rust - 如何将枚举值与整数匹配？

Question

我可以像这样获取枚举的整数值:

enum MyEnum {
    A = 1,
    B,
    C,
}

let x = MyEnum::C as i32;

但我似乎做不到:

match x {
    MyEnum::A => {}
    MyEnum::B => {}
    MyEnum::C => {}
    _ => {}
}

我如何匹配枚举的值或尝试将 x 转换回 MyEnum？

我可以看到像这样的函数对枚举很有用，但它可能不存在:

impl MyEnum {
    fn from<T>(val: &T) -> Option<MyEnum>;
}

Answer 1

从 Rust 1.34 开始，我建议实现 TryFrom :

use std::convert::TryFrom;

impl TryFrom<i32> for MyEnum {
    type Error = ();

    fn try_from(v: i32) -> Result<Self, Self::Error> {
        match v {
            x if x == MyEnum::A as i32 => Ok(MyEnum::A),
            x if x == MyEnum::B as i32 => Ok(MyEnum::B),
            x if x == MyEnum::C as i32 => Ok(MyEnum::C),
            _ => Err(()),
        }
    }
}

然后你可以使用TryInto并处理可能的错误:

use std::convert::TryInto;

fn main() {
    let x = MyEnum::C as i32;

    match x.try_into() {
        Ok(MyEnum::A) => println!("a"),
        Ok(MyEnum::B) => println!("b"),
        Ok(MyEnum::C) => println!("c"),
        Err(_) => eprintln!("unknown number"),
    }
}

如果您有大量变体，可以使用宏根据枚举的定义自动创建 TryFrom 的并行实现:

macro_rules! back_to_enum {
    ($(#[$meta:meta])* $vis:vis enum $name:ident {
        $($(#[$vmeta:meta])* $vname:ident $(= $val:expr)?,)*
    }) => {
        $(#[$meta])*
        $vis enum $name {
            $($(#[$vmeta])* $vname $(= $val)?,)*
        }

        impl std::convert::TryFrom<i32> for $name {
            type Error = ();

            fn try_from(v: i32) -> Result<Self, Self::Error> {
                match v {
                    $(x if x == $name::$vname as i32 => Ok($name::$vname),)*
                    _ => Err(()),
                }
            }
        }
    }
}

back_to_enum! {
    enum MyEnum {
        A = 1,
        B,
        C,
    }
}

另见:

• How to ensure every enum variant can be returned from a specific function at compile time?
• Generating documentation in macros