4 iota enum examples

Iota basic example

• The iota keyword represents successive integer constants 0, 1, 2,…
• It resets to 0 whenever the word const appears in the source code,
• and increments after each const specification.

const (
    C0 = iota
    C1 = iota
    C2 = iota
)
fmt.Println(C0, C1, C2) // "0 1 2"

This can be simplified to

const (
	C0 = iota
	C1
	C2
)

Here we rely on the fact that expressions are implicitly repeated in a paren­thesized const declaration – this indicates a repetition of the preceding expression and its type.

Start from one

To start a list of constants at 1 instead of 0, you can use iota in an arithmetic expression.

const (
    C1 = iota + 1
    C2
    C3
)
fmt.Println(C1, C2, C3) // "1 2 3"

Skip value

You can use the blank identifier to skip a value in a list of constants.

const (
    C1 = iota + 1
    _
    C3
    C4
)
fmt.Println(C1, C3, C4) // "1 3 4"

Complete enum type with strings [best practice]

Here’s an idiomatic way to implement an enumerated type:

• create a new integer type,
• list its values using iota,
• give the type a String function.

type Direction int

const (
    North Direction = iota
    East
    South
    West
)

func (d Direction) String() string {
    return [...]string{"North", "East", "South", "West"}[d]
}

In use:

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.")
}
// Output: North goes up.

Naming convention

The standard naming convention is to use mixed caps also for constants. For example, an exported constant is NorthWest, not NORTH_WEST.

Further reading

Another common application for iota is to implement a bitmask, a small set of booleans, often called flags, represented by the bits in a single number.

See Bitmasks and flags for a complete example.

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