1.0.0[−][src]Trait std::marker::Copy
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.
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.
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 implementCopy
(even if the referent doesn't), while variables captured by mutable reference never implementCopy
.
Implementations on Foreign Types
impl Copy for __m512
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impl Copy for __m512d
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impl Copy for __m256d
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impl Copy for CpuidResult
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impl Copy for __m128d
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impl Copy for __m128
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impl Copy for __m64
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impl Copy for __m256
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impl Copy for __m256i
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impl Copy for __m512i
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impl Copy for __m128i
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Implementors
impl Copy for std::cmp::Ordering
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impl Copy for Infallible
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impl Copy for ErrorKind
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impl Copy for SeekFrom
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impl Copy for IpAddr
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impl Copy for Ipv6MulticastScope
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impl Copy for Shutdown
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impl Copy for SocketAddr
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impl Copy for FpCategory
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impl Copy for SearchStep
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impl Copy for std::sync::atomic::Ordering
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impl Copy for RecvTimeoutError
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impl Copy for TryRecvError
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impl Copy for bool
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impl Copy for char
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impl Copy for f32
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impl Copy for f64
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impl Copy for i128
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impl Copy for i16
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impl Copy for i32
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impl Copy for i64
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impl Copy for i8
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impl Copy for isize
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impl Copy for !
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impl Copy for u128
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impl Copy for u16
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impl Copy for u32
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impl Copy for u64
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impl Copy for u8
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impl Copy for usize
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impl Copy for Global
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impl Copy for Layout
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impl Copy for System
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impl Copy for TypeId
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impl Copy for TryFromSliceError
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impl Copy for CharTryFromError
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impl Copy for UnicodeVersion
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impl Copy for Error
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impl Copy for FileType
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impl Copy for PhantomPinned
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impl Copy for Ipv4Addr
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impl Copy for Ipv6Addr
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impl Copy for SocketAddrV4
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impl Copy for SocketAddrV6
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impl Copy for NonZeroI128
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impl Copy for NonZeroI16
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impl Copy for NonZeroI32
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impl Copy for NonZeroI64
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impl Copy for NonZeroI8
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impl Copy for NonZeroIsize
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impl Copy for NonZeroU128
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impl Copy for NonZeroU16
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impl Copy for NonZeroU32
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impl Copy for NonZeroU64
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impl Copy for NonZeroU8
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impl Copy for NonZeroUsize
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impl Copy for TryFromIntError
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impl Copy for RangeFull
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impl Copy for NoneError
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impl Copy for ExitCode
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impl Copy for ExitStatus
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impl Copy for TraitObject
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impl Copy for Utf8Error
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impl Copy for RecvError
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impl Copy for WaitTimeoutResult
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impl Copy for RawWakerVTable
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impl Copy for ThreadId
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impl Copy for Duration
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impl Copy for Instant
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impl Copy for SystemTime
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impl<'_, T> Copy for &'_ T where
T: ?Sized,
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T: ?Sized,
impl<'a> Copy for Component<'a>
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impl<'a> Copy for Prefix<'a>
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impl<'a> Copy for ErrorIter<'a>
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impl<'a> Copy for Arguments<'a>
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impl<'a> Copy for Ancestors<'a>
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impl<'a> Copy for PrefixComponent<'a>
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impl<F> Copy for OnceWith<F> where
F: Copy,
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F: Copy,
impl<F> Copy for RepeatWith<F> where
F: Copy,
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F: Copy,
impl<Idx> Copy for RangeTo<Idx> where
Idx: Copy,
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Idx: Copy,
impl<Idx> Copy for RangeToInclusive<Idx> where
Idx: Copy,
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Idx: Copy,
impl<P> Copy for Pin<P> where
P: Copy,
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P: Copy,
impl<T> Copy for Bound<T> where
T: Copy,
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T: Copy,
impl<T> Copy for Option<T> where
T: Copy,
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T: Copy,
impl<T> Copy for Poll<T> where
T: Copy,
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T: Copy,
impl<T> Copy for *const T where
T: ?Sized,
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T: ?Sized,
impl<T> Copy for *mut T where
T: ?Sized,
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T: ?Sized,
impl<T> Copy for Reverse<T> where
T: Copy,
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T: Copy,
impl<T> Copy for PhantomData<T> where
T: ?Sized,
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T: ?Sized,
impl<T> Copy for Discriminant<T>
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impl<T> Copy for ManuallyDrop<T> where
T: Copy + ?Sized,
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T: Copy + ?Sized,
impl<T> Copy for Wrapping<T> where
T: Copy,
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T: Copy,
impl<T> Copy for NonNull<T> where
T: ?Sized,
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T: ?Sized,
impl<T> Copy for MaybeUninit<T> where
T: Copy,
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T: Copy,
impl<T, E> Copy for Result<T, E> where
E: Copy,
T: Copy,
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E: Copy,
T: Copy,
impl<T: Copy> Copy for TrySendError<T>
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impl<T: Copy> Copy for SendError<T>
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impl<Y, R> Copy for GeneratorState<Y, R> where
R: Copy,
Y: Copy,
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R: Copy,
Y: Copy,