Trait wasmtime_environ::__core::prelude::rust_2024::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.

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.

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

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

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

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

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

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

Constructs a new RandomState.

Creates a new empty cell.

Example
#![feature(once_cell)]

use std::lazy::SyncOnceCell;

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

Creates an empty OsStr.

Creates a new lazy value using Default as the initializing function.

Constructs an empty OsString.

Creates an empty slice.

Returns the default value of 0

Returns the default value of 0

Creates an empty str

Returns the default value of 0

Creates a mutable empty slice.

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 ()

Returns the default value of 0.0

Returns the default value of false

Returns the default value of \x00

Returns the default value of 0

Returns the default value of 0

Returns the default value of 0

Returns the default value of 0.0

Returns the default value of 0

Creates an empty deque.

Creates an empty BinaryHeap<T>.

Creates an empty BTreeSet.

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);

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());

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 an empty BTreeMap.

Creates an empty Vec<T>.

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

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

Creates an empty LinkedList<T>.

Creates an empty CString.

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 String.

Return an empty IndexSet

Return an empty IndexMap

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.