Methods and pointer indirection

Comparing the previous two programs, you might notice that functions with a pointer argument must take a pointer: 

  1. var v Vertex
  2. ScaleFunc(v, 5)  // Compile error!
  3. ScaleFunc(&v, 5) // OK

while methods with pointer receivers take either a value or a pointer as the receiver when they are called: 

  1. var v Vertex
  2. v.Scale(5)  // OK
  3. p := &v
  4. p.Scale(10) // OK

For the statement v.Scale(5), even though v is a value and not a pointer, the method with the pointer receiver is called automatically. That is, as a convenience, Go interprets the statement v.Scale(5) as (&v).Scale(5) since the Scale method has a pointer receiver.

indirection.go

  1. package main
  3. import "fmt"
  5. type Vertex struct {
  6.     X, Y float64
  7. }
  9. func (v *Vertex) Scale(f float64) {
  10.     v.X = v.X * f
  11.     v.Y = v.Y * f
  12. }
  14. func ScaleFunc(v *Vertex, f float64) {
  15.     v.X = v.X * f
  16.     v.Y = v.Y * f
  17. }
  19. func main() {
  20.     v := Vertex{3, 4}
  21.     v.Scale(2)
  22.     ScaleFunc(&v, 10)
  24.     p := &Vertex{4, 3}
  25.     p.Scale(3)
  26.     ScaleFunc(p, 8)
  28.     fmt.Println(v, p)
  29. }