Default 特征

Default 特征会为类型生成默认值。

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();
19. println!("{default_struct:#?}");

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

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

This slide should take about 5 minutes.

• 系统可以直接实现它，也可以通过 #[derive(Default)] 派生出它。
• A derived implementation will produce a value where all fields are set to their default values.
• 这意味着，该结构体中的所有类型也都必须实现 Default。
• 标准的 Rust 类型通常会以合理的值（例如 0“” `等）实现Default`。
• 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.