Full Slice Expression.md

tags
Go

Overview

In Go, slicing with three indexes is done using s[i:j:k]

• Length: j - i
• Capacity: k - i

Usage

This slicing technique is useful for preventing access to elements beyond a specified range in the backing array

When using a slice without the third index (s[i:j]), the append operation may reuse the backing array, modifying its content. By setting the capacity to 1 (e.g., s[2:3:3]), append allocates a new backing array, ensuring that the initial array remains unmodified

Consider this example:

func main() {
	s := []int{1, 2, 3}
	f(s[:2])
	fmt.Println(s) // [1,2,10]
}

func f(s []int) {
	_ = append(s, 10)
}

Here, the third element of the slice is modified, even though we pass only two elements. To protect the third element we can use full slice expression like so:

func main() {
	s := []int{1, 2, 3}
	f(s[:2:2])
	fmt.Println(s) // [1,2,3]
}

func f(s []int) {
	_ = append(s, 10)
}

Memory Usage

The inaccessible space (elements in the backing array with indices larger than capacity) is not reclaimed by GC

References

• Three-Index Slices in Go 1.2
• 100 Go Mistakes and How to Avoid Them. Teiva Harsanyi