Trait std::marker::Copy
[−]
[src]
pub trait Copy: Clone { }
Types that can be copied by simply copying bits (i.e. memcpy
).
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 just derive a `Copy` implementation #[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 if you are allowed to
access x
after the assignment: a move is also a bitwise copy under the hood.
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, }
A struct
can be Copy
, and i32
is Copy
, so therefore, Point
is eligible to be Copy
.
struct PointList { points: Vec<Point>, }
The PointList
struct
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`
How can I implement Copy
?
There are two ways to implement Copy
on your type:
#[derive(Copy, Clone)] struct MyStruct;
and
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.
When can my type not be Copy
?
Some types can't be copied safely. For example, copying &mut T
would create an aliased
mutable reference, and copying String
would result in two attempts to free the same buffer.
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.
When should my type be Copy
?
Generally speaking, if your type can implement Copy
, it should. There's one important thing
to consider though: if you think your type may not be able to implement Copy
in the future,
then it might be prudent to not implement Copy
. This is because removing Copy
is a breaking
change: that second example would fail to compile if we made Foo
non-Copy
.
Derivable
This trait can be used with #[derive]
.
Implementors
impl<T> Copy for Wrapping<T> where T: Copy
impl Copy for Decoded
impl Copy for FullDecoded
impl<'a> Copy for Part<'a>
impl Copy for Sign
impl Copy for Unpacked
impl Copy for Fp
impl Copy for FpCategory
impl<T> Copy for NonZero<T> where T: Copy + Zeroable
impl<T> Copy for Shared<T> where T: ?Sized
impl<T> Copy for PhantomData<T> where T: ?Sized
impl Copy for RangeFull
impl<Idx> Copy for RangeTo<Idx> where Idx: Copy
impl Copy for Ordering
impl Copy for TypeId
impl Copy for Ordering
impl Copy for BorrowState
impl<T> Copy for Option<T> where T: Copy
impl<T> Copy for Slice<T>
impl Copy for TraitObject
impl<T, E> Copy for Result<T, E> where E: Copy, T: Copy
impl Copy for SearchStep
impl Copy for Utf8Error
impl Copy for CharRange
impl Copy for Radix
impl<T, R> Copy for RadixFmt<T, R> where R: Copy, T: Copy
impl Copy for Argument
impl Copy for FormatSpec
impl Copy for Alignment
impl Copy for Count
impl Copy for Position
impl Copy for Error
impl<'a> Copy for ArgumentV1<'a>
impl Copy for FlagV1
impl<'a> Copy for Arguments<'a>
impl Copy for ExchangeHeapSingleton
impl<'a, K, V, Type> Copy for NodeRef<Immut<'a>, K, V, Type> where K: 'a, V: 'a
impl<Node, Type> Copy for Handle<Node, Type> where Node: Copy
impl<E> Copy for EnumSet<E>
impl<T> Copy for Rawlink<T>
impl Copy for ParseError
impl<T> Copy for Bound<T> where T: Copy
impl Copy for StandardNormal
impl Copy for Normal
impl Copy for LogNormal
impl Copy for Exp1
impl Copy for Exp
impl Copy for IsaacRng
impl Copy for Isaac64Rng
impl Copy for ChaChaRng
impl Copy for ReseedWithDefault
impl<K, V> Copy for RawBucket<K, V>
impl<K, V, M: Copy> Copy for Bucket<K, V, M>
impl Copy for SafeHash
impl Copy for Packet
impl Copy for n16
impl Copy for n32
impl Copy for MACAddr
impl Copy for IPv4Addr
impl Copy for IPv6Addr
impl Copy for Checksum
impl Copy for Duration