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copied!<p>Gtk::Widget::get_window() returns a null Glib::RefPtr, since the widget has not been realized just yet.</p> <p>Based on the <a href="http://developer.gnome.org/gtk3/3.2/GtkDrawingArea.html" rel="nofollow">GtkDrawingArea documentation</a>, you need to hook onto the "draw" signal to handle drawing, where your Cairo context is already created and handed to you. Going back to the <a href="http://developer.gnome.org/gtkmm/unstable/classGtk_1_1Widget.html" rel="nofollow">Gtkmm reference</a>, you would use Gtk::Widget::signal_draw() to hook onto that, or you could overload the virtual on_draw() function to handle your drawing.</p> <p>Additionally, you also need to call .show() on each widget, i.e. your DrawingArea and your Window, and call ccontext->stroke() to get the line actually drawn.</p> <p>The result would look something like:</p> <pre><code>#include <gtkmm.h> bool draw (const Cairo::RefPtr<Cairo::Context> &ccontext, Gtk::DrawingArea *drawarea) { Gtk::Allocation allocation = drawarea->get_allocation(); const int width = allocation.get_width(); const int height = allocation.get_height(); ccontext->set_source_rgb(1.0, 0.0, 0.0); ccontext->set_line_width(2.0); ccontext->move_to(0,0); ccontext->line_to(width, height); ccontext->stroke (); return true; } int main(int argc, char * argv[]) { Gtk::Main kit(argc, argv); Gtk::Window window; Gtk::DrawingArea drawarea; drawarea.signal_draw ().connect (sigc::bind (sigc::ptr_fun (&draw), &drawarea)); window.add(drawarea); window.show_all (); Gtk::Main::run(window); return 0; } </code></pre> <p>or alternatively:</p> <pre><code>#include <gtkmm.h> class LineBox : public Gtk::DrawingArea { protected: virtual bool on_draw (const Cairo::RefPtr<Cairo::Context> &ccontext); }; bool LineBox::on_draw (const Cairo::RefPtr<Cairo::Context> &ccontext) { Gtk::Allocation allocation = get_allocation(); const int width = allocation.get_width(); const int height = allocation.get_height(); ccontext->set_source_rgb(1.0, 0.0, 0.0); ccontext->set_line_width(2.0); ccontext->move_to(0,0); ccontext->line_to(width, height); ccontext->stroke (); return true; } int main(int argc, char * argv[]) { Gtk::Main kit(argc, argv); Gtk::Window window; LineBox drawarea; window.add(drawarea); window.show_all (); Gtk::Main::run(window); return 0; } </code></pre>

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