

impl Copy for CallConv

impl Copy for LookupError

impl Copy for Detail

impl Copy for LibcallCallConv

impl Copy for OptLevel

impl Copy for ProbestackStrategy

impl Copy for cranelift_codegen::settings::SettingKind

impl Copy for TlsModel

impl Copy for LabelValueLoc

impl Copy for GlobalVariable

impl Copy for cranelift_wasm::translation_utils::TableElementType

impl Copy for crossbeam_channel::err::RecvTimeoutError

impl Copy for crossbeam_channel::err::TryRecvError

impl Copy for der::error::ErrorKind

impl Copy for Class

impl Copy for der::tag::Tag

impl Copy for TagMode

impl Copy for TruncSide

impl Copy for ed25519_zebra::error::Error

impl Copy for TimestampPrecision

impl Copy for WriteStyle

impl Copy for PollNext

impl Copy for gimli::common::DwarfFileType

impl Copy for gimli::common::DwarfFileType

impl Copy for gimli::common::Format

impl Copy for gimli::common::Format

impl Copy for gimli::common::SectionId

impl Copy for gimli::common::SectionId

impl Copy for gimli::endianity::RunTimeEndian

impl Copy for gimli::endianity::RunTimeEndian

impl Copy for gimli::read::cfi::Pointer

impl Copy for gimli::read::cfi::Pointer

impl Copy for gimli::read::Error

impl Copy for gimli::read::Error

impl Copy for gimli::read::line::ColumnType

impl Copy for gimli::read::line::ColumnType

impl Copy for gimli::read::value::Value

impl Copy for gimli::read::value::Value

impl Copy for gimli::read::value::ValueType

impl Copy for gimli::read::value::ValueType

impl Copy for ConvertError