Trait core::marker::Copy 1.0.0
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[src]
#[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 valueRun
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.
Closures
Closure types automatically implement Copy
if they capture no value from the environment
or if all such captured values implement Copy
themselves.
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;Run
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, }Run
A struct can be Copy
, and i32
is Copy
, therefore Point
is eligible to be Copy
.
By contrast, consider
struct PointList { points: Vec<Point>, }Run
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.
Implementors
impl Copy for NonZeroU8
impl Copy for NonZeroU16
impl Copy for NonZeroU32
impl Copy for NonZeroU64
impl Copy for NonZeroU128
impl Copy for NonZeroUsize
impl Copy for NonZeroI8
impl Copy for NonZeroI16
impl Copy for NonZeroI32
impl Copy for NonZeroI64
impl Copy for NonZeroI128
impl Copy for NonZeroIsize
impl<T: Copy> Copy for Wrapping<T>
impl Copy for FpCategory
impl Copy for TryFromIntError
impl<T> Copy for Discriminant<T>
impl<T: Copy> Copy for ManuallyDrop<T>
impl<T: Copy + Zeroable> Copy for NonZero<T>
impl<T: ?Sized> Copy for Unique<T>
impl<T: ?Sized> Copy for NonNull<T>
impl<T: ?Sized> Copy for PhantomData<T>
impl<Y: Copy, R: Copy> Copy for GeneratorState<Y, R>
impl Copy for RangeFull
impl<Idx: Copy> Copy for RangeTo<Idx>
impl<Idx: Copy> Copy for RangeToInclusive<Idx>
impl<T: Copy> Copy for Bound<T>
impl Copy for core::cmp::Ordering
impl<T: Copy> Copy for Reverse<T>
impl Copy for TypeId
impl Copy for TryFromSliceError
impl Copy for core::sync::atomic::Ordering
impl Copy for CharTryFromError
impl<F: Copy> Copy for RepeatWith<F>
impl<T: Copy> Copy for Option<T>
impl Copy for NoneError
impl Copy for TraitObject
impl<T: Copy, E: Copy> Copy for Result<T, E>
impl Copy for SearchStep
impl Copy for Utf8Error
impl Copy for Error
impl<'a> Copy for Arguments<'a>
impl Copy for Duration
impl Copy for i8x2
impl Copy for u8x2
impl Copy for b8x2
impl Copy for i16x2
impl Copy for u16x2
impl Copy for i8x4
impl Copy for u8x4
impl Copy for b8x4
impl Copy for i8x8
impl Copy for u8x8
impl Copy for b8x8
impl Copy for i16x4
impl Copy for u16x4
impl Copy for i32x2
impl Copy for u32x2
impl Copy for f32x2
impl Copy for i8x16
impl Copy for u8x16
impl Copy for b8x16
impl Copy for i16x8
impl Copy for u16x8
impl Copy for i32x4
impl Copy for u32x4
impl Copy for f32x4
impl Copy for i64x2
impl Copy for u64x2
impl Copy for f64x2
impl Copy for i8x32
impl Copy for u8x32
impl Copy for b8x32
impl Copy for i16x16
impl Copy for u16x16
impl Copy for i32x8
impl Copy for u32x8
impl Copy for f32x8
impl Copy for i64x4
impl Copy for u64x4
impl Copy for f64x4
impl Copy for i8x64
impl Copy for u8x64
impl Copy for b8x64
impl Copy for i16x32
impl Copy for u16x32
impl Copy for i32x16
impl Copy for u32x16
impl Copy for f32x16
impl Copy for i64x8
impl Copy for u64x8
impl Copy for f64x8
impl Copy for CpuidResult
impl Copy for __m64
impl Copy for __m128i
impl Copy for __m128
impl Copy for __m128d
impl Copy for __m256i
impl Copy for __m256
impl Copy for __m256d