Slice storage

March 6, 2023

Slice types

The length of a slice s can be discovered by the built-in function len; unlike with arrays it may change during execution. The elements can be addressed by integer indices 0 through len(s)-1. The slice index of a given element may be less than the index of the same element in the underlying array.

A slice, once initialized, is always associated with an underlying array that holds its elements. A slice therefore shares storage with its array and with other slices of the same array; by contrast, distinct arrays always represent distinct storage.

Last week we saw that a slice is effectively a view into an array.

These two paragraphs expand on that concept, pointing out a couple of important implications this has.

  1. There’s no relationship between the index of an element in a slice, and the index of the same element in the underlying array. Consider:

    	// Initialize a as an array of 10 integers
    	var a = [10]int{0, 1, 2, 3, 4, 5, 6, 7, 8, 9}
    	// Initialize s as a slice "view" of a, of indexes 0, 1, and 2
    	var s = a[3:6]
    	fmt.Println(a, s) // Prints: [0 1 2 3 4 5 6 7 8 9] [3 4 5]

    s[0] is the same as a[3], and a[0] is not present in the slice s at all.

  2. Because a slice and array use the same backing memory, modifying one will modify the other as well:

    	var a = [10]int{0, 1, 2, 3, 4, 5, 6, 7, 8, 9}
    	var s = a[3:6]
    	s[0] = 99
    	fmt.Println(a, s) // Prints [0 1 2 99 4 5 6 7 8 9] [99 4 5]

    Notice that both the slice and the underlying array are modified.

Quotes from The Go Programming Language Specification Version of December 15, 2022

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