How can i use gtk_window_set_transient_for ()?

I am using the library Gtkmm with C++. My code is almost perfect I think because it compiles and I can achieve to execute it. But in the terminal when I clicked on open a file in my software that I made with gtkmm I can read this message :

Gtk-Message: GtkDialog mapped without a transient parent. This is discouraged.

So I looked for on this forum how can I solve it and I understood I have to use this method : gtk_window_set_transient_for().

Actually i have a Gtk::Window and a Gtk::FileChooserDialog. Can you put an example which use gtk_window_set_transient_for() ?

Thank you very much !