Trait core::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 Decodedimpl Copy for FullDecodedimpl<'a> Copy for Part<'a>impl Copy for Signimpl Copy for Unpackedimpl Copy for Fpimpl<T: Copy> Copy for Wrapping<T>impl Copy for FpCategoryimpl<T: Copy + Zeroable> Copy for NonZero<T>impl<T: ?Sized> Copy for Shared<T>impl<T: ?Sized> Copy for PhantomData<T>impl Copy for RangeFullimpl<Idx: Copy> Copy for RangeTo<Idx>impl<Idx: Copy> Copy for RangeInclusive<Idx>impl<Idx: Copy> Copy for RangeToInclusive<Idx>impl Copy for Orderingimpl Copy for TypeIdimpl Copy for Orderingimpl Copy for BorrowStateimpl<T: Copy> Copy for Option<T>impl<T> Copy for Slice<T>impl Copy for TraitObjectimpl<T: Copy, E: Copy> Copy for Result<T, E>impl Copy for SearchStepimpl Copy for Utf8Errorimpl Copy for CharRangeimpl Copy for Argumentimpl Copy for FormatSpecimpl Copy for Alignmentimpl Copy for Countimpl Copy for Positionimpl Copy for Errorimpl<'a> Copy for Arguments<'a>