Why is glade not being loaded into my class?

Question

GTK+

Guys, I ask you not to hurt me, I'm learning the pros and faced such a disaster. I create a window with markup from a glade file like this:

#include <gtkmm.h>
#include <iostream>
#include "./win.cpp"

int main (int argc, char *argv[]){
  auto app = Gtk::Application::create(argc, argv, "org.gtkmm.example");
  auto refBuilder = Gtk::Builder::create();
  Gtk::Window* TestWin;
  //MainWindow* TestWin;

  try{
      refBuilder->add_from_file("./test.glade");
  }
  catch(const Glib::FileError& ex){
      std::cerr << "!! FileError: " << ex.what() << std::endl;
  }
  catch(const Glib::MarkupError& ex){
      std::cerr << "!! MarkupError: " << ex.what() << std::endl;
  }
  catch(const Gtk::BuilderError& ex){
      std::cerr << "!! BuilderError: " << ex.what() << std::endl;
  }

  refBuilder->get_widget("window_id", TestWin);
  if(TestWin)
    return app->run(*TestWin);

  else{
    std::cout << "Ошибка загрузки виджета." << std::endl;
    return 1;
  }
}

And oddly enough, it compiles the code and even works, loads the markup without any problems and displays the desired window.
The problem appears when I try to inherit my class and already pass the markup file to it. The class itself so far is practically no different, created solely for educational purposes:

class MainWindow : public Gtk::Window{
public:
    virtual ~MainWindow(){
        std::cout << "Окно закрыто." << std::endl;
    }

protected:
    void on_button_clicked(){ //Обработчик сигналов от кнопки
        std::cout << "Hello World" << std::endl;
    }
};

And now if in the main() function we comment out the line "Gtk::Window* TestWin;" and uncomment "MainWindow* TestWin;", in an attempt to create a window from its class, the compiler issues the following:

** (program:22060): CRITICAL **: 18:50:50.811: Gtk::Builder::get_widget(): dynamic_cast<> failed.
Widget loading error.

He sincerely tried to google, but something didn’t work at all to understand what he needed from me ...

Answer 1

In case someone runs into the same problem, I read the official manuals for a long time and was able to understand what the jamb is. The constructor must be inherited too. Approximately like this:

MainWindow(BaseObjectType* cobject, const Glib::RefPtr<Gtk::Builder>& refGlade) : Gtk::Window(cobject);

After that, everything started! :)