Golang go Pointer Gotcha

gotcha.go

package main

import (
	"fmt"
	"time"
)

type foo struct {
	i int
}

func (f *foo) bar(tag string) {
	fmt.Printf("%v: %v\n", tag, f)
}

func main() {
	fs := []foo{
		{i: 1},
		{i: 2},
	}

	for _, f := range fs {
		// go resolves the parameters. Because the bar method expects pointer
		// type, it is resolved to the address of the loop variable.

		// wrong
		go f.bar("1")
		// go (&f).bar("1") // same as above

		// ok
		// go fs[i].bar("1")

		// ok. This works because f2 is a fresh copy of f in a new memory address
		// f2 := f
		// go f2.bar("1")

	}

	fs2 := []*foo{
		{i: 1},
		{i: 2},
	}

	for _, f := range fs2 {
		// ok. f is a pointer
		go f.bar("2")
	}

	time.Sleep(1 * time.Second)
}

// Output:
// 1: &{2}
// 1: &{2}
// 2: &{1}
// 2: &{2}