How to maintain a reference to Cairo::Context or Cairo::Surface?

["BC Zhu" <zhubicen gmail com>]

Now  i am using Gtkmm. I want to maintain a reference to Cairo::Context or Cairo::Surface, so i can draw something on it anywhere.
I am trying to do this , but failed.
With following code , i am trying to keep a reference to Cairo:::Surface，but when on_expose_event was called secondly , it will fail.
And i have tried to maintain a reference to Cairo::Context too, but failed either.
Could some body give me an advice?
Thanks in advance!


class MyArea : public Gtk::DrawingArea
{
public:
    virtual ~MyArea();
protected:
//Override default signal handler:
    virtual bool on_expose_event(GdkEventExpose* event);
private:
    Cairo::RefPtr<Cairo::Surface> surf_;
};
MyArea::MyArea()
{
}
MyArea::~MyArea()
{
}
bool MyArea::on_expose_event(GdkEventExpose* event)
{
// This is where we draw on the window
    if(!surf_){
        Glib::RefPtr<Gdk::Window> window = get_window();
        Cairo::RefPtr<Cairo::Context> cr = window->create_cairo_context();
        surf_ = cr->get_target();
    }
    Cairo::RefPtr<Cairo::Context> cr = Cairo::Context::create(surf_);
    cr->set_line_width(10.0);
    cr->move_to(0, 0);
    cr->line_to(100, 100);
    cr->stroke();    
    return true;
}


-- 
Best Regards
Bicen.Zhu