[gtkmm] How to force widget draw

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I've a Gtk::Statusbar widget that I use to display status info text to the 
user (what an strange use :-)).
The problem that I'm having is that I push a text to the Gtk::Statusbar just 
before a call to a function that make a "hard" process (lot of CPU use).
The text is displayed after the "hard" process functions ends.
I've tried to force the graphic server to draw the statusbar by calling the 
Gtk::Widget->show and Gtk::Widget->show_now function of the statusbar, but 
nothing.
How can I show the text to the statusbar before the "hard" process?

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