How to best clear a slice: empty vs. nil

Remove all elements

To remove all elements, simply set the slice to nil.

a := []string{"A", "B", "C", "D", "E"}
a = nil
fmt.Println(a, len(a), cap(a)) // [] 0 0

This will release the underlying array to the garbage collector (assuming there are no other references).

Keep allocated memory

To keep the underlying array, slice the slice to zero length.

a := []string{"A", "B", "C", "D", "E"}
a = a[:0]
fmt.Println(a, len(a), cap(a)) // [] 0 5

If the slice is extended again, the original data reappears.

fmt.Println(a[:2]) // [A B]

Empty slice vs. nil slice

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

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

Further reading

Slices and arrays in 6 easy steps

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