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
Copy. For example, this
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,
Pointis eligible to be
Copy`.
struct PointList { points: Vec<Point>, }
The PointList
struct
`structcannot implement
Copy, because
Vecis not
` is not Copy
`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
`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
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
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
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
Copyin the future, then it might be prudent to not implement
Copy. This is because removing
Copyis a breaking change: that second example would fail to compile if we made
Foonon-
` non-Copy
`Copy`.
Implementors
impl<T: Copy> Copy for Wrapping<T>
impl Copy for FpCategory
impl<T: Copy + Zeroable> Copy for NonZero<T>
impl<T: ?Sized> Copy for PhantomData<T>
impl Copy for RangeFull
impl<Idx: Copy> Copy for RangeTo<Idx>
impl Copy for Ordering
impl Copy for TypeId
impl Copy for Ordering
impl Copy for BorrowState
impl<T: Copy> Copy for Option<T>
impl<T> Copy for Slice<T>
impl Copy for TraitObject
impl<T: Copy, E: Copy> Copy for Result<T, E>
impl Copy for i8x16
impl Copy for i16x8
impl Copy for i32x4
impl Copy for i64x2
impl Copy for u8x16
impl Copy for u16x8
impl Copy for u32x4
impl Copy for u64x2
impl Copy for f32x4
impl Copy for f64x2
impl Copy for SearchStep
impl Copy for Utf8Error
impl Copy for CharRange
impl Copy for Radix
impl<T: Copy, R: Copy> Copy for RadixFmt<T, R>
impl Copy for Error
impl<'a> Copy for Arguments<'a>