Trait core::marker::Copy 1.0.0[−][src]
#[lang = "copy"]pub trait Copy: Clone { }
Types whose values can be duplicated simply by copying bits.
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 valueRun
However, if a type implements Copy, it instead has 'copy semantics':
// We can derive a `Copy` implementation. `Clone` is also required, as it's // a supertrait of `Copy`. #[derive(Debug, Copy, Clone)] struct Foo; let x = Foo; let y = x; // `y` is a copy of `x` println!("{:?}", x); // A-OK!Run
It's important to note that in these two examples, the only difference is whether you
are allowed to access x after the assignment. Under the hood, both a copy and a move
can result in bits being copied in memory, although this is sometimes optimized away.
How can I implement Copy?
There are two ways to implement Copy on your type. The simplest is to use derive:
#[derive(Copy, Clone)] struct MyStruct;Run
You can also implement Copy and Clone manually:
struct MyStruct; impl Copy for MyStruct { } impl Clone for MyStruct { fn clone(&self) -> MyStruct { *self } }Run
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.
What's the difference between Copy and Clone?
Copies happen implicitly, for example as part of an assignment y = x. The behavior of
Copy is not overloadable; it is always a simple bit-wise copy.
Cloning is an explicit action, x.clone(). The implementation of Clone can
provide any type-specific behavior necessary to duplicate values safely. For example,
the implementation of Clone for String needs to copy the pointed-to string
buffer in the heap. A simple bitwise copy of String values would merely copy the
pointer, leading to a double free down the line. For this reason, String is Clone
but not Copy.
Clone is a supertrait of Copy, so everything which is Copy must also implement
Clone. If a type is Copy then its Clone implementation only needs to return *self
(see the example above).
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, }Run
A struct can be Copy, and i32 is Copy, therefore Point is eligible to be Copy.
By contrast, consider
struct PointList { points: Vec<Point>, }Run
The struct PointList 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`
When can't my type be Copy?
Some types can't be copied safely. For example, copying &mut T would create an aliased
mutable reference. Copying String would duplicate responsibility for managing the
String's buffer, leading to a double free.
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.
If you try to implement Copy on a struct or enum containing non-Copy data, you will get
the error E0204.
When should my type be Copy?
Generally speaking, if your type can implement Copy, it should. Keep in mind, though,
that implementing Copy is part of the public API of your type. If the type might become
non-Copy in the future, it could be prudent to omit the Copy implementation now, to
avoid a breaking API change.
Additional implementors
In addition to the implementors listed below,
the following types also implement Copy:
- Function item types (i.e. the distinct types defined for each function)
- Function pointer types (e.g.
fn() -> i32) - Array types, for all sizes, if the item type also implements
Copy(e.g.[i32; 123456]) - Tuple types, if each component also implements
Copy(e.g.(),(i32, bool)) - Closure types, if they capture no value from the environment
or if all such captured values implement
Copythemselves. Note that variables captured by shared reference always implementCopy(even if the referent doesn't), while variables captured by mutable reference never implementCopy.
Implementors
impl Copy for NonZeroU8impl Copy for NonZeroU16impl Copy for NonZeroU32impl Copy for NonZeroU64impl Copy for NonZeroU128impl Copy for NonZeroUsizeimpl<T: Copy> Copy for Wrapping<T>impl Copy for FpCategoryimpl Copy for TryFromIntErrorimpl<T> Copy for Discriminant<T>impl<T: Copy> Copy for ManuallyDrop<T>impl<T: ?Sized> Copy for NonNull<T>impl Copy for usizeimpl Copy for u8impl Copy for u16impl Copy for u32impl Copy for u64impl Copy for u128impl Copy for isizeimpl Copy for i8impl Copy for i16impl Copy for i32impl Copy for i64impl Copy for i128impl Copy for f32impl Copy for f64impl Copy for boolimpl Copy for charimpl Copy for !impl<T: ?Sized> Copy for *const Timpl<T: ?Sized> Copy for *mut Timpl<'a, T: ?Sized> Copy for &'a Timpl<T: ?Sized> Copy for PhantomData<T>impl Copy for Pinnedimpl<Y: Copy, R: Copy> Copy for GeneratorState<Y, R>impl Copy for RangeFullimpl<Idx: Copy> Copy for RangeTo<Idx>impl<Idx: Copy> Copy for RangeToInclusive<Idx>impl<T: Copy> Copy for Bound<T>impl Copy for core::cmp::Orderingimpl<T: Copy> Copy for Reverse<T>impl Copy for TypeIdimpl Copy for TryFromSliceErrorimpl Copy for core::sync::atomic::Orderingimpl Copy for CharTryFromErrorimpl<F: Copy> Copy for RepeatWith<F>impl<T: Copy> Copy for Option<T>impl Copy for NoneErrorimpl Copy for TraitObjectimpl<T: Copy, E: Copy> Copy for Result<T, E>impl Copy for SearchStepimpl Copy for Utf8Errorimpl Copy for Errorimpl<'a> Copy for Arguments<'a>impl Copy for Durationimpl Copy for UnicodeVersionimpl<T: Copy> Copy for Poll<T>impl Copy for Layoutimpl Copy for CpuidResultimpl Copy for __m64impl Copy for __m128iimpl Copy for __m128impl Copy for __m128dimpl Copy for __m256iimpl Copy for __m256impl Copy for __m256dimpl Copy for float64x1_timpl Copy for float64x2_timpl Copy for int8x4_timpl Copy for uint8x4_timpl Copy for int16x2_timpl Copy for uint16x2_timpl Copy for int8x8_timpl Copy for uint8x8_timpl Copy for poly8x8_timpl Copy for int16x4_timpl Copy for uint16x4_timpl Copy for poly16x4_timpl Copy for int32x2_timpl Copy for uint32x2_timpl Copy for float32x2_timpl Copy for int64x1_timpl Copy for uint64x1_timpl Copy for int8x16_timpl Copy for uint8x16_timpl Copy for poly8x16_timpl Copy for int16x8_timpl Copy for uint16x8_timpl Copy for poly16x8_timpl Copy for int32x4_timpl Copy for uint32x4_timpl Copy for float32x4_timpl Copy for int64x2_timpl Copy for uint64x2_timpl Copy for i8x16impl Copy for vector_signed_longimpl Copy for vector_unsigned_longimpl Copy for vector_bool_longimpl Copy for vector_double