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枚舉的本質(zhì)是什么，我們天天寫(xiě)代碼用枚舉，那啥是枚舉啊。wiki上是這么說(shuō)的
- In computer programming, an enumerated type (also called enumeration, enum, or factor in the R programming language, and a categorical variable in statistics) is a data type consisting of a set of named values called elements, members, enumeral, or enumerators of the type. The enumerator names are usually identifiers that behave as constants in the language. An enumerated type can be seen as a degenerate tagged union of unit type. A variable that has been declared as having an enumerated type can be assigned any of the enumerators as a value. In other words, an enumerated type has values that are different from each other, and that can be compared and assigned, but are not specified by the programmer as having any particular concrete representation in the computer’s memory; compilers and interpreters can represent them arbitrarily.
用人話說(shuō)就是
枚舉是強(qiáng)類(lèi)型編程語(yǔ)言中的一種類(lèi)型，由一組名稱和值組成。通常用來(lái)在編程語(yǔ)言中充當(dāng)常量的標(biāo)識(shí)符。
沒(méi)毛病，我們也確實(shí)是這樣使用的。比如上學(xué)的時(shí)候，經(jīng)常寫(xiě)c的小玩具代碼，c標(biāo)準(zhǔn)里面提供了enum關(guān)鍵字，寫(xiě)起來(lái)比較直白，使用的時(shí)候和struct類(lèi)似,需要enum week這樣寫(xiě)，c里面默認(rèn)枚舉值是從0開(kāi)始，int類(lèi)型，其實(shí)c里面就是把枚舉當(dāng)做int類(lèi)型來(lái)用的。
- #include
- enum week{Mon, Tue, Wed, Thur, Fri, Sat, Sun};
- int main()
- {
- enum week day; // 需要加 enum 關(guān)鍵字
- day = Wed;
- printf("%d",day); // 輸出 2
- int i;
- for (i=Mon; i<=Sun; i++){ // 可以直接把枚舉類(lèi)型賦值給int類(lèi)型
- printf("%d ", i); // 輸出 0，1，2，3，4，5，6
- }
- return 0;
- }
上面的例子沒(méi)問(wèn)題，在初始化的時(shí)候，枚舉值默認(rèn)情況下，編譯器會(huì)從分配0開(kāi)始的值，例如上面的Mon=0，Tue=1…，但是也會(huì)想不按照編譯器的默認(rèn)分配方式，由我自己分配，那怎么寫(xiě)呢，看下面的例子：
- #include
- enum day {sunday = 1, monday, tuesday = 5,
- wednesday, thursday = 10, friday, saturday};
- int main()
- {
- printf("%d %d %d %d %d %d %d", sunday, monday, tuesday,
- wednesday, thursday, friday, saturday); // 輸出1 2 5 6 10 11 12
- return 0;
- }
也就是說(shuō)，枚舉里面可以按任何順序?qū)⒅捣峙浣o某個(gè)名稱。所有未分配的名稱都會(huì)把值作為前一個(gè)名稱的值加一。
其實(shí)，定義幾個(gè)常量的事，是不是用宏這個(gè)東西更好呢，比如這么寫(xiě)
- #define sunday 0
- #define monday 1
- #define tuesday 2
但是老師說(shuō)了，盡量別用宏，不是說(shuō)宏不好，宏也好，編譯器替換，沒(méi)有運(yùn)行期啥事，多快啊，但是有幾個(gè)問(wèn)題：
1)宏沒(méi)有作用域一說(shuō) 2)枚舉是類(lèi)型安全的
扯的有點(diǎn)遠(yuǎn)了，現(xiàn)在回來(lái)看看Go里面的枚舉怎么寫(xiě)。當(dāng)然也很簡(jiǎn)單了，官方教導(dǎo)我們這么寫(xiě)：
- type ByteSize float64
- const (
- _ = iota
- KB ByteSize = 1 << (10 * iota)
- MB
- GB
- )
Go里面更簡(jiǎn)潔了，直接把enum關(guān)鍵字去掉了，其實(shí)從Go的角度看，枚舉不就是常量么，搞這么多語(yǔ)法糖干嘛，Go里面提供了一個(gè)關(guān)鍵字iota可以實(shí)現(xiàn)常量的遞增，同時(shí)也支持手動(dòng)賦值，iota和手動(dòng)賦值結(jié)合起來(lái)，就可以實(shí)現(xiàn)類(lèi)似c里面的效果
- const (
- A0 = iota
- A1 = iota
- A2 = iota
- )
- fmt.Println(A0, A1, A2) // "0 1 2"
可以 簡(jiǎn)寫(xiě)成這樣
- const (
- A0 = iota
- A1
- A2
- )
也可以從1開(kāi)始
- const (
- A1 = iota + 1
- A2
- A3
- )
- fmt.Println(A1, A2, A3) // "1 2 3"
或者跳過(guò)某個(gè)值
- const (
- C1 = iota + 1
- _
- C3
- C4
- )
- fmt.Println(C1, C3, C4) // "1 3 4"
看到這里你或許有個(gè)疑問(wèn)，這里的枚舉其實(shí)就是常量么，那怎么寫(xiě)是字符串類(lèi)型的枚舉呢，你可能會(huì)說(shuō)，當(dāng)然是用字符串常量了。但是那只是字符串常量了，沒(méi)有枚舉的性質(zhì)。我可能想要的是一種字符串到值的枚舉類(lèi)型。思考再三，看我這種寫(xiě)法是否可以：
- 步驟一：創(chuàng)建一個(gè)新的int類(lèi)型
- 步驟二：使用iota表示值
- 步驟三：給這個(gè)新的類(lèi)型一個(gè)String的方法
- type Direction int
- const (
- North Direction = iota
- East
- South
- West
- )
- func (d Direction) String() string {
- return [...]string{"North", "East", "South", "West"}[d]
- }
使用的時(shí)候
- var d Direction = North
- fmt.Print(d)
- switch d {
- case North:
- fmt.Println(" goes up.")
- case South:
- fmt.Println(" goes down.")
- default:
- fmt.Println(" stays put.")
- }
當(dāng)然還有一種方法，比較stupid
- type weekday string
- func (w weekday) isWeekday() weekday {
- return w
- }
- type Weekday interface {
- isWeekday() weekday
- }
- const (
- Monday = weekday("Monday")
- Tuesday = weekday("Tuesday")
- Wendsday = weekday("Wendsday")
- Thursday = weekday("Thursday")
- Friday = weekday("Friday")
- Saturday = weekday("Saturday")
- Sunday = weekday("Sunday")
- )
- // 使用
- func main() {
- var d1 = weekday.Monday
- var d2 = weekday.Tuesday
- fmt.Println(d1, d2, d1 == d2, d1 == weekday.Monday)
- }
如果使用struct表示枚舉，那其實(shí)還可以使用反射的方式，比如下面這樣寫(xiě)：
- import (
- "reflect"
- )
- type weekday struct {
- Monday, Tuesday, Wendsday, Thursday, Friday, Saturday, Sunday int
- }
- func (c weekday) Get(id string) int {
- vo := reflect.ValueOf(c)
- typeVo := vo.Type()
- for i := 0; i < vo.NumField(); i++ {
- if typeVo.Field(i).Name == id {
- return vo.Field(i).Interface().(int)
- }
- }
- return 0
- }
- var weekdayEnum = weekday {
- Monday: 1,
- Tuesday: 2,
- Wendsday: 3,
- Thursday: 4,
- Friday: 5，
- Saturday: 6,
- Sunday: 7
- }
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文章名稱：Go中你用對(duì)枚舉了么？
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