How to write modular code in c++ with gtkmm?

Question

How to write modular code in gtkmm3 I want to create objects in a window that I have using the different classes I created (inherited from Gtk::Grid) and put them in the mainGrid window

I do this but it shows me nothing

class MainWindow:public Gtk::Window{
    public:
        MainWindow();
    private:
        Gtk::Grid mainGrid;
};

class FirstGrid:public Gtk::Grid{
    public:
        FirstGrid();
        Gtk::Grid getGrid();
    private:
        Gtk::Grid mainGrid;
        Gtk::Button button{"button"};
};

FirstGrid::FirstGrid(){
    mainGrid.attach(button,1,1);
}
FirstGrid::getGrid(){
    return std::move(mainGrid);
}

MainWindow::MainWindow(){
    set_size_request(500,500);
    add(mainGrid);
    FirstGrid firstGrid;
    mainGrid = firstGrid.getGrid();
    show_all();
}

Answer 1

The problem here seems to be that you are not using inheritance and polymorphism, even though you made it clear in your classes that they are indeed inheriting.

Here is a working example:

#include <gtkmm.h>

class FirstGrid : public Gtk::Grid
{

public:

    FirstGrid()
    {
        // Since FirstGrid is a Gtk::Grid, the `attach` method is
        // inherited, you can use it directly:
        attach(button, 0, 0, 1, 1);
    }

private:

    // No need for an extra Gtk::Grid here.
    Gtk::Button button{"button"};
}; 

class MainWindow : public Gtk::Window
{

public:

    MainWindow()
    {
        set_size_request(500, 500);

        // Use FirstGrid as a Gtk::Grid here:
        add(mainGrid);
        show_all();
    }

private:

    FirstGrid mainGrid;
};

int main(int argc, char *argv[])
{
    auto app = Gtk::Application::create(argc, argv, "org.gtkmm.examples.base");
  
    MainWindow window;
    window.set_default_size(200, 200);
  
    return app->run(window);
}

which you ca build with:

g++ main.cpp -o example.out `pkg-config --cflags --libs gtkmm-3.0`

assuming you copy it in a file named main.cpp.