The Default Trait

Default trait provides a default implementation of a trait.

1. #[derive(Debug, Default)]
2. struct Derived {
3. x: u32,
4. y: String,
5. z: Implemented,
6. }

8. #[derive(Debug)]
9. struct Implemented(String);

11. impl Default for Implemented {
12. fn default() -> Self {
13. Self("John Smith".into())
14. }
15. }

17. fn main() {
18. let default_struct: Derived = Default::default();
19. println!("{default_struct:#?}");

21. let almost_default_struct = Derived {
22. y: "Y is set!".into(),
23. ..Default::default()
24. };
25. println!("{almost_default_struct:#?}");

27. let nothing: Option<Derived> = None;
28. println!("{:#?}", nothing.unwrap_or_default());
29. }

• It can be implemented directly or it can be derived via #[derive(Default)].
• Derived implementation will produce an instance 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 copy works nicely with default.
• Rust standard library is aware that types can implement Default and provides convenience methods that use it.