Tour of Go 2.0

Interface values with nil underlying values

What if the concrete value inside an interface is nil? The method still gets called — just with a nil receiver.

In most languages that’s a crash waiting to happen. In Go, it’s fine as long as your method handles it. See how M checks for nil and returns early instead of dereferencing a nil pointer.

One important distinction: an interface holding a nil concrete value is not itself nil. The interface variable has a type — it just has no value. That’s a subtlety worth remembering.

main.go ▶ Run in Playground 
package main

import "fmt"

type I interface {
	M()
}

type T struct {
	S string
}

func (t *T) M() {
	if t == nil {
		fmt.Println("<nil>")
		return
	}
	fmt.Println(t.S)
}

func main() {
	var i I

	var t *T
	i = t
	describe(i)
	i.M()

	i = &T{"hello"}
	describe(i)
	i.M()
}

func describe(i I) {
	fmt.Printf("(%v, %T)\n", i, i)
}