Declare slice or make slice?

Question

In Go, what is the difference between var s []int and s := make([]int, 0)?

I find that both works, but which one is better?

Answer 1

Simple declaration

var s []int

does not allocate memory and s points to nil, while

s := make([]int, 0)

allocates memory and s points to memory to a slice with 0 elements.

Usually, the first one is more idiomatic if you don't know the exact size of your use case.

Answer 2

A bit more completely (one more argument in make) example:

slice := make([]int, 2, 5)
fmt.Printf("length:  %d - capacity %d - content:  %d", len(slice), cap(slice), slice)

Out:

length:  2 - capacity 5 - content:  [0 0]

---

Or with dynamic type of slice:

slice := make([]interface{}, 2, 5)
fmt.Printf("length:  %d - capacity %d - content:  %d", len(slice), cap(slice), slice)

Out:

length:  2 - capacity 5 - content:  [<nil> <nil>]

Answer 3

Just found a difference. If you use

var list []MyObjects

and then you encode the output as JSON, you get null.

list := make([]MyObjects, 0)

results in [] as expected.

Answer 4

In addition to fabriziom's answer, you can see more examples at "Go Slices: usage and internals", where a use for []int is mentioned:

Since the zero value of a slice (nil) acts like a zero-length slice, you can declare a slice variable and then append to it in a loop:

// Filter returns a new slice holding only
// the elements of s that satisfy f()
func Filter(s []int, fn func(int) bool) []int {
    var p []int // == nil
    for _, v := range s {
        if fn(v) {
            p = append(p, v)
        }
    }
    return p
}

It means that, to append to a slice, you don't have to allocate memory first: the nil slice p int[] is enough as a slice to add to.