Assign a new value to a struct field

Question

I have a problem with structure fields.

I've created a class Point with one method Move() that increases or decreases object variable x by dx. Another method Print is used to output results.

In main() a new instance is created with default x = 3 and dx = 2, then I call Move() and Print(). I expect that value of x is changed during Move() and Print() will produce Final x=5, but instead of it displays this:

2014/07/28 15:49:44 New X=5
2014/07/28 15:49:44 Final X=3

What's wrong with my code?

type Point struct {
  x, dx int
}

func (s Point) Move() {
  s.x += s.dx
  log.Printf("New X=%d", s.x)
}

func (s Point) Print() {
  log.Printf("Final X=%d", s.x)
}

func main() {
  st := Point{ 3, 2 };
  st.Move()
  st.Print()
}

Answer 1

You need to use a pointer receiver here or you are only changing a copy of the original object every time. Everything is passed by value in go.

type Point struct {
  x, dx int
}

func (s *Point) Move() {
  s.x += s.dx
  log.Printf("New X=%d", s.x)
}

func (s *Point) Print() {
  log.Printf("Final X=%d", s.x)
}

func main() {
  st := Point{ 3, 2 };
  st.Move()
  st.Print()
}

Answer 2

That's the difference between pointer receiver and value receiver

http://golang.org/doc/faq#methods_on_values_or_pointers