1.0.0[][src]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 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:

Implementations on Foreign Types

impl Copy for TryFromSliceError
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Implementors

impl Copy for std::cmp::Ordering
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impl Copy for ErrorKind
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impl Copy for SeekFrom
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impl Copy for IpAddr
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impl Copy for Ipv6MulticastScope
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impl Copy for Shutdown
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impl Copy for SocketAddr
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impl Copy for FpCategory
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impl Copy for SearchStep
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impl Copy for ParseError
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impl Copy for std::sync::atomic::Ordering
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impl Copy for RecvTimeoutError
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impl Copy for TryRecvError
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impl Copy for bool
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impl Copy for char
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impl Copy for f32
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impl Copy for f64
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impl Copy for i128
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impl Copy for i16
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impl Copy for i32
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impl Copy for i64
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impl Copy for i8
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impl Copy for isize
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impl Copy for !
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impl Copy for u128
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impl Copy for u16
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impl Copy for u32
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impl Copy for u64
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impl Copy for u8
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impl Copy for usize
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impl Copy for Global
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impl Copy for Layout
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impl Copy for TypeId
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impl Copy for CharTryFromError
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impl Copy for UnicodeVersion
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impl Copy for Error
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impl Copy for FileType
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impl Copy for Pinned
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impl Copy for Ipv4Addr
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impl Copy for Ipv6Addr
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impl Copy for SocketAddrV4
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impl Copy for SocketAddrV6
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impl Copy for NonZeroU128
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impl Copy for NonZeroU16
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impl Copy for NonZeroU32
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impl Copy for NonZeroU64
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impl Copy for NonZeroU8
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impl Copy for NonZeroUsize
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impl Copy for TryFromIntError
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impl Copy for RangeFull
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impl Copy for NoneError
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impl Copy for ExitCode
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impl Copy for ExitStatus
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impl Copy for TraitObject
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impl Copy for Utf8Error
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impl Copy for RecvError
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impl Copy for WaitTimeoutResult
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impl Copy for ThreadId
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impl Copy for Duration
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impl Copy for Instant
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impl Copy for SystemTime
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impl<'_, T> Copy for &'_ T where
    T: ?Sized
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impl<'a> Copy for Component<'a>
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impl<'a> Copy for Prefix<'a>
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impl<'a> Copy for Arguments<'a>
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impl<'a> Copy for Ancestors<'a>
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impl<'a> Copy for PrefixComponent<'a>
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impl<F> Copy for RepeatWith<F> where
    F: Copy
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impl<Idx> Copy for RangeTo<Idx> where
    Idx: Copy
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impl<Idx> Copy for RangeToInclusive<Idx> where
    Idx: Copy
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impl<P> Copy for Pin<P> where
    P: Copy
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impl<T> Copy for Bound<T> where
    T: Copy
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impl<T> Copy for Option<T> where
    T: Copy
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impl<T> Copy for Poll<T> where
    T: Copy
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impl<T> Copy for *const T where
    T: ?Sized
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impl<T> Copy for *mut T where
    T: ?Sized
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impl<T> Copy for Reverse<T> where
    T: Copy
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impl<T> Copy for PhantomData<T> where
    T: ?Sized
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impl<T> Copy for Discriminant<T>
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impl<T> Copy for ManuallyDrop<T> where
    T: Copy + ?Sized
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impl<T> Copy for Wrapping<T> where
    T: Copy
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impl<T> Copy for NonNull<T> where
    T: ?Sized
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impl<T, E> Copy for Result<T, E> where
    E: Copy,
    T: Copy
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impl<T: Copy> Copy for TrySendError<T>
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impl<T: Copy> Copy for SendError<T>
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impl<Y, R> Copy for GeneratorState<Y, R> where
    R: Copy,
    Y: Copy
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