Go Enum和iota

Golang没有内置的enum类型，通常都是用常量来模拟。如下例所示:

package enum1

import (
    "fmt"
    "testing"
)

type Hero int

const (
    IRONMAN   Hero = 0
    SPIDERMAN Hero = 1
    BATMAN    Hero = 2
)

func TestName(t *testing.T) {
    ironman := IRONMAN
    fmt.Println(ironman == SPIDERMAN)
    fmt.Println(ironman == IRONMAN)
}

输出为false、true。我们可以用iota这个go的语法糖来简化这个代码

import (
    "fmt"
    "testing"
)

type Hero int

const (
    IRONMAN   Hero = iota
    SPIDERMAN
    BATMAN
)

func TestName(t *testing.T) {
    ironman := IRONMAN
    fmt.Println(ironman == SPIDERMAN)
    fmt.Println(ironman == IRONMAN)
}

输出结果和上面一样。下面介绍一下iota的语法

• iota的初始值为0
• 每经过一个ConstSpec语句，iota会递增。在同一个ConstSpec中出现的iota的值一样。如 a, b = iota, iota，a和b的值相同。
• 每当const关键字出现，iota会重置为0

const (
    // iota reset to 0
    // c0 = 0
    c0 = iota
    // c1 = 1
    c1 = iota
    // c2 = 2
    c2 = iota
)

const (
    // iota reset to 0
    // a = 1
    a = 1 << iota
    // b = 2
    b = 1 << iota
    // c = 4
    c = 1 << iota
)

const (
    // u = 0
    u = iota * 42
    // v = 42.0
    v float64 = iota * 42
    // w = 84
    w = iota * 42
)

const x = 0
const y = 0

// 在表达式列表中，每个iota的值都相同，iota仅仅在每个ConstSpec之后递增
const (
    bit0, mask0 = 1 << iota, 1<<iota - 1 // bit0 == 1, mask0 == 0
    bit1, mask1                          // bit1 == 2, mask1 == 1
    _, _                                 // skips iota == 2
    bit3, mask3                          // bit3 == 8, mask3 == 7
)

参考资料

https://stackoverflow.com/questions/14426366/what-is-an-idiomatic-way-of-representing-enums-in-go