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.
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]
}