Trait std::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();
}
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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,
}
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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 }
}
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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();
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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

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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 (const: unstable) · 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

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impl Default for Error

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impl Default for FileTimes

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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

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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

1.70.0 · source§

impl<'a, K, V> Default for std::collections::btree_map::Iter<'a, K, V>where K: 'a, V: 'a,

1.70.0 · source§

impl<'a, K, V> Default for std::collections::btree_map::IterMut<'a, K, V>where K: 'a, V: 'a,

1.70.0 · source§

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

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>

1.70.0 · source§

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

1.70.0 · source§

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

1.70.0 · source§

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

1.70.0 · source§

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

1.70.0 · source§

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

1.70.0 · source§

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

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

1.70.0 · source§

impl<K, V> Default for Keys<'_, K, V>

1.70.0 · source§

impl<K, V> Default for std::collections::btree_map::Range<'_, K, V>

1.70.0 · source§

impl<K, V> Default for Values<'_, K, V>

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

1.70.0 · source§

impl<K, V, A> Default for std::collections::btree_map::IntoIter<K, V, A>where A: Allocator + Default + Clone,

1.70.0 · source§

impl<K, V, A> Default for IntoKeys<K, V, A>where A: Allocator + Default + Clone,

1.70.0 · source§

impl<K, V, A> Default for IntoValues<K, V, A>where A: Allocator + Default + Clone,

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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 (const: unstable) · source§

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

1.4.0 (const: unstable) · source§

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

1.4.0 (const: unstable) · source§

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

1.4.0 (const: unstable) · source§

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

1.4.0 (const: unstable) · source§

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

1.4.0 (const: unstable) · source§

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

1.4.0 (const: unstable) · source§

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

1.4.0 (const: unstable) · source§

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

1.4.0 (const: unstable) · source§

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

1.4.0 (const: unstable) · source§

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

1.4.0 (const: unstable) · source§

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

1.4.0 (const: unstable) · source§

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

1.4.0 (const: unstable) · source§

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

1.4.0 (const: unstable) · source§

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

1.4.0 (const: unstable) · source§

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

1.4.0 (const: unstable) · source§

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

1.4.0 (const: unstable) · source§

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

1.4.0 (const: unstable) · source§

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

1.4.0 (const: unstable) · source§

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

1.4.0 (const: unstable) · source§

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

1.4.0 (const: unstable) · source§

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

1.4.0 (const: unstable) · source§

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

1.4.0 (const: unstable) · source§

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

1.4.0 (const: unstable) · source§

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

1.4.0 (const: unstable) · source§

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

1.4.0 (const: unstable) · source§

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

1.4.0 (const: unstable) · source§

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

1.4.0 (const: unstable) · source§

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

1.4.0 (const: unstable) · source§

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

1.4.0 (const: unstable) · source§

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

1.4.0 (const: unstable) · source§

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

1.4.0 (const: unstable) · source§

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

const: unstable · 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>

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

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

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

1.70.0 · source§

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,

1.70.0 · source§

impl<T> Default for std::collections::binary_heap::IntoIter<T>

1.70.0 · source§

impl<T> Default for std::collections::btree_set::Iter<'_, T>

1.70.0 · source§

impl<T> Default for std::collections::btree_set::Range<'_, T>

1.70.0 · source§

impl<T> Default for std::collections::linked_list::IntoIter<T>

1.70.0 · source§

impl<T> Default for std::collections::linked_list::Iter<'_, T>

1.70.0 · source§

impl<T> Default for std::collections::linked_list::IterMut<'_, T>

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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,

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

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

1.62.0 · source§

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

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

1.10.0 · source§

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

1.70.0 · source§

impl<T> Default for std::slice::Iter<'_, T>

1.70.0 · source§

impl<T> Default for std::slice::IterMut<'_, T>

const: unstable · source§

impl<T> Default for AtomicPtr<T>

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

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

1.70.0 (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>

1.70.0 · source§

impl<T, A> Default for std::collections::btree_set::IntoIter<T, A>where A: Allocator + Default + Clone,

1.70.0 · source§

impl<T, A> Default for std::vec::IntoIter<T, A>where A: Allocator + Default,

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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 LaneCount<LANES>: SupportedLaneCount, T: SimdElement + Default,

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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>