访问函数内部结构的指针值

Question

I want to pass a struct object to a function & be able to access its pointer value from that function. I am not able to understand why the following is resulting in error.

func GetStructFieldPointers(u interface{}, jsonFields []string) []interface{} {
    structVal := reflect.ValueOf(&u).Elem()
    structType := reflect.TypeOf(u)
    numberOfFields := structVal.NumField() // getting error here reflect:
                                           // call of reflect.Value.NumField
                                           // on interface Value
    numberOfJSONFields := len(jsonFields)
    res := make([]interface{}, numberOfJSONFields)
    fmt.Println(jsonFields)
    for fieldIndex, field := range jsonFields {
        for i := 0; i < numberOfFields; i++ {
            if structType.Field(i).Tag.Get("json") == field {
                valueField := structVal.Field(i)
                res[fieldIndex] = valueField.Addr().Interface()
            }
        }
    }

    return res
}

type User struct {
    Id             int      `json:"id"`
    Name           string   `json:"name"`
    Address        string   `json:"address"`
}
user := User{}
res := GetStructFieldPointers(user, []string{"id", "name"}) 

To make this work, I had to make structType as a parameter like following:

func GetStructFieldPointers(u interface{}, structType reflect.Type, jsonFields []string) []interface{} {
    structVal := reflect.ValueOf(u).Elem()
    // structType := reflect.TypeOf(u)
    numberOfFields := structVal.NumField() 
    numberOfJSONFields := len(jsonFields)
    res := make([]interface{}, numberOfJSONFields)
    fmt.Println(jsonFields)
    for fieldIndex, field := range jsonFields {
        for i := 0; i < numberOfFields; i++ {
            if structType.Field(i).Tag.Get("json") == field {
                valueField := structVal.Field(i)
                res[fieldIndex] = valueField.Addr().Interface()
            }
        }
    }

    return res
}

user := User{}
res := GetStructFieldPointers(&user, reflect.TypeOf(user), []string{"id", "name"}) 

I like to know how to pass User{} as a parameter & use in both reflect.ValueOf & reflect.TypeOf calls.

Answer 1

On this line: structVal := reflect.ValueOf(&u).Elem() you're taking the address of an interface (the argument of your func) and not an address to the interface's underlying value, and then you're passing the pointer to ValueOf, so the .Elem() call returns the "elem value" to which the pointer points to, which is the interface, not struct.

If you know for a fact that the passed in value is a struct and not a pointer, all you need is this: structVal := reflect.ValueOf(u).

If a pointer was passed to your func, eg GetStructFieldPointers(&u, ... then this is what you want: structVal := reflect.ValueOf(u).Elem().

But also you can handle both cases by checking the value's kind.

rv := reflect.ValueOf(u)
if rv.Kind() == reflect.Ptr {
    rv = rv.Elem()
}
if rv.Kind() == reflect.Struct {
    fmt.Println(rv.NumField())
}

https://play.golang.org/p/9F9LNnwEaH

Update: Took a better look at your code... If you want to be able to get the addresses of your struct's fields, you need to pass a pointer to the struct as the argument, or else those fields are going to be unaddressable.

https://play.golang.org/p/RaA2rau3s-