Calling struct method with reflection

Question

I'm having trouble calling a method in my recursive reflection function. Here it is:

func setPropertiesFromFlags(v reflect.Value, viper *viper.Viper) {
    t := v.Type()
    method := v.MethodByName("Parse")
    fmt.Println(method)
    for i := 0; i < t.NumField(); i++ {
        field := t.Field(i)
        switch field.Type.Kind() {
        case reflect.Struct:
            setPropertiesFromFlags(v.Field(i), viper)
        case reflect.String:
            v.Field(i).SetString(viper.GetString(field.Tag.Get("name")))
    }
}

I'm calling the function with:

// Config struct passed to all services
type Config struct {
    common.Config
    common.ServerConfig
    common.AuthConfig
}
// Parse the thing already!
func (c *Config) Parse() {
    fmt.Println("RUN THIS THING")
}
int main() {
   setPropertiesFromFlags(reflect.ValueOf(c).Elem(), viper)
}

What I'm hoping for is to get my parse method in the place where I'm printing method and run .Call() against it. Instead it's printing out: <invalid reflect.Value> which I cannot call against.

I suppose I'm having trouble wrapping my head around the return values of each method. I know I have to use ValueOf to pull the value but it seems any permutation I try I'm getting the methods from the reflection class itself :-p sigh

I believe it's because it's declared on a pointer, the Parse method that is, but you are passing in a value to your function. That is, to fix this, declare Parse on a non-pointer. Or pass in the pointer, i.e. setPropertiesFromFlags(reflect.ValueOf(c), viper) (without .Elem()) and then when you need to manipulate the struct inside setPropertiesFromFlags call Elem there. — mkopriva
– mkopriva, Commented Nov 6, 2019 at 18:53
... alternatively keep doing what you're doing but retrieve the method from v's "address", i.e. v.Addr().MethodByName("Parse"). — mkopriva
– mkopriva, Commented Nov 6, 2019 at 18:59
@mkopriva you beat me to the answer in these comments and your last comment is perhaps the best answer. Should you write an actual answer instead of a comment? — user12258482
– user12258482, Commented Nov 6, 2019 at 19:03
@ddibiase couple demonstrations of what you can and cannot do depending on whether you have a pointer or a value. play.golang.com/p/vxyIIvrsBZt (updated) — mkopriva
– mkopriva, Commented Nov 6, 2019 at 19:11

user12258482 · Accepted Answer · 2019-11-06 19:00:01Z

The problem is that the method is on the pointer receiver, but the function is working with a valuer receiver. Rewrite the function to work with a pointer to a struct:

func setPropertiesFromFlags(vp reflect.Value, viper *viper.Viper) {
    method := vp.MethodByName("Parse")
    fmt.Println(method)

    v := vp.Elem()
    t := v.Type()
    for i := 0; i < t.NumField(); i++ {
        field := t.Field(i)
        switch field.Type.Kind() {
        case reflect.Struct:
            setPropertiesFromFlags(v.Field(i).Addr(), viper) // <-- take address of field here
        case reflect.String:
            v.Field(i).SetString(viper.GetString(field.Tag.Get("name")))
        }
    }
}

Call like this:

   setPropertiesFromFlags(reflect.ValueOf(c), viper) // <-- do not call Elem()

I see now. I wasn't passing in .Addr() THAT was the major issue. Thanks gents.

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