Where should I put these code?

["chunlong84" <chunlong84 gmail com>]

 For efficiency,i don't want put these codes in MyDrawingArea::on_expose_event().
I need a fixed size window,so i only need execute these code once,not every time.
but i still cann't put them in construction method,where should i put them?

	allocation=get_allocation();     //allocation,cr,gc are data members of MyDrawingArea:public Gtk::DrawingArea
	cr = get_window()->create_cairo_context();   
   	gc=Gdk::GC::create(get_window());