// I am omiting (tons of) cleanup code and where I set the display and toplevel variables.

Display* display;
Widget toplevel;

bool has_name(Window window)
{
    XTextProperty data = XTextProperty ();
    return (!XGetWMName (display, window, &data));
}

bool has_client_leader(Window window)
{
    unsigned long nitems = 0;
    unsigned char* data = 0;
    Atom actual_type;
    int actual_format;
    unsigned long bytes;
    // WM_CLIENT_LEADER is an interned Atom for the WM_CLIENT_LEADER property
    int status = XGetWindowProperty (display, window, WM_CLIENT_LEADER, 0L, (~0L), False,
    	AnyPropertyType, &actual_type, &actual_format, &nitems, &bytes, &data);
    if (status != Success || acutal_type == None) return false;
    Window* leader = reinterpret_cast<Window*> (data);
    return (*leader != 0);
}

bool has_class(Window window)
{
    XClassHint data = XClassHint ();
    return (!GetClassHint (display, window, &data));
}

void handle_event(Widget widget, XtPointer, XEvent* event, Boolean*)
{
    if (event->type != CreateNotify) return;

    Window w = event->xcreatewindow.window;

    // confirm window has a name
    if (!has_name (w)) return;

    // confirm window is a client window
    Window client = XmuClientWindow (display, w);
    if (!client || client != w) return;

    // confirm window has a client leader that is not 0x0
    if (!has_client_leader (client)) return;

    // confirm window has a class
    if (!has_class (client)) return;

    // The window has passed all our checks!
    // Go on to do stuff with the window ...
}

int main(int argc, char* argv[])
{
    // ...

    // Setting up the event handler for SubstructureNotify on root window
    Window root_window = XDefaultRootWindow (display);
    Widget dummy = XtCreateWidget ("dummy", coreWidgetClass, toplevel, 0, 0);
    XtRegisterDrawable (display, root_window, dummy);
    XSelectInput (display, root_window, SubstructureNotifyMask);
    XtAddRawEventHandler (dummy, SubstructureNotifyMask, False, handle_event, 0);

// ...
}