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,
}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§
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§
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 AsciiChar
impl Default for LineEnding
impl Default for BigEndian
impl Default for LittleEndian
impl Default for PrefilterConfig
impl Default for Encoding
impl Default for Value
The default value is Value::Null.
This is useful for handling omitted Value fields when deserializing.
§Examples
use serde_json::Value;
#[derive(Deserialize)]
struct Settings {
level: i32,
#[serde(default)]
extras: Value,
}
let data = r#" { "level": 42 } "#;
let s: Settings = serde_json::from_str(data)?;
assert_eq!(s.level, 42);
assert_eq!(s.extras, Value::Null);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 DictManager
impl Default for ApTracking
impl Default for ReferenceManager
impl Default for cairo_vm::stdlib::prelude::Box<str>
impl Default for cairo_vm::stdlib::prelude::Box<CStr>
impl Default for cairo_vm::stdlib::prelude::Box<OsStr>
impl Default for String
impl Default for Felt
Defaults to Felt::ZERO.
impl Default for ExecutionScopes
impl Default for Program
impl Default for Relocatable
impl Default for OutputBuiltinRunner
impl Default for ExecutionResources
impl Default for RunResources
impl Default for DeducedOperands
impl Default for VirtualMachineBuilder
impl Default for Memory
impl Default for MemorySegmentManager
impl Default for cairo_vm::with_std::alloc::Global
impl Default for System
impl Default for Error
impl Default for DefaultHasher
impl Default for RandomState
impl Default for SipHasher
impl Default for PhantomPinned
impl Default for RangeFull
impl Default for Alignment
Returns Alignment::MIN, which is valid for any type.
impl Default for Rc<str>
impl Default for Rc<CStr>
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 Arc<str>
impl Default for Arc<CStr>
impl Default for Condvar
impl Default for cairo_vm::with_std::time::Duration
impl Default for CString
impl Default for OsString
impl Default for FileTimes
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 Adler32
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 allocator_api2::stable::alloc::global::Global
impl Default for anyhow::Chain<'_>
impl Default for EmptyFlags
impl Default for SizeWriter
impl Default for Lsb0
impl Default for Msb0
impl Default for Eager
impl Default for block_buffer::Lazy
impl Default for bzip2::Compression
impl Default for Hasher
impl Default for Limb
impl Default for Reciprocal
impl Default for MacError
impl Default for InvalidBufferSize
impl Default for InvalidOutputSize
impl Default for Crc
impl Default for GzBuilder
impl Default for GzHeader
impl Default for flate2::Compression
impl Default for itoa::Buffer
impl Default for PedersenStarkCurve
impl Default for PoseidonCairoStark252
impl Default for FinderBuilder
impl Default for CompressorOxide
impl Default for DecompressorOxide
impl Default for InflateState
impl Default for num_bigint::bigint::BigInt
impl Default for BigUint
impl Default for FactorizationConfig
impl Default for PrimalityTestConfig
impl Default for OnceBool
impl Default for OnceNonZeroUsize
impl Default for ParamsString
impl Default for Params
impl Default for FormatterOptions
impl Default for ThreadRng
impl Default for OsRng
impl Default for Decimal
impl Default for ryu::buffer::Buffer
impl Default for IgnoredAny
impl Default for Map<String, Value>
impl Default for Sha1Core
impl Default for Keccak224Core
impl Default for Keccak256Core
impl Default for Keccak256FullCore
impl Default for Keccak384Core
impl Default for Keccak512Core
impl Default for Sha3_224Core
impl Default for Sha3_256Core
impl Default for Sha3_384Core
impl Default for Sha3_512Core
impl Default for Shake128Core
impl Default for Shake256Core
impl Default for PoseidonHasher
impl Default for starknet_ff::FieldElement
impl Default for time::duration::Duration
impl Default for B0
impl Default for B1
impl Default for Z0
impl Default for Equal
impl Default for Greater
impl Default for Less
impl Default for UTerm
impl Default for Const
impl Default for Mut
impl Default for NullPtrError
impl Default for DateTime
impl Default for FileOptions
impl Default for CCtx<'_>
impl Default for DCtx<'_>
impl Default for vec128_storage
impl Default for vec256_storage
impl Default for vec512_storage
impl<'a> Default for CairoRunConfig<'a>
impl<'a> Default for PrettyFormatter<'a>
impl<'a> Default for Compressor<'a>
impl<'a> Default for Decompressor<'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, M, T, O> Default for BitDomain<'a, M, T, O>where
M: Mutability,
T: 'a + BitStore,
O: BitOrder,
Address<M, BitSlice<T, O>>: Referential<'a>,
Address<M, BitSlice<<T as BitStore>::Unalias, O>>: Referential<'a>,
<Address<M, BitSlice<T, O>> as Referential<'a>>::Ref: Default,
<Address<M, BitSlice<<T as BitStore>::Unalias, O>> as Referential<'a>>::Ref: Default,
impl<'a, M, T, O> Default for Domain<'a, M, T, O>where
M: Mutability,
T: 'a + BitStore,
O: BitOrder,
Address<M, T>: Referential<'a>,
Address<M, [<T as BitStore>::Unalias]>: SliceReferential<'a>,
<Address<M, [<T as BitStore>::Unalias]> as Referential<'a>>::Ref: Default,
impl<'a, T> Default for OnceRef<'a, T>
impl<A> Default for allocator_api2::stable::boxed::Box<str, A>
impl<A, B> Default for cairo_vm::with_std::iter::Chain<A, B>
impl<A, O> Default for BitArray<A, O>where
A: BitViewSized,
O: BitOrder,
impl<B> Default for Cow<'_, B>
impl<BlockSize, Kind> Default for BlockBuffer<BlockSize, Kind>
impl<E, I, L> Default for Configuration<E, I, L>
impl<F> Default for DefaultTranscript<F>
impl<F> Default for lambdaworks_math::field::element::FieldElement<F>where
F: IsField,
impl<F, D, const NUM_BYTES: usize> Default for FieldElementBackend<F, D, NUM_BYTES>where
D: Digest,
impl<F, D, const NUM_BYTES: usize> Default for FieldElementVectorBackend<F, D, NUM_BYTES>where
D: Digest,
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 cairo_vm::with_std::ops::Range<Idx>where
Idx: Default,
impl<Idx> Default for core::range::Range<Idx>where
Idx: Default,
impl<Inner> Default for Frozen<Inner>where
Inner: Default + Mutability,
impl<K, V> Default for BTreeMap<K, V>
impl<K, V> Default for Keys<'_, K, V>
impl<K, V> Default for alloc::collections::btree::map::Range<'_, K, V>
impl<K, V> Default for Values<'_, K, V>
impl<K, V, A> Default for alloc::collections::btree::map::IntoIter<K, V, A>
impl<K, V, A> Default for IntoKeys<K, V, A>
impl<K, V, A> Default for IntoValues<K, V, A>
impl<K, V, S> Default for cairo_vm::with_std::collections::HashMap<K, V, S>where
S: Default,
impl<K, V, S, A> Default for hashbrown::map::HashMap<K, V, S, A>
impl<M, T, O> Default for BitPtrRange<M, T, O>
impl<MOD, const LIMBS: usize> Default for Residue<MOD, LIMBS>where
MOD: ResidueParams<LIMBS>,
impl<P> Default for CubicExtField<P>where
P: CubicExtConfig,
impl<P> Default for QuadExtField<P>where
P: QuadExtConfig,
impl<P> Default for TreePoseidon<P>
impl<P> Default for BatchPoseidonTree<P>
impl<P, const N: usize> Default for Fp<P, N>where
P: FpConfig<N>,
impl<R> Default for BitEnd<R>where
R: Default + BitRegister,
impl<R> Default for BitIdx<R>where
R: Default + BitRegister,
impl<R> Default for BitIdxError<R>where
R: Default + BitRegister,
impl<R> Default for BitMask<R>where
R: Default + BitRegister,
impl<R> Default for BitPos<R>where
R: Default + BitRegister,
impl<R> Default for BitSel<R>where
R: Default + BitRegister,
impl<T> Default for &[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.