How to get the widget's size before it has been shown?

["Lazy Fox" <lazy fox wu gmail com>]

/* How to get the widget's size before it has been shown?
 *
 * There are two "printf" statements below. I want to get the widget's size at the first printf.
 * How to do this?
 */
#include <gtk/gtk.h>

int main( int   argc, char *argv[] )
{
    GtkWidget *window;
    gtk_init (&argc, &argv);

    window = gtk_window_new (GTK_WINDOW_TOPLEVEL);
printf("width=%d \n", window->allocation.width); // return 1
    gtk_widget_show  (window);
printf("width=%d \n", window->allocation.width); // return 200
    gtk_main ();
    return 0;
}