GoConstants

Remarks

Go supports constants of character, string, boolean, and numeric values.

Declaring a constant

Constants are declared like variables, but using the const keyword:

const Greeting string = "Hello World"
const Years int = 10
const Truth bool = true

Like for variables, names starting with an upper case letter are exported (public), names starting with lower case are not.

// not exported
const alpha string = "Alpha"
// exported
const Beta string = "Beta"

Constants can be used like any other variable, except for the fact that the value cannot be changed. Here's an example:

package main

import (
    "fmt"
    "math"
)

const s string = "constant"

func main() {
    fmt.Println(s) // constant

    // A `const` statement can appear anywhere a `var` statement can.
    const n = 10
    fmt.Println(n)                           // 10
    fmt.Printf("n=%d is of type %T\n", n, n) // n=10 is of type int

    const m float64 = 4.3
    fmt.Println(m) // 4.3

    // An untyped constant takes the type needed by its context.
    // For example, here `math.Sin` expects a `float64`.
    const x = 10
    fmt.Println(math.Sin(x)) // -0.5440211108893699
}

Playground

Multiple constants declaration

You can declare multiple constants within the same const block:

const (
    Alpha = "alpha"
    Beta  = "beta"
    Gamma = "gamma"
)

And automatically increment constants with the iota keyword:

const (
    Zero = iota // Zero == 0
    One         // One  == 1
    Two         // Two  == 2
)

For more examples of using iota to declare constants, see Iota.

You can also declare multiple constants using the multiple assignment. However, this syntax may be harder to read and it is generally avoided.

const Foo, Bar = "foo", "bar"

Typed vs. Untyped Constants

Constants in Go may be typed or untyped. For instance, given the following string literal:

"bar"

one might say that the type of the literal is string, however, this is not semantically correct. Instead, literals are Untyped string constants. It is a string (more correctly, its default type is string), but it is not a Go value and therefore has no type until it is assigned or used in a context that is typed. This is a subtle distinction, but a useful one to understand.

Similarly, if we assign the literal to a constant:

const foo = "bar"

It remains untyped since, by default, constants are untyped. It is possible to declare it as a typed string constant as well:

const typedFoo string = "bar"

The difference comes into play when we attempt to assign these constants in a context that does have type. For instance, consider the following:

var s string
s = foo      // This works just fine
s = typedFoo // As does this

type MyString string
var mys MyString
mys = foo      // This works just fine
mys = typedFoo // cannot use typedFoo (type string) as type MyString in assignment