Go function with dynamic types of structures

Question

Does anybody can help me with it: I need to set up field values for different types of structures. I have a map with data, extracted from the database. In this particular function, I want to create an object of any structure, fields of which are matched with the map

type Member struct {
     firstName  string    `xml: "FIRST_NAME"`
     lastName string      `xml: "LAST_NAME"`
}

type CardData struct {
     cardType string     `xml: "CARD_TYPE"`
     cardNumber string   `xml: "CARD_NUMBER"`
}

func main() {
   fields := make(map[string]string)
   fields['CARD_TYPE'] = "VISA"
   fields['FIRS_NAME'] = "Aria Stark"
   member := Combiner(fields, Member{})
   card := Combiner(fields, CardData{})

}

func Combiner(m map[string]string, obj interface{}) interface{} {
    ff := reflect.ValueOf(obj)
    typeOfS := ff.Type() 
    for i := 0; i< ff.NumField(); i++ { 
        tag := typeOfS.Field(i).Tag.Get("xml") 
        if _, ok := m[tag]; ok { 
           n := typeOfS.Field(i).Name 
           reflections.SetField(&obj, n, m[tag]) 
        } else { 
            fmt.Printf("The field %s is not found \n", tag) 
        } 
   } 
   return obj 
}

but I get an error in this string "reflections.SetField(&obj, n, m[tag])" It doesn't work because "obj" is not a struct

Many thanks for all yours answers!

Erwin Bolwidt · Accepted Answer · 2021-02-07 20:17:07Z

1

You have to pass a pointer to the object to your Combiner function. From the documentation of reflections.SetFields:

obj param has to be a pointer to a struct, otherwise it will soundly fail.

So:

member := Combiner(fields, &Member{})
card := Combiner(fields, &CardData{})

and inside Combiner:

reflections.SetField(obj, n, m[tag])

answered Feb 7, 2021 at 20:17

Erwin Bolwidt

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3 revs user12258482 · Accepted Answer · 2021-02-15 02:00:15Z

You are close to a correct solution. The problems with the code are:

The string quote character is ", not '.
There is no space after the colon in standard struct field tags.
Non-exported fields cannot be set.
To set a value on a field, the reflect value must have an addressable value.

Here's the code with the problems fixed:

type Member struct {
    // NOTE: Export by capitalizing first letter in field name.
    // NOTE: Remove space after the :
    FirstName string `xml:"FIRST_NAME"`
    LastName  string `xml:"LAST_NAME"`
}

type CardData struct {
    CardType   string `xml:"CARD_TYPE"`
    CardNumber string `xml:"CARD_NUMBER"`
}

func main() {
    fields := make(map[string]string)

    // NOTE: use " instead of '
    fields["CARD_TYPE"] = "VISA"
    fields["FIRST_NAME"] = "Aria Stark"

    // NOTE: Pass pointer to struct so that combiner
    // has an addressable value.
    member := Combiner(fields, &Member{})
    card := Combiner(fields, &CardData{})

    fmt.Println(member)
    fmt.Println(card)
}

func Combiner(m map[string]string, obj interface{}) interface{} {
    // NOTE: dereference the pointer by calling Elem()
    ff := reflect.ValueOf(obj).Elem()
    typeOfS := ff.Type()
    for i := 0; i < ff.NumField(); i++ {
        tag := typeOfS.Field(i).Tag.Get("xml")
        if _, ok := m[tag]; ok {
            // NOTE: Set the field directly using the reflect package.
            ff.Field(i).Set(reflect.ValueOf(m[tag]))
        } else {
            fmt.Printf("The field %s is not found \n", tag)
        }
    }
    return obj
}

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