Trait std::default::Default

1.0.0 · source ·

pub trait Default {
 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.

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

Implementors

const: unstable · source

impl Default for &str

1.10.0 · source

impl Default for &CStr

1.9.0 · source

impl Default for &OsStr

1.28.0 · source

impl Default for &mut str

const: unstable · source

impl Default for bool

const: unstable · source

impl Default for char

const: unstable · source

impl Default for f32

const: unstable · source

impl Default for f64

const: unstable · source

impl Default for i8

const: unstable · source

impl Default for i16

const: unstable · source

impl Default for i32

const: unstable · source

impl Default for i64

const: unstable · source

impl Default for i128

const: unstable · source

impl Default for isize

const: unstable · source

impl Default for u8

const: unstable · source

impl Default for u16

const: unstable · source

impl Default for u32

const: unstable · source

impl Default for u64

const: unstable · source

impl Default for u128

const: unstable · source

impl Default for ()

const: unstable · source

impl Default for usize

source

impl Default for Global

1.28.0 · source

impl Default for System

1.17.0 (const: unstable) · source

impl Default for Box<str, Global>

1.17.0 · source

impl Default for Box<CStr, Global>

1.17.0 · source

impl Default for Box<OsStr>

1.13.0 · source

impl Default for DefaultHasher

1.7.0 · source

impl Default for RandomState

1.10.0 · source

impl Default for CString

1.9.0 · source

impl Default for OsString

source

impl Default for Error

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impl Default for FileTimes

source

impl Default for SipHasher

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impl Default for std::io::Empty

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impl Default for Sink

1.33.0 · source

impl Default for PhantomPinned

source

impl Default for RangeFull

1.17.0 · source

impl Default for PathBuf

const: unstable · source

impl Default for String

const: unstable · source

impl Default for AtomicBool

1.34.0 (const: unstable) · source

impl Default for AtomicI8

1.34.0 (const: unstable) · source

impl Default for AtomicI16

1.34.0 (const: unstable) · source

impl Default for AtomicI32

1.34.0 (const: unstable) · source

impl Default for AtomicI64

const: unstable · source

impl Default for AtomicIsize

1.34.0 (const: unstable) · source

impl Default for AtomicU8

1.34.0 (const: unstable) · source

impl Default for AtomicU16

1.34.0 (const: unstable) · source

impl Default for AtomicU32

1.34.0 (const: unstable) · source

impl Default for AtomicU64

const: unstable · source

impl Default for AtomicUsize

1.10.0 · source

impl Default for Condvar

1.3.0 · source

impl Default for Duration

1.11.0 · source

impl<B> Default for Cow<'_, B> where
B: ToOwned + ?Sized,
<B as ToOwned>::Owned: Default,

1.7.0 (const: unstable) · source

impl<H> Default for BuildHasherDefault<H>

source

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

source

impl<K, V> Default for BTreeMap<K, V, Global>

source

impl<K, V, S> Default for HashMap<K, V, S> where
S: Default,

const: unstable · source

impl<T> Default for &[T]

1.5.0 (const: unstable) · source

impl<T> Default for &mut [T]

const: unstable · source

impl<T> Default for Option<T>

1.4.0 (const: unstable) · 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,

source

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

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

const: unstable · source

impl<T> Default for Box<[T], Global>

source

impl<T> Default for Box<T, Global> where
T: Default,

source

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

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impl<T> Default for LazyCell<T, fn() -> T> where
T: Default,

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impl<T> Default for OnceCell<T>

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

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

1.10.0 · source

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

1.19.0 · source

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

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impl<T> Default for BTreeSet<T, Global>

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impl<T> Default for BinaryHeap<T> where
T: Ord,

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impl<T> Default for LinkedList<T>

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impl<T> Default for VecDeque<T, Global>

1.2.0 (const: unstable) · source

impl<T> Default for std::iter::Empty<T>

const: unstable · source

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

1.20.0 · source

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

source

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

source

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

1.62.0 · source

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

source

impl<T> Default for Rc<T> where
T: Default,

1.10.0 · source

impl<T> Default for std::rc::Weak<T>

const: unstable · source

impl<T> Default for AtomicPtr<T>

source

impl<T> Default for Arc<T> where
T: Default,

source

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

const: unstable · source

impl<T> Default for OnceLock<T>

1.10.0 · source

impl<T> Default for std::sync::Weak<T>

const: unstable · source

impl<T> Default for Vec<T, Global>

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impl<T, S> Default for HashSet<T, S> where
S: Default,

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

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

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impl<T: Default> Default for Cursor<T>

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impl<T: Default> Default for LazyLock<T>

1.10.0 · source

impl<T: Default> Default for RwLock<T>

1.10.0 · source

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