comprehensive-rust/src/std-traits/default.md

minutes
5

The Default Trait

Default trait produces a default value for a type.

#[derive(Debug, Default)]
struct Derived {
    x: u32,
    y: String,
    z: Implemented,
}

#[derive(Debug)]
struct Implemented(String);

impl Default for Implemented {
    fn default() -> Self {
        Self("John Smith".into())
    }
}

fn main() {
    let default_struct = Derived::default();
    println!("{default_struct:#?}");

    let almost_default_struct = Derived {
        y: "Y is set!".into(),
        ..Derived::default()
    };
    println!("{almost_default_struct:#?}");

    let nothing: Option<Derived> = None;
    println!("{:#?}", nothing.unwrap_or_default());
}

• It can be implemented directly or it can be derived via #[derive(Default)].
• A derived implementation will produce a value where all fields are set to their default values.
  • This means all types in the struct must implement Default too.
• Standard Rust types often implement Default with reasonable values (e.g. 0, "", etc).
• The partial struct initialization works nicely with default.
• The Rust standard library is aware that types can implement Default and provides convenience methods that use it.
• The .. syntax is called struct update syntax.