# Trait core::marker::Copy 1.0.0 [−] [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 valueRun

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!Run

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;Run

You can also implement `Copy` and `Clone` manually:

    struct MyStruct;

    impl Copy for MyStruct { }

    impl Clone for MyStruct {
        fn clone(&self) -> MyStruct {
            *self
        }
    }Run

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 need only 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,
    }Run

A struct can be `Copy`, and `i32` is `Copy`, therefore `Point` is eligible to be `Copy`. By contrast, consider

    struct PointList {
        points: Vec<Point>,
    }Run

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.

## Implementors

- `impl<T: Copy> Copy for Wrapping<T>`
- `impl Copy for FpCategory`
- `impl Copy for TryFromIntError`
- `impl<T> Copy for Discriminant<T>`
- `impl<T: Copy + Zeroable> Copy for NonZero<T>`
- `impl<T: ?Sized> Copy for Shared<T>`
- `impl<T: ?Sized> Copy for PhantomData<T>`
- `impl Copy for RangeFull`
- `impl<Idx: Copy> Copy for RangeTo<Idx>`
- `impl<Idx: Copy> Copy for RangeToInclusive<Idx>`
- `impl Copy for core::cmp::Ordering`
- `impl Copy for TypeId`
- `impl Copy for core::sync::atomic::Ordering`
- `impl Copy for BorrowState`
- `impl Copy for CharTryFromError`
- `impl<T: Copy> Copy for Option<T>`
- `impl Copy for TraitObject`
- `impl<T: Copy, E: Copy> Copy for Result<T, E>`
- `impl Copy for SearchStep`
- `impl Copy for Utf8Error`
- `impl Copy for Error`
- `impl<'a> Copy for Arguments<'a>`

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