1.0.0[−][src]Trait wasmer_types::entity::__core::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 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 { *self } }
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
:
- 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 ThreadId
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impl Copy for SocketAddr
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impl Copy for AccessError
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impl Copy for SeekFrom
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impl<T> Copy for SendError<T> where
T: Copy,
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T: Copy,
impl Copy for Shutdown
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impl Copy for Ipv4Addr
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impl Copy for System
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impl<T> Copy for TrySendError<T> where
T: Copy,
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T: Copy,
impl Copy for WaitTimeoutResult
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impl Copy for FileType
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impl Copy for ErrorKind
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impl Copy for SocketAddrV4
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impl Copy for Ipv6MulticastScope
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impl Copy for ExitStatus
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impl<'a> Copy for IoSlice<'a>
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impl Copy for UCred
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impl<'a> Copy for Prefix<'a>
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impl Copy for TryRecvError
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impl Copy for SocketAddrV6
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impl<'a> Copy for Component<'a>
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impl Copy for Ipv6Addr
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impl Copy for Instant
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impl Copy for RecvTimeoutError
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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 Copy for RecvError
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impl Copy for IpAddr
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impl Copy for ExitCode
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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,
Shared references can be copied, but mutable references cannot!
impl Copy for i128
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impl Copy for u64
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impl Copy for u8
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impl Copy for char
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impl<T> Copy for *const T where
T: ?Sized,
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T: ?Sized,
impl Copy for i64
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impl Copy for u32
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impl Copy for isize
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impl Copy for f32
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impl Copy for usize
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impl Copy for !
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impl<T> Copy for *mut T where
T: ?Sized,
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T: ?Sized,
impl Copy for i16
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impl Copy for u16
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impl Copy for i32
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impl Copy for u128
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impl Copy for i8
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impl Copy for f64
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impl Copy for bool
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impl Copy for Global
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impl Copy for _Unwind_Reason_Code
impl Copy for _Unwind_Action
impl<E> Copy for U128Deserializer<E>
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impl<'de, E> Copy for StrDeserializer<'de, E>
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impl<E> Copy for I32Deserializer<E>
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impl<E> Copy for U8Deserializer<E>
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impl<'de, E> Copy for BorrowedStrDeserializer<'de, E>
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impl<E> Copy for IsizeDeserializer<E>
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impl<E> Copy for UnitDeserializer<E>
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impl<E> Copy for UsizeDeserializer<E>
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impl<E> Copy for U32Deserializer<E>
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impl<E> Copy for U64Deserializer<E>
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impl<E> Copy for I128Deserializer<E>
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impl<'a> Copy for Unexpected<'a>
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impl<E> Copy for I8Deserializer<E>
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impl<E> Copy for CharDeserializer<E>
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impl<E> Copy for I64Deserializer<E>
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impl<E> Copy for BoolDeserializer<E>
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impl<'de, E> Copy for BorrowedBytesDeserializer<'de, E>
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impl<E> Copy for U16Deserializer<E>
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impl<E> Copy for F64Deserializer<E>
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impl<E> Copy for F32Deserializer<E>
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impl Copy for IgnoredAny
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impl<E> Copy for I16Deserializer<E>
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Loading content...Implementors
impl Copy for wasmer_types::entity::__core::cmp::Ordering
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impl Copy for Infallible
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impl Copy for FpCategory
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impl Copy for SearchStep
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impl Copy for wasmer_types::entity::__core::sync::atomic::Ordering
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impl Copy for GlobalInit
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impl Copy for Mutability
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impl Copy for Type
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impl Copy for AllocError
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impl Copy for Layout
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impl Copy for TypeId
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impl Copy for CpuidResult
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impl Copy for __m128
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impl Copy for __m128d
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impl Copy for __m128i
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impl Copy for __m256
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impl Copy for __m256d
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impl Copy for __m256i
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impl Copy for __m512
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impl Copy for __m512d
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impl Copy for __m512i
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impl Copy for TryFromSliceError
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impl Copy for CharTryFromError
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impl Copy for Error
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impl Copy for PhantomPinned
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impl Copy for NonZeroI8
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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 NonZeroI128
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impl Copy for NonZeroIsize
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impl Copy for NonZeroU8
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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 NonZeroU128
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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 TraitObject
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impl Copy for Utf8Error
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impl Copy for RawWakerVTable
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impl Copy for Duration
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impl Copy for Bytes
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impl Copy for CustomSectionIndex
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impl Copy for DataIndex
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impl Copy for ElemIndex
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impl Copy for FunctionIndex
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impl Copy for GlobalIndex
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impl Copy for GlobalType
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impl Copy for LocalFunctionIndex
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impl Copy for LocalGlobalIndex
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impl Copy for LocalMemoryIndex
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impl Copy for LocalTableIndex
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impl Copy for MemoryIndex
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impl Copy for MemoryType
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impl Copy for Pages
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impl Copy for SignatureIndex
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impl Copy for TableIndex
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impl Copy for TableType
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impl Copy for V128
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impl<'a> Copy for Arguments<'a>
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impl<'a> Copy for Location<'a>
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impl<'a, T, const N: usize> Copy for ArrayWindows<'a, T, N> where
T: 'a + Copy,
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T: 'a + Copy,
impl<C, B> Copy for ControlFlow<C, B> where
B: Copy,
C: Copy,
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B: Copy,
C: 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 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 PackedOption<T> where
T: ReservedValue + Copy,
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T: ReservedValue + Copy,
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<Y, R> Copy for GeneratorState<Y, R> where
R: Copy,
Y: Copy,
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R: Copy,
Y: Copy,