去：结构体中的数组失去了它的内容

Question

The following go code (shared: link) should insert two positions in an order (both order and position are structs:

package main

import "fmt"

type orderPosition struct{
   art string
   qty string   
}

type order struct{
   posList []orderPosition
}

func main() {
  o := new(order)
  o.loadPos()
  fmt.Printf("# pos: %d
",len(o.posList))

}

func (o order) loadPos() {
  o.posList = append(o.posList, orderPosition {art: "art 1", qty: "2 pc"})
  o.posList = append(o.posList, orderPosition {art: "art 2", qty: "7 pc"})
  fmt.Printf("# pos: %d
",len(o.posList))
}

The output is:

# pos: 2
# pos: 0    

The method loadPos fills the positions in the order. But as soon as the method is left the list of positions is lost.

Why is the array loosing its content?

Any help would be greatly appreciated.

Answer 1

Your loadPos() method is using a value receiver (as opposed to a pointer), meaning that it's operating on a copy of the order object.
So when you call o.loadPos(), go is copying o, and calling loadPos() on that copy.

The solution is to simply change the receiver to a pointer:

func (o *order) loadPos() {

^{https://play.golang.org/p/PATpBCZLT7}