GTKMM/C++11: How to create a custom composite widget out of other widgets?

I want to derive my own widget class and add standard widgets to this class to create a composite widget. Does anybody have an examples or suggesions on how to do this? For example, Suppose I want to create custom composite widget of 4 buttons. I'm guessing its something like the code below:

//First Question:  Is this the best way to create composite widget? (see below)

//Second Question:  How do you make a widget container expand in 
// the horizontal direction while at the same time shrink in 
// the vertical direction?  because i wanted the boxes to expand horizontally
// to fill the window, and at the same time shrink to minimum width in the vertical
// direction


#include <iostream>

using namespace std;
#include <gtkmm.h>

class MyWidget : public Gtk::Frame {
  public:
    MyWidget() {
        add(m_hbox1);
        m_hbox1.pack_start  (m_vbox1,    Gtk::PackOptions::PACK_SHRINK);
        m_vbox1.pack_start  (m_hbox2,    Gtk::PackOptions::PACK_EXPAND_WIDGET);
        m_hbox2.pack_start  (m_btn_fwd,  Gtk::PackOptions::PACK_EXPAND_WIDGET);
        m_hbox2.pack_start  (m_btn_play, Gtk::PackOptions::PACK_EXPAND_WIDGET);
        m_hbox2.pack_start  (m_btn_stop, Gtk::PackOptions::PACK_EXPAND_WIDGET);
        m_hbox2.pack_start  (m_btn_back, Gtk::PackOptions::PACK_EXPAND_WIDGET);

        show_all_children();
    }
    ~MyWidget() {
    }

  private:

    Gtk::Box    m_vbox1    {Gtk::ORIENTATION_VERTICAL};
    Gtk::Box    m_hbox1    {Gtk::ORIENTATION_HORIZONTAL};
    Gtk::Box    m_hbox2    {Gtk::ORIENTATION_HORIZONTAL};
    Gtk::Button m_btn_fwd  {"Fwd"};
    Gtk::Button m_btn_back {"Back"};
    Gtk::Button m_btn_play {"Play"};
    Gtk::Button m_btn_stop {"Stop"};
};

class MyWindow : public Gtk::Window {
  public:
    MyWindow(string name) {
       set_title(name);
       add(m_vbox);

       // Shrink in Vertical Direction
       m_vbox.pack_start(m_mywidget, Gtk::PackOptions::PACK_SHRINK);

       show_all_children();
    }

  private:
    Gtk::Box    m_vbox      {Gtk::ORIENTATION_VERTICAL};
    MyWidget    m_mywidget;
};

int main(int argc, char *argv[])
{
  auto app = Gtk::Application::create(argc, argv,
     "org.gtkmm.example.actionbar");

  MyWindow window {"Testing Custom Composite Widget"};

  // Shows the window and returns when it is closed.
  return app->run(window);
}