Trait concordium_std::marker::Copy 1.0.0[−][src]
pub trait Copy: Clone { }
Expand description
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
:
#[derive(Copy, Clone)] 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`
Shared references (&T
) are also Copy
, so a type can be Copy
, even when it holds
shared references of types T
that are not Copy
. Consider the following struct,
which can implement Copy
, because it only holds a shared reference to our non-Copy
type PointList
from above:
#[derive(Copy, Clone)] struct PointListWrapper<'a> { point_list_ref: &'a PointList, }
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 implementCopy
(even if the referent doesn’t), while variables captured by mutable reference never implementCopy
.
Implementations on Foreign Types
impl Copy for SocketAddr
[src]
impl Copy for ExitStatus
[src]
impl<'a> Copy for IoSlice<'a>
[src]
impl Copy for SystemTime
[src]
impl Copy for ThreadId
[src]
impl<T> Copy for SendError<T> where
T: Copy,
[src]
T: Copy,
impl<T> Copy for TrySendError<T> where
T: Copy,
[src]
T: Copy,
impl Copy for SocketAddrV4
[src]
impl Copy for SocketAddrV6
[src]
impl Copy for FileType
[src]
impl<'a> Copy for Ancestors<'a>
[src]
impl Copy for RecvError
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impl Copy for RecvTimeoutError
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impl Copy for Instant
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impl<'a> Copy for Component<'a>
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impl Copy for Ipv6Addr
[src]
impl Copy for TryRecvError
[src]
impl Copy for Shutdown
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impl Copy for Ipv6MulticastScope
[src]
impl<'a> Copy for PrefixComponent<'a>
[src]
impl Copy for ExitCode
[src]
impl Copy for SeekFrom
[src]
impl Copy for UCred
[src]
impl<'a> Copy for Prefix<'a>
[src]
impl Copy for Ipv4Addr
[src]
impl Copy for AccessError
[src]
impl Copy for ErrorKind
[src]
impl Copy for WaitTimeoutResult
[src]
impl Copy for System
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impl Copy for IpAddr
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impl Copy for AllocError
[src]
impl Copy for __m512bh
[src]
impl Copy for __m256d
[src]
impl Copy for i128
[src]
impl Copy for !
[src]
impl Copy for usize
[src]
impl Copy for isize
[src]
impl Copy for __m256bh
[src]
impl Copy for __m256i
[src]
impl Copy for i32
[src]
impl Copy for __m128bh
[src]
impl<Dyn> Copy for DynMetadata<Dyn> where
Dyn: ?Sized,
[src]
Dyn: ?Sized,
impl Copy for TraitObject
[src]
impl Copy for u32
[src]
impl Copy for u128
[src]
impl Copy for __m128
[src]
impl Copy for __m128d
[src]
impl<T, E> Copy for Result<T, E> where
T: Copy,
E: Copy,
[src]
T: Copy,
E: Copy,
impl Copy for CpuidResult
[src]
impl Copy for u64
[src]
impl Copy for u8
[src]
impl<T> Copy for *const T where
T: ?Sized,
[src]
T: ?Sized,
impl<Idx> Copy for RangeTo<Idx> where
Idx: Copy,
[src]
Idx: Copy,
impl<T> Copy for NonNull<T> where
T: ?Sized,
[src]
T: ?Sized,
impl<'_, T> Copy for &'_ T where
T: ?Sized,
[src]
T: ?Sized,
Shared references can be copied, but mutable references cannot!
impl Copy for RangeFull
[src]
impl Copy for TryFromSliceError
[src]
impl<T> Copy for Option<T> where
T: Copy,
[src]
T: Copy,
impl Copy for CharTryFromError
[src]
impl Copy for __m128i
[src]
impl Copy for Ordering
[src]
impl Copy for Error
[src]
impl<'a> Copy for Location<'a>
[src]
impl<T> Copy for Poll<T> where
T: Copy,
[src]
T: Copy,
impl<'a, T, const N: usize> Copy for ArrayWindows<'a, T, N> where
T: 'a + Copy,
[src]
T: 'a + Copy,
impl Copy for i8
[src]
impl Copy for RawWakerVTable
[src]
impl Copy for u16
[src]
impl Copy for Ordering
[src]
impl<T> Copy for Reverse<T> where
T: Copy,
[src]
T: Copy,
impl Copy for SearchStep
[src]
impl Copy for Utf8Error
[src]
impl Copy for f32
[src]
impl<B, C> Copy for ControlFlow<B, C> where
C: Copy,
B: Copy,
[src]
C: Copy,
B: Copy,
impl Copy for __m512i
[src]
impl Copy for __m512
[src]
impl<Y, R> Copy for GeneratorState<Y, R> where
R: Copy,
Y: Copy,
[src]
R: Copy,
Y: Copy,
impl<T> Copy for *mut T where
T: ?Sized,
[src]
T: ?Sized,
impl Copy for i64
[src]
impl<F> Copy for RepeatWith<F> where
F: Copy,
[src]
F: Copy,
impl<'a> Copy for Arguments<'a>
[src]
impl Copy for f64
[src]
impl Copy for __m512d
[src]
impl Copy for __m256
[src]
impl Copy for Duration
[src]
impl<Idx> Copy for RangeToInclusive<Idx> where
Idx: Copy,
[src]
Idx: Copy,
impl Copy for NoneError
[src]
impl Copy for bool
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impl Copy for Layout
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impl<P> Copy for Pin<P> where
P: Copy,
[src]
P: Copy,
impl Copy for char
[src]
impl Copy for TypeId
[src]
impl Copy for i16
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impl Copy for Global
[src]
impl Copy for _Unwind_Reason_Code
impl Copy for _Unwind_Action
impl Copy for Bytes
impl Copy for Words
impl Copy for Pages
impl Copy for Words
impl Copy for Pages
impl Copy for mach_header
impl Copy for lconv
impl Copy for glob_t
impl Copy for processor_cpu_load_info
impl Copy for linger
impl Copy for sockaddr_inarp
impl Copy for aiocb
impl Copy for utimbuf
impl Copy for timex
impl Copy for semid_ds
impl Copy for segment_command_64
impl Copy for sockaddr_dl
impl Copy for utmpx
impl Copy for utsname
impl Copy for sigval
impl Copy for passwd
impl Copy for sembuf
impl Copy for ipc_perm
impl Copy for mach_header_64
impl Copy for if_data
impl Copy for ntptimeval
impl Copy for ucontext_t
impl Copy for sockaddr
impl Copy for ipv6_mreq
impl Copy for statvfs
impl Copy for group
impl Copy for servent
impl Copy for fd_set
impl Copy for bpf_hdr
impl Copy for statfs
impl Copy for proc_taskinfo
impl Copy for proc_threadinfo
impl Copy for sa_endpoints_t
impl Copy for fpos_t
impl Copy for radvisory
impl Copy for timeval32
impl Copy for stack_t
impl Copy for ip_mreq
impl Copy for segment_command
impl Copy for __darwin_x86_thread_state64
impl Copy for fsid_t
impl Copy for in_addr
impl Copy for cmsghdr
impl Copy for load_command
impl Copy for hostent
impl Copy for in6_addr
impl Copy for pthread_cond_t
impl Copy for pthread_mutex_t
impl Copy for pthread_rwlock_t
impl Copy for tms
impl Copy for winsize
impl Copy for sockaddr_in6
impl Copy for __darwin_mmst_reg
impl Copy for termios
impl Copy for pthread_mutexattr_t
impl Copy for kevent
impl Copy for timeval
impl Copy for siginfo_t
impl Copy for in_pktinfo
impl Copy for sockaddr_ctl
impl Copy for __darwin_xmm_reg
impl Copy for tm
impl Copy for processor_set_basic_info
impl Copy for sockaddr_storage
impl Copy for pthread_attr_t
impl Copy for __darwin_mcontext64
impl Copy for xucred
impl Copy for sigaction
impl Copy for rusage
impl Copy for if_msghdr
impl Copy for pollfd
impl Copy for semun
impl Copy for ifaddrs
impl Copy for arphdr
impl Copy for rlimit
impl Copy for in6_pktinfo
impl Copy for proc_bsdinfo
impl Copy for protoent
impl Copy for regex_t
impl Copy for flock
impl Copy for __darwin_x86_exception_state64
impl Copy for proc_taskallinfo
impl Copy for regmatch_t
impl Copy for timespec
impl Copy for __darwin_x86_float_state64
impl Copy for xsw_usage
impl Copy for pthread_rwlockattr_t
impl Copy for pthread_condattr_t
impl Copy for fstore_t
impl Copy for msghdr
impl Copy for stat
impl Copy for processor_set_load_info
impl Copy for timezone
impl Copy for sockaddr_un
impl Copy for FILE
impl Copy for kevent64_s
impl Copy for iovec
impl Copy for processor_basic_info
impl Copy for shmid_ds
impl Copy for dqblk
impl Copy for sockaddr_in
impl Copy for sf_hdtr
impl Copy for sigevent
impl Copy for max_align_t
impl Copy for mach_timebase_info
impl Copy for itimerval
impl Copy for addrinfo
impl Copy for if_nameindex
impl Copy for dirent
impl Copy for Dl_info
impl Copy for DIR
Loading content...Implementors
impl Copy for Infallible
1.34.0[src]
impl Copy for Address
[src]
impl Copy for AmountParseError
[src]
impl Copy for LogError
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impl Copy for FpCategory
[src]
impl Copy for NonZeroI8
1.34.0[src]
impl Copy for NonZeroI16
1.34.0[src]
impl Copy for NonZeroI32
1.34.0[src]
impl Copy for NonZeroI64
1.34.0[src]
impl Copy for NonZeroI128
1.34.0[src]
impl Copy for NonZeroIsize
1.34.0[src]
impl Copy for NonZeroU8
1.28.0[src]
impl Copy for NonZeroU16
1.28.0[src]
impl Copy for NonZeroU32
1.28.0[src]
impl Copy for NonZeroU64
1.28.0[src]
impl Copy for NonZeroU128
1.28.0[src]
impl Copy for NonZeroUsize
1.28.0[src]
impl Copy for TryFromIntError
1.34.0[src]
impl Copy for AccountAddress
[src]
impl Copy for Amount
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impl Copy for AttributeTag
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impl Copy for ContractAddress
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impl Copy for concordium_std::Duration
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impl Copy for Timestamp
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impl Copy for PhantomPinned
1.33.0[src]
impl<'a> Copy for ContractName<'a>
[src]
impl<'a> Copy for ReceiveName<'a>
[src]
impl<T> Copy for Bound<T> where
T: Copy,
1.17.0[src]
T: Copy,
impl<T> Copy for Discriminant<T>
1.21.0[src]
impl<T> Copy for ManuallyDrop<T> where
T: Copy + ?Sized,
1.20.0[src]
T: Copy + ?Sized,
impl<T> Copy for Wrapping<T> where
T: Copy,
[src]
T: Copy,
impl<T> Copy for PhantomData<T> where
T: ?Sized,
[src]
T: ?Sized,
impl<T> Copy for MaybeUninit<T> where
T: Copy,
1.36.0[src]
T: Copy,