声明
const identifier type = value
Normal side of Go constant:
- declare and assign at the same time, can not be done separately.
- Value has to be determined in complie time, not run time
- The type of constant can be implicit, or explicit if you need to
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| const pi = 3 // type not assigned
fmt.Println(pi) // pi is treated as int
fmt.Println(pi + 0.14) // pi is treated as float
pi = 3.1415 // you can NOT change value of a constant（这是与VAR的区别）
//./main.go:10:5: cannot assign to pi
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在K8s中用法示例： 类型重命名
These list of const have been limited to a new string type “ServiceType”, so you will never have a typo on the strings.
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| # 使用 type 关键字定义新的类型， ServiceType 是一个自定义类型，其基础类型是 string。
# 通过为不同的概念和实体引入自定义类型，可以使代码更具表达力和可读性
type ServiceType string
const (
ServiceTypeClusterIP ServiceType = "ClusterIP"
ServiceTypeNodePort ServiceType = "NodePort"
ServiceTypeLoadBalancer ServiceType = "LoadBalancer"
ServiceTypeExternalName ServiceType = "ExternalName"
)
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Iota
Constant block, you can have multiple of them and iota will start from 0 in each block.
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| const (
pi = 3
greeting = "hello"
)
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You can build long chain of constants using iota.
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| const (
Monday = iota // 0 iota starts at 0 and increase by 1 every time it is used
Tuesday // 1 reuse the constant expression above,
Wednesday // 2 which is simply iota
Thursday // 3 其实就是行数
Friday // 4
Saturday // 5
Sunday // 6
)
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You can also use complex constant expresssion
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| const (
_ = iota // Ignore the first iota, which is 0
KB = 1 << (10 * iota) // 1 << (10 * 1)，same as 1 times 2^10
MB 、// 1 << (10 * 2)，same as 1 times 2^20，（2^10 = 1024）
GB // 1 << (10 * 3)，This is called
TB // 1 << (10 * 4)，bit shift operator
)
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The purpose of iota is for convenient.
跨包全局常量（或变量）
名字首字母大写即可