「Golang系列」可視化圖解 Go Enums 和 iota（二）

每天三分鐘，知識更輕鬆。
歡迎關注同名微信公眾賬號極客24h。

基本表達式

type Timezone int
const (
 // iota: 0, EST: -5 
 EST Timezone = -(5 + iota)
 
 // iota: 1, CST: -6
 CST
 
 // iota: 2, MST: -7
 MST
 
 // iota: 3, MST: -8
 PST
)

重置iota

// iota reset: it will be 0.
const (
 Zero = iota // Zero = 0
 One // One = 1
)
// iota reset: will be 0 again
const (
 Two = iota // Two = 0
)
// iota: reset
const Three = iota // Three = 0

跳過一些值

type Timezone int
const (
 // iota: 0, EST: -5
 EST Timezone = -(5 + iota)
 // _ is the blank identifier
 // iota: 1
 _
 // iota: 2, MST: -7
 MST
 // iota: 3, MST: -8
 PST
)

關於iota在評論和空白行上的行為

type Timezone int
const (
 // iota: 0, EST: -5
 EST Timezone = -(5 + iota)
 // On a comment or an empty
 // line, iota will not 
 // increase
 // iota: 1, CST: -6
 CST
 // iota: 2, MST: -7
 MST
 // iota: 3, MST: -8
 PST
)

在中間使用iota

const (
 One = 1
 Two = 2
 // Three = 2 + 1 => 3
 // iota in the middle
 Three = iota + 1
 // Four = 3 + 1 => 4
 Four
)

單行多個iota

const (
 // Active = 0, Moving = 0,
 // Running = 0
 Active, Moving, Running = iota, iota, iota
 // Passive = 1, Stopped = 1,
 // Stale = 1
 Passive, Stopped, Stale
)

在同一行中，所有常量將獲得相同的iota值。

const (
 // Active = 0, Running = 100
 Active, Running = iota, iota + 100
 // Passive = 1, Stopped = 101
 Passive, Stopped
 // You can't declare like this.
 // The last expression will be
 // repeated
 CantDeclare
 // But, you can reset 
 // the last expression
 Reset = iota
 // You can use any other
 // expression even without iota
 AnyOther = 10
)

重複和取消表達式

const (
 // iota: 0, One: 1 (type: int64)
 One int64 = iota + 1
 // iota: 1, Two: 2 (type: int64)
 // Two will be declared as if:
 // Two int64 = iota + 1
 Two
 // iota: 2, Four: 4 (type: int32)
 Four int32 = iota + 2
 // iota: 3, Five: 5 (type: int32)
 // Five will be declared as if:
 // Five int32 = iota + 2
 Five
 // (type: int)
 Six = 6
 // (type: int)
 // Seven will be declared as if:
 // Seven = 6
 Seven
)

最後使用的表達式和類型將被重複

奇數和偶數

type Even bool
const (
 // 0 % 2 == 0 ==> Even(true)
 a = Even(iota % 2 == 0)
 // 1 % 2 == 0 ==> Even(false)
 b
 // 2 % 2 == 0 ==> Even(true)
 c
 // 3 % 2 == 0 ==> Even(false)
 d
)

倒數

const (
 max = 10
)
const (
 a = (max - iota) // 10
 b // 9
 c // 8
)

生成字母

const (
 // string will convert the
 // expression into string.
 //
 // or, it'll assign character
 // codes.
 a = string(iota + 'a') // a
 b // b
 c // c
 d // d
 e // e
)

按位運算

type Month int
const (
 // 1 << 0 ==> 1
 January Month = 1 << iota
 February // 1 << 1 ==> 2
 March // 1 << 2 ==> 4
 April // 1 << 3 ==> 8
 May // 1 << 4 ==> 16
 June // ...
 July
 August
 September
 October
 November
 December
 // Break the iota chain here.
 // AllMonths will have only
 // the assigned month values, 
 // not the iota's.
 AllMonths = January | February |
 March | April | May | June |
 July | August | September |
 October | November |
 December
)

注意0值

type Activity int
const (
 Sleeping = iota
 Walking
 Running
)
func main() {
 var activity Activity
 // activity initialized to 
 // its zero-value of int
 // which is Sleeping
}

iota + 1 小技巧

const (
 Sleeping = iota + 1
 Walking
 Running
)
func main() {
 var activity Activity
 // activity will be zero, 
 // so it's not initialized
 activity = Sleeping
 // now you know that it's been
 // initialized
}

使用“ iota + 1”來確保枚舉類型已初始化。

未知狀態模式

const (
 Unknown = iota
 Sleeping
 Walking
 Running
)

以“未知”開始，以確保枚舉已初始化。

總結

大結局！

閱讀更多 極客24h 的文章

關鍵字: 微信 極客 可視化技術