Get Gtk::DrawingArea size, before signaling draw

I want to draw some frames, one by one in a Gtk::DrawingArea. At the beginning of application, I need to get the size of DrawingArea, to allocate a RGB buffer, hewever, DrawingArea::get_width() and DrawingArea::get_height() return 1.

My code is based on https://developer.gnome.org/gtkmm-tutorial/stable/sec-drawing-clock-example.html.en with an extra line to get the size of DrawingArea:

#include "clock.h"
#include <gtkmm/application.h>
#include <gtkmm/window.h>

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

   Gtk::Window win;
   win.set_title("Cairomm Clock");

   Clock c;
   win.add(c);
   c.show();

   int w = c.get_width();
   // w is 1;

   return app->run(win);
}

How can I get the size of DrawingArea before actually drawing into it?

I was able to trace size changes based on Scheff's comments:

#include "clock.h"
#include <gtkmm/application.h>
#include <gtkmm/window.h>

int width_ = 0;
int height_ = 0;
bool resized_ = false;

void check_size()
{
   if (resized_)
   {
      resized_ = false;
      // Apply size changes to resources using width_ and height_
   }
}

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

   Gtk::Window win;
   win.set_title("Cairomm Clock");

   Clock c;
   c.signal_configure_event().connect([&](GdkEventConfigure* event)
   {
      width_ = event->width;
      height_ = event->height;
      resized_ = true;
      return true;
   });
   win.add(c);
   c.show();

   return app->run(win);
}

Where to call check_size function is dependent on the design of the application. In my case I call it right before c.queue_draw().