Example showing difference between storing struct and *struct in a string

map_immutability.go

package main

import "fmt"

type S struct {
        I int
}

func main() {
        m1 := map[string]S{"0": S{0}, "1": S{1}}
        fmt.Println("map[string]struct")
        fmt.Println("\tm1 =", m1)
        fmt.Println("\tchange field I via reference")
        v := m1["0"]
        v.I = 2
        fmt.Println("\tm1 =", m1)
        fmt.Println()

        fmt.Println("\treplace struct in map")
        m1["0"] = S{2}
        fmt.Println("\tm1 =", m1)
        fmt.Println()

        m2 := map[string]*S{"0": &S{0}, "1": &S{1}}
        fmt.Println("map[string]*struct")
        fmt.Printf("\tm2 = map[0:{%v} 1:{%v}]\n", m2["0"].I, m2["1"].I)
        fmt.Println("\tchange field I via reference")
        r := m2["0"]
        r.I = 2
        fmt.Printf("\tm2 = map[0:{%v} 1:{%v}]\n", m2["0"].I, m2["1"].I)
        fmt.Println()

        fmt.Println("\treplace struct in map")
        m2["0"] = &S{3}
        fmt.Printf("\tm2 = map[0:{%v} 1:{%v}]\n", m2["0"].I, m2["1"].I)
        fmt.Println()
}