Trait Default

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

🔬This is a nightly-only experimental API. (prelude_2024)

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 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() };
}

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

🔬This is a nightly-only experimental API. (prelude_2024)

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 }
}

Object Safety

This trait is not object safe.

Implementors

impl Default for &str

impl Default for &CStr

impl Default for &mut str

impl Default for MemoryInitialization

impl Default for AsciiChar

impl Default for Prefilter

impl Default for Endianness

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 Box<str>

impl Default for Box<CStr>

impl Default for String

impl Default for FilePos

impl Default for Module

impl Default for ModuleTypes

impl Default for TableInitialization

impl Default for VMSharedTypeIndex

impl Default for WasmFunctionInfo

impl Default for RandomState

impl Default for FuncValidatorAllocations

impl Default for Parser

impl Default for SegmentFlags

impl Default for SymbolFlags

impl Default for Validator

impl Default for ValidatorId

impl Default for WasmFeatures

impl Default for Remapping

impl Default for Error

impl Default for SipHasher

impl Default for PhantomPinned

impl Default for Alignment

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

impl Default for RangeFull

impl Default for AtomicBool

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

impl Default for Global

impl Default for CString

impl Default for Rc<str>

impl Default for Rc<CStr>

impl Default for Arc<str>

impl Default for Arc<CStr>

impl Default for AHasher

Provides a default Hasher with fixed keys. This is typically used in conjunction with BuildHasherDefault to create AHashers in order to hash the keys of the map.

Generally it is preferable to use RandomState instead, so that different hashmaps will have different keys. However if fixed keys are desirable this may be used instead.

Example

use std::hash::BuildHasherDefault;
use ahash::{AHasher, RandomState};
use std::collections::HashMap;

let mut map: HashMap<i32, i32, BuildHasherDefault<AHasher>> = HashMap::default();
map.insert(12, 34);

impl Default for FinderBuilder

impl Default for BigEndian

impl Default for LittleEndian

impl Default for VersionIndex

impl Default for OnceBool

impl Default for OnceNonZeroUsize

impl Default for BuildMetadata

impl Default for Prerelease

impl Default for VersionReq

The default VersionReq is the same as VersionReq::STAR.

impl Default for IgnoredAny

impl Default for DefaultToHost

impl Default for DefaultToUnknown

impl<'a, K, V> Default for alloc::collections::btree::map::Iter<'a, K, V>

where K: 'a, V: 'a,

impl<'a, K, V> Default for alloc::collections::btree::map::IterMut<'a, K, V>

where K: 'a, V: 'a,

impl<'a, T> Default for OnceRef<'a, T>

impl<'data> Default for Version<'data>

impl<'data> Default for ObjectMap<'data>

impl<'data> Default for ObjectMapEntry<'data>

impl<'data> Default for Bytes<'data>

impl<'data, Elf> Default for VersionTable<'data, Elf>

where Elf: FileHeader,

impl<'data, Elf, R> Default for SectionTable<'data, Elf, R>

where Elf: Default + FileHeader, R: Default + ReadRef<'data>, <Elf as FileHeader>::SectionHeader: Default,

impl<'data, Elf, R> Default for SymbolTable<'data, Elf, R>

where Elf: FileHeader, R: ReadRef<'data>,

impl<'data, R> Default for StringTable<'data, R>

where R: ReadRef<'data>,

impl<A> Default for SmallVec<A>

where A: Array,

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

where A: Default, B: Default,

impl<B> Default for Cow<'_, B>

where B: ToOwned + ?Sized, <B as ToOwned>::Owned: Default,

impl<E> Default for CompressionHeader32<E>

where E: Default + Endian,

impl<E> Default for CompressionHeader64<E>

where E: Default + Endian,

impl<E> Default for Sym32<E>

where E: Default + Endian,

impl<E> Default for Sym64<E>

where E: Default + Endian,

impl<E> Default for I16<E>

where E: Default + Endian,

impl<E> Default for I32<E>

where E: Default + Endian,

impl<E> Default for I64<E>

where E: Default + Endian,

impl<E> Default for U16<E>

where E: Default + Endian,

impl<E> Default for U32<E>

where E: Default + Endian,

impl<E> Default for U64<E>

where E: Default + Endian,

impl<E> Default for I16Bytes<E>

where E: Default + Endian,

impl<E> Default for I32Bytes<E>

where E: Default + Endian,

impl<E> Default for I64Bytes<E>

where E: Default + Endian,

impl<E> Default for U16Bytes<E>

where E: Default + Endian,

impl<E> Default for U32Bytes<E>

where E: Default + Endian,

impl<E> Default for U64Bytes<E>

where E: Default + Endian,

impl<H> Default for BuildHasherDefault<H>

impl<I> Default for Cloned<I>

where I: Default,

impl<I> Default for Copied<I>

where I: Default,

impl<I> Default for Enumerate<I>

where I: Default,

impl<I> Default for Flatten<I>

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

impl<I> Default for Fuse<I>

where I: Default,

impl<I> Default for Rev<I>

where I: Default,

impl<Idx> Default for wasmtime_environ::__core::range::legacy::Range<Idx>

where Idx: Default,

impl<Idx> Default for wasmtime_environ::__core::range::Range<Idx>

where Idx: Default,

impl<K> Default for EntitySet<K>

where K: EntityRef,

impl<K, V> Default for &indexmap::map::slice::Slice<K, V>

impl<K, V> Default for &mut indexmap::map::slice::Slice<K, V>

impl<K, V> Default for Box<Slice<K, V>>

impl<K, V> Default for PrimaryMap<K, V>

where K: EntityRef,

impl<K, V> Default for SecondaryMap<K, V>

where K: EntityRef, V: Clone + Default,

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

impl<K, V> Default for alloc::collections::btree::map::Keys<'_, K, V>

impl<K, V> Default for alloc::collections::btree::map::Range<'_, K, V>

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

impl<K, V> Default for alloc::collections::btree::map::Values<'_, K, V>

impl<K, V> Default for alloc::collections::btree::map::ValuesMut<'_, K, V>

impl<K, V> Default for indexmap::map::iter::IntoIter<K, V>

impl<K, V> Default for indexmap::map::iter::IntoKeys<K, V>

impl<K, V> Default for indexmap::map::iter::IntoValues<K, V>

impl<K, V> Default for indexmap::map::iter::Iter<'_, K, V>

impl<K, V> Default for indexmap::map::iter::IterMut<'_, K, V>

impl<K, V> Default for indexmap::map::iter::Keys<'_, K, V>

impl<K, V> Default for indexmap::map::iter::Values<'_, K, V>

impl<K, V> Default for indexmap::map::iter::ValuesMut<'_, K, V>

impl<K, V, A> Default for alloc::collections::btree::map::IntoIter<K, V, A>

where A: Allocator + Default + Clone,

impl<K, V, A> Default for alloc::collections::btree::map::IntoKeys<K, V, A>

where A: Allocator + Default + Clone,

impl<K, V, A> Default for alloc::collections::btree::map::IntoValues<K, V, A>

where A: Allocator + Default + Clone,

impl<K, V, S> Default for IndexMap<K, V, S>

where S: Default,

impl<K, V, S, A> Default for HashMap<K, V, S, A>

where S: Default, A: Default + Allocator,

impl<T> Default for &[T]

impl<T> Default for &indexmap::set::slice::Slice<T>

impl<T> Default for &mut [T]

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 (T₁, T₂, …, Tₙ)

where T: Default,

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

impl<T> Default for PackedOption<T>

where T: ReservedValue,

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

impl<T> Default for Box<Slice<T>>

impl<T> Default for Box<T>

where T: Default,

impl<T> Default for Vec<T>

impl<T> Default for EntityList<T>

where T: EntityRef + ReservedValue,

Create an empty list.

impl<T> Default for ListPool<T>

where T: EntityRef + ReservedValue,

impl<T> Default for Cell<T>

where T: Default,

impl<T> Default for LazyCell<T>

where T: Default,

impl<T> Default for wasmtime_environ::__core::cell::OnceCell<T>

impl<T> Default for RefCell<T>

where T: Default,

impl<T> Default for SyncUnsafeCell<T>

where T: Default,

impl<T> Default for UnsafeCell<T>

where T: Default,

impl<T> Default for Reverse<T>

where T: Default,

impl<T> Default for Empty<T>

impl<T> Default for PhantomData<T>

where T: ?Sized,

impl<T> Default for ManuallyDrop<T>

where T: Default + ?Sized,

impl<T> Default for Saturating<T>

where T: Default,

impl<T> Default for Wrapping<T>

where T: Default,

impl<T> Default for AssertUnwindSafe<T>

where T: Default,

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

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

impl<T> Default for AtomicPtr<T>

impl<T> Default for Exclusive<T>

where T: Default + ?Sized,

impl<T> Default for BinaryHeap<T>

where T: Ord,

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

impl<T> Default for alloc::collections::binary_heap::Iter<'_, T>

impl<T> Default for BTreeSet<T>

impl<T> Default for alloc::collections::btree::set::Iter<'_, T>

impl<T> Default for alloc::collections::btree::set::Range<'_, T>

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

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

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

impl<T> Default for LinkedList<T>

impl<T> Default for alloc::collections::vec_deque::iter::Iter<'_, T>

impl<T> Default for alloc::collections::vec_deque::iter_mut::IterMut<'_, T>

impl<T> Default for VecDeque<T>

impl<T> Default for Rc<[T]>

impl<T> Default for Rc<T>

where T: Default,

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

impl<T> Default for Arc<[T]>

impl<T> Default for Arc<T>

where T: Default,

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

impl<T> Default for indexmap::set::iter::IntoIter<T>

impl<T> Default for indexmap::set::iter::Iter<'_, T>

impl<T> Default for SymbolMap<T>

where T: Default + SymbolMapEntry,

impl<T> Default for OnceBox<T>

impl<T> Default for Lazy<T>

where T: Default,

impl<T> Default for once_cell::unsync::OnceCell<T>

impl<T> Default for Unalign<T>

where T: Default,

impl<T, A> Default for wasmtime_environ::prelude::vec::IntoIter<T, A>

where A: Allocator + Default,

impl<T, A> Default for alloc::collections::btree::set::IntoIter<T, A>

where A: Allocator + Default + Clone,

impl<T, A> Default for RawTable<T, A>

where A: Allocator + Default,

impl<T, A> Default for HashTable<T, A>

where A: Allocator + Default,

impl<T, S> Default for IndexSet<T, S>

where S: Default,

impl<T, S, A> Default for HashSet<T, S, A>

where S: Default, A: Default + Allocator,

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

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

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

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