golang "cannot assign to" http://play.golang.org/p/GJWp1G8H-E

file.go

package main

import (
	"fmt"
)

type A struct {
	name string
	s    []string
}

type B struct {
	name string
	as   map[int]A
}

type C struct {
	name string
	as   map[int]*A
}

func main() {
	b := &B{}
	b.name = "b"
	b.as = make(map[int]A)

	// b.as[1] = A{name: "hu"}
	// error cannot assign to, because b.as[1] is a
	// copy of A{name: "table 1"} and not addressable
	// b.as[1].s = append(b.as[1].s, "a")
	// proof, the value of b.as[1].s will be nil
	// see: https://golang.org/ref/spec#Address_operators
	// b2 := b.as[1]
	// b2.s = append(b2.s, "hello")
	// b2.s = append(b2.s, "world")
	// fmt.Printf("%#v\n", b)
	// fmt.Printf("%#v\n", b.as[1].s)

	// modification must perform on the reference of object
	a := A{}
	a.name = "a"
	a.s = append(a.s, "a")
	b.as[1] = a
	fmt.Printf("b: %#v\n", b)
	
	// pass A by reference
	c := &C{}
	c.name = "c"
	c.as = make(map[int]*A)
	c.as[1] = &A{name: "1"}
	c.as[1].s = append(c.as[1].s, "hello")
	c.as[1].s = append(c.as[1].s, "world")

	fmt.Printf("c: %#v\n", c)
}