1.0.0[−][src]Trait std::prelude::v1::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 u64`[src]

`impl Copy for u32`[src]

`impl Copy for !`[src]

`impl Copy for f64`[src]

`impl Copy for i8`[src]

`impl Copy for isize`[src]

`impl<'_, T> Copy for &'_ T where T: ?Sized,` [src]

`impl Copy for u128`[src]

`impl<T> Copy for *mut T where T: ?Sized,` [src]

`impl Copy for u8`[src]

`impl Copy for i64`[src]

`impl Copy for usize`[src]

`impl Copy for char`[src]

`impl Copy for f32`[src]

`impl Copy for i128`[src]

`impl Copy for u16`[src]

`impl Copy for bool`[src]

`impl Copy for i16`[src]

`impl Copy for i32`[src]

`impl<T> Copy for *const T where T: ?Sized,` [src]

`impl Copy for epoll_event`

`impl Copy for protoent`

`impl Copy for dirent`

`impl Copy for user_regs_struct`

`impl Copy for sockaddr_in`

`impl Copy for ipc_perm`

`impl Copy for stat`

`impl Copy for sigaction`

`impl Copy for rusage`

`impl Copy for passwd`

`impl Copy for packet_mreq`

`impl Copy for fd_set`

`impl Copy for Elf32_Shdr`

`impl Copy for ucred`

`impl Copy for sched_param`

`impl Copy for fpos64_t`

`impl Copy for Elf32_Sym`

`impl Copy for nl_mmap_req`

`impl Copy for sysinfo`

`impl Copy for aiocb`

`impl Copy for msginfo`

`impl Copy for arpreq_old`

`impl Copy for termios`

`impl Copy for signalfd_siginfo`

`impl Copy for Dl_info`

`impl Copy for flock`

`impl Copy for lconv`

`impl Copy for input_id`

`impl Copy for __exit_status`

`impl Copy for mcontext_t`

`impl Copy for genlmsghdr`

`impl Copy for spwd`

`impl Copy for Elf64_Phdr`

`impl Copy for linger`

`impl Copy for Elf64_Sym`

`impl Copy for iovec`

`impl Copy for Elf64_Chdr`

`impl Copy for sigset_t`

`impl Copy for sockaddr_nl`

`impl Copy for DIR`

`impl Copy for timezone`

`impl Copy for msqid_ds`

`impl Copy for fsid_t`

`impl Copy for sembuf`

`impl Copy for in6_addr`

`impl Copy for Elf64_Ehdr`

`impl Copy for in6_pktinfo`

`impl Copy for sockaddr_alg`

`impl Copy for winsize`

`impl Copy for in6_rtmsg`

`impl Copy for arphdr`

`impl Copy for mallinfo`

`impl Copy for stat64`

`impl Copy for nlmsghdr`

`impl Copy for sigevent`

`impl Copy for msghdr`

`impl Copy for utimbuf`

`impl Copy for statfs`

`impl Copy for FILE`

`impl Copy for rtentry`

`impl Copy for input_event`

`impl Copy for Elf32_Ehdr`

`impl Copy for arpd_request`

`impl Copy for posix_spawn_file_actions_t`

`impl Copy for fpos_t`

`impl Copy for sockaddr_ll`

`impl Copy for ff_replay`

`impl Copy for arpreq`

`impl Copy for __timeval`

`impl Copy for statx_timestamp`

`impl Copy for itimerspec`

`impl Copy for user`

`impl Copy for stack_t`

`impl Copy for _libc_xmmreg`

`impl Copy for sockaddr`

`impl Copy for ff_ramp_effect`

`impl Copy for nlmsgerr`

`impl Copy for sigval`

`impl Copy for Elf64_Shdr`

`impl Copy for dl_phdr_info`

`impl Copy for servent`

`impl Copy for cmsghdr`

`impl Copy for input_absinfo`

`impl Copy for statx`

`impl Copy for statfs64`

`impl Copy for mq_attr`

`impl Copy for group`

`impl Copy for nlattr`

`impl Copy for pthread_condattr_t`

`impl Copy for dqblk`

`impl Copy for in_pktinfo`

`impl Copy for pthread_mutexattr_t`

`impl Copy for termios2`

`impl Copy for utsname`

`impl Copy for ff_rumble_effect`

`impl Copy for dirent64`

`impl Copy for ff_trigger`

`impl Copy for mmsghdr`

`impl Copy for nl_mmap_hdr`

`impl Copy for hostent`

`impl Copy for _libc_fpstate`

`impl Copy for ff_periodic_effect`

`impl Copy for pollfd`

`impl Copy for ipv6_mreq`

`impl Copy for inotify_event`

`impl Copy for sem_t`

`impl Copy for ff_effect`

`impl Copy for user_fpregs_struct`

`impl Copy for mntent`

`impl Copy for itimerval`

`impl Copy for in_addr`

`impl Copy for statvfs`

`impl Copy for siginfo_t`

`impl Copy for sockaddr_in6`

`impl Copy for af_alg_iv`

`impl Copy for ff_constant_effect`

`impl Copy for pthread_cond_t`

`impl Copy for tm`

`impl Copy for _libc_fpxreg`

`impl Copy for ifaddrs`

`impl Copy for rlimit`

`impl Copy for cpu_set_t`

`impl Copy for Elf32_Phdr`

`impl Copy for glob64_t`

`impl Copy for timespec`

`impl Copy for addrinfo`

`impl Copy for posix_spawnattr_t`

`impl Copy for sockaddr_vm`

`impl Copy for input_mask`

`impl Copy for ff_condition_effect`

`impl Copy for statvfs64`

`impl Copy for tms`

`impl Copy for utmpx`

`impl Copy for pthread_rwlockattr_t`

`impl Copy for sockaddr_storage`

`impl Copy for pthread_rwlock_t`

`impl Copy for pthread_mutex_t`

`impl Copy for input_keymap_entry`

`impl Copy for shmid_ds`

`impl Copy for sockaddr_un`

`impl Copy for glob_t`

`impl Copy for ff_envelope`

`impl Copy for if_nameindex`

`impl Copy for ip_mreq`

`impl Copy for nl_pktinfo`

`impl Copy for pthread_attr_t`

`impl Copy for ucontext_t`

`impl Copy for rlimit64`

`impl Copy for timeval`

`impl Copy for Elf32_Chdr`

`impl Copy for _Unwind_Action`

`impl Copy for _Unwind_Reason_Code`

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Implementors

`impl Copy for ExitCode`[src]

`impl Copy for SocketAddr`[src]

`impl Copy for RecvError`[src]

`impl Copy for IpAddr`[src]

`impl Copy for System`[src]

`impl Copy for ThreadId`[src]

`impl Copy for ExitStatus`[src]

`impl Copy for Shutdown`[src]

`impl Copy for ErrorKind`[src]

`impl Copy for FileType`[src]

`impl Copy for WaitTimeoutResult`[src]

`impl Copy for Instant`[src]

`impl Copy for RecvTimeoutError`[src]

`impl Copy for SystemTime`[src]

`impl Copy for TryRecvError`[src]

`impl Copy for SeekFrom`[src]

`impl Copy for AccessError`[src]

`impl Copy for Infallible`[src]

`impl Copy for Duration`[src]

`impl Copy for NonZeroUsize`[src]

`impl Copy for RangeFull`[src]

`impl Copy for std::cmp::Ordering`[src]

`impl Copy for NoneError`[src]

`impl Copy for TryFromIntError`[src]

`impl Copy for CharTryFromError`[src]

`impl Copy for FpCategory`[src]

`impl Copy for PhantomPinned`[src]

`impl Copy for NonZeroIsize`[src]

`impl Copy for TraitObject`[src]

`impl Copy for TypeId`[src]

`impl Copy for Error`[src]

`impl Copy for SearchStep`[src]

`impl Copy for UnicodeVersion`[src]

`impl Copy for Layout`[src]

`impl Copy for RawWakerVTable`[src]

`impl Copy for std::sync::atomic::Ordering`[src]

`impl Copy for TryFromSliceError`[src]

`impl Copy for Global`[src]

`impl Copy for Ipv6MulticastScope`[src]

`impl Copy for CpuidResult`[src]

`impl Copy for __m128`[src]

`impl Copy for __m128d`[src]

`impl Copy for __m128i`[src]

`impl Copy for __m256`[src]

`impl Copy for __m256d`[src]

`impl Copy for __m256i`[src]

`impl Copy for __m512`[src]

`impl Copy for __m512d`[src]

`impl Copy for __m512i`[src]

`impl Copy for __m64`[src]

`impl Copy for Ipv4Addr`[src]

`impl Copy for Ipv6Addr`[src]

`impl Copy for SocketAddrV4`[src]

`impl Copy for SocketAddrV6`[src]

`impl Copy for NonZeroI128`[src]

`impl Copy for NonZeroI16`[src]

`impl Copy for NonZeroI32`[src]

`impl Copy for NonZeroI64`[src]

`impl Copy for NonZeroI8`[src]

`impl Copy for NonZeroU128`[src]

`impl Copy for NonZeroU16`[src]

`impl Copy for NonZeroU32`[src]

`impl Copy for NonZeroU64`[src]

`impl Copy for NonZeroU8`[src]

`impl Copy for Utf8Error`[src]

`impl<'a> Copy for ErrorIter<'a>`[src]

`impl<'a> Copy for Ancestors<'a>`[src]

`impl<'a> Copy for PrefixComponent<'a>`[src]

`impl<'a> Copy for Prefix<'a>`[src]

`impl<'a> Copy for Component<'a>`[src]

`impl<'a> Copy for Arguments<'a>`[src]

`impl<F> Copy for OnceWith<F> where F: Copy,` [src]

`impl<F> Copy for RepeatWith<F> where F: Copy,` [src]

`impl<Idx> Copy for RangeTo<Idx> where Idx: Copy,` [src]

`impl<Idx> Copy for RangeToInclusive<Idx> where Idx: Copy,` [src]

`impl<P> Copy for Pin<P> where P: Copy,` [src]

`impl<T> Copy for Discriminant<T>`[src]

`impl<T> Copy for Bound<T> where T: Copy,` [src]

`impl<T> Copy for Option<T> where T: Copy,` [src]

`impl<T> Copy for TrySendError<T> where T: Copy,` [src]

`impl<T> Copy for Poll<T> where T: Copy,` [src]

`impl<T> Copy for Reverse<T> where T: Copy,` [src]

`impl<T> Copy for PhantomData<T> where T: ?Sized,` [src]

`impl<T> Copy for ManuallyDrop<T> where T: Copy + ?Sized,` [src]

`impl<T> Copy for Wrapping<T> where T: Copy,` [src]

`impl<T> Copy for NonNull<T> where T: ?Sized,` [src]

`impl<T> Copy for SendError<T> where T: Copy,` [src]

`impl<T> Copy for MaybeUninit<T> where T: Copy,` [src]

`impl<T, E> Copy for Result<T, E> where E: Copy, T: Copy,` [src]

`impl<Y, R> Copy for GeneratorState<Y, R> where R: Copy, Y: Copy,` [src]

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1.0.0[−][src]Trait std::prelude::v1::Copy

How can I implement Copy?

What's the difference between Copy and Clone?

When can my type be Copy?

When can't my type be Copy?

When should my type be Copy?

Additional implementors

Implementations on Foreign Types

impl Copy for u64[src]

impl Copy for u32[src]

impl Copy for ![src]

impl Copy for f64[src]

impl Copy for i8[src]

impl Copy for isize[src]

impl<'_, T> Copy for &'_ T where T: ?Sized, [src]

impl Copy for u128[src]

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

impl Copy for u8[src]

impl Copy for i64[src]

impl Copy for usize[src]

impl Copy for char[src]

impl Copy for f32[src]

impl Copy for i128[src]

impl Copy for u16[src]

impl Copy for bool[src]

impl Copy for i16[src]

impl Copy for i32[src]

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

impl Copy for epoll_event

impl Copy for protoent

impl Copy for dirent

impl Copy for user_regs_struct

impl Copy for sockaddr_in

impl Copy for ipc_perm

impl Copy for stat

impl Copy for sigaction

impl Copy for rusage

impl Copy for passwd

impl Copy for packet_mreq

impl Copy for fd_set

impl Copy for Elf32_Shdr

impl Copy for ucred

impl Copy for sched_param

impl Copy for fpos64_t

impl Copy for Elf32_Sym

impl Copy for nl_mmap_req

impl Copy for sysinfo

impl Copy for aiocb

impl Copy for msginfo

impl Copy for arpreq_old

impl Copy for termios

impl Copy for signalfd_siginfo

impl Copy for Dl_info

impl Copy for flock

impl Copy for lconv

impl Copy for input_id

impl Copy for __exit_status

impl Copy for mcontext_t

impl Copy for genlmsghdr

impl Copy for spwd

impl Copy for Elf64_Phdr

impl Copy for linger

impl Copy for Elf64_Sym

impl Copy for iovec

impl Copy for Elf64_Chdr

impl Copy for sigset_t

impl Copy for sockaddr_nl

impl Copy for DIR

impl Copy for timezone

impl Copy for msqid_ds

impl Copy for fsid_t

impl Copy for sembuf

impl Copy for in6_addr

impl Copy for Elf64_Ehdr

impl Copy for in6_pktinfo

impl Copy for sockaddr_alg

impl Copy for winsize

impl Copy for in6_rtmsg

impl Copy for arphdr

impl Copy for mallinfo