Call a unknown constructor (Replacement for reflection) c++

I'm porting a networking api I made in java/as3 to c++. I'm quite new to c++ so I'm not that familiar with all the tools that are available.

I have a class called Packet. In my java/as3 class that packet has 2 hashmaps, one for classToId and one for idToClass.

Then I register a derived packet with an id and its class object.

When a packet is received I get its packet ID and then create a new packet with that ID.

In as3 code I then use:

    public static function getNewPacket(id:int):Packet
    {
        try
        {
            var clazz:Class = getClassById[id];
            return (clazz == null ? null : Packet(new clazz()));
        }
        catch (e:Error)
        {
            trace("Skipping packet with id " + id + ": " + e.message);
        }
        return null;
    }

As you can see, reflection is used to call the constructor. In c++ however, I have no idea how I could create this. I can't use a static list of packets because this is a API class.(So this class will be extended in an application using this API)