Exploring how interfaces interact with methods that have pointer receivers.

interface_star.go

// Run this with go run.
package main

import "fmt"

type Counter interface {
	bumpAndCount() int
	sayMyName() string
}

type AnObject struct {
	callCount int
	name string
}

func (ao *AnObject) bumpAndCount() int {
	fmt.Println("COUNTING", ao.callCount)
	ao.callCount += 1
	return ao.callCount
}

func (ao AnObject) sayMyName() string {
	return ao.name
}

type AnValue struct {
	callCount int
	name string
}

func (av AnValue) bumpAndCount() int {
	fmt.Println("VOUNTING", av.callCount)
	av.callCount += 1
	return av.callCount
}

func (av AnValue) sayMyName() string {
	return av.name
}

// this is the part of this code that seems strange to me.
// Depending on wether you implemented the counter interface with a pointer receiver or a value receiver
// you pass either a pointer or a value to this function. The interface accepts both, but will only let you send
// the one that matches your interface.

// So, for AnObject, you must pass a pointer, but for AnValue you must pass a value
// Don't try and have the method take a *Counter, that's a pointer to the interface and apparenly never what you want.
// But why does it not care about sayMyName()? The pointer * seems to be a one way street.
func interfaceCounter(counter Counter) {
	counter.bumpAndCount()
	counter.bumpAndCount()

	fmt.Println(counter.bumpAndCount())
	fmt.Println(counter.sayMyName())
}


func pointerObjCounter(counter *AnObject) {
	counter.bumpAndCount()
	counter.bumpAndCount()

	fmt.Println(counter.bumpAndCount())
	fmt.Println(counter.sayMyName())
}

func valueObjCounter(counter AnObject) {
	counter.bumpAndCount()
	counter.bumpAndCount()

	fmt.Println(counter.bumpAndCount())
	fmt.Println(counter.sayMyName())
}


func valueValueCounter(counter AnValue) {
	counter.bumpAndCount()
	counter.bumpAndCount()

	fmt.Println(counter.bumpAndCount())
	fmt.Println(counter.sayMyName())
}

func main() {
    fmt.Println("hello")

    pointobj := &AnObject{name: "This object is passed by reference"}
    valobj := AnObject{name: "This object is passed by value"}
    valval := AnValue{name: "This value passed by value"}

    interfaceCounter(pointobj)
    valueObjCounter(*pointobj)
    pointerObjCounter(pointobj)

    interfaceCounter(&valobj)
    valueObjCounter(valobj)
    pointerObjCounter(&valobj)

    valueValueCounter(valval)
    interfaceCounter(valval)
    interfaceCounter(&valval) // ...

}