通过反射创建然后修改地图

Question

I have a new type derived from map such:

type mapp map[string]interface{}

with a small function on it

func (c mapp) Set() error {
    // c is nil
    c["a"] = "b"
    return nil
}

type Setter interface {
  Set() error
}

func main() {
    var aa mapp
    out := reflect.ValueOf(&aa)
    s := out.Interface().(Setter)
    s.Set()
}

This code works on a struct, why this code fails when it comes to a type of a map?

Here's a playground: https://play.golang.org/p/Z1LFqb6kF7

Many thanks,

Asaf.

Answer 1

Go maps (and slices) are created via make. The equivalent function in reflect is reflect.MakeMap

out := reflect.ValueOf(&aa).Elem()
out.Set(reflect.MakeMap(out.Type()))
s := out.Interface().(Setter)
s.Set()