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How to reference pointers to the same struct object from outside the scope in which they were defined

October 6, 2022

In the below code, two pointer variables, r1 and r2 (of type *Rect), reference the same struct object (of type Rect):

type Rect struct {
    width int
}

func main() {
    r1 := new(Rect)
    r2 := new(Rect)
    r1 = r2

    fmt.Printf("%p, %p", r1, r2) // prints the addresses of the Rects being pointed to by each variable
}

0xc00001c038, 0xc00001c038 (GOOD OUTPUT)

How would you reference r1 and r2 to the same struct object from outside of the function in which they were defined? In other words, how would you create a function to replace r1 = r2? My attempt at dereferencing and then assigning the variables from within an external function did not succeed at referencing r1 to r2‘s referenced struct object:

func assign(r1, r2 *Rect) {
    *r1 = *r2
}

func main() {
    r1 := new(Rect)
    r2 := new(Rect)
    assign(r1, r2)

    fmt.Printf("%p, %p", r1, r2) // prints the addresses of the Rects being pointed to by each variable
}

0xc00001c030, 0xc00001c038 (BAD OUTPUT)

PLAYGROUND: https://go.dev/play/p/ld0C5Bkmxo3

>Solution :

If you need to change where a pointer points to within a function, you have to pass the address of it:

func assign(r1, r2 **Rect) {
    *r1 = *r2
}

func main() {
    r1 := new(Rect)
    r2 := new(Rect)
    assign(&r1, &r2)

    fmt.Printf("%p, %p", r1, r2) // prints the addresses of the Rects being pointed to by each variable
}

PLAYGROUND: https://go.dev/play/p/5fAakjB50JJ