setting fields of object using traits

[Ram_B via Digitalmars-d-learn]

I'm trying to set fields of object from JSON with traits library. 
How i can to it properly?

import std.stdio;
import std.json;
import std.traits;
import std.meta:  Alias;
class Obj{
        void fromJSON(this T)(JSONValue j){
                foreach(field; FieldNameTuple!T){
                        alias member = Alias!(__traits(getMember, T, field));
                        static if (__traits(hasMember, member, "fromJSON")){
                                member.fromJSON(j[field]);
                        } else {
                                member = j[field];
                        }
                }
        }
}

class A : Obj{
        int a,b;
        C c;
        this(){
                c = new C();
        }
        
}

class C : Obj{
        int a;
        this(){
                a = 0;
        };
}

int main(string[] argv)
{
        string s = "{\"a\": 1, \"b\": 2, \"c\": {\"a\": 3} }";
        JSONValue j = parseJSON(s);
        A a = new A();
        a.fromJSON(j);
        writeln(a.b);
        readln();
    return 0;
}

main.d(14): Error: need 'this' for 'a' of type 'int'
main.d(14): Error: need 'this' for 'b' of type 'int'
main.d(12): Error: template main.Obj.fromJSON cannot deduce 
function from argument types !(c)(JSONValue), candidates are:
main.d(8):        main.Obj.fromJSON(this T)(JSONValue j)
main.d(41): Error: template instance main.Obj.fromJSON!(A) error 
instantiating