Trait core::marker::Copy
[−]
[src]
pub trait Copy: Clone { }Types that can be copied by simply copying bits (i.e. memcpy`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`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`x` after the assignment: a move is also a bitwise copy under the hood.
When can my type be Copy`Copy`?
A type can implement Copy`Copyif all of its components implement` if all of its components implement Copy`Copy. For example, this`. For example, this
struct`structcan be` can be Copy`Copy`:
struct Point { x: i32, y: i32, }
A struct`structcan be` can be Copy`Copy, and`, and i32`i32is` is Copy`Copy, so therefore,`, so therefore, Point`Pointis eligible to be` is eligible to be Copy`Copy`.
struct PointList { points: Vec<Point>, }
The PointList`PointList` struct`structcannot implement` cannot implement Copy`Copy, because`, because Vec<T>`Vecis not` is not Copy`Copy. If we attempt to derive a`. If we
attempt to derive a Copy`Copy` implementation, we'll get an error.
error: the trait `Copy` may not be implemented for this type; field `points` does not implement
`Copy`
How can I implement Copy`Copy`?
There are two ways to implement Copy`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`derivestrategy will also place a` strategy will also place a Copy`Copy`
bound on type parameters, which isn't always desired.
When can my type not be Copy`Copy`?
Some types can't be copied safely. For example, copying &mut T`&mut Twould create an aliased mutable reference, and copying` would create an aliased
mutable reference, and copying String`String` would result in two attempts to free the same buffer.
Generalizing the latter case, any type implementing Drop`Dropcan't be` can't be Copy`Copy, because it's managing some resource besides its own`, because it's
managing some resource besides its own size_of::<T>()`size_of::
When should my type be Copy`Copy`?
Generally speaking, if your type can implement Copy`Copy, it should. There's one important thing to consider though: if you think your type may _not_ be able to implement`, it should. There's one important thing
to consider though: if you think your type may not be able to implement Copy`Copyin the future, then it might be prudent to not implement` in the future,
then it might be prudent to not implement Copy`Copy. This is because removing`. This is because removing Copy`Copyis a breaking change: that second example would fail to compile if we made` is a breaking
change: that second example would fail to compile if we made Foo`Foonon-` non-Copy`Copy`.
Implementors
impl<T: Copy> Copy for Wrapping<T> where T: Copyimpl Copy for FpCategoryimpl<T: Copy + Zeroable> Copy for NonZero<T> where T: Copyimpl<T: ?Sized> Copy for PhantomData<T>impl Copy for RangeFullimpl<Idx: Copy> Copy for RangeTo<Idx> where Idx: Copyimpl Copy for Orderingimpl Copy for TypeIdimpl Copy for Orderingimpl Copy for BorrowStateimpl<T: Copy> Copy for Option<T> where T: Copyimpl<T> Copy for Slice<T>impl Copy for TraitObjectimpl<T: Copy, E: Copy> Copy for Result<T, E> where T: Copy, E: Copyimpl Copy for i8x16impl Copy for i16x8impl Copy for i32x4impl Copy for i64x2impl Copy for u8x16impl Copy for u16x8impl Copy for u32x4impl Copy for u64x2impl Copy for f32x4impl Copy for f64x2impl Copy for SearchStepimpl Copy for Utf8Errorimpl Copy for CharRangeimpl Copy for Radiximpl<T: Copy, R: Copy> Copy for RadixFmt<T, R> where T: Copy, R: Copyimpl Copy for Errorimpl<'a> Copy for Arguments<'a>