Ern i want to update my window using flush. However i encounter error message like flush() is protected etc. My primary purpose is that i want my window to update itself after i change some of the parameter. However i seem unable to achieve this. Can anyone advice me on this.

Fl_Window *win = new....
some coding for implementation
finally win->flush();

You rarely need to call flush() yourself. One example would be an extremely long calculation which freezes the display. But there are much better ways than flush().

You are probably looking for redraw(). That function signals to the window that it wants to be redrawn at the next occasion.

flush() btw. is a function of Fl, so you need to call Fl::flush().