Interfaces

An interface type is defined as a set of method signatures.

A value of interface type can hold any value that implements those methods.

Note: There is an error in the example code on line 22. Vertex (the value type) doesn't implement Abser because the Abs method is defined only on *Vertex (the pointer type).

interfaces.go

  1. package main
  3. import (
  4.     "fmt"
  5.     "math"
  6. )
  8. type Abser interface {
  9.     Abs() float64
  10. }
  12. func main() {
  13.     var a Abser
  14.     f := MyFloat(-math.Sqrt2)
  15.     v := Vertex{3, 4}
  17.     a = f  // a MyFloat implements Abser
  18.     a = &v // a *Vertex implements Abser
  20.     // In the following line, v is a Vertex (not *Vertex)
  21.     // and does NOT implement Abser.
  22.     a = v
  24.     fmt.Println(a.Abs())
  25. }
  27. type MyFloat float64
  29. func (f MyFloat) Abs() float64 {
  30.     if f < 0 {
  31.         return float64(-f)
  32.     }
  33.     return float64(f)
  34. }
  36. type Vertex struct {
  37.     X, Y float64
  38. }
  40. func (v *Vertex) Abs() float64 {
  41.     return math.Sqrt(v.X*v.X + v.Y*v.Y)
  42. }