Trait cretonne::prelude::codegen::entity::__core::prelude::v1::Copy1.0.0[−][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 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<T> Copy for SendError<T> where
    T: Copy, 
[src]

impl Copy for Ipv4Addr
[src]

impl Copy for Ipv6Addr
[src]

impl Copy for Shutdown
[src]

impl Copy for ExitCode
[src]

impl Copy for RecvTimeoutError
[src]

impl Copy for SeekFrom
[src]

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

impl Copy for ErrorKind
[src]

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

impl Copy for RecvError
[src]

impl Copy for SocketAddr
[src]

impl Copy for ThreadId
[src]

impl Copy for ExitStatus
[src]

impl Copy for Ipv6MulticastScope
[src]

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

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

impl Copy for IpAddr
[src]

impl Copy for SocketAddrV4
[src]

impl Copy for SocketAddrV6
[src]

impl Copy for WaitTimeoutResult
[src]

impl Copy for SystemTime
[src]

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

impl Copy for TryRecvError
[src]

impl Copy for FileType
[src]

impl Copy for Instant
[src]

impl Copy for u8
[src]

impl Copy for usize
[src]

impl Copy for u128
[src]

impl Copy for isize
[src]

impl Copy for i32
[src]

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

impl Copy for char
[src]

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

impl Copy for i8
[src]

impl Copy for i128
[src]

impl Copy for !
[src]

impl Copy for f64
[src]

impl Copy for i16
[src]

impl Copy for u32
[src]

impl Copy for f32
[src]

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

impl Copy for i64
[src]

impl Copy for u16
[src]

impl Copy for u64
[src]

impl Copy for bool
[src]

impl Copy for Global
[src]

impl Copy for ParseError
[src]

impl Copy for stat
[src]

impl Copy for tm
[src]

impl Copy for timeval
[src]

impl Copy for timespec
[src]

impl Copy for utimbuf
[src]

impl<E> Copy for Compat<E> where
    E: Copy, 

impl Copy for BinaryFormat

impl Copy for PointerWidth

impl Copy for OperatingSystem

impl Copy for Environment

impl Copy for Architecture

impl Copy for Vendor

impl Copy for Endianness

Implementors