void Gtk::Widget::get_preferred_width pass nullptr

[Matthew Morgan <matthew lifandi org>]

I'm using the function Gtk::Widget::get_preferred_width() in a program, 
and I noticed something odd in it's documentation on 
developer.gnome.org.  It specifies it this way:

void Gtk::Widget::get_preferred_width(int &minimum_width, int 
&natural_width)
minimum_width    Location to store the minimum width, or nullptr.
natural_width    Location to store the natural width, or nullptr.

As I use it I really only want natural_width, but everything I read 
indicates that passing a nullptr reference is invalid; in fact, I can't 
even come up with code to do so that gcc will compile.  What is the 
intended way to pass nullptr to this function?  For now I'm just giving 
a real int to minimum_width and not using it.