Trait flo_render_gl_offscreen::winapi::_core::prelude::rust_2015::Copy

1.0.0 · source ·

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` cannot 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)
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.

Trait flo_render_gl_offscreen::winapi::_core::prelude::rust_2015::Copy

Implementors§

impl Copy for flo_render_gl_offscreen::winapi::_core::cmp::Ordering

impl Copy for Infallible

impl Copy for flo_render_gl_offscreen::winapi::_core::fmt::Alignment

impl Copy for IpAddr

impl Copy for Ipv6MulticastScope

impl Copy for SocketAddr

impl Copy for FpCategory

impl Copy for Which

impl Copy for SearchStep

impl Copy for flo_render_gl_offscreen::winapi::_core::sync::atomic::Ordering

impl Copy for SeekFrom

impl Copy for ErrorKind

impl Copy for Shutdown

impl Copy for BacktraceStyle

impl Copy for RecvTimeoutError

impl Copy for TryRecvError

impl Copy for bool

impl Copy for char

impl Copy for f32

impl Copy for f64

impl Copy for i8

impl Copy for i16

impl Copy for i32

impl Copy for i64

impl Copy for i128

impl Copy for isize

impl Copy for !

impl Copy for u8

impl Copy for u16

impl Copy for u32

impl Copy for u64

impl Copy for u128

impl Copy for usize

impl Copy for DEVPROPCOMPKEY

impl Copy for DEVPROPERTY

impl Copy for DEVPROPKEY

impl Copy for GUID

impl Copy for KCRM_MARSHAL_HEADER

impl Copy for KCRM_PROTOCOL_BLOB

impl Copy for KCRM_TRANSACTION_BLOB

impl Copy for TRANSACTION_NOTIFICATION

impl Copy for TRANSACTION_NOTIFICATION_MARSHAL_ARGUMENT

impl Copy for TRANSACTION_NOTIFICATION_PROPAGATE_ARGUMENT

impl Copy for TRANSACTION_NOTIFICATION_RECOVERY_ARGUMENT

impl Copy for TRANSACTION_NOTIFICATION_SAVEPOINT_ARGUMENT

impl Copy for TRANSACTION_NOTIFICATION_TM_ONLINE_ARGUMENT

impl Copy for FILETIME

impl Copy for CSTRING

impl Copy for flo_render_gl_offscreen::winapi::shared::ntdef::FLOAT128

impl Copy for flo_render_gl_offscreen::winapi::shared::ntdef::GROUP_AFFINITY

impl Copy for LARGE_INTEGER_s

impl Copy for LARGE_INTEGER_u

impl Copy for flo_render_gl_offscreen::winapi::shared::ntdef::LIST_ENTRY32

impl Copy for flo_render_gl_offscreen::winapi::shared::ntdef::LIST_ENTRY64

impl Copy for flo_render_gl_offscreen::winapi::shared::ntdef::LIST_ENTRY

impl Copy for flo_render_gl_offscreen::winapi::shared::ntdef::OBJECTID

impl Copy for OBJECT_ATTRIBUTES32

impl Copy for OBJECT_ATTRIBUTES64

impl Copy for OBJECT_ATTRIBUTES

impl Copy for flo_render_gl_offscreen::winapi::shared::ntdef::PROCESSOR_NUMBER

impl Copy for QUAD

impl Copy for RTL_BALANCED_NODE

impl Copy for RTL_BALANCED_NODE_s

impl Copy for RTL_BALANCED_NODE_u

impl Copy for SINGLE_LIST_ENTRY32

impl Copy for flo_render_gl_offscreen::winapi::shared::ntdef::SINGLE_LIST_ENTRY

impl Copy for STRING32

impl Copy for STRING64

impl Copy for STRING

impl Copy for ULARGE_INTEGER_s

impl Copy for ULARGE_INTEGER_u

impl Copy for UNICODE_STRING

impl Copy for WNF_STATE_NAME

impl Copy for POINT

impl Copy for POINTL

impl Copy for POINTS

impl Copy for RECT

impl Copy for RECTL