pub trait Copy: Clone { }
Expand description
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
RunHowever, 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!
RunIt’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;
RunYou can also implement Copy
and Clone
manually:
struct MyStruct;
impl Copy for MyStruct { }
impl Clone for MyStruct {
fn clone(&self) -> MyStruct {
*self
}
}
RunThere 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,
}
RunA struct can be Copy
, and i32
is Copy
, therefore Point
is eligible to be Copy
.
By contrast, consider
struct PointList {
points: Vec<Point>,
}
RunThe 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` cannot 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,
}
Run§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
) - 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
.
Object Safety§
Implementors§
impl Copy for AsciiChar
impl Copy for std::cmp::Ordering
impl Copy for Infallible
impl Copy for std::fmt::Alignment
impl Copy for ErrorKind
impl Copy for SeekFrom
impl Copy for IpAddr
impl Copy for Ipv6MulticastScope
impl Copy for Shutdown
impl Copy for SocketAddr
impl Copy for FpCategory
impl Copy for BacktraceStyle
impl Copy for SearchStep
impl Copy for std::sync::atomic::Ordering
impl Copy for RecvTimeoutError
impl Copy for TryRecvError
impl Copy for bool
impl Copy for char
impl Copy for f32
impl Copy for f64
impl Copy for i8
impl Copy for i16
impl Copy for i32
impl Copy for i64
impl Copy for i128
impl Copy for isize
impl Copy for !
impl Copy for u8
impl Copy for u16
impl Copy for u32
impl Copy for u64
impl Copy for u128
impl Copy for ()
impl Copy for usize
impl Copy for CpuidResult
impl Copy for __m128
impl Copy for __m128bh
impl Copy for __m128d
impl Copy for __m128i
impl Copy for __m256
impl Copy for __m256bh
impl Copy for __m256d
impl Copy for __m256i
impl Copy for __m512
impl Copy for __m512bh
impl Copy for __m512d
impl Copy for __m512i
impl Copy for TimSortRun
impl Copy for AllocError
impl Copy for Global
impl Copy for Layout
impl Copy for System
impl Copy for TypeId
impl Copy for TryFromSliceError
impl Copy for CharTryFromError
impl Copy for TryFromCharError
impl Copy for Error
impl Copy for FileTimes
impl Copy for FileType
impl Copy for Empty
impl Copy for Sink
impl Copy for Assume
impl Copy for Ipv4Addr
impl Copy for Ipv6Addr
impl Copy for SocketAddrV4
impl Copy for SocketAddrV6
impl Copy for NonZero<i8>
impl Copy for NonZero<i16>
impl Copy for NonZero<i32>
impl Copy for NonZero<i64>
impl Copy for NonZero<i128>
impl Copy for NonZero<isize>
impl Copy for NonZero<u8>
impl Copy for NonZero<u16>
impl Copy for NonZero<u32>
impl Copy for NonZero<u64>
impl Copy for NonZero<u128>
impl Copy for NonZero<usize>
impl Copy for TryFromIntError
impl Copy for RangeFull
impl Copy for UCred
impl Copy for ExitCode
impl Copy for ExitStatus
impl Copy for ExitStatusError
impl Copy for std::ptr::Alignment
impl Copy for Utf8Error
impl Copy for RecvError
impl Copy for WaitTimeoutResult
impl Copy for RawWakerVTable
impl Copy for AccessError
impl Copy for ThreadId
impl Copy for Duration
impl Copy for Instant
impl Copy for SystemTime
impl Copy for PhantomPinned
impl<'a> Copy for Component<'a>
impl<'a> Copy for Prefix<'a>
impl<'a> Copy for Arguments<'a>
impl<'a> Copy for IoSlice<'a>
impl<'a> Copy for Location<'a>
impl<'a> Copy for Ancestors<'a>
impl<'a> Copy for PrefixComponent<'a>
impl<'a, T, const N: usize> Copy for ArrayWindows<'a, T, N>where
T: Copy + 'a,
impl<'fd> Copy for BorrowedFd<'fd>
impl<'handle> Copy for BorrowedHandle<'handle>
impl<'socket> Copy for BorrowedSocket<'socket>
impl<B, C> Copy for ControlFlow<B, C>
impl<Dyn> Copy for DynMetadata<Dyn>where
Dyn: ?Sized,
impl<F> Copy for RepeatWith<F>where
F: Copy,
impl<Idx> Copy for RangeTo<Idx>where
Idx: Copy,
impl<Idx> Copy for RangeToInclusive<Idx>where
Idx: Copy,
impl<Ptr> Copy for Pin<Ptr>where
Ptr: Copy,
impl<Ret, T> Copy for fn(T₁, T₂, …, Tₙ) -> Ret
This trait is implemented on function pointers with any number of arguments.
impl<T> Copy for Bound<T>where
T: Copy,
impl<T> Copy for Option<T>where
T: Copy,
impl<T> Copy for Poll<T>where
T: Copy,
impl<T> Copy for *const Twhere
T: ?Sized,
impl<T> Copy for *mut Twhere
T: ?Sized,
impl<T> Copy for &Twhere
T: ?Sized,
Shared references can be copied, but mutable references cannot!