Trait druid::piet::cairo::glib::bitflags::_core::marker::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` 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)
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.

impl<T> Copy for &Twhere T: ?Sized,

impl<const CAP: usize> Copy for ArrayString<CAP>

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Trait druid::piet::cairo::glib::bitflags::_core::marker::Copy

Implementors§

impl Copy for Code

impl Copy for druid::Color

impl Copy for Handled

impl Copy for druid::Location

impl Copy for MouseButton

impl Copy for RawMods

impl Copy for RawWindowHandle

impl Copy for SysMods

impl Copy for Target

impl Copy for WindowSizePolicy

impl Copy for druid::WindowState

impl Copy for BarHeldState

impl Copy for BarHoveredState

impl Copy for ScrollbarsEnabled

impl Copy for druid::text::Direction

impl Copy for Event

impl Copy for Movement

impl Copy for Action

impl Copy for VerticalMovement

impl Copy for WritingDirection

impl Copy for Axis

impl Copy for CrossAxisAlignment

impl Copy for FillStrat

impl Copy for KnobStyle

impl Copy for LineBreaking

impl Copy for MainAxisAlignment

impl Copy for TabsEdge

impl Copy for TabsTransition

impl Copy for FontStyle

impl Copy for druid::piet::ImageFormat

impl Copy for InterpolationMode

impl Copy for druid::piet::LineCap

impl Copy for druid::piet::LineJoin

impl Copy for TextAlignment

impl Copy for druid::piet::image_crate::ColorType

impl Copy for ExtendedColorType

impl Copy for druid::piet::image_crate::ImageFormat

impl Copy for druid::piet::image_crate::flat::Error

impl Copy for NormalForm

impl Copy for druid::piet::image_crate::imageops::FilterType

impl Copy for druid::piet::kurbo::PathEl

impl Copy for druid::piet::kurbo::PathSeg

impl Copy for Antialias

impl Copy for Content

impl Copy for druid::piet::cairo::DeviceType

impl Copy for druid::piet::cairo::Error

impl Copy for Extend

impl Copy for druid::piet::cairo::FillRule

impl Copy for Filter

impl Copy for FontSlant

impl Copy for FontType

impl Copy for druid::piet::cairo::FontWeight

impl Copy for druid::piet::cairo::Format

impl Copy for HintMetrics

impl Copy for HintStyle

impl Copy for druid::piet::cairo::LineCap

impl Copy for druid::piet::cairo::LineJoin

impl Copy for MeshCorner

impl Copy for Operator

impl Copy for PathDataType

impl Copy for druid::piet::cairo::PathSegment

impl Copy for PatternType

impl Copy for RegionOverlap

impl Copy for SubpixelOrder

impl Copy for SurfaceType

impl Copy for TextClusterFlags

impl Copy for ChecksumType

impl Copy for ConvertError

impl Copy for DateMonth

impl Copy for DateWeekday

impl Copy for FileError

impl Copy for KeyFileError

impl Copy for LogLevel

impl Copy for LogWriterOutput

impl Copy for MarkupError

impl Copy for OptionArg

impl Copy for SeekType

impl Copy for TimeType