The Default Trait

Default trait produces a default value for a type.

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

struct Implemented(String);

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

fn main() {
    let default_struct = Derived::default();

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

    let nothing: Option<Derived> = None;
    println!("{:#?}", nothing.unwrap_or_default());
This slide should take about 5 minutes.
  • 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.