Length and capacity

Slices have both length and capacity. The length of a slice is the number of elements currently in the slice, while the capacity is the number of elements the slice can hold before needing to be reallocated.

When creating a slice using the built-in make() function, you can specify its length, and optionally its capacity. You can check capacity and length with len() and cap():

var s = make([]int, 3, 5)
fmt.Printf("Length:   %d\n", len(s))
fmt.Printf("Capacity: %d\n", cap(s))

Length:   3
Capacity: 5

If the capacity is not explicitly specified, it will default to the value of the specified length.

var s = make([]int, 4)
fmt.Printf("Length:   %d\n", len(s))
fmt.Printf("Capacity: %d\n", cap(s))

Length:   4
Capacity: 4

Elements created by make() are set to the zero value for the element type of the slice:

var s = make([]int, 3)
for idx, val := range s {
	fmt.Println(idx, val)
}

0 0
1 0
2 0

You cannot access elements beyond the length of a slice, even if the index is within capacity:

s := make([]int, 3, 20)
fmt.Println(s[5])

panic: runtime error: index out of range [5] with length 3

goroutine 1 [running]:
main.main()
	/tmp/src679752982/main.go:9 +0x1d
exit status 2

Capacity allow us to optimize performance.