Trait std::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

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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!

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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;

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You can also implement Copy and Clone manually:

struct MyStruct;

impl Copy for MyStruct { }

impl Clone for MyStruct {
    fn clone(&self) -> MyStruct {
        *self
    }
}

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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,
}

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A struct can be Copy, and i32 is Copy, therefore Point is eligible to be Copy. By contrast, consider

struct PointList {
    points: Vec<Point>,
}

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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,
}

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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 std::marker::Copy

Implementors§

impl Copy for AsciiChar

impl Copy for StackFrame

impl Copy for PrintFmt

impl Copy for std::cmp::Ordering

impl Copy for Infallible

impl Copy for std::fmt::Alignment

impl Copy for SeekFrom

impl Copy for ErrorKind

impl Copy for IpAddr

impl Copy for Ipv6MulticastScope

impl Copy for Shutdown

impl Copy for SocketAddr

impl Copy for FpCategory

impl Copy for BacktraceStyle

impl Copy for State

impl Copy for Which

impl Copy for SearchStep

impl Copy for std::sync::atomic::Ordering

impl Copy for Selected

impl Copy for RecvTimeoutError

impl Copy for TryRecvError

impl Copy for ReparsePoint

impl Copy for bool

impl Copy for char

impl Copy for f32

impl Copy for f64

impl Copy for i8

impl Copy for i16

impl Copy for i32

impl Copy for i64

impl Copy for i128

impl Copy for isize

impl Copy for !

impl Copy for u8

impl Copy for u16

impl Copy for u32

impl Copy for u64

impl Copy for u128

impl Copy for ()

impl Copy for usize

impl Copy for CpuidResult

impl Copy for __m128

impl Copy for __m128bh

impl Copy for __m128d

impl Copy for __m128i

impl Copy for __m256

impl Copy for __m256bh

impl Copy for __m256d

impl Copy for __m256i

impl Copy for __m512

impl Copy for __m512bh

impl Copy for __m512d

impl Copy for __m512i

impl Copy for TimSortRun

impl Copy for AllocError

impl Copy for Global

impl Copy for Layout

impl Copy for System

impl Copy for TypeId

impl Copy for TryFromSliceError

impl Copy for Frame

impl Copy for ADDRESS64

impl Copy for std::backtrace_rs::windows::CONTEXT

impl Copy for std::backtrace_rs::windows::FLOATING_SAVE_AREA

impl Copy for IMAGEHLP_LINEW64

impl Copy for KDHELP64

impl Copy for std::backtrace_rs::windows::M128A

impl Copy for MODULEENTRY32W

impl Copy for STACKFRAME64

impl Copy for STACKFRAME_EX

impl Copy for SYMBOL_INFOW

impl Copy for CharTryFromError

impl Copy for TryFromCharError

impl Copy for Error

impl Copy for std::fs::FileTimes

impl Copy for std::fs::FileType

impl Copy for Empty

impl Copy for Sink