1.0.0[−][src]Trait nom::lib::std::prelude::v1::v1::Copy
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 implementCopy
(even if the referent doesn't), while variables captured by mutable reference never implementCopy
.
Implementations on Foreign Types
impl Copy for Ipv6MulticastScope
[src]
impl Copy for RecvTimeoutError
[src]
impl Copy for IpAddr
[src]
impl Copy for ErrorKind
[src]
impl Copy for ExitStatus
[src]
impl Copy for Instant
[src]
impl Copy for SocketAddrV6
[src]
impl<'a> Copy for PrefixComponent<'a>
[src]
impl Copy for FileType
[src]
impl Copy for SocketAddr
[src]
impl<'a> Copy for ErrorIter<'a>
[src]
impl Copy for Ipv4Addr
[src]
impl<T> Copy for TrySendError<T> where
T: Copy,
[src]
T: Copy,
impl Copy for TryRecvError
[src]
impl<T> Copy for SendError<T> where
T: Copy,
[src]
T: Copy,
impl Copy for SystemTime
[src]
impl Copy for SeekFrom
[src]
impl Copy for WaitTimeoutResult
[src]
impl Copy for SocketAddrV4
[src]
impl Copy for RecvError
[src]
impl Copy for Shutdown
[src]
impl<'a> Copy for Prefix<'a>
[src]
impl Copy for Ipv6Addr
[src]
impl<'a> Copy for Ancestors<'a>
[src]
impl Copy for ExitCode
[src]
impl Copy for ThreadId
[src]
impl<'a> Copy for Component<'a>
[src]
impl Copy for __m512i
[src]
impl Copy for i32
[src]
impl Copy for TypeId
[src]
impl Copy for __m128d
[src]
impl Copy for NonZeroU64
[src]
impl<T> Copy for NonNull<T> where
T: ?Sized,
[src]
T: ?Sized,
impl Copy for u32
[src]
impl Copy for UnicodeVersion
[src]
impl Copy for __m512d
[src]
impl Copy for i128
[src]
impl Copy for __m256
[src]
impl Copy for __m512
[src]
impl Copy for NonZeroU32
[src]
impl Copy for u128
[src]
impl Copy for __m128
[src]
impl<T> Copy for Wrapping<T> where
T: Copy,
[src]
T: Copy,
impl Copy for NonZeroU128
[src]
impl<T> Copy for PhantomData<T> where
T: ?Sized,
[src]
T: ?Sized,
impl<T> Copy for *const T where
T: ?Sized,
[src]
T: ?Sized,
impl Copy for NonZeroU16
[src]
impl Copy for TryFromSliceError
[src]
impl Copy for u8
[src]
impl Copy for __m256d
[src]
impl Copy for NonZeroI16
[src]
impl Copy for f32
[src]
impl Copy for !
[src]
impl<T> Copy for Poll<T> where
T: Copy,
[src]
T: Copy,
impl Copy for NonZeroI64
[src]
impl Copy for char
[src]
impl Copy for NonZeroU8
[src]
impl Copy for TraitObject
[src]
impl Copy for f64
[src]
impl Copy for usize
[src]
impl Copy for NonZeroI8
[src]
impl Copy for bool
[src]
impl Copy for FpCategory
[src]
impl Copy for __m64
[src]
impl Copy for RawWakerVTable
[src]
impl Copy for __m256i
[src]
impl Copy for __m128i
[src]
impl Copy for u16
[src]
impl Copy for i8
[src]
impl Copy for TryFromIntError
[src]
impl Copy for i64
[src]
impl Copy for NonZeroIsize
[src]
impl Copy for Duration
[src]
impl<P> Copy for Pin<P> where
P: Copy,
[src]
P: Copy,
impl<'_, T> Copy for &'_ T where
T: ?Sized,
[src]
T: ?Sized,
impl Copy for i16
[src]
impl Copy for NonZeroI32
[src]
impl Copy for PhantomPinned
[src]
impl Copy for CharTryFromError
[src]
impl Copy for NonZeroUsize
[src]
impl Copy for CpuidResult
[src]
impl Copy for isize
[src]
impl Copy for NonZeroI128
[src]
impl<T> Copy for *mut T where
T: ?Sized,
[src]
T: ?Sized,
impl Copy for Ordering
[src]
impl Copy for u64
[src]
impl Copy for stack_t
impl Copy for mach_header_64
impl Copy for xsw_usage
impl Copy for load_command
impl Copy for servent
impl Copy for pthread_rwlock_t
impl Copy for semid_ds
impl Copy for sigevent
impl Copy for group
impl Copy for fsid_t
impl Copy for lconv
impl Copy for pthread_rwlockattr_t
impl Copy for sockaddr_ctl
impl Copy for msghdr
impl Copy for segment_command
impl Copy for timezone
impl Copy for sigaction
impl Copy for FILE
impl Copy for proc_taskinfo
impl Copy for dirent
impl Copy for glob_t
impl Copy for sockaddr_inarp
impl Copy for sockaddr
impl Copy for utimbuf
impl Copy for proc_threadinfo
impl Copy for if_nameindex
impl Copy for radvisory
impl Copy for pthread_condattr_t
impl Copy for timespec
impl Copy for pollfd
impl Copy for passwd
impl Copy for pthread_cond_t
impl Copy for protoent
impl Copy for xucred
impl Copy for pthread_attr_t
impl Copy for rlimit
impl Copy for stat
impl Copy for fstore_t
impl Copy for DIR
impl Copy for iovec
impl Copy for addrinfo
impl Copy for sembuf
impl Copy for fpos_t
impl Copy for locale_t
impl Copy for kevent64_s
impl Copy for termios
impl Copy for cmsghdr
impl Copy for itimerval
impl Copy for pthread_mutex_t
impl Copy for tm
impl Copy for aiocb
impl Copy for proc_bsdinfo
impl Copy for sigval
impl Copy for in_pktinfo
impl Copy for sockaddr_dl
impl Copy for dqblk
impl Copy for utmpx
impl Copy for bpf_hdr
impl Copy for in_addr
impl Copy for rusage
impl Copy for flock
impl Copy for ip_mreq
impl Copy for shmid_ds
impl Copy for ifaddrs
impl Copy for tms
impl Copy for ipc_perm
impl Copy for ipv6_mreq
impl Copy for if_data
impl Copy for timeval
impl Copy for if_msghdr
impl Copy for sockaddr_storage
impl Copy for sockaddr_un
impl Copy for winsize
impl Copy for kevent
impl Copy for proc_taskallinfo
impl Copy for sf_hdtr
impl Copy for siginfo_t
impl Copy for in6_addr
impl Copy for sockaddr_in
impl Copy for Dl_info
impl Copy for linger
impl Copy for fd_set
impl Copy for in6_pktinfo
impl Copy for timeval32
impl Copy for hostent
impl Copy for sockaddr_in6
impl Copy for pthread_mutexattr_t
impl Copy for mach_timebase_info
impl Copy for arphdr
impl Copy for utsname
impl Copy for statvfs
impl Copy for segment_command_64
impl Copy for semun
impl Copy for mach_header
impl Copy for statfs
impl Copy for _Unwind_Action
impl Copy for _Unwind_Reason_Code
impl<'t> Copy for Match<'t>
[src]
impl<'t> Copy for Match<'t>
[src]
impl Copy for MatchKind
impl Copy for ClassBytesRange
impl Copy for Span
impl Copy for ClassUnicodeRange
impl Copy for ClassSetBinaryOpKind
impl Copy for Position
impl Copy for Flag
impl Copy for Utf8Sequence
impl Copy for Utf8Range
impl Copy for RoundingKind
[src]
impl Copy for Error
[src]
impl Copy for ErrorCode
[src]
impl<T> Copy for Result<T> where
T: Copy,
[src]
T: Copy,
impl Copy for Void
impl Copy for Buffer
[src]
Loading content...
Implementors
impl Copy for System
[src]
impl Copy for SearchStep
[src]
impl Copy for NoneError
[src]
impl Copy for nom::lib::std::cmp::Ordering
[src]
impl Copy for Infallible
[src]
impl Copy for nom::lib::std::fmt::Error
[src]
impl Copy for RangeFull
[src]
impl Copy for Layout
[src]
impl Copy for Global
[src]
impl Copy for nom::error::ErrorKind
[src]
impl Copy for Needed
[src]
impl Copy for Endianness
[src]
impl Copy for Utf8Error
[src]
impl<'a> Copy for Arguments<'a>
[src]
impl<F> Copy for OnceWith<F> where
F: Copy,
[src]
F: Copy,
impl<F> Copy for RepeatWith<F> where
F: Copy,
[src]
F: Copy,
impl<Idx> Copy for RangeTo<Idx> where
Idx: Copy,
[src]
Idx: Copy,
impl<Idx> Copy for RangeToInclusive<Idx> where
Idx: Copy,
[src]
Idx: Copy,
impl<T> Copy for Discriminant<T>
[src]
impl<T> Copy for Bound<T> where
T: Copy,
[src]
T: Copy,
impl<T> Copy for Option<T> where
T: Copy,
[src]
T: Copy,
impl<T> Copy for Reverse<T> where
T: Copy,
[src]
T: Copy,
impl<T> Copy for ManuallyDrop<T> where
T: Copy + ?Sized,
[src]
T: Copy + ?Sized,
impl<T> Copy for MaybeUninit<T> where
T: Copy,
[src]
T: Copy,
impl<T, E> Copy for nom::lib::std::prelude::v1::v1::Result<T, E> where
E: Copy,
T: Copy,
[src]
E: Copy,
T: Copy,
impl<Y, R> Copy for GeneratorState<Y, R> where
R: Copy,
Y: Copy,
[src]
R: Copy,
Y: Copy,