How to change pointer slice in golang - go

Question

I'm trying to get a better understanding of go. I created a little exercise for myself: pass a pointer slice to a function and modify it.
This is what I came up with:
package main
import (
"fmt"
"unsafe"
)
func main() {
var data *[]byte
fillData(data)
fmt.Println((*data)[0:5])
}
func fillData(data *[]byte) {
b := []byte("hello")
fmt.Println(b[0:5])
fmt.Println(string(b[0:5]))
data = (*[]byte)(unsafe.Pointer(&b[0]))
}
But it gives an invalid memory address or nil pointer dereference error. I know I wouldn't use something like this in real code but I was just curious how to pass a slice and modify it without returning it.
https://play.golang.org/p/_K5ltKKKNV

Answer 1

When you try to update data in fillData, you make two errors. First, you update the pointer rather than what it's pointed to. Second, data is a nil pointer, so writing through that pointer will cause a nil pointer error.
Here's one possible way to write the code. data starts as a zero'ed slice, and gets updated inside fillData. This will copy the slice information (len, cap, and pointer to array) from b to *data which means that data will share information with b (importantly, including sharing the underlying array).
package main
import "fmt"
func main() {
var data []byte
fillData(&data)
fmt.Println(data, data[0:5])
}
func fillData(data *[]byte) {
b := []byte("hello")
*data = b[0:1]
}
Another way would be to have data being a pointer, and updating it. Then you have to pass a double pointer into fillData. That would look like this:
package main
import "fmt"
func main() {
var data *[]byte
fillData(&data)
fmt.Println((*data)[0:5])
}
func fillData(data **[]byte) {
b := []byte("hello")
*data = &b
}
Finally, the best way to write this code isn't to use pointers at all, and just return the slice. Unlike C or C++, it's rarely needed to use "output" parameters to functions. That's because go allows multiple return values.
package main
import "fmt"
func main() {
data := getData()
fmt.Println(data, data[0:5])
}
func getData() []byte {
return []byte("hello")[:1]
}