Pass interface slice in function and unmarshal into its items

Can i pass into function a slice of structs, converted to []interface{}, fill it and use after function end work?

Here is full example of the problem https://play.golang.org/p/iPijsawEEg

Short describe:

type DBResponse struct {
	Rows  int             `json:"rows"`
	Error string          `json:"error"`
	Value json.RawMessage `json:"value"`
}
type User struct {
	Id   int    `json:"id"`
	Name string `json:"name"`
}

func loadDBRows(p []interface{}) {
	var response DBResponse
	someDataFromDB := []byte("{\"rows\":1, \"error\": \"\", \"value\": {\"name\":\"John\", \"id\":2}}")
	json.Unmarshal(someDataFromDB, &response)
	json.Unmarshal(response.Value, &p[0])
    fmt.Println(p)//p[0] filled with map, not object
}

func main() {
	users := make([]User, 5)
	data := make([]interface{}, 5)
	for i := range users {
		data[i] = users[i]
	}
	loadDBRows(data)
}

This problem can be easily solved for single interface{}, u can test in at full example. Why i can't solve it for slice?

I want to do it without reflect! Is there any "true way" to write universal json parser to selected data struct without reflect and map[string]interface{}? Don't want complicated code and extra operations

Thank you for help!

Since p is a slice of interfaces, on this line json.Unmarshal(response.Value, &p[0]) you're passing a pointer to an interface{} not to a User and since json.Unmarshal allows interfaces as the destination to which to unmarshal the data, it doesn't look under the interface{} for another type and just decodes the json into a map.

What you can do is to have the interface{} already be a pointer to a concrete type e.g. data[i] = &users[i] then you just pass the interface{} without & to json.Unmarshal.

func loadDBRows(p []interface{}) {
    var response DBResponse
    someDataFromDB := []byte("{\"rows\":1, \"error\": \"\", \"value\": {\"name\":\"John\", \"id\":2}}")
    json.Unmarshal(someDataFromDB, &response)
    json.Unmarshal(response.Value, p[0]) // notice the missing &
    fmt.Println(p)
}

users := make([]User, 5)
data := make([]interface{}, 5)
for i := range users {
	data[i] = &users[i] // notice the added &
}

https://play.golang.org/p/GEbIq9febY

One option is to use the reflect package to access the slice elements.

The function assumes that p is a slice:

func loadDBRows(p interface{}) {
  var response DBResponse
  someDataFromDB := []byte("{\"rows\":1, \"error\": \"\", \"value\": {\"name\":\"John\", \"id\":2}}")
  json.Unmarshal(someDataFromDB, &response)
  v := reflect.ValueOf(p). // get reflect.Value for argument
	           Index(0).   // get first element assuming that p is a slice
	           Addr().     // take address of the element
               Interface() // get the pointer to element as an interface{}
  json.Unmarshal(response.Value, v)
}

Use loadDBRows like this. There's no need to create the []interface{} as in the question:

func main() {
  users := make([]User, 5)
  loadDBRows(users)
  fmt.Println(users)
}

playground example