Jakub Kopyś Jakub Kopyś - 1 month ago 13
C++ Question

Golang passing arrays to the function and modifying it

In most languages (like c++) passing arrays result in implicitly passing it by a reference, so any changes to the passed array in the function will result in changing the original one. I am learning Golang, and In the book "The Go Programming Language" by Alan A.A. Donovan and Brian W. Kernighan It is said, that its behaviour is different from other languages - It does not implicitly pass array by reference.

It is confusing me a bit - doesn't that mean that passing an array without the reference should not modify the array itself? Let me illustrate that:

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]
}
}


In code above array is not passed by the reference, but the reverse function modifies the original one, so It works somewhat like C++ program would do. Could anyone explain me the difference?

PS: Sorry if it is a dummy question, I am totally new to the Golang and trying to understand the basics well.

Answer

The explanation is rather simple: there isn't a single array declared or used explicitly in the code above. Your tab local variable and the tab parameter are slices.

In Go the length of an array is part of the type, e.g. [3]int (this is true to an extent that for example [2]int and [3]int are 2 different / distinct array types). If the length is not present (either explicit like [2]int or implicit like in the composite literal [...]int{1, 2, 3}), then that is not an array type but a slice type.

Yes, as you read, an array value means all its elements, and when passed around (or assigned), all its elements are copied. Slices however are just small descriptors, headers, describing a contiguous section of an array, so when slices are passed around (or assigned), only this header is copied (the pointer included), which will point to the same underlying array. And so if you modify elements of the slice copy, the changes will be reflected in the original slice as there is only one backing array that holds the elements.

If you want to know exactly what's in a slice header, you may check out the reflect.SliceHeader type: it's a struct containing the pointer, the length and the capacity of the slice.

Please read the following blog posts which explain this in great details:

Go Slices: usage and internals

Arrays, slices (and strings): The mechanics of 'append'

Also see these related questions for more details:

Why have arrays in Go?

Are golang slices pass by value?