Go Pointers: Value vs. Pointer Receivers - Does it Matter?

Go Pointers: A Closer Look at Value and Pointer Receivers

In Go, pointers play a crucial role in effectively managing memory and creating reusable data structures. Beginners often grapple with the concept of pointers, especially as it differs from languages like C/C . This article aims to clarify the nuances of Go pointers and address common misconceptions.

Consider the code snippet below, borrowed from the Go Tour #52:

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())
}

Here, we define a struct Vertex, and a method Abs which calculates the absolute value of a vertex. The receiver v of Abs is a pointer to Vertex. This means that Abs operates on pointers to vertices, allowing for modifications to the original vertex.

Now, let's consider a slight modification to the code:

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

Interestingly, this modification leads to the same result. Our question arises: is there a difference between using a *Vertex or Vertex as the receiver for Abs?

The answer lies in two fundamental rules of Go:

1. Value and Pointer Receiver Conversion: Go allows deriving a method with a pointer receiver from one with a value receiver. Thus, func (v Vertex) Abs() float64 automatically generates an additional 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

   The compiler seamlessly finds the generated method, explaining why v.Abs() still works even though v is not a pointer.

2. Automatic Address Taking: Go can automatically take the address of a variable. This means when calling v.Abs() in the modified code without the pointer receiver, it's equivalent to:

   vp := &v
   vp.Abs()

   Thus, the function still receives a pointer to the vertex, regardless of whether we explicitly use &.

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