Go: Do Arrays Pass by Reference or Value?

Golang: Unraveling the Mystery of Array Passing

In this article, we delve into the enigma surrounding passing arrays in Golang. Unlike some other languages where arrays are implicitly passed by reference, Golang's approach is distinctive.

Exploring the Confusion

Alan A.A. Donovan and Brian W. Kernighan in "The Go Programming Language" suggest that arrays in Go are not implicitly passed by reference. However, a close examination of the following code may raise questions:

<code class="go">func main() {
    tab := []int{1, 2, 3}
    fmt.Println(tab)
    // Results in [1 2 3]
    reverse(tab)
    fmt.Println(tab)
    // Results in [3 2 1]
}

func reverse(tab []int) {
    for i, j := 0, len(tab)-1; i < j; i, j = i+1, j-1 {
        tab[i], tab[j] = tab[j], tab[i]
    }
}</code>

Despite not passing the array by reference, the reverse function alters the original array. This behavior seems akin to C .

The Truth Revealed

The key lies in recognizing that the variables tab (in main) and the parameter tab (in reverse) are not arrays but rather slices of an underlying array. Unlike arrays, slices are just headers that describe a portion of an array. When slices are passed around, only these headers are copied, pointing to the same backing array.

Essential Distinctions

In Go, array lengths are inherent to their type (e.g., [3]int), indicating that actual arrays are passed by value. Slices, however, being mere descriptors, are passed by value, but modifications to their elements affect the underlying array because it is the same for all slices.

Additional Insights

For a deeper understanding, refer to the following resources:

• Go Slices: Usage and Internals
• Arrays, Slices (and Strings): The Mechanics of 'append'
• Why Have Arrays in Go?
• Are Go Slices Pass by Value?

Conclusion

Go's approach to passing arrays is distinct from some other languages. Understanding the difference between arrays and slices is crucial to comprehending this behavior. Slices, serving as flexible headers, allow for modifying the underlying array through different slice copies.

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