#[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
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.
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.
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).
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, }
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`
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.
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.
impl<T> Copy for NonZero<T> where
T: Zeroable + Copy,
[src]
impl Copy for TryFromSliceError
[src]
impl<T> Copy for Bound<T> where
T: Copy,
impl Copy for ParseError
impl Copy for ExchangeHeapSingleton
impl Copy for Heap
impl<T> Copy for Discriminant<T>
impl Copy for TraitObject
impl Copy for TryFromIntError
impl Copy for std::sync::atomic::Ordering
impl Copy for std::cmp::Ordering
impl Copy for CharTryFromError
impl<T> Copy for Shared<T> where
T: ?Sized,
impl<T, E> Copy for Result<T, E> where
E: Copy,
T: Copy,
impl<Y, R> Copy for GeneratorState<Y, R> where
R: Copy,
Y: Copy,
impl Copy for RangeFull
impl Copy for TypeId
impl<Idx> Copy for RangeToInclusive<Idx> where
Idx: Copy,
impl Copy for NoneError
impl Copy for Error
impl Copy for Utf8Error
impl Copy for SearchStep
impl Copy for FpCategory
impl<T> Copy for ManuallyDrop<T> where
T: Copy,
impl<'a> Copy for Arguments<'a>
impl Copy for Infallible
impl<T> Copy for PhantomData<T> where
T: ?Sized,
impl<T> Copy for Unique<T> where
T: ?Sized,
impl<Idx> Copy for RangeTo<Idx> where
Idx: Copy,
impl<T> Copy for Option<T> where
T: Copy,
impl<T> Copy for Wrapping<T> where
T: Copy,
impl Copy for UnicodeVersion
impl Copy for LocalKeyState
impl Copy for ThreadId
impl Copy for FileType
impl Copy for ErrorKind
impl Copy for SeekFrom
impl Copy for IpAddr
impl Copy for Ipv4Addr
impl Copy for Ipv6Addr
impl Copy for Ipv6MulticastScope
impl Copy for SocketAddr
impl Copy for SocketAddrV4
impl Copy for SocketAddrV6
impl Copy for Shutdown
impl<'a> Copy for Prefix<'a>
impl<'a> Copy for PrefixComponent<'a>
impl<'a> Copy for Component<'a>
impl Copy for ExitStatus
impl<T: Copy> Copy for SendError<T>
impl Copy for RecvError
impl Copy for TryRecvError
impl Copy for RecvTimeoutError
impl<T: Copy> Copy for TrySendError<T>
impl Copy for WaitTimeoutResult
impl Copy for Duration
impl Copy for Instant
impl Copy for SystemTime
© 2010 The Rust Project Developers
Licensed under the Apache License, Version 2.0 or the MIT license, at your option.
https://doc.rust-lang.org/std/marker/trait.Copy.html