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:
fn main() { #[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 }#[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
fn main() { struct MyStruct; impl Copy for MyStruct {} impl Clone for MyStruct { fn clone(&self) -> MyStruct { *self } } }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 Copy for TypeIdimpl Copy for BorrowStateimpl Copy for Orderingimpl<T> Copy for PhantomData<T> where T: ?Sizedimpl Copy for RangeFullimpl<Idx> Copy for RangeTo<Idx> where Idx: Copyimpl<Idx> Copy for RangeInclusive<Idx> where Idx: Copyimpl<Idx> Copy for RangeToInclusive<Idx> where Idx: Copyimpl<T> Copy for Shared<T> where T: ?Sizedimpl<T> Copy for Slice<T>impl Copy for TraitObjectimpl<T, E> Copy for Result<T, E> where T: Copy, E: Copyimpl<T> Copy for Option<T> where T: Copyimpl Copy for ExchangeHeapSingletonimpl Copy for Errorimpl<'a> Copy for Arguments<'a>impl Copy for CharRangeimpl Copy for SearchStepimpl Copy for Utf8Errorimpl Copy for ParseErrorimpl Copy for FpCategoryimpl<T> Copy for Wrapping<T> where T: Copyimpl Copy for LocalKeyStateimpl<T> Copy for Bound<T> where T: Copyimpl Copy for FileTypeimpl Copy for ErrorKindimpl Copy for SeekFromimpl Copy for IpAddrimpl Copy for Ipv4Addrimpl Copy for Ipv6Addrimpl Copy for Ipv6MulticastScopeimpl Copy for SocketAddrimpl Copy for SocketAddrV4impl Copy for SocketAddrV6impl Copy for Shutdownimpl<'a> Copy for Prefix<'a>impl<'a> Copy for PrefixComponent<'a>impl<'a> Copy for Component<'a>impl Copy for ExitStatusimpl Copy for Orderingimpl<T: Copy> Copy for SendError<T>impl Copy for RecvErrorimpl Copy for TryRecvErrorimpl<T: Copy> Copy for TrySendError<T>impl Copy for WaitTimeoutResultimpl Copy for Durationimpl Copy for Instantimpl Copy for SystemTimeimpl Copy for IsaacRngimpl Copy for Isaac64Rngimpl Copy for ReseedWithDefault