枚举

Like tuples, enums can also be destructured by matching:

模式还可用于将变量绑定到值的某些部分。这是您检查类型结构的方式。我们先从简单的“enum”类型开始：

enum Result {
    Ok(i32),
    Err(String),
}

fn divide_in_two(n: i32) -> Result {
    if n % 2 == 0 {
        Result::Ok(n / 2)
    } else {
        Result::Err(format!("cannot divide {n} into two equal parts"))
    }
}

fn main() {
    let n = 100;
    match divide_in_two(n) {
        Result::Ok(half) => println!("{n} divided in two is {half}"),
        Result::Err(msg) => println!("sorry, an error happened: {msg}"),
    }
}

在这里，我们使用了分支来解构“Result”值。在第一个分支中，“half”被绑定到“Ok”变体中的值。在第二个分支中，“msg”被绑定到错误消息。

“if”/“else”表达式将返回一个枚举，该枚举之后会使用“match”进行解封装。
您可以尝试在枚举定义中添加第三个变体，并在运行代码时显示错误。指出代码现在有哪些地方还不详尽，并说明编译器会如何尝试给予提示。
The values in the enum variants can only be accessed after being pattern matched.
Demonstrate what happens when the search is inexhaustive. Note the advantage the Rust compiler provides by confirming when all cases are handled.
Save the result of divide_in_two in the result variable and match it in a loop. That won’t compile because msg is consumed when matched. To fix it, match &result instead of result. That will make msg a reference so it won’t be consumed. This “match ergonomics” appeared in Rust 2018. If you want to support older Rust, replace msg with ref msg in the pattern.

Comprehensive Rust 🦀

枚举