Go Pointers: Why Does It Seem Like I Can Pass Either a Value or a Pointer to My Method?

Go Pointers: Demystifying Pointer Semantics

Poinrers are a fundamental concept in Go programming that enables the manipulation of memory addresses and values by reference. While pointers share similarities with other programming languages, Go's pointer semantics can be unique and may cause confusion for beginners.

Consider the following code:

type Vertex struct {
    X, Y float64
}

func (v *Vertex) Abs() float64 {
    return math.Sqrt(v.X*v.X + v.Y*v.Y)
}

func main() {
    v := &Vertex{3, 4}
    fmt.Println(v.Abs())
}

The code snippet defines a Vertex struct and a func (v *Vertex) Abs() method, which calculates the absolute value of the vertex's coordinates. By declaring the method receiver as a pointer *Vertex, we are indicating that the method operates on a pointer to a Vertex value.

If we modify the code as follows:

func (v Vertex) Abs() float64 {
    [...]
    v := &Vertex{3, 4}
}

where the receiver is no longer a pointer but a value (Vertex), we will obtain the same result. The reason for this is two-fold:

Method Derivation: Go allows for the derivation of a method with a pointer receiver from a method with a value receiver. This implies that the method func (v Vertex) Abs() automatically generates an additional method implementation:

func (v Vertex) Abs() float64 { return math.Sqrt(v.X*v.X+v.Y*v.Y) }
func (v *Vertex) Abs() float64 { return Vertex.Abs(*v) } // GENERATED METHOD

When you call v.Abs(), where v is not a pointer, the compiler will automatically find and use the generated method.

Automatic Address Taking: Go has the ability to automatically take the address of a variable. In the code below:

func (v *Vertex) Abs() float64 { return math.Sqrt(v.X*v.X+v.Y*v.Y) }
func main() {
    v := Vertex{3, 4}
    v.Abs()
}

the expression v.Abs() is equivalent to the code:

vp := &v
vp.Abs()

In this scenario, the compiler automatically takes the address of the v variable and passes it to the pointer receiver method func (v *Vertex) Abs().

Therefore, in the case of the code modifications you mentioned, regardless of whether you pass a value or a pointer to the Abs() method, Go's pointer semantics ensure the correct behavior. However, it's generally considered good practice to use pointers when working with reference types such as structs in Go.

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