Trait core::default::Default1.0.0[][src]

pub trait Default: Sized {
    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,
}
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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();
}
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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() };
}
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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 }
}
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Examples

#[derive(Default)]
struct SomeOptions {
    foo: i32,
    bar: f32,
}
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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();
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Making your own:

enum Kind {
    A,
    B,
    C,
}

impl Default for Kind {
    fn default() -> Self { Kind::A }
}
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Implementors

Creates an empty mutable str

Creates an empty str

Returns the default value of ()

Creates an AtomicBool initialized to false.

Returns the default value of false

Returns the default value of \x00

Returns the default value of 0.0

Returns the default value of 0.0

Returns the default value of 0

Returns the default value of 0

Returns the default value of 0

Returns the default value of 0

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

Returns the default value of 0

Returns the default value of 0

Returns the default value of 0

Creates an empty slice.

Creates a mutable empty slice.

Returns None.

Examples

let opt: Option<u32> = Option::default();
assert!(opt.is_none());
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Creates a null AtomicPtr<T>.

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

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

Creates an UnsafeCell, with the Default value for T.

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