Tour of Go 2.0

Pointer receivers

Want a method to actually change its receiver? Use a pointer receiver.

The syntax is *T — a pointer to the type. Here, Scale is defined on *Vertex, so it can modify v.X and v.Y directly.

Without a pointer receiver, the method gets a copy. Changes stay local and disappear. Since methods frequently need to mutate state, pointer receivers are more common than value receivers.

Try it: remove the * from Scale’s receiver on line 16 and re-run. Notice the output changes — Scale no longer affects the original Vertex.

main.go ▶ Run in Playground 
package main

import (
	"fmt"
	"math"
)

type Vertex struct {
	X, Y float64
}

func (v Vertex) Abs() float64 {
	return math.Sqrt(v.X*v.X + v.Y*v.Y)
}

func (v *Vertex) Scale(f float64) {
	v.X = v.X * f
	v.Y = v.Y * f
}

func main() {
	v := Vertex{3, 4}
	v.Scale(10)
	fmt.Println(v.Abs())
}