Q: golang pointer to map[string]interface{}

Question

I (golang newbie) am trying to create a map[string]interfaces{} in a function. The code compiles and runs but the map is empty.

package main

import (
    "fmt"   
    "encoding/json" 
)


func main() {
    var f interface{}
    var sJson string                    // JSON string from VT
    var err error                       // errors
    var b []byte                        // bytearray of JSON string
    var rootMap map[string]interface{}

    rootMap = make(map[string]interface{})

    sJson=`{"key": "foo"}`

    fmt.Println(sJson)

    err = json2map(&b, &sJson, f, rootMap)

    if err != nil { return }

    switch v := rootMap["key"].(type) {
        case float64:
            fmt.Printf("Value: %d",v)
        case string:
            fmt.Printf("Value: %s", v)
        case nil:
            fmt.Println("key is nil")           
        default:
            fmt.Println("type is unknown")          
    }       
}

func json2map(b *[]byte, sJson *string, f interface{}, myMap map[string]interface{}) error {
    var err error
    *b = []byte(*sJson) 
    err = json.Unmarshal(*b,&f) 
    myMap = f.(map[string]interface{})
    return err
}

The output is:

{"key": "foo"}
key is nil

I found this article which describes how to use map[string]string. This code works as expected:

package main

import (
    "fmt"
)

type MyType struct {
    Value1 int
    Value2 string
}

func main() {
    myMap := make(map[string]string)
    myMap["key"] = "foo"
    ChangeMyMap(myMap)  
    fmt.Printf("Value : %s\n",  myMap["key"])    
}

func ChangeMyMap(TheMap map[string]string) {
    TheMap["key"] = "bar"
}

So I think my problem has to do with the map being of type interface instead of string but I cannot tell why the first code doesn't work, while the 2nd does.

Answer 1

There's a number of things causing confusion here:

• You don't need b at all. You're passing a pointer to a byte slice that you're reassigning inside the function. Just use the string directly.
• There's no need to us a pointer to the sJson string. Strings are immutable, and you're not trying to reassign the sJson variable.
• You're unmarshaling into an empty interface, and then trying to reassign the myMap variable to the contents of f . Since myMap isn't a pointer, that assignment is only scoped to within the function. Just unmarshal directly into myMap

If you change those things, you'll find the json2map function ends up being one line, and can be dropped altogether:

func json2map(sJson string, myMap map[string]interface{}) error {
    return json.Unmarshal([]byte(sJson), &myMap)
}