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:
- 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