Trait std::marker::Copy

#[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 value

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!

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;

You can also implement Copy and Clone manually:

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.

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,
}

A struct can be Copy, and i32 is Copy, therefore Point is eligible to be Copy. By contrast, consider

struct PointList {
    points: Vec<Point>,
}

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 Copy themselves. Note that variables captured by shared reference always implement Copy (even if the referent doesn't), while variables captured by mutable reference never implement Copy.

Implementations on Foreign Types

impl Copy for TryFromSliceError

Implementors

impl Copy for std::cmp::Ordering

impl Copy for ErrorKind

impl Copy for SeekFrom

impl Copy for IpAddr

impl Copy for Ipv6MulticastScope

impl Copy for Shutdown

impl Copy for SocketAddr

impl Copy for FpCategory

impl Copy for SearchStep

impl Copy for ParseError

impl Copy for std::sync::atomic::Ordering

impl Copy for RecvTimeoutError

impl Copy for TryRecvError

impl Copy for bool

impl Copy for char

impl Copy for f32

impl Copy for f64

impl Copy for i128

impl Copy for i16

impl Copy for i32

impl Copy for i64

impl Copy for i8

impl Copy for isize

impl Copy for !

impl Copy for u128

impl Copy for u16

impl Copy for u32

impl Copy for u64

impl Copy for u8

impl Copy for usize

impl Copy for Global

impl Copy for Layout

impl Copy for TypeId

impl Copy for CharTryFromError

impl Copy for UnicodeVersion

impl Copy for Error

impl Copy for FileType

impl Copy for Pinned

impl Copy for Ipv4Addr

impl Copy for Ipv6Addr

impl Copy for SocketAddrV4

impl Copy for SocketAddrV6

impl Copy for NonZeroU128

impl Copy for NonZeroU16

impl Copy for NonZeroU32

impl Copy for NonZeroU64

impl Copy for NonZeroU8

impl Copy for NonZeroUsize

impl Copy for TryFromIntError

impl Copy for RangeFull

impl Copy for NoneError

impl Copy for ExitCode

impl Copy for ExitStatus

impl Copy for TraitObject

impl Copy for Utf8Error

impl Copy for RecvError

impl Copy for WaitTimeoutResult

impl Copy for ThreadId

impl Copy for Duration

impl Copy for Instant

impl Copy for SystemTime

impl<'a> Copy for Component<'a>

impl<'a> Copy for Prefix<'a>

impl<'a> Copy for Arguments<'a>

impl<'a> Copy for Ancestors<'a>

impl<'a> Copy for PrefixComponent<'a>

impl<'a, T> Copy for &'a T where     T: ?Sized,

impl<F> Copy for RepeatWith<F> where     F: Copy,

impl<Idx> Copy for RangeTo<Idx> where     Idx: Copy,

impl<Idx> Copy for RangeToInclusive<Idx> where     Idx: Copy,

impl<P> Copy for Pin<P> where     P: Copy,

impl<T> Copy for Bound<T> where     T: Copy,

impl<T> Copy for Option<T> where     T: Copy,

impl<T> Copy for Poll<T> where     T: Copy,

impl<T> Copy for *const T where     T: ?Sized,

impl<T> Copy for *mut T where     T: ?Sized,

impl<T> Copy for Reverse<T> where     T: Copy,

impl<T> Copy for PhantomData<T> where     T: ?Sized,

impl<T> Copy for Discriminant<T>

impl<T> Copy for ManuallyDrop<T> where     T: Copy + ?Sized,

impl<T> Copy for Wrapping<T> where     T: Copy,

impl<T> Copy for NonNull<T> where     T: ?Sized,

impl<T, E> Copy for Result<T, E> where     E: Copy,     T: Copy,

impl<T: Copy> Copy for TrySendError<T>

impl<T: Copy> Copy for SendError<T>

impl<Y, R> Copy for GeneratorState<Y, R> where     R: Copy,     Y: Copy,