Default in core::default - Rust
Description: A trait for giving a type a useful default value.

# Trait Default

1.6.0 · Source

    pub trait Default: Sized {
        // Required method
        fn default() -> Self;
    }

Expand description

A trait for giving a type a useful default value.

Sometimes, you want to fall back to some kind of default value, and don’t particularly care what it is. This comes up often with `struct`s that define a set of options:

    struct SomeOptions {
        foo: i32,
        bar: f32,
    }

How can we define some default values? You can use `Default`:

    #[derive(Default)]
    struct SomeOptions {
        foo: i32,
        bar: f32,
    }

    fn main() {
        let options: SomeOptions = Default::default();
    }

Now, you get all of the default values. Rust implements `Default` for various primitives types.

If you want to override a particular option, but still retain the other defaults:

    fn main() {
        let options = SomeOptions { foo: 42, ..Default::default() };
    }

### §Derivable

This trait can be used with `#[derive]` if all of the type’s fields implement `Default`. When `derive`d, it will use the default value for each field’s type.

#### §`enum`s

When using `#[derive(Default)]` on an `enum`, you need to choose which unit variant will be default. You do this by placing the `#[default]` attribute on the variant.

    #[derive(Default)]
    enum Kind {
        #[default]
        A,
        B,
        C,
    }

You cannot use the `#[default]` attribute on non-unit or non-exhaustive variants.

The `#[default]` attribute was stabilized in Rust 1.62.0.

### §How can I implement `Default`?

Provide an implementation for the `default()` method that returns the value of your type that should be the default:

    enum Kind {
        A,
        B,
        C,
    }

    impl Default for Kind {
        fn default() -> Self { Kind::A }
    }

## §Examples

    #[derive(Default)]
    struct SomeOptions {
        foo: i32,
        bar: f32,
    }

## Required Methods§

1.0.0 · Source

#### fn default() -> Self

Returns the “default value” for a type.

Default values are often some kind of initial value, identity value, or anything else that may make sense as a default.

##### §Examples

Using built-in default values:

    let i: i8 = Default::default();
    let (x, y): (Option<String>, f64) = Default::default();
    let (a, b, (c, d)): (i32, u32, (bool, bool)) = Default::default();

Making your own:

    enum Kind {
        A,
        B,
        C,
    }

    impl Default for Kind {
        fn default() -> Self { Kind::A }
    }

## 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.

## Implementors§

1.0.0 · Source§

### impl Default for &str

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### impl Default for &CStr

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### impl Default for &mut str

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### impl Default for Char

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### impl Default for bool

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### impl Default for char

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### impl Default for f16

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### impl Default for f32

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### impl Default for f64

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### impl Default for f128

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### impl Default for i8

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### impl Default for i16

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### impl Default for i32

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### impl Default for i64

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### impl Default for i128

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### impl Default for isize

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### impl Default for u8

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### impl Default for u16

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### impl Default for u32

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### impl Default for u64

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### impl Default for u128

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### impl Default for ()

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### impl Default for usize

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### impl Default for Error

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### impl Default for FormattingOptions

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### impl Default for SipHasher

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### impl Default for PhantomPinned

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### impl Default for RangeFull

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### impl Default for Alignment

Returns `Alignment::MIN`, which is valid for any type.

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### impl Default for AtomicBool

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### impl Default for AtomicI8

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### impl Default for AtomicI16

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### impl Default for AtomicI32

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### impl Default for AtomicI64

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### impl Default for AtomicIsize

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### impl Default for AtomicU8

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### impl Default for AtomicU16

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### impl Default for AtomicU32

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### impl Default for AtomicU64

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### impl Default for AtomicUsize

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### impl Default for Duration

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### impl<'a> Default for &'a ByteStr

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### impl<'a> Default for &'a mut ByteStr

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### impl<'a> Default for PhantomContravariantLifetime<'a>

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### impl<'a> Default for PhantomCovariantLifetime<'a>

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### impl<'a> Default for PhantomInvariantLifetime<'a>

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### impl<A: Default, B: Default> Default for Chain<A, B>

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### impl<H> Default for BuildHasherDefault<H>

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### impl<I> Default for Flatten<I>
where I: Default + Iterator<Item: IntoIterator>,

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### impl<I: Default> Default for Cloned<I>

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### impl<I: Default> Default for Copied<I>

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### impl<I: Default> Default for Enumerate<I>

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### impl<I: Default> Default for Fuse<I>

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### impl<I: Default> Default for Rev<I>

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### impl<Idx: Default> Default for core::ops::Range<Idx>

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### impl<Idx: Default> Default for core::range::Range<Idx>

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### impl<T> Default for &[T]

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### impl<T> Default for &mut [T]

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### impl<T> Default for Option<T>

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### impl<T> Default for [T; 0]

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### impl<T> Default for [T; 1]
where T: Default,

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### impl<T> Default for [T; 2]
where T: Default,

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### impl<T> Default for [T; 3]
where T: Default,

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### impl<T> Default for [T; 4]
where T: Default,

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### impl<T> Default for [T; 5]
where T: Default,

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### impl<T> Default for [T; 6]
where T: Default,

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### impl<T> Default for [T; 7]
where T: Default,

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### impl<T> Default for [T; 8]
where T: Default,

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### impl<T> Default for [T; 9]
where T: Default,

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### impl<T> Default for [T; 10]
where T: Default,

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### impl<T> Default for [T; 11]
where T: Default,

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### impl<T> Default for [T; 12]
where T: Default,

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### impl<T> Default for [T; 13]
where T: Default,

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### impl<T> Default for [T; 14]
where T: Default,

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### impl<T> Default for [T; 15]
where T: Default,

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### impl<T> Default for [T; 16]
where T: Default,

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### impl<T> Default for [T; 17]
where T: Default,

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### impl<T> Default for [T; 18]
where T: Default,

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### impl<T> Default for [T; 19]
where T: Default,

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### impl<T> Default for [T; 20]
where T: Default,

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### impl<T> Default for [T; 21]
where T: Default,

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### impl<T> Default for [T; 22]
where T: Default,

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### impl<T> Default for [T; 23]
where T: Default,

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### impl<T> Default for [T; 24]
where T: Default,

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### impl<T> Default for [T; 25]
where T: Default,

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### impl<T> Default for [T; 26]
where T: Default,

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### impl<T> Default for [T; 27]
where T: Default,

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### impl<T> Default for [T; 28]
where T: Default,

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### impl<T> Default for [T; 29]
where T: Default,

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### impl<T> Default for [T; 30]
where T: Default,

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### impl<T> Default for [T; 31]
where T: Default,

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### impl<T> Default for [T; 32]
where T: Default,

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### impl<T> Default for OnceCell<T>

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### impl<T> Default for Empty<T>

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### impl<T> Default for PhantomContravariant<T>
where T: ?Sized,

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### impl<T> Default for PhantomCovariant<T>
where T: ?Sized,

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### impl<T> Default for PhantomInvariant<T>
where T: ?Sized,

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### impl<T> Default for Iter<'_, T>

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### impl<T> Default for IterMut<'_, T>

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### impl<T> Default for AtomicPtr<T>

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### impl<T, const N: usize> Default for Mask<T, N>
where T: MaskElement, LaneCount<N>: SupportedLaneCount,

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### impl<T, const N: usize> Default for Simd<T, N>
where LaneCount<N>: SupportedLaneCount, T: SimdElement + Default,

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### impl<T: Default + ?Sized> Default for ManuallyDrop<T>

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### impl<T: Default + ?Sized> Default for Exclusive<T>

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### impl<T: Default> Default for (T₁, T₂, …, Tₙ)

This trait is implemented for tuples up to twelve items long.

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### impl<T: Default> Default for Cell<T>

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### impl<T: Default> Default for LazyCell<T>

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### impl<T: Default> Default for RefCell<T>

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### impl<T: Default> Default for SyncUnsafeCell<T>

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### impl<T: Default> Default for UnsafeCell<T>

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### impl<T: Default> Default for Reverse<T>

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### impl<T: Default> Default for Saturating<T>

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### impl<T: Default> Default for Wrapping<T>

1.62.0 · Source§

### impl<T: Default> Default for AssertUnwindSafe<T>

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### impl<T: Default> Default for UnsafePinned<T>

1.88.0 · Source§

### impl<T: ?Sized + Thin> Default for *const T

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### impl<T: ?Sized + Thin> Default for *mut T

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### impl<T: ?Sized> Default for PhantomData<T>