core::traits::Default

A trait for giving a type a useful default value. Cairo implements Default for various primitives types.

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.

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 can even use the #[default] attribute even on non-unit variants, provided that the associated type implements Default.

Signature

pub trait Default

Examples

#[derive(Drop, Default, PartialEq)]
struct SomeOptions {
    foo: i32,
    bar: u32,
}

assert!(Default::default() == SomeOptions { foo: 0, bar: 0 });

To implement Default, provide an implementation for the default() method that returns the value of your type that should be the default:

#[derive(Copy, Drop)]
enum Kind {
    A,
    B,
    C,
}

impl DefaultKind of Default {
    fn default() -> Kind { Kind::A }
}

Trait functions

default

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.

Signature

fn default() -> T

Examples

let i: i8 = Default::default();
let (x, y): (Option, u64) = Default::default();
let (a, b, (c, d)): (i32, u32, (bool, bool)) = Default::default();