Trait wasmtime_environ::__core::default::Default

1.0.0 · source · [−]
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 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.

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 a new lazy value using Default as the initializing function.

Creates an empty HashMap<K, V, S>, with the Default value for the hasher.

Creates a Condvar which is ready to be waited on and notified.

Creates a new empty cell.

Example
#![feature(once_cell)]

use std::lazy::SyncOnceCell;

fn main() {
    assert_eq!(SyncOnceCell::<()>::new(), SyncOnceCell::default());
}

Creates a new RwLock<T>, with the Default value for T.

Creates a Mutex<T>, with the Default value for T.

Constructs an empty OsString.

Constructs a new RandomState.

Creates an empty HashSet<T, S> with the Default value for the hasher.

Creates an empty CString.

Creates an empty OsStr.

Creates a new DefaultHasher using new. See its documentation for more.

Returns the default value of 0.0

Returns the default value of \x00

Returns the default value of ()

Returns the default value of 0.0

Returns the default value of false

Returns the default value of 0

Returns the default value of 0

Returns the default value of 0

Returns the default value of 0

Returns the default value of 0

Creates an empty mutable str

Returns the default value of 0

Returns the default value of 0

Creates an empty slice.

Returns the default value of 0

Returns the default value of 0

Returns the default value of 0

Creates a mutable empty slice.

Returns the default value of 0

Creates an empty str

Returns the default value of 0

Creates a new Rc<T>, with the Default value for T.

Examples
use std::rc::Rc;

let x: Rc<i32> = Default::default();
assert_eq!(*x, 0);

Creates an empty String.

Creates an empty Vec<T>.

Creates an owned Cow<’a, B> with the default value for the contained owned value.

Creates an empty BTreeMap.

Creates a Box<T>, with the Default value for T.

Constructs a new Weak<T>, without allocating any memory. Calling upgrade on the return value always gives None.

Examples
use std::rc::Weak;

let empty: Weak<i64> = Default::default();
assert!(empty.upgrade().is_none());

Creates a new Arc<T>, with the Default value for T.

Examples
use std::sync::Arc;

let x: Arc<i32> = Default::default();
assert_eq!(*x, 0);

Creates an empty BinaryHeap<T>.

Creates an empty BTreeSet.

Constructs a new Weak<T>, without allocating memory. Calling upgrade on the return value always gives None.

Examples
use std::sync::Weak;

let empty: Weak<i64> = Default::default();
assert!(empty.upgrade().is_none());

Creates an empty deque.

Creates an empty LinkedList<T>.

Return an empty IndexMap

Return an empty IndexSet

Creates an empty HashMap<K, V, S, A>, with the Default value for the hasher and allocator.

Creates an empty HashSet<T, S> with the Default value for the hasher.

Implementors

Create an empty list.