Re: wy signal handler could not be override

["Klaus Rudolph" <lts-rudolph gmx de>]

To override the signal handler is part of the documentation. Looking in the sources shows the virtual 
functions:
virtual bool on_<Signal_Name>(...);
 
But it seems simply not to work as expected and I have no idea why.
 
I would provide a full example:
 

#include <iostream>
using namespace std;
#include <gtkmm.h>
#include <goocanvasmm.h>
bool MyExternalHandler( const Glib::RefPtr<Goocanvas::Item>& item, GdkEventButton* ev )
{
    cout << "External Handler" << endl;
    return false;
}
class MyRect : public Goocanvas::Rect
{
    public:
        MyRect( double x, double y, double w, double h)
            //: Goocanvas::Rect( x,y,w,h)
    {
        property_x()=x;
        property_y()=y;
        property_width()=w;
        property_height()=h;
    }
    public:
        virtual void nonsens() {}
        bool on_button_press_event(const Glib::RefPtr<Item>& target, GdkEventButton* event) override
        {
            cout << "override handler" << endl;
            return false;
        }
        bool Handler( const Glib::RefPtr<Goocanvas::Item>& item, GdkEventButton* ev )
        {
            cout << "via mem_fun" << endl;
            return false;
        }
        bool on_enter_notify_event(const Glib::RefPtr<Item>& target, GdkEventCrossing* event) override
        {
            cout << "override enter notify" << endl;
            return false;
        }
};
int main(int argc, char* argv[])
{
    Gtk::Main app(&argc, &argv);
    Goocanvas::init("example", "0.1", argc, argv);
    Gtk::Window win;
    Goocanvas::Canvas m_canvas;
    m_canvas.set_size_request(640, 480);
    m_canvas.set_bounds(0, 0, 1000, 1000);
    MyRect* ptr;
    Glib::RefPtr<MyRect> m_rect_own(ptr=new MyRect(225, 225, 150, 150));
    m_rect_own->property_line_width() = 1.0;
    m_rect_own->property_stroke_color() = "black";
    m_rect_own->property_fill_color_rgba() = 0x555555ff;     
    Glib::RefPtr<Goocanvas::Item> root = m_canvas.get_root_item();
    root->add_child( m_rect_own);
    
((Glib::RefPtr<Goocanvas::Item>&)m_rect_own)->signal_button_press_event().connect(sigc::ptr_fun(&MyExternalHandler));
    ((Glib::RefPtr<Goocanvas::Item>&)m_rect_own)->signal_button_press_event().connect(sigc::mem_fun(*ptr, 
&MyRect::Handler));
    win.add(m_canvas);
    win.show_all_children();
    Gtk::Main::run(win);
    return 0;
}
 
 
 
> In C++, declaring the method as "virtual" is needed if you want to
> override anything by polymorphism.

> However, in Gtkmm, I never override methods. Signals do the same job and
> are more flexible, so I connect methods to the signals I want to handle
> in the constructor:

> signal_button_press_event().connect(sigc::mem_fun(this,
> &MyClass::button_pressed));

Yes, the slot mechanic works, but I simply want to test what is written in the docs. And I dive into the 
source and also find the on_XXX(...) virtual function call. But there is something wrong with them or my 
usage.