Cannot take the address of map element

Question

I want to find out why

x:= odsMap[segRef]
x.GetValue(\"@OriginDestinationKey\")

works, but this does not:

odsMap[segRef].GetVal         


        

Answer 1

@icza's answer is the correct one.

Here is an example to illustrate how "value receiver" vs "pointer receiver" interact with "pointer map" vs "values map" :

https://play.golang.org/p/JVp6DirgPkU

package main

import (
    "fmt"
)

// a simple type, with two methods : one with a value receiver, one with a pointer receiver
type Item struct {
    name string
}

func (i Item) GetNameByValue() string {
    return i.name
}

func (i *Item) GetNameByRef() string {
    return i.name
}

func main() {
    {
        // in this map, we store *pointers* to Item values
        mapByRef := make(map[int]*Item)

        mapByRef[0] = &Item{"I am stored as a pointer"}

        // GetNameByRef will work on a *Item : "mapByRef[0]" is already a pointer
        fmt.Println("GetByRef   :", mapByRef[0].GetNameByRef())

        // GetNameByValue will work on a *Item :   go automatically turns this into '(*mapByRef[0]).GetNameByValue()', and this is valid
        fmt.Println("GetByValue :", mapByRef[0].GetNameByValue())
    }

    {
        // in this map, we store Item values (no pointers)
        mapByValue := make(map[int]Item)

        mapByValue[0] = Item{"I am stored as a value"}

        // GetNameByValue will work on a Item :  "mapByValue[0]" has the right type
        fmt.Println("GetByValue :", mapByValue[0].GetNameByValue())

        // GetNameByRef will not work :  go tries to turn this into :  (&mapByValue[0]).GetNameByRef(),
        // and go refuses to let you take the address of a value inside a map

        // fmt.Println("GetByRef   :", mapByValue[0].GetNameByRef())

        // compiler error :
        //   ./prog.go:47:46: cannot call pointer method on mapByValue[0]
        //   ./prog.go:47:46: cannot take the address of mapByValue[0]

        // you will need some way to copy the value before taking its address :
        item := mapByValue[0]
        fmt.Println("item.GetByRef    :", item.GetNameByRef())
        // same as :
        fmt.Println("(&item).GetByRef :", (&item).GetNameByRef())
    }
}

// Output :
//
// GetByRef   : I am stored as a pointer
// GetByValue : I am stored as a pointer
// GetByValue : I am stored as a value
// item.GetByRef    : I am stored as a value
// (&item).GetByRef : I am stored as a value