Can Pointer Methods Be Called on Non-Pointer Values in Go?

Pointer Methods on Non-Pointer Types

The response to a previous question states that pointer methods can only be invoked on pointers. However, this seems to be incorrect, as demonstrated by the following code that successfully calls a pointer method on a non-pointer value:

package main

import "fmt"

type car struct {
    wheels int
}

func (c *car) fourWheels() {
    c.wheels = 4
}

func main() {
    var c = car{}
    fmt.Println("Wheels:", c.wheels)
    c.fourWheels()
    fmt.Println("Wheels:", c.wheels)
}

Explanation

The key to understanding this behavior lies in the fact that the method receiver is a pointer. When you call c.fourWheels(), the receiver value, c, is a non-pointer. However, since the receiver type is a pointer, this is interpreted as a shorthand for (&c).fourWheels().

According to the Go specification, when calling a method, if the receiver value is addressable and the receiver type is a pointer, the method call is interpreted as a pointer method call. In our case, the receiver value c is addressable, and the receiver type *car is a pointer, so the method call c.fourWheels() is effectively calling (&c).fourWheels().

This behavior is consistent with the statement that value methods can be invoked on pointers and values, while pointer methods can only be invoked on pointers. In the case of pointer methods, if the receiver value is not a pointer, it must be dereferenced to obtain a pointer value that can be used as the receiver.

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