Go gotcha: What's a nil pointer dereference?

Why does this program crash?

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 explained for more about pointers in Go.

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