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 }
}

Dyn Compatibility

This trait is not dyn compatible.

In older versions of Rust, dyn compatibility was called "object safety", so this trait is not object safe.

Implementors

impl Default for &str

impl Default for &CStr

impl Default for &OsStr

impl Default for &mut str

impl Default for MemoryInitialization

impl Default for AsciiChar

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 ComponentDfg

impl Default for CanonicalAbiInfo

impl Default for wasmtime_environ::component::Component

impl Default for ComponentTypes

impl Default for ResourcesBuilder

impl Default for TypeComponent

impl Default for TypeComponentInstance

impl Default for TypeModule

impl Default for DrcTypeLayouts

impl Default for NullTypeLayouts

impl Default for Box<str>

impl Default for Box<CStr>

impl Default for Box<OsStr>

impl Default for String

impl Default for AddressMapSection

impl Default for ConfigTunables

impl Default for FilePos

impl Default for wasmtime_environ::Module

impl Default for ModuleTypes

impl Default for TableInitialization

impl Default for TrapEncodingBuilder

impl Default for VMSharedTypeIndex

impl Default for WasmFileInfo

impl Default for WasmFunctionInfo

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 System

impl Default for OsString

impl Default for FileTimes

impl Default for DefaultHasher

impl Default for std::hash::random::RandomState

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

impl Default for Sink

impl Default for PathBuf

impl Default for ExitCode

The default value is ExitCode::SUCCESS

impl Default for ExitStatus

The default value is one which indicates successful completion.

impl Default for DefaultRandomSource

impl Default for Condvar

impl Default for anyhow::Chain<'_>

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 AHasher

Provides a default Hasher with fixed keys. This is typically used in conjunction with BuildHasherDefault to create [AHasher]s 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 Abbreviations

impl Default for AbbreviationsCache

impl Default for AllocVec

impl Default for Augmentation

impl Default for AuxSymbolSection

impl Default for BaseAddresses

impl Default for BigEndian

impl Default for BigEndian

impl Default for BranchHints

impl Default for CanonicalFunctionSection

impl Default for Class

impl Default for CodeSection

impl Default for ColorChoice

The default is Auto.

impl Default for ColorSpec

impl Default for Component

impl Default for ComponentAliasSection

impl Default for ComponentBuilder

impl Default for ComponentExportSection

impl Default for ComponentImportSection

impl Default for ComponentInstanceSection

impl Default for ComponentNameSection

impl Default for ComponentType

impl Default for ComponentTypeSection

impl Default for CompoundBitSet

impl Default for Config

impl Default for CoreDumpSection

impl Default for CoreDumpStackSection

impl Default for CoreTypeSection

impl Default for CvQualifiers

impl Default for DataSection

impl Default for DebugInfoOffsets

impl Default for DefaultToHost

impl Default for DefaultToUnknown

impl Default for DemangleOptions

impl Default for Dwarf

impl Default for DwarfFileType

impl Default for ElementSection

impl Default for EncoderState

impl Default for Endianness

impl Default for ExportSection

impl Default for Expression

impl Default for FileHeader

impl Default for FileInfo

impl Default for FinderBuilder

impl Default for FrameTable

impl Default for FuncValidatorAllocations

impl Default for FunctionSection

impl Default for GlobalSection

impl Default for Hasher

impl Default for ImageSectionHeader

impl Default for ImportSection

impl Default for IndirectNameMap

impl Default for InstanceSection

impl Default for InstanceType

impl Default for LineEncoding

impl Default for LineStringTable

impl Default for LinkingSection

impl Default for LittleEndian

impl Default for LittleEndian

impl Default for LocationListTable

impl Default for MachOBuildVersion

impl Default for MemorySection

impl Default for Module

impl Default for ModuleType

impl Default for Name

impl Default for NameMap

impl Default for NameSection

impl Default for OnceBool

impl Default for OnceNonZeroUsize

impl Default for ParseOptions

impl Default for Parser

impl Default for Pointer

impl Default for Prefilter

impl Default for ProducersField

impl Default for ProducersSection

impl Default for RandomState

impl Default for RangeListTable

impl Default for Relocation

impl Default for Relocation

impl Default for RelocationMap

impl Default for RelocationSections

impl Default for Remapping

impl Default for RunTimeEndian

impl Default for SectionBaseAddresses

impl Default for SectionHeader

impl Default for SectionIndex

impl Default for SectionRange

impl Default for SegmentFlags

impl Default for Size

impl Default for StringTable

impl Default for Symbol

impl Default for SymbolFlags

impl Default for SymbolIndex

impl Default for SymbolTable

impl Default for TableSection

impl Default for TagSection

impl Default for TypeSection

impl Default for UnitTable

impl Default for Validator

impl Default for ValidatorId

impl Default for VersionIndex

impl Default for WasmFeatures

impl<'a> Default for DebugInfoData<'a>

impl<'a> Default for wasmtime_environ::NameSection<'a>

impl<'a> Default for MetadataBuilder<'a>

impl<'a> Default for RecordBuilder<'a>

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 ModuleTranslation<'data>

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

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

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

impl<'data> Default for RelocationBlockIterator<'data>

impl<'data> Default for SectionTable<'data>

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

impl<'data, E> Default for LoadCommandIterator<'data, E>

where E: Default + Endian,

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

where Elf: FileHeader,

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

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

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

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

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

where Mach: MachHeader, R: ReadRef<'data>,

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

where R: ReadRef<'data>,

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

where R: ReadRef<'data>, Coff: CoffHeader,

impl<'data, Xcoff> Default for SectionTable<'data, Xcoff>

where Xcoff: FileHeader,

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

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

impl<'input, Endian> Default for EndianSlice<'input, Endian>

where Endian: Default + Endianity,

impl<A> Default for SmallVec<A>

where A: Array,

impl<A, B> Default for wasmtime_environ::__core::iter::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 I16<E>

where E: Default + Endian,

impl<E> Default for I16Bytes<E>

where E: Default + Endian,

impl<E> Default for I32<E>

where E: Default + Endian,

impl<E> Default for I32Bytes<E>

where E: Default + Endian,

impl<E> Default for I64<E>

where E: Default + Endian,

impl<E> Default for I64Bytes<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 U16<E>

where E: Default + Endian,

impl<E> Default for U16Bytes<E>

where E: Default + Endian,

impl<E> Default for U32<E>

where E: Default + Endian,

impl<E> Default for U32Bytes<E>

where E: Default + Endian,

impl<E> Default for U64<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> Default for std::collections::hash::set::IntoIter<K>

impl<K> Default for std::collections::hash::set::Iter<'_, K>

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

impl<K, V> Default for &mut Slice<K, V>

impl<K, V> Default for wasmtime_environ::component::NameMap<K, V>

where K: Clone + Hash + Eq + Ord,

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

impl<K, V> Default for wasmtime_environ::prelude::IndexMap<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 std::collections::hash::map::IntoIter<K, V>

impl<K, V> Default for std::collections::hash::map::IntoKeys<K, V>

impl<K, V> Default for std::collections::hash::map::IntoValues<K, V>

impl<K, V> Default for std::collections::hash::map::Iter<'_, K, V>

impl<K, V> Default for std::collections::hash::map::IterMut<'_, K, V>

impl<K, V> Default for std::collections::hash::map::Keys<'_, K, V>

impl<K, V> Default for std::collections::hash::map::Values<'_, K, V>

impl<K, V> Default for std::collections::hash::map::ValuesMut<'_, K, V>

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

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

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

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

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

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

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

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

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

where S: Default,

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<K: EntityRef, V> Default for Intern<K, V>

impl<R> Default for DebugAbbrev<R>

where R: Default,

impl<R> Default for DebugAddr<R>

where R: Default,

impl<R> Default for DebugAranges<R>

where R: Default,

impl<R> Default for DebugCuIndex<R>

where R: Default,

impl<R> Default for DebugInfo<R>

where R: Default,

impl<R> Default for DebugLine<R>

where R: Default,

impl<R> Default for DebugLineStr<R>

where R: Default,

impl<R> Default for DebugLoc<R>

where R: Default,

impl<R> Default for DebugLocLists<R>

where R: Default,

impl<R> Default for DebugRanges<R>

where R: Default,

impl<R> Default for DebugRngLists<R>

where R: Default,

impl<R> Default for DebugStr<R>

where R: Default,

impl<R> Default for DebugStrOffsets<R>

where R: Default,

impl<R> Default for DebugTuIndex<R>

where R: Default,

impl<R> Default for DebugTypes<R>

where R: Default,

impl<R> Default for Dwarf<R>

where R: Default,

impl<R> Default for LocationLists<R>

where R: Default,

impl<R> Default for RangeIter<R>

where R: Reader,

impl<R> Default for RangeLists<R>

where R: Default,

impl<T> Default for &[T]

impl<T> Default for &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 wasmtime_environ::prelude::IndexSet<T>

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 wasmtime_environ::__core::iter::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 Cursor<T>

where T: Default,

impl<T> Default for LazyLock<T>

where T: Default,

impl<T> Default for Mutex<T>

where T: Default + ?Sized,

impl<T> Default for OnceLock<T>

impl<T> Default for ReentrantLock<T>

where T: Default,

impl<T> Default for RwLock<T>

where T: Default,

impl<T> Default for CfaRule<T>

where T: ReaderOffset,

impl<T> Default for IntoIter<T>

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

impl<T> Default for Lazy<T>

where T: Default,

impl<T> Default for Lazy<T>

where T: Default,

impl<T> Default for OnceBox<T>

impl<T> Default for OnceCell<T>

impl<T> Default for OnceCell<T>

impl<T> Default for ScalarBitSet<T>

where T: ScalarBitSetStorage,

impl<T> Default for Set<T>

impl<T> Default for SymbolMap<T>

where T: Default + SymbolMapEntry,

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 HashTable<T, A>

where A: Allocator + Default,

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

where A: Allocator + Default,

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

where S: Default,

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

where S: Default,

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

where T: ReaderOffset, S: UnwindContextStorage<T>,

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

where T: ReaderOffset, S: UnwindContextStorage<T>,

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,

impl<T: Default> Default for AllCallFunc<T>

impl<W> Default for DebugAbbrev<W>

where W: Default + Writer,

impl<W> Default for DebugFrame<W>

where W: Default + Writer,

impl<W> Default for DebugInfo<W>

where W: Default + Writer,

impl<W> Default for DebugLine<W>

where W: Default + Writer,

impl<W> Default for DebugLineStr<W>

where W: Default + Writer,

impl<W> Default for DebugLoc<W>

where W: Default + Writer,

impl<W> Default for DebugLocLists<W>

where W: Default + Writer,

impl<W> Default for DebugRanges<W>

where W: Default + Writer,

impl<W> Default for DebugRngLists<W>

where W: Default + Writer,

impl<W> Default for DebugStr<W>

where W: Default + Writer,

impl<W> Default for EhFrame<W>

where W: Default + Writer,

impl<W> Default for Sections<W>

where W: Default + Writer,