Trait Copy

1.55.0 · 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:

#[derive(Clone)]
struct MyStruct<T>(T);

impl<T: Copy> Copy for MyStruct<T> { }

This isn’t always desired. For example, shared references (&T) can be copied regardless of whether T is Copy. Likewise, a generic struct containing markers such as PhantomData could potentially be duplicated with a bit-wise copy.

§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.

Dyn Compatibility§

This trait is not dyn compatible.

In older versions of Rust, dyn compatibility was called "object safety", so this trait is not object safe.

Copy

Trait Copy Copy item path

§How can I implement Copy?

§What’s the difference between Copy and Clone?

§When can my type be Copy?

§When can’t my type be Copy?

§When should my type be Copy?

§Additional implementors

Dyn Compatibility§

Implementors§

impl Copy for ProblemType

impl Copy for CoordinateSystem

impl Copy for TensorType

impl Copy for MonomialOrder

impl Copy for CheckpointingFrequency

impl Copy for ObserverMode

impl Copy for rssn::prelude::argmin::distr::BernoulliError

impl Copy for rssn::prelude::argmin::distr::uniform::Error

impl Copy for rssn::prelude::argmin::seq::WeightError

impl Copy for rssn::prelude::length::Units

impl Copy for rssn::prelude::mass::Units

impl Copy for rssn::prelude::nalgebra::TAffine

impl Copy for rssn::prelude::nalgebra::TGeneral

impl Copy for rssn::prelude::nalgebra::TProjective

impl Copy for rssn::prelude::ndarray::ErrorKind

impl Copy for Order

impl Copy for SliceInfoElem

impl Copy for rssn::prelude::num::Sign

impl Copy for rssn::prelude::rand::distributions::BernoulliError

impl Copy for WeightedError

impl Copy for FftDirection

impl Copy for CompressedStorage

impl Copy for FillInReduction

impl Copy for PermutationCheck

impl Copy for SymmetryCheck

impl Copy for StructureError

impl Copy for StructureErrorKind

impl Copy for SymmetryMode

impl Copy for NumKind

impl Copy for ThreadingStrategy

impl Copy for rssn::prelude::statrs::distribution::BetaError

impl Copy for BinomialError

impl Copy for CategoricalError

impl Copy for CauchyError

impl Copy for ChiError

impl Copy for DiracError

impl Copy for DirichletError

impl Copy for DiscreteUniformError

impl Copy for ExpError

impl Copy for FisherSnedecorError

impl Copy for GammaError

impl Copy for GeometricError

impl Copy for GumbelError

impl Copy for HypergeometricError

impl Copy for InverseGammaError

impl Copy for LaplaceError

impl Copy for LogNormalError

impl Copy for MultinomialError

impl Copy for MultivariateNormalError

impl Copy for MultivariateStudentError

impl Copy for NegativeBinomialError

impl Copy for NormalError

impl Copy for ParetoError

impl Copy for PoissonError

impl Copy for StudentsTError

impl Copy for rssn::prelude::statrs::distribution::TriangularError

impl Copy for UniformError

impl Copy for WeibullError

impl Copy for BetaFuncError

impl Copy for GammaFuncError

impl Copy for RankTieBreaker

impl Copy for Alternative

impl Copy for FishersExactTestError

impl Copy for TryFromError

impl Copy for rssn::prelude::time::Units

impl Copy for AsciiChar

impl Copy for core::cmp::Ordering

impl Copy for Infallible

impl Copy for FromBytesWithNulError