在 Rust 中如何将值推送到枚举 struct 内的 vec

发布于03月11日

如何将值推送到Rust中枚举 struct 中的vec？

我正在try 弄清楚如何将值推送到定义为 struct 的枚举内的vec.

以下是我try 过的一些设置:

enum Widget {
    Alfa { strings: Vec<String> },
}

fn main() {
    let wa = Widget::Alfa { strings: vec![] };

    // wa.strings.push("a".to_string()); 
    // no field `strings` on type `Widget`

    // wa.Alfa.strings.push("a".to_string()); 
    // no field `Alfa` on type `Widget`

    // wa.alfa.strings.push("a".to_string()); 
    // no field `alfa` on type `Widget`

    // wa.Widget::Alfa.strings.push("a".to_string()); 
    // expected one of `(`, `.`, `;`, `?`, `}`, or an operator, found `::`

    // wa["strings"].push("a".to_string()); 
    // cannot index into a value of type `Widget`
}

是否可以在eMum中创建VEC后对其进行更新？如果是这样的话，我们该如何go 做呢？

推荐答案

您不能直接访问枚举变量上的字段，因为编译器只知道值是枚举类型(Widget)，而不知道它具有哪个枚举变量.您必须对枚举进行 struct 分析，例如使用match:

let mut wa = Widget::Alfa { strings: vec![] };

match &mut wa {
    Widget::Alfa { strings /*: &mut Vec<String> */ } => {
        strings.push("a".to_string());
    }

    // if the enum has more variants, you must have branches for these as well.
    // if you only care about `Widget::Alfa`, a wildcard branch like this is often a
    // good choice.
    _ => unreachable!(), // panics if ever reached, which we know in this case it won't
                         // because we just assigned `wa` before the `match`.
}

或者，您也可以改用if let:

let mut wa = Widget::Alfa { strings: vec![] };

if let Widget::Alfa { strings } = &mut wa {
    strings.push("a".to_string());
} else {
    // some other variant than `Widget::Alfa`, equivalent to the wildcard branch
    // of the `match`. you can omit this, which would just do nothing
    // if it doesn't match.
    unreachable!()
}