pub trait Default {
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.
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
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 }
}
Implementations on Foreign Types
Creates an UnsafeCell
, with the Default
value for T.
impl<T, const LANES: usize> Default for Simd<T, LANES> where
T: SimdElement + Default,
LaneCount<LANES>: SupportedLaneCount,
impl<T, const LANES: usize> Default for Simd<T, LANES> where
T: SimdElement + Default,
LaneCount<LANES>: SupportedLaneCount,
Creates an AtomicBool
initialized to false
.
Creates a mutable empty slice.
impl<T, const LANES: usize> Default for Mask<T, LANES> where
T: MaskElement,
LaneCount<LANES>: SupportedLaneCount,
impl<T, const LANES: usize> Default for Mask<T, LANES> where
T: MaskElement,
LaneCount<LANES>: SupportedLaneCount,
pub fn default() -> GenericArray<T, N>
Creates an empty LinkedList<T>
.
Creates an empty BinaryHeap<T>
.
pub fn default() -> BuildHasherDefault<H>
impl Default for AHasher
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 desireable 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);
pub fn default() -> AHasher
pub fn default() -> AHasher
Constructs a new [AHasher] with fixed keys.
If std
is enabled these will be generated upon first invocation.
Otherwise if the compile-time-rng
feature is enabled these will be generated at compile time.
If neither of these features are available, hardcoded constants will be used.
Because the values are fixed, different hashers will all hash elements the same way. This could make hash values predictable, if DOS attacks are a concern. If this behaviour is not required, it may be preferable to use [RandomState] instead.
Examples
use ahash::AHasher;
use std::hash::Hasher;
let mut hasher_1 = AHasher::default();
let mut hasher_2 = AHasher::default();
hasher_1.write_u32(1234);
hasher_2.write_u32(1234);
assert_eq!(hasher_1.finish(), hasher_2.finish());