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Exploring and Mastering the Full Go Featureset

Nil Slices

Learn about nil slices in Go.

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Nil slices

Slices don’t have to be checked for nil values and don’t always need to be initialized. Functions such as len, cap, and append work fine on a nil slice:

Go (1.16.5)
package main
import "fmt"
func main() {
    var s []int // nil slice
    fmt.Println(s, len(s), cap(s)) // [] 0 0
    s = append(s, 1)
    fmt.Println(s, len(s), cap(s)) // [1] 1 1
}

An empty slice is not the same thing as a nil slice:

Go (1.16.5)
package main
import "fmt"
func main() {
    var s []int // this is a nil slice
    s2 := []int{} // this is an empty slice
    // looks like the same thing here:
    fmt.Println(s, len(s), cap(s)) // [] 0 0
    fmt.Println(s2, len(s2), cap(s2)) // [] 0 0
    // but s2 is actually allocated somewhere
    fmt.Printf("%p %p", s, s2) // 0x0 0x65ca90(any address)
}

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