Go: Empty slice vs. nil slice

In practice, nil slices and empty slices can often be treated in the same way:

  • they have zero length and capacity,
  • they can be used with the same effect in for loops and append functions,
  • and they even look the same when printed.
var a []int = nil
fmt.Println(len(a)) // 0
fmt.Println(cap(a)) // 0
fmt.Println(a)      // []

However, if needed, you can tell the difference:

var a []int = nil
var a0 []int = make([]int, 0)

fmt.Println(a == nil)  // true
fmt.Println(a0 == nil) // false

fmt.Printf("%#v\n", a)  // []int(nil)
fmt.Printf("%#v\n", a0) // []int{}

The official Go wiki recommends using nil slices over empty slices:

[…] the nil slice is the preferred style.

Note that there are limited circumstances where a non-nil but zero-length slice is preferred, such as when encoding JSON objects (a nil slice encodes to null, while []string{} encodes to the JSON array []).

When designing interfaces, avoid making a distinction between a nil slice and a non-nil, zero-length slice, as this can lead to subtle programming errors. The Go wiki: Declaring empty slices