从另一个函数追加到数组？

Question

I have this code in which I am appending to an array of a struct in one function. The change does not appear in the other function.

type my struct{
arr []int
}

func New_my() *my {
  m := new (my)
  return m
}

func (m my) Dosomething(){
  m.arr = append(m.arr,1)
  m.arr = append(m.arr,2)
  m.arr = append(m.arr,3)
}

func (m my) Dosomethingelse(){
  fmt.Println(m.arr)
}

func main(){

  m:= New_my()
  m.Dosomething()
  m.Dosomethingelse()
}

The output is:

[]

Please, explain what is happening? Why does the change not appear in the array?

Answer 1

If you are new to go you should totally do the tour of go and the effective go document. Go is a new language and with a strange combination of ideas so the official documentation is the best place to start.

First of all you are using a slice not an array. (Read this to understand slices)

The error in your code is because Dosomething() is defined for my instead of *my. This is explained here.

Just change it to:

func (m *my) Dosomething(){
  m.arr = append(m.arr,1)
  m.arr = append(m.arr,2)
  m.arr = append(m.arr,3)
}

In go everything is passed by value so in your code you are passing a copy of the struct to the function Dosomething(), and because the capacity of the slice is 0, the append function creates a new underlying array and returns a reference to it, and when yo do:

m.arr = append(...)

the new slice (using the new array) is lost because it is stored in m that is a copy of the original struct, if m were a *my the new slice would replace the previous in the arr property.