`Default` 特徵

Default 特徵會產生型別的預設值。

#[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());
}

This slide should take about 5 minutes.

這可以直接實作，也可以透過 #[derive(Default)] 衍生得出。
A derived implementation will produce a value where all fields are set to their default values.
- 也就是說，該結構體中的所有型別也都必須實作 Default。
標準的 Rust 型別通常會以合理的值 (例如 0、"" 等等) 實作 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.

Comprehensive Rust 🦀

Default 特徵

`Default` 特徵