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 struct
s
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 derive
d, it will use the default value for each field’s type.
§enum
s
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§
1.0.0 · sourcefn default() -> Self
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();
Making your own:
enum Kind {
A,
B,
C,
}
impl Default for Kind {
fn default() -> Self { Kind::A }
}
Object Safety§
Implementors§
impl Default for &str
impl Default for &CStr
impl Default for &OsStr
impl Default for &mut str
impl Default for byteorder::BigEndian
impl Default for byteorder::LittleEndian
impl Default for DwarfFileType
impl Default for RunTimeEndian
impl Default for Pointer
impl Default for PrefilterConfig
impl Default for Endianness
impl Default for Name
impl Default for ColorChoice
The default is Auto
.
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 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 anyhow::Chain<'_>
impl Default for EncoderState
impl Default for CompoundBitSet
impl Default for Hasher
impl Default for LineEncoding
impl Default for gimli::endianity::BigEndian
impl Default for gimli::endianity::LittleEndian
impl Default for Abbreviations
impl Default for AbbreviationsCache
impl Default for Augmentation
impl Default for BaseAddresses
impl Default for SectionBaseAddresses
impl Default for FrameTable
impl Default for gimli::write::dwarf::Dwarf
impl Default for FileInfo
impl Default for LocationListTable
impl Default for Expression
impl Default for RangeListTable
impl Default for LineStringTable
impl Default for gimli::write::str::StringTable
impl Default for DebugInfoOffsets
impl Default for UnitTable
impl Default for FinderBuilder
impl Default for object::endian::BigEndian
impl Default for object::endian::LittleEndian
impl Default for ImageSectionHeader
impl Default for RelocationSections
impl Default for VersionIndex
impl Default for object::read::pe::relocation::Relocation
impl Default for RelocationMap
impl Default for AuxSymbolSection
impl Default for FileHeader
impl Default for object::write::coff::writer::Relocation
impl Default for SectionHeader
impl Default for Symbol
impl Default for Class
impl Default for SectionIndex
impl Default for SymbolIndex
impl Default for MachOBuildVersion
impl Default for SectionRange
impl Default for OnceBool
impl Default for OnceNonZeroUsize
impl Default for AllocVec
impl Default for Size
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 Default for ColorSpec
impl Default for ComponentAliasSection
impl Default for ComponentBuilder
impl Default for CanonicalFunctionSection
impl Default for ComponentExportSection
impl Default for ComponentImportSection
impl Default for ComponentInstanceSection
impl Default for InstanceSection
impl Default for ComponentNameSection
impl Default for wasm_encoder::component::Component
impl Default for ComponentType
impl Default for ComponentTypeSection
impl Default for CoreTypeSection
impl Default for InstanceType
impl Default for ModuleType
impl Default for CodeSection
impl Default for DataSection
impl Default for CoreDumpSection
impl Default for CoreDumpStackSection
impl Default for ElementSection
impl Default for ExportSection
impl Default for FunctionSection
impl Default for GlobalSection
impl Default for ImportSection
impl Default for LinkingSection
impl Default for wasm_encoder::core::linking::SymbolTable
impl Default for MemorySection
impl Default for IndirectNameMap
impl Default for wasm_encoder::core::names::NameMap
impl Default for wasm_encoder::core::names::NameSection
impl Default for ProducersField
impl Default for ProducersSection
impl Default for wasm_encoder::core::Module
impl Default for TableSection
impl Default for TagSection
impl Default for TypeSection
impl Default for Config
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 wasmtime_environ::prelude::Box<str>
impl Default for wasmtime_environ::prelude::Box<CStr>
impl Default for wasmtime_environ::prelude::Box<OsStr>
impl Default for String
impl Default for AddressMapSection
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 WasmFileInfo
impl Default for WasmFunctionInfo
impl Default for wasmtime_environ::wasmparser::collections::hash::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 alloc::alloc::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 Condvar
impl Default for Global
impl<'a> Default for MetadataBuilder<'a>
impl<'a> Default for RecordBuilder<'a>
impl<'a> Default for DebugInfoData<'a>
impl<'a> Default for wasmtime_environ::NameSection<'a>
impl<'a, K, V> Default for hashbrown::map::Iter<'a, K, V>
impl<'a, K, V> Default for hashbrown::map::IterMut<'a, K, V>
impl<'a, K, V> Default for hashbrown::map::Keys<'a, K, V>
impl<'a, K, V> Default for hashbrown::map::Values<'a, K, V>
impl<'a, K, V> Default for hashbrown::map::ValuesMut<'a, K, V>
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 hashbrown::table::Iter<'a, T>
impl<'a, T> Default for IterHash<'a, T>
impl<'a, T> Default for IterHashMut<'a, T>
impl<'a, T> Default for hashbrown::table::IterMut<'a, T>
impl<'a, T> Default for OnceRef<'a, T>
impl<'data> Default for object::read::coff::section::SectionTable<'data>
impl<'data> Default for Version<'data>
impl<'data> Default for RelocationBlockIterator<'data>
impl<'data> Default for ObjectMap<'data>
impl<'data> Default for ObjectMapEntry<'data>
impl<'data> Default for Bytes<'data>
impl<'data> Default for ModuleTranslation<'data>
impl<'data, E> Default for LoadCommandIterator<'data, E>
impl<'data, Elf> Default for VersionTable<'data, Elf>where
Elf: FileHeader,
impl<'data, Elf, R> Default for object::read::elf::section::SectionTable<'data, Elf, R>where
Elf: FileHeader,
R: ReadRef<'data>,
impl<'data, Elf, R> Default for object::read::elf::symbol::SymbolTable<'data, Elf, R>where
Elf: FileHeader,
R: ReadRef<'data>,
impl<'data, Mach, R> Default for object::read::macho::symbol::SymbolTable<'data, Mach, R>where
Mach: MachHeader,
R: ReadRef<'data>,
impl<'data, R> Default for object::read::util::StringTable<'data, R>where
R: ReadRef<'data>,
impl<'data, R, Coff> Default for object::read::coff::symbol::SymbolTable<'data, R, Coff>where
R: ReadRef<'data>,
Coff: CoffHeader,
impl<'data, Xcoff> Default for object::read::xcoff::section::SectionTable<'data, Xcoff>where
Xcoff: FileHeader,
impl<'data, Xcoff, R> Default for object::read::xcoff::symbol::SymbolTable<'data, Xcoff, R>where
Xcoff: FileHeader,
R: ReadRef<'data>,
impl<'input, Endian> Default for EndianSlice<'input, Endian>
impl<A> Default for SmallVec<A>where
A: Array,
impl<A> Default for Box<str, A>where
A: Allocator + Default,
impl<A, B> Default for wasmtime_environ::__core::iter::Chain<A, B>
impl<B> Default for Cow<'_, B>
impl<E> Default for CompressionHeader32<E>
impl<E> Default for CompressionHeader64<E>
impl<E> Default for Sym32<E>
impl<E> Default for Sym64<E>
impl<E> Default for object::endian::aligned::I16<E>
impl<E> Default for object::endian::aligned::I32<E>
impl<E> Default for object::endian::aligned::I64<E>
impl<E> Default for object::endian::aligned::U16<E>
impl<E> Default for object::endian::aligned::U32<E>
impl<E> Default for object::endian::aligned::U64<E>
impl<E> Default for I16Bytes<E>
impl<E> Default for I32Bytes<E>
impl<E> Default for I64Bytes<E>
impl<E> Default for U16Bytes<E>
impl<E> Default for U32Bytes<E>
impl<E> Default for U64Bytes<E>
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>
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 hashbrown::set::Iter<'_, K>
impl<K> Default for EntitySet<K>where
K: EntityRef,
impl<K, A> Default for hashbrown::set::IntoIter<K, A>where
A: Allocator,
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 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 IterMut2<'_, 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> Default for wasmtime_environ::component::NameMap<K, V>
impl<K, V> Default for wasmtime_environ::prelude::Box<Slice<K, V>>
impl<K, V> Default for PrimaryMap<K, V>where
K: EntityRef,
impl<K, V> Default for SecondaryMap<K, V>
impl<K, V> Default for wasmtime_environ::wasmparser::collections::IndexMap<K, V>
impl<K, V> Default for Map<K, V>
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, A> Default for hashbrown::map::IntoIter<K, V, A>where
A: Allocator,
impl<K, V, A> Default for hashbrown::map::IntoKeys<K, V, A>where
A: Allocator,
impl<K, V, A> Default for hashbrown::map::IntoValues<K, V, A>where
A: Allocator,
impl<K, V, A> Default for alloc::collections::btree::map::IntoIter<K, V, A>
impl<K, V, A> Default for alloc::collections::btree::map::IntoKeys<K, V, A>
impl<K, V, A> Default for alloc::collections::btree::map::IntoValues<K, V, A>
impl<K, V, S> Default for indexmap::map::IndexMap<K, V, S>where
S: Default,
impl<K, V, S> Default for std::collections::hash::map::HashMap<K, V, S>where
S: Default,
impl<K, V, S, A> Default for hashbrown::map::HashMap<K, V, S, A>
impl<K, V, S, A> Default for hashbrown::map::HashMap<K, V, S, A>
impl<K: EntityRef, V> Default for Intern<K, V>
impl<O> Default for F32<O>
impl<O> Default for F64<O>
impl<O> Default for zerocopy::byteorder::I16<O>
impl<O> Default for zerocopy::byteorder::I32<O>
impl<O> Default for zerocopy::byteorder::I64<O>
impl<O> Default for I128<O>
impl<O> Default for zerocopy::byteorder::U16<O>
impl<O> Default for zerocopy::byteorder::U32<O>
impl<O> Default for zerocopy::byteorder::U64<O>
impl<O> Default for U128<O>
impl<R> Default for gimli::read::abbrev::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 gimli::read::dwarf::Dwarf<R>where
R: Default,
impl<R> Default for RangeIter<R>where
R: Reader,
impl<R> Default for DebugCuIndex<R>where
R: Default,
impl<R> Default for DebugTuIndex<R>where
R: Default,
impl<R> Default for gimli::read::line::DebugLine<R>where
R: Default,
impl<R> Default for gimli::read::loclists::DebugLoc<R>where
R: Default,
impl<R> Default for gimli::read::loclists::DebugLocLists<R>where
R: Default,
impl<R> Default for LocationLists<R>where
R: Default,
impl<R> Default for gimli::read::rnglists::DebugRanges<R>where
R: Default,
impl<R> Default for gimli::read::rnglists::DebugRngLists<R>where
R: Default,
impl<R> Default for RangeLists<R>where
R: Default,
impl<R> Default for gimli::read::str::DebugLineStr<R>where
R: Default,
impl<R> Default for gimli::read::str::DebugStr<R>where
R: Default,
impl<R> Default for DebugStrOffsets<R>where
R: Default,
impl<R> Default for gimli::read::unit::DebugInfo<R>where
R: Default,
impl<R> Default for DebugTypes<R>where
R: Default,
impl<T> Default for &[T]
impl<T> Default for &indexmap::set::slice::Slice<T>
impl<T> Default for &mut [T]
impl<T> Default for CfaRule<T>where
T: ReaderOffset,
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 ScalarBitSet<T>where
T: ScalarBitSetStorage,
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 once_cell::sync::Lazy<T>where
T: Default,
impl<T> Default for once_cell::sync::OnceCell<T>
impl<T> Default for once_cell::unsync::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> Default for PackedOption<T>where
T: ReservedValue,
impl<T> Default for wasmtime_environ::prelude::Box<[T]>
impl<T> Default for wasmtime_environ::prelude::Box<Slice<T>>
impl<T> Default for wasmtime_environ::prelude::Box<T>where
T: Default,
impl<T> Default for wasmtime_environ::prelude::Vec<T>
impl<T> Default for EntityList<T>where
T: EntityRef + ReservedValue,
Create an empty list.