如何在 Rust 中创建具有常量值的枚举? [英] How can I create enums with constant values in Rust?

问题描述

我可以这样做:

enum MyEnum {
    A(i32),
    B(i32),
}

但不是这个:

enum MyEnum {
    A(123), // 123 is a constant
    B(456), // 456 is a constant
}

我可以使用单个字段为 A 和 B 创建结构，然后实现该字段，但我认为可能有更简单的方法.有吗?

I can create the structures for A and B with a single field and then implement that field, but I think there might be an easier way. Is there any?

推荐答案

回答这个问题的最好方法是弄清楚为什么要在枚举中使用常量:您是将值与每个变体相关联，还是希望每个变体都是那个值(就像 C 或 C++ 中的 enum 一样)?

The best way to answer this is working out why you want constants in an enum: are you associating a value with each variant, or do you want each variant to be that value (like an enum in C or C++)?

对于第一种情况，让枚举变体不带数据并创建一个函数可能更有意义:

For the first case, it probably makes more sense to just leave the enum variants with no data, and make a function:

enum MyEnum {
    A,
    B,
}

impl MyEnum {
    fn value(&self) -> i32 {
        match *self {
            MyEnum::A => 123,
            MyEnum::B => 456,
        }
    }
}
// call like some_myenum_value.value()

这种方法可以多次应用，将许多单独的信息与每个变体相关联，例如也许你想要一个 .name() ->&'static str 方法也是如此.将来，这些函数甚至可以标记为const 函数.

This approach can be applied many times, to associate many separate pieces of information with each variant, e.g. maybe you want a .name() -> &'static str method too. In the future, these functions can even be marked as const functions.

对于第二种情况，您可以指定显式整数标记值，就像 C/C++ 一样:

For the second case, you can assign explicit integer tag values, just like C/C++:

enum MyEnum {
    A = 123,
    B = 456,
}

这可以以所有相同的方式match，但也可以转换为整数MyEnum::A as i32.(请注意，像 MyEnum::A | MyEnum::B 这样的计算在 Rust 中并不自动合法:枚举具有特定值，它们不是位标志.)

This can be matched on in all the same ways, but can also be cast to an integer MyEnum::A as i32. (Note that computations like MyEnum::A | MyEnum::B are not automatically legal in Rust: enums have specific values, they're not bit-flags.)

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