1.0.0[−][src]Trait maybe_std::default::Default

pub trait Default {
    fn default() -> Self;
}

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

`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 }
}

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Implementors

`impl Default for ()`[src]

`fn default()`[src]

Returns the default value of ()

`impl Default for Error`[src]

`fn default() -> Error`[src]

`impl Default for SipHasher`[src]

`fn default() -> SipHasher`[src]

`impl Default for RangeFull`[src]

`fn default() -> RangeFull`[src]

`impl Default for AtomicBool`[src]

`fn default() -> AtomicBool`[src]

Creates an AtomicBool initialized to false.

`impl Default for AtomicI8`[src]

`fn default() -> AtomicI8`[src]

`impl Default for AtomicI16`[src]

`fn default() -> AtomicI16`[src]

`impl Default for AtomicI32`[src]

`fn default() -> AtomicI32`[src]

`impl Default for AtomicI64`[src]

`fn default() -> AtomicI64`[src]

`impl Default for AtomicIsize`[src]

`fn default() -> AtomicIsize`[src]

`impl Default for AtomicU8`[src]

`fn default() -> AtomicU8`[src]

`impl Default for AtomicU16`[src]

`fn default() -> AtomicU16`[src]

`impl Default for AtomicU32`[src]

`fn default() -> AtomicU32`[src]

`impl Default for AtomicU64`[src]

`fn default() -> AtomicU64`[src]

`impl Default for AtomicUsize`[src]

`fn default() -> AtomicUsize`[src]

`impl Default for Duration`[src]

`fn default() -> Duration`[src]

`impl Default for bool`[src]

`fn default() -> bool`[src]

Returns the default value of false

`impl Default for char`[src]

`fn default() -> char`[src]

Returns the default value of \x00

`impl Default for f32`[src]

`fn default() -> f32`[src]

Returns the default value of 0.0

`impl Default for f64`[src]