Golang pointers

gistfile1.go

// This is a Gist to try to help you understand go pointers.
// You can run it in an interactive environment over on the go
// playground at: http://play.golang.org/p/pYTsfo6uhz

package main

import (
	"fmt"
)

type MyStruct struct {
	myField int
}

// This is a function that takes a pointer to a struct
// i.e. it takes the memory address of a struct
func SetFieldByPointer(s *MyStruct, newFieldVal int) {
	fmt.Println("[SET POINTER] Incoming value: ", s)
	s.myField = newFieldVal
	fmt.Println("[SET POINTER] After assignment: ", s)
}

// This is a function that takes the actual values of a struct
func SetFieldByValue(s MyStruct, newFieldVal int) {
	fmt.Println("[SET VALUE] Incoming value: ", s)
	s.myField = newFieldVal
	fmt.Println("[SET VALUE] After assignment: ", s)
}

func main() {
	// new() returns a pointer to a struct, which is just the memory
	// address of that struct
	s := new(MyStruct)
	s.myField = 5

	// What happens to the local object when we pass the struct value?
	fmt.Println("[MAIN FUNC] Before setting by value: ", s)
	// Putting a '*' before an address will return the actual value
	// storred in memory
	SetFieldByValue(*s, 10)
	fmt.Println("[MAIN FUNC] After setting by value: ", s)

	// What happens to the local object when we pass the struct pointer?
	fmt.Println("[MAIN FUNC] Before setting by pointer: ", s)
	SetFieldByPointer(s, 13)
	fmt.Println("[MAIN FUNC] After setting by pointer: ", s)
}


// OUTPUT:
// [MAIN FUNC] Before setting by value:  &{5}
// [SET VALUE] Incoming value:  {5}
// [SET VALUE] After assignment:  {10}
// [MAIN FUNC] After setting by value:  &{5}
// [MAIN FUNC] Before setting by pointer:  &{5}
// [SET POINTER] Incoming value:  &{5}
// [SET POINTER] After assignment:  &{13}
// [MAIN FUNC] After setting by pointer:  &{13}