Trait core::default::Default

1.0.0 · 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 structs that define a set of options:

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

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();
}
Run

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() };
}
Run

§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,
}
Run

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 }
}
Run

§Examples

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

Required Methods§

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();
Run

Making your own:

enum Kind {
    A,
    B,
    C,
}

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

Object Safety§

This trait is not object safe.

Implementors§

source§

impl Default for &str

1.10.0 · source§

impl Default for &CStr

1.28.0 · source§

impl Default for &mut str

source§

impl Default for Char

source§

impl Default for bool

source§

impl Default for char

source§

impl Default for f16

source§

impl Default for f32

source§

impl Default for f64

source§

impl Default for f128

source§

impl Default for i8

source§

impl Default for i16

source§

impl Default for i32

source§

impl Default for i64

source§

impl Default for i128

source§

impl Default for isize

source§

impl Default for u8

source§

impl Default for u16

source§

impl Default for u32

source§

impl Default for u64

source§

impl Default for u128

source§

impl Default for ()

source§

impl Default for usize

source§

impl Default for Error

source§

impl Default for SipHasher

1.33.0 · source§

impl Default for PhantomPinned

source§

impl Default for RangeFull

source§

impl Default for Alignment

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

source§

impl Default for AtomicBool

1.34.0 · source§

impl Default for AtomicI8

1.34.0 · source§

impl Default for AtomicI16

1.34.0 · source§

impl Default for AtomicI32

1.34.0 · source§

impl Default for AtomicI64

source§

impl Default for AtomicIsize

1.34.0 · source§

impl Default for AtomicU8

1.34.0 · source§

impl Default for AtomicU16

1.34.0 · source§

impl Default for AtomicU32

1.34.0 · source§

impl Default for AtomicU64

source§

impl Default for AtomicUsize

1.3.0 · source§

impl Default for Duration

1.70.0 · source§

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

1.7.0 · source§

impl<H> Default for BuildHasherDefault<H>

1.70.0 · source§

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

1.70.0 · source§

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

1.70.0 · source§

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

1.70.0 · source§

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

1.70.0 · source§

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

1.70.0 · source§

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

source§

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

source§

impl<T> Default for &[T]

1.5.0 · source§

impl<T> Default for &mut [T]

source§

impl<T> Default for Option<T>

1.4.0 · source§

impl<T> Default for [T; 0]

1.4.0 · source§

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

1.4.0 · source§

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

1.4.0 · source§

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

1.4.0 · source§

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

1.4.0 · source§

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

1.4.0 · source§

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

1.4.0 · source§

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

1.4.0 · source§

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

1.4.0 · source§

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

1.4.0 · source§

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

1.4.0 · source§

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

1.4.0 · source§

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

1.4.0 · source§

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

1.4.0 · source§

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

1.4.0 · source§

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

1.4.0 · source§

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

1.4.0 · source§

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

1.4.0 · source§

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

1.4.0 · source§

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

1.4.0 · source§

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

1.4.0 · source§

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

1.4.0 · source§

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

1.4.0 · source§

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

1.4.0 · source§

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

1.4.0 · source§

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

1.4.0 · source§

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

1.4.0 · source§

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

1.4.0 · source§

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

1.4.0 · source§

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

1.4.0 · source§

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

1.4.0 · source§

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

1.4.0 · source§

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

1.70.0 · source§

impl<T> Default for OnceCell<T>

1.2.0 · source§

impl<T> Default for Empty<T>

1.70.0 · source§

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

1.70.0 · source§

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

source§

impl<T> Default for AtomicPtr<T>

source§

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

source§

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

1.20.0 · source§

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

source§

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

source§

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

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

source§

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

source§

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

source§

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

source§

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

1.10.0 · source§

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

1.19.0 · source§

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

1.74.0 · source§

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

source§

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

1.62.0 · source§

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

source§

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