class GUI_Object_Builder  {    // ...  };    class Image_Builder  {    public:        Image_Builder();                virtual void Handle_Field_Change(const std::string& data);    private:        image_ptr image;  };  class Button_Builder {};  // etc.    Image_Builder::Do_Registration()  {        GEM.Register(this, Trigger("property_grid", INPUT / PROPERTY_FIELD_CHANGED));  }    Image_Builder::Notify(const GUI_Event& event_)  {        if(Has_Focus() && event_.Name() == "property_grid" && event_.Type() == INPUT / PROPERTY_FIELD_CHANGED)        {            GUI_Object_Builder::Handle_Field_Change(event_.Data());                        Handle_Field_Change(event_);        }  }    Image_Builder::Handle_Field_Change(const std::string& data)  {        std::string field = Get_Field(data);        std::string value = Get_Value(data);                if(field == "size")        {            image->Set_Size(from_string<PointI>(data));        }        else if(field == "position")        {            image->Set_Position(from_string<PointI>(data));        }        // etc.  }