Why Can I Call a Go Method with a Pointer Receiver Using an Object Instead of a Pointer?

Calling a Method Intended for Pointers with an Object

In Go, it is possible to call a method intended for a pointer receiver using an object instead of a pointer to it. This may seem unusual, but it is permitted according to the Go specification.

Consider the following code:

package main

import "math"

type Vertex struct {
    X, Y float64
}

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

func (v *Vertex) Scale(f float64) {
    v.X = v.X * f
    v.Y = v.Y * f
}

func main() {
    v := Vertex{3, 4}
    v.Scale(10) // v is not a pointer to a Vertex object
    fmt.Println(v.Abs())
}

In this example, v is an object of type Vertex, and Scale is a method intended for a pointer to Vertex. However, instead of passing a pointer to v, we directly call v.Scale(10).

Why is this not an error? The answer lies in a rule defined in the Go specification for method calls:

If x is addressable and &x's method set contains m, x.m() is shorthand for (&x).m().

This rule states that if the value being called is addressable (i.e., it is a local variable or a pointer), then calling a method on it is equivalent to calling the method on its address. In this case, v is a local variable, so v.Scale(10) is interpreted as if we had written (&v).Scale(10) instead.

This convenience allows us to write cleaner and more concise code without having to explicitly create a pointer to the object before calling the method. However, it is important to note that this only works for methods with pointer receivers. For methods with value receivers, passing a pointer to the object is not allowed.

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