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 valueHowever, 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:
#[derive(Clone)]
struct MyStruct<T>(T);
impl<T: Copy> Copy for MyStruct<T> { }This isn’t always desired. For example, shared references (&T) can be copied regardless of
whether T is Copy. Likewise, a generic struct containing markers such as PhantomData
could potentially be duplicated with a bit-wise copy.
§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` 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,
}§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
Copythemselves. Note that variables captured by shared reference always implementCopy(even if the referent doesn’t), while variables captured by mutable reference never implementCopy.
Dyn Compatibility§
This trait is not dyn compatible.
In older versions of Rust, dyn compatibility was called "object safety", so this trait is not object safe.
Implementors§
impl Copy for AsciiChar
impl Copy for no_std_compat::cmp::Ordering
impl Copy for Infallible
impl Copy for no_std_compat::fmt::Alignment
impl Copy for FpCategory
impl Copy for SearchStep
impl Copy for no_std_compat::sync::atomic::Ordering
impl Copy for IpAddr
impl Copy for Ipv6MulticastScope
impl Copy for SocketAddr
impl Copy for bool
impl Copy for char
impl Copy for f16
impl Copy for f32
impl Copy for f64
impl Copy for f128
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 usize
impl Copy for AllocError
impl Copy for Layout
impl Copy for TypeId
impl Copy for CpuidResult
impl Copy for __m128
impl Copy for __m128bh
impl Copy for __m128d
impl Copy for __m128h
impl Copy for __m128i
impl Copy for __m256
impl Copy for __m256bh
impl Copy for __m256d
impl Copy for __m256h
impl Copy for __m256i
impl Copy for __m512
impl Copy for __m512bh
impl Copy for __m512d
impl Copy for __m512h
impl Copy for __m512i
impl Copy for bf16
impl Copy for TryFromSliceError
impl Copy for CharTryFromError
impl Copy for TryFromCharError
impl Copy for Error
impl Copy for PhantomPinned
impl Copy for Assume
impl Copy for TryFromIntError
impl Copy for RangeFull
impl Copy for no_std_compat::ptr::Alignment
impl Copy for Utf8Error
impl Copy for RawWakerVTable
impl Copy for Duration
impl Copy for Ipv4Addr
impl Copy for Ipv6Addr
impl Copy for SocketAddrV4
impl Copy for SocketAddrV6
impl<'a> Copy for Utf8Pattern<'a>
impl<'a> Copy for Arguments<'a>
impl<'a> Copy for Location<'a>
impl<'a, T, const N: usize> Copy for ArrayWindows<'a, T, N>where
T: Copy + 'a,
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<Idx> Copy for Range<Idx>where
Idx: Copy,
impl<Idx> Copy for RangeFrom<Idx>where
Idx: Copy,
impl<Idx> Copy for RangeInclusive<Idx>where
Idx: Copy,
impl<Ptr> Copy for Pin<Ptr>where
Ptr: Copy,
impl<T> Copy for Bound<T>where
T: Copy,
impl<T> Copy for Poll<T>where
T: Copy,
impl<T> Copy for Option<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!