[Go pointer receiver method avoids runtime panic] #go #golang

main.go

// https://play.golang.com/p/NOxcQO8U-G7
// 
// IMPORTANT: Even though you can call the pointer receiver method, you still can't access a field without triggering a runtime panic. But just being able to call the method is still useful/interesting.
//
// The explanation for this is as follows...
//
// There’s two things going on here. The first is calling a method. Go knows which method to call thanks to the name of the method and the type of the receiver. That’s all there is to knowing which method to call.
// e.g. The Go runtime evaluates the code `e.PointerMethod()` into something like `(*Example).PointerMethod(e)`.
// The trick to understand the rest is to remember that having a pointer be nil is not a reason to panic just yet. You only panic when you try to do something that needs to dereference that nil pointer.
// Now when the method gets called, the sugared version (e.g. `(*Example).PointerMethod(e)`) shows that the first argument is the receiver. 
// If that receiver is a pointer, then Go gives that even if it’s nil and that’s fine since it doesn’t get dereferenced (until you dereference it yourself and that panics). 
// If you try to pass a nil pointer by value, Go will need to dereference it and will panic before having a chance to call your method.
// This is why calling a method with a value receiver will trigger a runtime panic.

package main

import (
	"fmt"
)

type Example struct {
	Field int
}

func (e *Example) PointerMethod() {
	fmt.Printf("PointerMethod: %#v (%T)\n", e, e)
	fmt.Printf("PointerMethod field access: %#v (%T)\n", e.Field, e.Field) // this is a runtime panic
}

func (e Example) ValueMethod() {
	fmt.Printf("ValueMethod: %#v (%T)\n", e, e)
}

func NewExample() *Example {
	return nil
}

func main() {
	e := NewExample()
	fmt.Printf("main: %#v (%T)\n", e, e) // (*main.Example)(nil) (*main.Example)
	e.PointerMethod()                    // this is ok! :mindblown:
	e.ValueMethod()                      // this is a runtime panic
	fmt.Println(e.Field)                 // this is a runtime panic