Invalid memory address or nil pointer dereference

Why does this program panic?

type Point struct {	X, Y float64 } func (p *Point) Abs() float64 {	return math.Sqrt(p.X*p.X + p.Y*p.Y) } func main() {	var p *Point	fmt.Println(p.Abs()) }

panic: runtime error: invalid memory address or nil pointer dereference [signal SIGSEGV: segmentation violation code=0xffffffff addr=0x0 pc=0xd2c5a] goroutine 1 [running]: main.(*Point).Abs(...)	../main.go:6 main.main()	../main.go:11 +0x1a 

Answer

The uninitialized pointer p in the main function is nil, and you can’t follow the nil pointer.

If x is nil, an attempt to evaluate *x will cause a run-time panic. The Go Programming Language Specification: Address operators

You need to create a Point:

func main() {	var p *Point = new(Point)	fmt.Println(p.Abs()) }

Since methods with pointer receivers take either a value or a pointer, you could also skip the pointer altogether:

func main() {	var p Point // has zero value Point{X:0, Y:0}	fmt.Println(p.Abs()) }

See Pointers for more about pointers in Go.

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