1.0.0[][src]Trait core::marker::Copy

#[lang = "copy"]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:


impl Copy for ![src]

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

impl Copy for Infallible[src]

impl Copy for FpCategory[src]

impl Copy for SearchStep[src]

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

impl Copy for AllocError[src]

impl Copy for Layout[src]

impl Copy for TypeId[src]

impl Copy for float64x1_t[src]

impl Copy for float64x2_t[src]

impl Copy for int8x16x2_t[src]

impl Copy for int8x16x3_t[src]

impl Copy for int8x16x4_t[src]

impl Copy for poly8x16x2_t[src]

impl Copy for poly8x16x3_t[src]

impl Copy for poly8x16x4_t[src]

impl Copy for poly64_t[src]

impl Copy for poly64x1_t[src]

impl Copy for poly64x2_t[src]

impl Copy for poly128_t[src]

impl Copy for uint8x16x2_t[src]

impl Copy for uint8x16x3_t[src]

impl Copy for uint8x16x4_t[src]

impl Copy for float32x2_t[src]

impl Copy for float32x4_t[src]

impl Copy for int8x8_t[src]

impl Copy for int8x8x2_t[src]

impl Copy for int8x8x3_t[src]

impl Copy for int8x8x4_t[src]

impl Copy for int8x16_t[src]

impl Copy for int16x4_t[src]

impl Copy for int16x8_t[src]

impl Copy for int32x2_t[src]

impl Copy for int32x4_t[src]

impl Copy for int64x1_t[src]

impl Copy for int64x2_t[src]

impl Copy for poly8x8_t[src]

impl Copy for poly8x8x2_t[src]

impl Copy for poly8x8x3_t[src]

impl Copy for poly8x8x4_t[src]

impl Copy for poly8x16_t[src]

impl Copy for poly16x4_t[src]

impl Copy for poly16x8_t[src]

impl Copy for uint8x8_t[src]

impl Copy for uint8x8x2_t[src]

impl Copy for uint8x8x3_t[src]

impl Copy for uint8x8x4_t[src]

impl Copy for uint8x16_t[src]

impl Copy for uint16x4_t[src]

impl Copy for uint16x8_t[src]

impl Copy for uint32x2_t[src]

impl Copy for uint32x4_t[src]

impl Copy for uint64x1_t[src]

impl Copy for uint64x2_t[src]

impl Copy for vector_bool_long[src]

impl Copy for vector_double[src]

impl Copy for vector_signed_long[src]

impl Copy for vector_unsigned_long[src]

impl Copy for v128[src]

impl Copy for CpuidResult[src]

impl Copy for __m128[src]

impl Copy for __m128d[src]

impl Copy for __m128i[src]

impl Copy for __m256[src]

impl Copy for __m256d[src]

impl Copy for __m256i[src]

impl Copy for __m512[src]

impl Copy for __m512d[src]

impl Copy for __m512i[src]

impl Copy for TryFromSliceError[src]

impl Copy for CharTryFromError[src]

impl Copy for Error[src]

impl Copy for PhantomPinned[src]

impl Copy for NonZeroI8[src]

impl Copy for NonZeroI16[src]

impl Copy for NonZeroI32[src]

impl Copy for NonZeroI64[src]

impl Copy for NonZeroI128[src]

impl Copy for NonZeroIsize[src]

impl Copy for NonZeroU8[src]

impl Copy for NonZeroU16[src]

impl Copy for NonZeroU32[src]

impl Copy for NonZeroU64[src]

impl Copy for NonZeroU128[src]

impl Copy for NonZeroUsize[src]

impl Copy for TryFromIntError[src]

impl Copy for RangeFull[src]

impl Copy for NoneError[src]

impl Copy for TraitObject[src]

impl Copy for Utf8Error[src]

impl Copy for RawWakerVTable[src]

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

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

impl<'a, T: Copy + 'a, const N: usize> Copy for ArrayWindows<'a, T, N>[src]

impl<B: Copy, C: Copy> Copy for ControlFlow<B, C>[src]

impl<F: Copy> Copy for RepeatWith<F>[src]

impl<Idx: Copy> Copy for RangeTo<Idx>[src]

impl<Idx: Copy> Copy for RangeToInclusive<Idx>[src]

impl<P: Copy> Copy for Pin<P>[src]

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

impl<T: Copy + ?Sized> Copy for ManuallyDrop<T>[src]

impl<T: Copy> Copy for Bound<T>[src]

impl<T: Copy> Copy for Option<T>[src]

impl<T: Copy> Copy for Poll<T>[src]

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

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

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

impl<T: Copy, E: Copy> Copy for Result<T, E>[src]

impl<T: ?Sized> Copy for *const T[src]

impl<T: ?Sized> Copy for *mut T[src]

impl<T: ?Sized> Copy for PhantomData<T>[src]

impl<T: ?Sized> Copy for NonNull<T>[src]

impl<T: ?Sized, '_> Copy for &'_ T[src]

Shared references can be copied, but mutable references cannot!

impl<Y: Copy, R: Copy> Copy for GeneratorState<Y, R>[src]

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