Go言語のconstとiotaを活用した効率的な定数定義について解説

package main
import "fmt"
// 自動連番の例
const (
	First = iota  // First = 0
	Second        // Second = 1
	Third         // Third = 2
)
func main() {
	fmt.Println("First:", First)
	fmt.Println("Second:", Second)
	fmt.Println("Third:", Third)
}

First: 0
Second: 1
Third: 2

package main
import "fmt"
// 1つ目の定数ブロック
const (
	A = iota // A = 0
	B        // B = 1
)
// 2つ目の定数ブロック
const (
	C = 10  // C = 10
	D        // D = 10(前の値を継承)
)
func main() {
	fmt.Println("A:", A, "B:", B, "C:", C, "D:", D)
}

A: 0 B: 1 C: 10 D: 10

package main
import "fmt"
// 色の自動割り当て例
const (
	Red = iota   // Red = 0
	Green        // Green = 1
	Blue         // Blue = 2
)
func main() {
	fmt.Println("Red:", Red, "Green:", Green, "Blue:", Blue)
}

Red: 0 Green: 1 Blue: 2

package main
import "fmt"
// 曜日の定数定義
const (
	Monday = iota + 1  // Monday = 1
	Tuesday            // Tuesday = 2
	Wednesday          // Wednesday = 3
)
// 別ブロックでの10刻みの定数定義
const (
	DegreeCelsius = iota * 10  // DegreeCelsius = 0
	DegreeFahrenheit         // DegreeFahrenheit = 10
)
func main() {
	fmt.Printf("Monday: %d, Tuesday: %d, Wednesday: %d\n", Monday, Tuesday, Wednesday)
	fmt.Printf("DegreeCelsius: %d, DegreeFahrenheit: %d\n", DegreeCelsius, DegreeFahrenheit)
}

Monday: 1, Tuesday: 2, Wednesday: 3
DegreeCelsius: 0, DegreeFahrenheit: 10

package main
import "fmt"
// Weekday型のEnum例
type Weekday int
const (
	Sunday Weekday = iota  // Sunday = 0
	Monday                // Monday = 1
	Tuesday               // Tuesday = 2
	Wednesday             // Wednesday = 3
	Thursday              // Thursday = 4
	Friday                // Friday = 5
	Saturday              // Saturday = 6
)
func main() {
	fmt.Println("Sunday:", Sunday, "Monday:", Monday, "Tuesday:", Tuesday)
}

Sunday: 0 Monday: 1 Tuesday: 2

package main
import "fmt"
// Status型の定義
type Status int
const (
	Unknown Status = iota  // Unknown = 0
	Active               // Active = 1
	Inactive             // Inactive = 2
)
func main() {
	fmt.Println("Unknown:", Unknown, "Active:", Active, "Inactive:", Inactive)
}

Unknown: 0 Active: 1 Inactive: 2

package main
import "fmt"
// LogLevel型の定義
type LogLevel int
const (
	Debug LogLevel = iota  // Debug = 0
	Info                 // Info = 1
	Warning              // Warning = 2
	Error                // Error = 3
)
func main() {
	fmt.Println("Debug:", Debug, "Info:", Info, "Warning:", Warning, "Error:", Error)
}

Debug: 0 Info: 1 Warning: 2 Error: 3

package main
import "fmt"
// 最初のブロックでの定義
const (
	First = iota  // First = 0
	Second        // Second = 1
)
// 別ブロックで再度iotaが0から開始
const (
	Third = iota  // Third = 0(期待とは異なる値)
)
func main() {
	fmt.Println("First:", First, "Second:", Second, "Third:", Third)
}

First: 0 Second: 1 Third: 0

package main
import "fmt"
// グループ1: 数値の連番を定義
const (
	Value1 = iota  // Value1 = 0
	Value2         // Value2 = 1
	Value3         // Value3 = 2
)
// グループ2: 別の連番を必要とする定数群
const (
	CodeA = iota + 100  // CodeA = 100
	CodeB              // CodeB = 101
	CodeC              // CodeC = 102
)
func main() {
	fmt.Println("Value1:", Value1, "Value2:", Value2, "Value3:", Value3)
	fmt.Println("CodeA:", CodeA, "CodeB:", CodeB, "CodeC:", CodeC)
}

Value1: 0 Value2: 1 Value3: 2
CodeA: 100 CodeB: 101 CodeC: 102