Structs as keys in Go maps

Question

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?

Answer 1

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.

Answer 2

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