A trait for giving a type a useful default value.

Sometimes, you want to fall back to some kind of default value, anddon’t particularly care what it is. This comes up often withstructsthat define a set of options:

structSomeOptions {    foo: i32,    bar: f32,}

How can we define some default values? You can useDefault:

#[derive(Default)]structSomeOptions {    foo: i32,    bar: f32,}fnmain() {letoptions: SomeOptions = Default::default();}

Now, you get all of the default values. Rust implementsDefault for various primitive types.

If you want to override a particular option, but still retain the other defaults:

fnmain() {letoptions = SomeOptions { foo:42, ..Default::default() };}

§Derivable

This trait can be used with#[derive] if all of the type’s fields implementDefault. Whenderived, it will use the default value for each field’s type.

§enums

When using#[derive(Default)] on anenum, you need to choose which unit variant will bedefault. You do this by placing the#[default] attribute on the variant.

#[derive(Default)]enumKind {#[default]A,    B,    C,}

You cannot use the#[default] attribute on non-unit or non-exhaustive variants.

The#[default] attribute was stabilized in Rust 1.62.0.

§How can I implementDefault?

Provide an implementation for thedefault() method that returns the value ofyour type that should be the default:

enumKind {    A,    B,    C,}implDefaultforKind {fndefault() ->Self{ Kind::A }}

§Examples

#[derive(Default)]structSomeOptions {    foo: i32,    bar: f32,}