core::marker::Copy - Rust
Description: API documentation for the Rust `Copy` trait in crate `core`.
Keywords: rust, rustlang, rust-lang, Copy

# 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:

    #[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

Run

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!

Run

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
        }
    }

Run

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`:

    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` may not be implemented for this type; field `points` does not implement `Copy`

## 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`)
- Array types, for all sizes, if the item type also implements `Copy` (e.g., `[i32; 123456]`)
- Tuple types, if each component also implements `Copy` (e.g., `()`, `(i32, bool)`)
- 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`.

## Implementors

### `impl Copy for !`[src]

### `impl Copy for AllocInit`[src]

### `impl Copy for ReallocPlacement`[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 Layout`[src]

### `impl Copy for MemoryBlock`[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 poly128_t`[src]

### `impl Copy for poly64_t`[src]

### `impl Copy for poly64x1_t`[src]

### `impl Copy for poly64x2_t`[src]

### `impl Copy for poly8x16x2_t`[src]

### `impl Copy for poly8x16x3_t`[src]

### `impl Copy for poly8x16x4_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 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 int8x16_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 poly16x4_t`[src]

### `impl Copy for poly16x8_t`[src]

### `impl Copy for poly8x16_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 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 uint8x16_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 v16i8`[src]

### `impl Copy for v16u8`[src]

### `impl Copy for v2f64`[src]

### `impl Copy for v2i64`[src]

### `impl Copy for v2u64`[src]

### `impl Copy for v4f32`[src]

### `impl Copy for v4i32`[src]

### `impl Copy for v4u32`[src]

### `impl Copy for v8i16`[src]

### `impl Copy for v8u16`[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 __m64`[src]

### `impl Copy for TryFromSliceError`[src]

### `impl Copy for CharTryFromError`[src]

### `impl Copy for Error`[src]

### `impl Copy for PhantomPinned`[src]

### `impl Copy for NonZeroI128`[src]

### `impl Copy for NonZeroI16`[src]

### `impl Copy for NonZeroI32`[src]

### `impl Copy for NonZeroI64`[src]

### `impl Copy for NonZeroI8`[src]

### `impl Copy for NonZeroIsize`[src]

### `impl Copy for NonZeroU128`[src]

### `impl Copy for NonZeroU16`[src]

### `impl Copy for NonZeroU32`[src]

### `impl Copy for NonZeroU64`[src]

### `impl Copy for NonZeroU8`[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<'_, T: ?Sized> Copy for &'_ T`[src]

Shared references can be copied, but mutable references cannot!

### `impl<'a> Copy for Arguments<'a>`[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<Y: Copy, R: Copy> Copy for GeneratorState<Y, R>`[src]

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