Trait Default

pub trait Default: Sized

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:

# #[allow(dead_code)]
struct SomeOptions {
    foo: i32,
    bar: f32,
}

How can we define some default values? You can use Default:

# #[allow(dead_code)]
#[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 primitive types.

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

# #[allow(dead_code)]
# #[derive(Default)]
# struct SomeOptions {
#     foo: i32,
#     bar: f32,
# }
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.

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

How can I implement Default?

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

# #![allow(dead_code)]
enum Kind {
    A,
    B,
    C,
}

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

Examples

# #[allow(dead_code)]
#[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:

# #[allow(dead_code)]
enum Kind {
    A,
    B,
    C,
}

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

Implementors