Trait std::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.
Implementors
impl Copy for Heapimpl<T> Copy for Bound<T> where
T: Copy,impl Copy for ExchangeHeapSingletonimpl Copy for ParseErrorimpl Copy for _Unwind_Reason_Codeimpl Copy for _Unwind_Stateimpl Copy for ChaChaRngimpl Copy for Expimpl Copy for Isaac64Rngimpl Copy for StandardNormalimpl Copy for Normalimpl Copy for LogNormalimpl Copy for ReseedWithDefaultimpl Copy for IsaacRngimpl Copy for Exp1impl Copy for glob_timpl Copy for ff_replayimpl Copy for msghdrimpl Copy for ff_ramp_effectimpl Copy for sem_timpl Copy for sockaddrimpl Copy for ff_condition_effectimpl Copy for addrinfoimpl Copy for rlimitimpl Copy for passwdimpl Copy for stat64impl Copy for itimervalimpl Copy for dqblkimpl Copy for utimbufimpl Copy for pthread_cond_timpl Copy for ifaddrsimpl Copy for tmimpl Copy for groupimpl Copy for ff_effectimpl Copy for pthread_mutex_timpl Copy for tmsimpl Copy for mmsghdrimpl Copy for if_nameindeximpl Copy for itimerspecimpl Copy for input_absinfoimpl Copy for sockaddr_nlimpl Copy for statfsimpl Copy for flockimpl Copy for sockaddr_in6impl Copy for rusageimpl Copy for ff_envelopeimpl Copy for pollfdimpl Copy for ipv6_mreqimpl Copy for ip_mreqimpl Copy for ff_constant_effectimpl Copy for lconvimpl Copy for fsid_timpl Copy for winsizeimpl Copy for input_eventimpl Copy for __timevalimpl Copy for pthread_rwlockattr_timpl Copy for sockaddr_inimpl Copy for sched_paramimpl Copy for cpu_set_timpl Copy for ff_periodic_effectimpl Copy for sysinfoimpl Copy for termiosimpl Copy for pthread_condattr_timpl Copy for timevalimpl Copy for sigactionimpl Copy for iovecimpl Copy for statvfs64impl Copy for rlimit64impl Copy for sockaddr_llimpl Copy for signalfd_siginfoimpl Copy for __exit_statusimpl Copy for input_idimpl Copy for ff_rumble_effectimpl Copy for dirent64impl Copy for in6_addrimpl Copy for statvfsimpl Copy for statfs64impl Copy for Dl_infoimpl Copy for pthread_mutexattr_timpl Copy for input_maskimpl Copy for timespecimpl Copy for utmpximpl Copy for siginfo_timpl Copy for pthread_attr_timpl Copy for in_addrimpl Copy for ff_triggerimpl Copy for fd_setimpl Copy for ipc_permimpl Copy for glob64_timpl Copy for sockaddr_storageimpl Copy for hostentimpl Copy for sigvalimpl Copy for pthread_rwlock_timpl Copy for cmsghdrimpl Copy for mq_attrimpl Copy for msginfoimpl Copy for sigeventimpl Copy for spwdimpl Copy for stack_timpl Copy for input_keymap_entryimpl Copy for aiocbimpl Copy for sockaddr_unimpl Copy for msqid_dsimpl Copy for utsnameimpl Copy for ucredimpl Copy for direntimpl Copy for statimpl Copy for shmid_dsimpl Copy for sigset_timpl Copy for epoll_eventimpl Copy for sembufimpl Copy for lingerimpl<Idx> Copy for RangeTo<Idx> where
Idx: Copy,impl Copy for std::cmp::Orderingimpl<T> Copy for NonZero<T> where
T: Zeroable + Copy,impl<T> Copy for Option<T> where
T: Copy,impl<T> Copy for Wrapping<T> where
T: Copy,impl Copy for SearchStepimpl Copy for RangeFullimpl Copy for CharTryFromErrorimpl<T> Copy for Shared<T> where
T: ?Sized,impl<T> Copy for PhantomData<T> where
T: ?Sized,impl<'a> Copy for Arguments<'a>impl Copy for Errorimpl<T, E> Copy for Result<T, E> where
E: Copy,
T: Copy,impl<Idx> Copy for RangeToInclusive<Idx> where
Idx: Copy,impl Copy for TypeIdimpl<T> Copy for Unique<T> where
T: ?Sized,impl Copy for TraitObjectimpl Copy for Utf8Errorimpl Copy for FpCategoryimpl Copy for std::sync::atomic::Orderingimpl Copy for TryFromIntErrorimpl<T> Copy for Discriminant<T>impl Copy for LocalKeyStateimpl Copy for ThreadIdimpl Copy for FileTypeimpl Copy for ErrorKindimpl Copy for SeekFromimpl Copy for IpAddrimpl Copy for Ipv4Addrimpl Copy for Ipv6Addrimpl Copy for Ipv6MulticastScopeimpl Copy for SocketAddrimpl Copy for SocketAddrV4impl Copy for SocketAddrV6impl Copy for Shutdownimpl<'a> Copy for Prefix<'a>impl<'a> Copy for PrefixComponent<'a>impl<'a> Copy for Component<'a>impl Copy for ExitStatusimpl<T: Copy> Copy for SendError<T>impl Copy for RecvErrorimpl Copy for TryRecvErrorimpl Copy for RecvTimeoutErrorimpl<T: Copy> Copy for TrySendError<T>impl Copy for WaitTimeoutResultimpl Copy for Durationimpl Copy for Instantimpl Copy for SystemTime