gdk_screen_get_default() returns null

Please help with this beginner's program for fetching gdk screen attributes. I have a small C++ program to find out the connected display units. I am using c++ on Linux Debian. gdk_screen_get_default() doesn't return Screen object. If I don't check for screen object then the following error occurs.

Error (process:8023): Gdk-CRITICAL **: gdk_screen_get_monitor_geometry: assertion 'GDK_IS_SCREEN (screen)' failed

I went through related posts and referred to Programmatically determining individual screen widths/heights in Linux (w/Xinerama, TwinView, and/or BigDesktop) for the below code snippet.

Thanks for your help. Any pointers/guidelines to resolve this would be helpful.

I have one monitor connected and the display settings are

$ echo $XDG_CURRENT_DESKTOP
GNOME

$ echo $DISPLAY
:0

CODE

    #include <gdk/gdk.h>
    #include <iostream>

    /*
    GTK version 3.14.5
    g++ getScreenInfo.cpp -o getScreenInfo `pkg-config gtk+-3.0 --cflags --libs`
    */

    int main()
    {
       GdkScreen *screen;
       screen = gdk_screen_get_default();           
       int num_monitors;
       int i;

       if (screen)
       {
          num_monitors = gdk_screen_get_n_monitors(screen);

          for (i = 0; i < num_monitors; i++)
          {
              GdkRectangle rect;
              gdk_screen_get_monitor_geometry (screen, i, &rect);
              std::cout << "monitor " << i << ": coordinates (" << rect.x << ","
              <<  rect.y << ", size (" << rect.width << "," << rect.height << ")"
              << std::endl;
           }
       }else
       {
            std::cout << "Couldn't obtain default screen object" << std::endl;
        }
    }