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