Trait Default 

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

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 structs 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 primitive 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 derived, it will use the default value for each field’s type.

enums

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

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

impl Default for &str

Available on non-crate feature ferrocene_certified only.

impl Default for &CStr

Available on non-crate feature ferrocene_certified only.

impl Default for &mut str

Available on non-crate feature ferrocene_certified only.

impl Default for Char

impl Default for bool

impl Default for char

impl Default for f16

impl Default for f32

impl Default for f64

impl Default for f128

impl Default for i8

impl Default for i16

impl Default for i32

impl Default for i64

impl Default for i128

impl Default for isize

impl Default for u8

impl Default for u16

impl Default for u32

impl Default for u64

impl Default for u128

impl Default for ()

impl Default for usize

impl Default for Error

impl Default for FormattingOptions

Available on non-crate feature ferrocene_certified only.

impl Default for SipHasher

Available on non-crate feature ferrocene_certified only.

impl Default for PhantomPinned

impl Default for RangeFull

impl Default for Alignment

Available on non-crate feature ferrocene_certified only.

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

impl Default for AtomicBool

Available on non-crate feature ferrocene_certified only.

impl Default for AtomicI8

impl Default for AtomicI16

impl Default for AtomicI32

impl Default for AtomicI64

impl Default for AtomicIsize

impl Default for AtomicU8

impl Default for AtomicU16

impl Default for AtomicU32

impl Default for AtomicU64

impl Default for AtomicUsize

impl Default for Duration

Available on non-crate feature ferrocene_certified only.

impl<'a> Default for &'a ByteStr

Available on non-crate feature ferrocene_certified only.

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

Available on non-crate feature ferrocene_certified only.

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

Available on non-crate feature ferrocene_certified only.

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

Available on non-crate feature ferrocene_certified only.

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

Available on non-crate feature ferrocene_certified only.

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

Available on non-crate feature ferrocene_certified only.

impl<H> Default for BuildHasherDefault<H>

Available on non-crate feature ferrocene_certified only.

impl<I> Default for Flatten<I>

where I: Default + Iterator<Item: IntoIterator>,

Available on non-crate feature ferrocene_certified only.

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

Available on non-crate feature ferrocene_certified only.

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

Available on non-crate feature ferrocene_certified only.

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

Available on non-crate feature ferrocene_certified only.

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

Available on non-crate feature ferrocene_certified only.

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

Available on non-crate feature ferrocene_certified only.

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

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

Available on non-crate feature ferrocene_certified only.

impl<T> Default for &[T]

Available on non-crate feature ferrocene_certified only.

impl<T> Default for &mut [T]

Available on non-crate feature ferrocene_certified only.

impl<T> Default for Option<T>

impl<T> Default for [T; 0]

impl<T> Default for [T; 1]

where T: Default,

impl<T> Default for [T; 2]

where T: Default,

impl<T> Default for [T; 3]

where T: Default,

impl<T> Default for [T; 4]

where T: Default,

impl<T> Default for [T; 5]

where T: Default,

impl<T> Default for [T; 6]

where T: Default,

impl<T> Default for [T; 7]

where T: Default,

impl<T> Default for [T; 8]

where T: Default,

impl<T> Default for [T; 9]

where T: Default,

impl<T> Default for [T; 10]

where T: Default,

impl<T> Default for [T; 11]

where T: Default,

impl<T> Default for [T; 12]

where T: Default,

impl<T> Default for [T; 13]

where T: Default,

impl<T> Default for [T; 14]

where T: Default,

impl<T> Default for [T; 15]

where T: Default,

impl<T> Default for [T; 16]

where T: Default,

impl<T> Default for [T; 17]

where T: Default,

impl<T> Default for [T; 18]

where T: Default,

impl<T> Default for [T; 19]

where T: Default,

impl<T> Default for [T; 20]

where T: Default,

impl<T> Default for [T; 21]

where T: Default,

impl<T> Default for [T; 22]

where T: Default,

impl<T> Default for [T; 23]

where T: Default,

impl<T> Default for [T; 24]

where T: Default,

impl<T> Default for [T; 25]

where T: Default,

impl<T> Default for [T; 26]

where T: Default,

impl<T> Default for [T; 27]

where T: Default,

impl<T> Default for [T; 28]

where T: Default,

impl<T> Default for [T; 29]

where T: Default,

impl<T> Default for [T; 30]

where T: Default,

impl<T> Default for [T; 31]

where T: Default,

impl<T> Default for [T; 32]

where T: Default,

impl<T> Default for OnceCell<T>

Available on non-crate feature ferrocene_certified only.

impl<T> Default for Empty<T>

Available on non-crate feature ferrocene_certified only.

impl<T> Default for PhantomContravariant<T>

where T: ?Sized,

Available on non-crate feature ferrocene_certified only.

impl<T> Default for PhantomCovariant<T>

where T: ?Sized,

Available on non-crate feature ferrocene_certified only.

impl<T> Default for PhantomInvariant<T>

where T: ?Sized,

Available on non-crate feature ferrocene_certified only.

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

Available on non-crate feature ferrocene_certified only.

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

Available on non-crate feature ferrocene_certified only.

impl<T> Default for AtomicPtr<T>

Available on non-crate feature ferrocene_certified only.

impl<T, const N: usize> Default for IntoIter<T, N>

Available on non-crate feature ferrocene_certified only.

impl<T, const N: usize> Default for Mask<T, N>

where T: MaskElement, LaneCount<N>: SupportedLaneCount,

Available on non-crate feature ferrocene_certified only.

impl<T, const N: usize> Default for Simd<T, N>

where LaneCount<N>: SupportedLaneCount, T: SimdElement + Default,

Available on non-crate feature ferrocene_certified only.

impl<T: PointeeSized> Default for PhantomData<T>

Available on non-crate feature ferrocene_certified only.

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

Available on non-crate feature ferrocene_certified only.

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

Available on non-crate feature ferrocene_certified only.

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

Available on non-crate feature ferrocene_certified only.

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

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

Available on non-crate feature ferrocene_certified only.

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

Available on non-crate feature ferrocene_certified only.

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

Available on non-crate feature ferrocene_certified only.

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

Available on non-crate feature ferrocene_certified only.

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

Available on non-crate feature ferrocene_certified only.

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

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

Available on non-crate feature ferrocene_certified only.

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

Available on non-crate feature ferrocene_certified only.

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

Available on non-crate feature ferrocene_certified only.

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

Available on non-crate feature ferrocene_certified only.

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

Available on non-crate feature ferrocene_certified only.

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

Available on non-crate feature ferrocene_certified only.