Interface values with nil underlying values

If the concrete value inside the interface itself is nil, the method will be called with a nil receiver.

In some languages this would trigger a null pointer exception, but in Go it is common to write methods that gracefully handle being called with a nil receiver (as with the method M in this example.)

Note that an interface value that holds a nil concrete value is itself non-nil.

interface-values-with-nil.go

  1. package main
  3. import "fmt"
  5. type I interface {
  6.     M()
  7. }
  9. type T struct {
  10.     S string
  11. }
  13. func (t *T) M() {
  14.     if t == nil {
  15.         fmt.Println("")
  16.         return
  17.     }
  18.     fmt.Println(t.S)
  19. }
  21. func main() {
  22.     var i I
  24.     var t *T
  25.     i = t
  26.     describe(i)
  27.     i.M()
  29.     i = &T{"hello"}
  30.     describe(i)
  31.     i.M()
  32. }
  34. func describe(i I) {
  35.     fmt.Printf("(%v, %T)\n", i, i)
  36. }