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.