Declare slice or make slice?

Solution 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.

Solution 2:

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.

Solution 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.