How to determine if a pointer passed as a function argument is being modified, or if the copy is being modified?

Question content

package main

import (
    "fmt"
)

type Numbers struct {
    x int
    y int
}

func initial(number *Numbers) {
    number.x = 1
    number.y = 1
}

func final(number *Numbers) {
    number = &Numbers{2, 2}
}

func main() {
    p := Numbers{0, 0}
    fmt.Println(p) //Prints {0 0}

    initial(&p)
    fmt.Println(p) //Prints {1 1}

    final(&p)
    fmt.Println(p) //Expected to print {2, 2} but prints {1, 1}
}

Why does the initial function modify the pointer, while the final function modifies a copy of the pointer?

The function parameters of

initial and final point to the memory address of p in main; initial p can be changed, final cannot.

Any explanation as to why this is happening would be greatly appreciated.

Correct answer

To modify the data pointed to by a pointer, the pointer must be dereferenced. The dereference operator is *. However, for ease of use, Go implicitly inserts dereference operations in some cases. For example, number.x = 1 is converted to (*number).x = 1.

This implicit translation can be confusing, but you should see that if the translation did not occur, the expression number.x = 1 would be meaningless because number is a pointer type, and pointers have no fields.

To summarize, initial functions have implicit pointer dereferences, while final does not.

If you change final to explicitly and correctly dereference, *number = Numbers{2, 2}, then it will also change p.

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