Reading nested structure using reflection

Question

I write a recursive function that iterate over deep nested struct like the following:

type Container struct {
    Name string
    Items []Item
}

type Item struct {
    Name string
    Info Info
    Vals []string
}
// recursively reads nested struct, prints string values
func ReadStruct(st interface{}) {
    val := reflect.ValueOf(st).Elem()
    for i := 0; i < val.NumField(); i++ {
        fmt.Println(val.Type().Field(i).Type.Kind())
        switch val.Type().Field(i).Type.Kind() {
        case reflect.Struct:
            ReadStruct(val.Field(i)) // panic: call of reflect.Value.Elem on struct Value
        case reflect.Slice:
            // How to iterate over the reflect.Slice? 
        case reflect.String:
            fmt.Printf("%v=%v", val.Type().Field(i).Name, val.Field(i))
        }
    }
  

how to get access to inner objects (slices, structs) to work with them using reflect? to iterate over slice i tried to use:

for i:= 0; i < val.Field(i).Slice(0, val.Field(i).Len()); i++ { //error: reflect.Value doesnt support indexing
//some work
} 

Answer 1

Couple of errors in your code.

First, you only have to call Value.Elem() if the passed value is a pointer. When you iterate over the fields and you find a field of struct type, and you recursively call ReadStruct() with that, that won't be a pointer and thus you mustn't call Elem() on that.

So do it like this:

val := reflect.ValueOf(st)
if val.Kind() == reflect.Ptr {
    val = val.Elem()
}

Next, since you start ReadStruct() by calling reflect.ValueOf(), that assumes you have to pass non-reflect values to ReadStruct() (that is, not values of type reflect.Value).

But when you iterate over the fields, calling Value.Field(), you get a reflect.Value wrapping the field. You have to call Value.Interface() to extract the non-reflect value form it, to be passed in recursive calls.

To iterate over slices, simply use Value.Len() to get the slice length, and Value.Index() to get the i^th element of the slice.

Here's the corrected version of your traversal function:

// I used this type as you didn't post it in your question.
type Info struct {
    Key, Value string
}

func ReadStruct(st interface{}) {
    val := reflect.ValueOf(st)
    if val.Kind() == reflect.Ptr {
        val = val.Elem()
    }
    for i := 0; i < val.NumField(); i++ {
        // fmt.Println(val.Type().Field(i).Type.Kind())
        f := val.Field(i)
        switch f.Kind() {
        case reflect.Struct:
            ReadStruct(f.Interface())
        case reflect.Slice:
            for j := 0; j < f.Len(); j++ {
                ReadStruct(f.Index(i).Interface())
            }
        case reflect.String:
            fmt.Printf("%v=%v\n", val.Type().Field(i).Name, val.Field(i).Interface())
        }
    }
}

Testing it:

c := &Container{
    Name: "c1",
    Items: []Item{
        {
            Name: "i1",
            Info: Info{Key: "k1", Value: "v1"},
        },
        {
            Name: "i2",
            Info: Info{Key: "k2", Value: "v2"},
        },
    },
}
ReadStruct(c)

Output (try it on the Go Playground):

Name=c1
Name=i2
Key=k2
Value=v2
Name=i2
Key=k2
Value=v2

Note: By using recursive calls, you are extracting and re-acquiring reflect.Value values. It would be more efficient to always work with reflect.Values, so you can avoid these unnecessary calls.

This is how you could do that:

func ReadStruct(st interface{}) {
    readStruct(reflect.ValueOf(st))
}

func readStruct(val reflect.Value) {
    if val.Kind() == reflect.Ptr {
        val = val.Elem()
    }
    for i := 0; i < val.NumField(); i++ {
        // fmt.Println(val.Type().Field(i).Type.Kind())
        f := val.Field(i)
        switch f.Kind() {
        case reflect.Struct:
            readStruct(f)
        case reflect.Slice:
            for j := 0; j < f.Len(); j++ {
                readStruct(f.Index(i))
            }
        case reflect.String:
            fmt.Printf("%v=%v\n", val.Type().Field(i).Name, val.Field(i))
        }
    }
}

This will output the same. Try this one on the Go Playground.