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();
}
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§

source§

impl Default for &str

1.9.0 · source§

impl Default for &OsStr

1.10.0 · source§

impl Default for &CStr

1.28.0 · source§

impl Default for &mut str

source§

impl Default for Hook

source§

impl Default for bool

source§

impl Default for char

source§

impl Default for f32

source§

impl Default for f64

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 Global

1.28.0 · source§

impl Default for System

1.17.0 · source§

impl Default for Box<str, Global>

1.17.0 · source§

impl Default for Box<OsStr>

1.17.0 · source§

impl Default for Box<CStr, Global>

1.13.0 · source§

impl Default for DefaultHasher

1.7.0 · source§

impl Default for RandomState

1.9.0 · source§

impl Default for OsString

1.10.0 · source§

impl Default for CString

source§

impl Default for Error

source§

impl Default for std::fs::FileTimes

source§

impl Default for SipHasher

source§

impl Default for std::io::util::Empty

source§

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

source§

impl Default for String

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

1.10.0 · source§

impl Default for Condvar

source§

impl Default for ArrayToken

source§

impl Default for ListToken

source§

impl Default for Token

source§

impl Default for ZeroToken

source§

impl Default for OBJECT_ATTRIBUTES

source§

impl Default for std::sys::windows::fs::FileTimes

source§

impl Default for CommandEnv

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,

source§

impl<A, Z, Y, X, W, V, U, T> Default for (A, Z, Y, X, W, V, U, T)where A: Default, Z: Default, Y: Default, X: Default, W: Default, V: Default, U: Default, T: Default,

1.11.0 · source§

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

source§

impl<B, A, Z, Y, X, W, V, U, T> Default for (B, A, Z, Y, X, W, V, U, T)where B: Default, A: Default, Z: Default, Y: Default, X: Default, W: Default, V: Default, U: Default, T: Default,

source§

impl<C, B, A, Z, Y, X, W, V, U, T> Default for (C, B, A, Z, Y, X, W, V, U, T)where C: Default, B: Default, A: Default, Z: Default, Y: Default, X: Default, W: Default, V: Default, U: Default, T: Default,

source§

impl<D, C, B, A, Z, Y, X, W, V, U, T> Default for (D, C, B, A, Z, Y, X, W, V, U, T)where D: Default, C: Default, B: Default, A: Default, Z: Default, Y: Default, X: Default, W: Default, V: Default, U: Default, T: Default,

source§

impl<E, D, C, B, A, Z, Y, X, W, V, U, T> Default for (E, D, C, B, A, Z, Y, X, W, V, U, T)where E: Default, D: Default, C: Default, B: Default, A: Default, Z: Default, Y: Default, X: Default, W: Default, V: Default, U: Default, T: Default,

1.7.0 · 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,

source§

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>

source§

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,

source§

impl<K, V, S> Default for std::collections::hash::map::HashMap<K, V, S>where S: Default,

source§

impl<K, V, S, A> Default for hashbrown::map::HashMap<K, V, S, A>where S: Default, A: Default + Allocator + Clone,

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,

source§

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

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

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,

source§

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

1.70.0 · source§

impl<T> Default for OnceCell<T>

source§

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

source§

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, Global>

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, Global>

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>

source§

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

source§

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

source§

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

source§

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

1.2.0 · source§

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

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>

1.70.0 · source§

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

1.70.0 · source§

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

source§

impl<T> Default for AtomicPtr<T>

1.70.0 · source§

impl<T> Default for OnceLock<T>

source§

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

source§

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

1.10.0 · source§

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

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,

source§

impl<T, S> Default for std::collections::hash::set::HashSet<T, S>where S: Default,

source§

impl<T, S, A> Default for hashbrown::set::HashSet<T, S, A>where S: Default, A: Default + Allocator + Clone,

source§

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

source§

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

source§

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

source§

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

source§

impl<T: Default> Default for CachePadded<T>

1.10.0 · source§

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

1.10.0 · source§

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

source§

impl<U, T> Default for (U, T)where U: Default, T: Default,

source§

impl<V, U, T> Default for (V, U, T)where V: Default, U: Default, T: Default,

source§

impl<W, V, U, T> Default for (W, V, U, T)where W: Default, V: Default, U: Default, T: Default,

source§

impl<X, W, V, U, T> Default for (X, W, V, U, T)where X: Default, W: Default, V: Default, U: Default, T: Default,

source§

impl<Y, X, W, V, U, T> Default for (Y, X, W, V, U, T)where Y: Default, X: Default, W: Default, V: Default, U: Default, T: Default,

source§

impl<Z, Y, X, W, V, U, T> Default for (Z, Y, X, W, V, U, T)where Z: Default, Y: Default, X: Default, W: Default, V: Default, U: Default, T: Default,