Trait std::marker::Copy1.0.0[][src]

pub trait Copy: Clone { }

Types whose values can be duplicated simply by copying bits.

By default, variable bindings have ‘move semantics.’ In other words:

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 {

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:

Implementations on Foreign Types

impl Copy for __m128[src]

impl Copy for __m256i[src]

impl Copy for __m512bh[src]

impl Copy for __m256[src]

impl Copy for CpuidResult[src]

impl Copy for __m256bh[src]

impl Copy for __m128i[src]

impl Copy for __m512[src]

impl Copy for __m128d[src]

impl Copy for __m128bh[src]

impl Copy for __m256d[src]

impl Copy for __m512i[src]

impl Copy for __m512d[src]

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impl Copy for std::cmp::Ordering[src]

impl Copy for Infallible1.34.0[src]

impl Copy for ErrorKind[src]

impl Copy for SeekFrom[src]

impl Copy for IpAddr1.7.0[src]

impl Copy for Ipv6MulticastScope[src]

impl Copy for Shutdown[src]

impl Copy for SocketAddr[src]

impl Copy for FpCategory[src]

impl Copy for SearchStep[src]

impl Copy for std::sync::atomic::Ordering[src]

impl Copy for RecvTimeoutError1.12.0[src]

impl Copy for TryRecvError[src]

impl Copy for bool[src]

impl Copy for char[src]

impl Copy for f32[src]

impl Copy for f64[src]

impl Copy for i8[src]

impl Copy for i16[src]

impl Copy for i32[src]

impl Copy for i64[src]

impl Copy for i128[src]

impl Copy for isize[src]

impl Copy for ![src]

impl Copy for u8[src]

impl Copy for u16[src]

impl Copy for u32[src]

impl Copy for u64[src]

impl Copy for u128[src]

impl Copy for usize[src]

impl Copy for AllocError[src]

impl Copy for Global[src]

impl Copy for Layout1.28.0[src]

impl Copy for System1.28.0[src]

impl Copy for TypeId[src]

impl Copy for TryFromSliceError1.34.0[src]

impl Copy for CharTryFromError1.34.0[src]

impl Copy for Error[src]

impl Copy for FileType1.1.0[src]

impl Copy for Ipv4Addr[src]

impl Copy for Ipv6Addr[src]

impl Copy for SocketAddrV4[src]

impl Copy for SocketAddrV6[src]

impl Copy for NonZeroI81.34.0[src]

impl Copy for NonZeroI161.34.0[src]

impl Copy for NonZeroI321.34.0[src]

impl Copy for NonZeroI641.34.0[src]

impl Copy for NonZeroI1281.34.0[src]

impl Copy for NonZeroIsize1.34.0[src]

impl Copy for NonZeroU81.28.0[src]

impl Copy for NonZeroU161.28.0[src]

impl Copy for NonZeroU321.28.0[src]

impl Copy for NonZeroU641.28.0[src]

impl Copy for NonZeroU1281.28.0[src]

impl Copy for NonZeroUsize1.28.0[src]

impl Copy for TryFromIntError1.34.0[src]

impl Copy for RangeFull[src]

impl Copy for NoneError[src]

impl Copy for UCred[src]

This is supported on Unix only.

impl Copy for ExitCode[src]

impl Copy for ExitStatus[src]

impl Copy for TraitObject[src]

impl Copy for Utf8Error[src]

impl Copy for RecvError[src]

impl Copy for WaitTimeoutResult1.5.0[src]

impl Copy for RawWakerVTable1.36.0[src]

impl Copy for AccessError1.26.0[src]

impl Copy for ThreadId1.19.0[src]

impl Copy for Duration1.3.0[src]

impl Copy for Instant1.8.0[src]

impl Copy for SystemTime1.8.0[src]

impl Copy for PhantomPinned1.33.0[src]

impl<'_, T> Copy for &'_ T where
    T: ?Sized

Shared references can be copied, but mutable references cannot!

impl<'a> Copy for Component<'a>[src]

impl<'a> Copy for Prefix<'a>[src]

impl<'a> Copy for Arguments<'a>[src]

impl<'a> Copy for IoSlice<'a>1.36.0[src]

impl<'a> Copy for Location<'a>1.10.0[src]

impl<'a> Copy for Ancestors<'a>1.28.0[src]

impl<'a> Copy for PrefixComponent<'a>[src]

impl<'a, T, const N: usize> Copy for ArrayWindows<'a, T, N> where
    T: 'a + Copy

impl<B, C> Copy for ControlFlow<B, C> where
    C: Copy,
    B: Copy

impl<Dyn> Copy for DynMetadata<Dyn> where
    Dyn: ?Sized

impl<F> Copy for RepeatWith<F> where
    F: Copy

impl<Idx> Copy for RangeTo<Idx> where
    Idx: Copy

impl<Idx> Copy for RangeToInclusive<Idx> where
    Idx: Copy

impl<P> Copy for Pin<P> where
    P: Copy

impl<T> Copy for Bound<T> where
    T: Copy

impl<T> Copy for Option<T> where
    T: Copy

impl<T> Copy for Poll<T> where
    T: Copy

impl<T> Copy for *const T where
    T: ?Sized

impl<T> Copy for *mut T where
    T: ?Sized

impl<T> Copy for Reverse<T> where
    T: Copy

impl<T> Copy for Discriminant<T>1.21.0[src]

impl<T> Copy for ManuallyDrop<T> where
    T: Copy + ?Sized

impl<T> Copy for Wrapping<T> where
    T: Copy

impl<T> Copy for NonNull<T> where
    T: ?Sized

impl<T> Copy for PhantomData<T> where
    T: ?Sized

impl<T> Copy for MaybeUninit<T> where
    T: Copy

impl<T, E> Copy for Result<T, E> where
    T: Copy,
    E: Copy

impl<T: Copy> Copy for TrySendError<T>[src]

impl<T: Copy> Copy for SendError<T>[src]

impl<Y, R> Copy for GeneratorState<Y, R> where
    R: Copy,
    Y: Copy

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