How to define a pointer in go, then pass this pointer in to a func to change its value?

Question

import "fmt"

func zeroptr(ptr *int) {
    *ptr = 0
}

func main() {
    oneptr * int
    *ptr = 1
    fmt.Println("ptr is :", *ptr)
    zeroptr(ptr)
    fmt.Println("after calling zeroptr, the value of ptr is :", *ptr)
}

This does not work, I am looking for output as follows:

ptr is :1

after calling zeroptr, the value of ptr is : 0

Please indent your code. Your code is very hard to read like this. — fuz, Jan 09 '15 at 18:51

score 1 · Answer 1 · edited May 23 '17 at 11:50

You should use pass an &int to zeroptr, as in this example:

package main

import "fmt"

func zeroptr(ptr *int) {
    *ptr = 0
}

func main() {

    var ptr int
    ptr = 1
    fmt.Println("ptr is :", ptr)
    zeroptr(&ptr)
    fmt.Println("after calling zeroptr, the value of ptr is :", ptr)
}

Output:

ptr is : 1
after calling zeroptr, the value of ptr is : 0

You can see a similar example in "What's the point of having pointers in Go?", from the golang book.

score 0 · Accepted Answer · edited Jan 09 '15 at 19:20

What does your pointer point to? In order to manipulate the memory a pointer points to, you first need to point the pointer somewhere. Right now, your ptr is pointing to nil which is invalid. You could for instance do this:

func main() {
    var foo int
    var oneptr *int = &foo
    *oneptr = 1
    fmt.Println("oneptr is :", *oneptr)
    zeroptr(oneptr)
    fmt.Println("after calling zeroptr, the value of oneptr is :", *ptr)
}

For the future, please indent your code before submitting it here. You can do this with the gofmt program.

How to define a pointer in go, then pass this pointer in to a func to change its value?

2 Answers2