Why Does My Go Struct Method Not Modify the Original Object?

Assigning New Values to Struct Fields

Consider the following scenario: A Point class is defined with a Move() method that adjusts the x coordinate and a Print() method to display results. However, the code exhibits unexpected behavior, displaying the initial x value instead of the adjusted one after calling Move().

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()
}

The issue stems from the fact that Move() is defined as a value receiver, meaning it operates on a copy of the original Point object. Therefore, modifications to s within Move() have no effect on the actual instance.

To resolve this, the Move() method should be defined as a pointer receiver, allowing it to directly interact with the original object:

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()
}

Now, when Move() is called, the s variable points to the original Point object, so changes to s.x directly affect the instance. Consequently, calling Print() after Move() correctly displays the updated x coordinate.

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