`Default` 特征

Default 特征会为类型生成默认值。

#[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.

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.

Comprehensive Rust 🦀

Default 特征

`Default` 特征