Go中的非指针错误，不确定是什么意思

Question

I have a function that takes a json decoder and an interface as its arguements, and i am decoding to a struct that is passed on the interface. Like so:

func DecodeJSON(decoder *json.Decoder, i interface{}) bool {
    if c, ok := i.(User); ok {
        err := decoder.Decode(c)
        if err != nil {
            fmt.Println(err)
            return false //err is not nil
        }
    }
    return false
}

Usage of the function:

// Register test
func HandleRequest(w rest.ResponseWriter, r *rest.Request) {

    decoder := json.NewDecoder(r.Body)
    user := User{}
    if DecodeJSON(decoder, user) {    
        fmt.Println("success")
}

Error i am getting:

json: Unmarshal(non-pointer main.User)

Bit confused by this message since my DecodeJSON is not taking a pointer for user . So am not sure what i have done wrong with my code. Hope some one can explain so i can understand my mistake.

Answer 1

You need to use a pointer to a user to decode the data in, otherwise the decoded data gets decoded in a copy of the object that gets deleted when the function returns.

func DecodeJSON(decoder *json.Decoder, i interface{}) bool {
    if c, ok := i.(*User); ok {
        err := decoder.Decode(c)
        if err != nil {
            fmt.Println(err)
            return false //err is not nil
        }
    }
    return false
}

// Register test
func HandleRequest(w rest.ResponseWriter, r *rest.Request) {

    decoder := json.NewDecoder(r.Body)
    user := &User{}
    if DecodeJSON(decoder, user) {    
        fmt.Println("success")
}

Leave the interface parameter as is, just use a pointer when you pass the User and when you get the user from the interface.

Answer 2

Based on your code, you'd better change your function signature to func DecodeJSON(decoder *json.Decoder, user *User) bool

This will (1) eliminate explicit runtime cast, and (2) make your code less ambiguous and safe and compile-time checked.