Why Doesn't Modifying a Go Struct Field in a Value Receiver Method Change the Original Value?

Assigning a New Value to a Struct Field

In Go, when dealing with structures, it's important to understand the concept of value receivers versus pointer receivers. A value receiver operates on a copy of the original value, while a pointer receiver operates directly on the original value.

Consider this code snippet, where the question is raised about unexpectedly unchanged struct field values:

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 expectation is that the Move() method modifies the x field of the Point struct, which should be reflected in the Print() method. However, the output shows that the x field remains unchanged.

The Solution: Using Pointer Receivers

The issue lies in the use of a value receiver in the Move() and Print() methods. In Go, everything is passed by value, meaning that a copy of the original value is created when passing a struct to a function. Therefore, any modifications made to the copy within the function do not affect the original value.

To solve this, we need to use pointer receivers. A pointer receiver allows the function to operate directly on the original value. Here's the corrected 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()
}

By using a pointer receiver for the Move() and Print() methods, we now operate directly on the original Point struct, and the changes to the x field are reflected correctly.

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