[2023-01-11] Don't use pointers in structs used for Go map keys (or know what you're getting into)

main.go

package main

import (
	"fmt"
	"os"

	"github.com/google/uuid"
)

type mapKey struct {
	SubscriptionID uuid.UUID
	SiteID         uuid.NullUUID
}

type mapKeyBad struct {
	SubscriptionID uuid.UUID
	SiteID         *uuid.UUID
}

func run() error {
	u1, err := uuid.Parse("0b9fbf97-924e-465a-8913-b1bdd35251c9")
	if err != nil {
		return err
	}

	u2, err := uuid.Parse("4417a084-2686-445d-8178-3fa7a6422bf9")
	if err != nil {
		return err
	}

	u3 := uuid.NullUUID{UUID: u2, Valid: true}
	u4 := uuid.NullUUID{Valid: false}

	k1 := mapKey{SubscriptionID: u1, SiteID: u3}
	k2 := mapKey{SubscriptionID: u2, SiteID: u3}
	k3 := mapKey{SubscriptionID: u1, SiteID: u4}
	k4 := mapKey{SubscriptionID: u2, SiteID: u4}

	m := map[mapKey]int{k1: 11, k2: 23, k3: 37, k4: 41}

	k5 := mapKey{SubscriptionID: u1, SiteID: u3}
	fmt.Println(m[k5])
	u5 := uuid.NullUUID{UUID: u1, Valid: true}
	k6 := mapKey{SubscriptionID: u2, SiteID: u5}
	fmt.Println(m[k6])

	/////////

	u2Copy, err := uuid.Parse("4417a084-2686-445d-8178-3fa7a6422bf9")
	if err != nil {
		return err
	}
	kb1 := mapKeyBad{SubscriptionID: u1, SiteID: &u2}
	kb2 := mapKeyBad{SubscriptionID: u1, SiteID: &u2Copy}
	mb := map[mapKeyBad]int{kb1: 11, kb2: 23}
	fmt.Println(mb)

	return nil
}

func main() {
	err := run()
	if err != nil {
		fmt.Fprintf(os.Stderr, "%v\n", err)
		os.Exit(1)
	}
}

Output:

11
0
map[{0b9fbf97-924e-465a-8913-b1bdd35251c9 4417a084-2686-445d-8178-3fa7a6422bf9}:11 {0b9fbf97-924e-465a-8913-b1bdd35251c9 4417a084-2686-445d-8178-3fa7a6422bf9}:23]