A Tour of Go Methods with pointer receivers

Methods can be associated with a named type or a pointer to a named type.

We just saw two Abs methods. One on the *Vertex pointer type and the other on the MyFloat value type.

There are two reasons to use a pointer receiver. First, to avoid copying the value on each method call (more efficient if the value type is a large struct). Second, so that the method can modify the value that its receiver points to.

Try changing the declarations of the Abs and Scale methods to use Vertex as the receiver, instead of *Vertex.

The Scale method has no effect when v is a Vertex. Scale mutates v. When v is a value (non-pointer) type, the method sees a copy of the Vertex and cannot mutate the original value.

Abs works either way. It only reads v. It doesn't matter whether it is reading the original value (through a pointer) or a copy of that value.

package main 

import (
    "fmt"
    "math"
)

type Vertex struct {
    X, Y float64
}

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

func main() {
    v := &Vertex{3, 4}
    v.Scale(5)
    fmt.Println(v, v.Abs())
}

package main 

import (
    "fmt"
    "math"
)

type Vertex struct {
    X, Y float64
}

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

func main() {
    v := &Vertex{3, 4}
    v.Scale(5)
    fmt.Println(v, v.Abs())
}