35

I was looking into using structs as keys in golang maps. A field in this struct is supposed to be a map also and this seems to go against the documentation provided here which says that only structs that have fields that can be compared with == and != can be in the fields of structs that are used as keys in maps. I however went ahead to try the following:

package main

import "fmt"
import "strings"

func main() {
    fmt.Println("Hello, 世界")
    fmt.Println(strings.Join([]string{"obi", "$", "56"}, ""))
    z := make(map[string]float64)

    z["obi"] = 0.003

    x := &test{
        name:"testing",
        code:z,
    }

    a := &test{
        name:"testing2",
        code:z,
    }

    y := make(map[*test] string)

    y[x] = "go home"
    y[a] = "come home"

    for key, val := range y{
        fmt.Println(key.name, key.code, val)
    }

}

type test struct{
    name string
    code map[string]float64
}

The output was:

Hello, 世界
obi$56
testing map[obi:0.003] go home
testing2 map[obi:0.003] come home

This seems to go against the documentation as a field in the struct used as a key is a map. What do I seem to be getting wrong?

cobie
  • 7,023
  • 11
  • 38
  • 60

2 Answers2

54

In your example the map key is a pointer to the struct, not the struct itself. Pointers can be compared for equality even when the items they point to can't be compared. This comparison is not based on the contents of the item, but only on its memory address.

Kokos
  • 442
  • 4
  • 12
andybalholm
  • 15,395
  • 3
  • 37
  • 41
  • question, would there be cases where its plausible to have a map's key to be pointer of x type? – Roy Lee May 09 '16 at 00:35
  • 2
    Sure, when you are more interested in the identity of the objects than in the equality of their contents. For example, you might store some extra per-request data in a web server in a map[*http.Request]string or something. – andybalholm May 09 '16 at 21:54
10

only comparable type can be used as a key (== ,!=). struct (not a pointer) is comparable in case it contains only comparable types.

15412s
  • 3,298
  • 5
  • 28
  • 38