The iota Constant Generator

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The iota Constant Generator

iota is a predeclared identifier that represents successive untyped integer constants. It starts at 0 and increments with each ConstSpec in a const block. This is perfect for creating enumerated constants, bit flags, and related values. Each new const block resets iota to 0. Within a block, every line without an explicit value implicitly uses the previous line's iota value plus one.

snippet.go
go
package main
 
import "fmt"
 
const (
    _ = iota
    KB ByteSize = 1 << (10 * iota) // 1024
    MB                             // 1048576
    GB                             // 1048576 * 1024
    TB                             // 1099511627776
)
 
const (
    FlagA Flags = 1 << iota // 1
    FlagB                   // 2
    FlagC                   // 4
    FlagD                   // 8
)
 
type ByteSize int64
type Flags int
 
func (b ByteSize) String() string {
    return fmt.Sprintf("%d B", b)
}
 
const (
    _          = iota
    January
    February
    March
    April
    May
    June
    July
    August
    September
    October
    November
    December
)
 
func main() {
    fmt.Printf("KB: %d, MB: %d\n", KB, MB)
    fmt.Printf("Flags: A=%d, B=%d, C=%d\n", FlagA, FlagB, FlagC)
    fmt.Printf("March is month number: %d\n", March)
}

Breakdown

const ( _ = iota; KB = 1 << (10 * iota) )

iota starts at 0, so _ ignores it. KB uses iota=1 to compute 1<<10 = 1024

MB // uses implicit iota+1 = 2

Line without explicit value continues the pattern: 1 << (10*2) = 1<<20

FlagA Flags = 1 << iota

Creates bit flags where each flag is a power of 2: 1, 2, 4, 8

January; February; March

Simple enumeration: iota produces 1, 2, 3 (since _ = 0 is skipped)

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The iota Constant Generator

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