Re: why is class HelloWorld : public Gtk::Window ?

[José Alburquerque <jaalburquerque cox net>]

Damon Register said the following:
> In the gtkmm tutorial
> http://www.gtkmm.org/docs/gtkmm-2.4/docs/tutorial/html/sec-helloworld.html 
>
> and elsewhere I have seen this example method used for a gtkmm app.
> There is the main() and a class that descends from Gtk::Window as in
>
> class HelloWorld : public Gtk::Window
>
> Why does HelloWorld descend from Gtk::Window?

The classes that derive from Gtk::Window are themselves a Gtk::Window 
which means they can be shown on the screen and can contain other 
widgets that can also be shown within the window to the user.  See the 
Gtk::Window API: 
http://www.gtkmm.org/docs/gtkmm-2.4/docs/reference/html/classGtk_1_1Window.html.

> I have seen examples for libglademm or gtk-builder which also have
> a class such as HelloWorld that descends from Gtk::Window.  So, I have
> two questions:
>
> 1. Is it necessary to descend from Gtk::Window
>    when _not_ using libglademm or gtk-builder such as in the tutorial?
>
> 2. Is it necessary to descend from Gtk::Window
>    when using libglademm or gtk-builder?

Deriving from Gtk::Window is necessary when the class being developed is 
to be used as a window to be shown on the screen.

> Why do I care?  In trying to find out why I get the
> "assertion `G_IS_OBJECT (object)' failed" message, I am trying
> different program configurations.  

Why not run the different configurations through a debugger?  It can 
help you pinpoint exactly where the warning is coming from.

> In the process I have noticed
> something interesting.  In an example where
> class HelloWorld : public Gtk::Window
> I create an instance of the class in the main() with this
>
>     HelloWorld window;
>
> and then in the class constructor I have
>
>     refXml = Gtk::Builder::create_from_file("../hello.xml");
>     refXml->get_widget("layout1", layout1);
>     layout1->reparent(*this);
>
> When using libglademm I have something similar but in either case I
> get the impression that I may have two Windows; one resulting from
>
>     refXml = Gtk::Builder::create_from_file("../hello.xml");
>
> and another because the class descends from Gtk::Window.  Is
> that really happening as I suspect?  

No.  Unless you've defined a Gtk::Window widget in your glade file, 
you'll only have the one window.  Also, it is not a programming error to 
have more than one Gtk::Window if you need them.

> I also noticed that the hello
> class is destroyed after Gtk::Main::run finishes.  Is that ok?

If you don't get any error messages or warnings, yes.  Debugging might 
help you pinpoint errors if there are any.

> As an experiment I tried
> connecting to Gtk::Main::signal_quit and in my signal handler
> I call remove().  When I run the program and then exit, I don't
> get the error message.
>
> I tried a second experiment which proved interesting (for me
> anyway).  Instead of having the hello class descend from
> Gtk::Window, I have it not descend from anything.  In the
> hello constructor I have
>
>     refXml = Gtk::Builder::create_from_file("../hello.xml");
>     refXml->get_widget("hello_main_window", pWindow);
>
> and in the main() I have
>
>     kit.run(*(window.pWindow));
>
> Am I on the right track so far?  At this point I favor making
> my class not descend from Gtk::Window as it seems to work but
> I would like to know if this makes any sense before I go off
> and change my applications.

Either way will work.  If you have a class that derives from Gtk::Window 
in your program you can use that class as a window instead of the 
"hello_main_window" widget defined in your glade file.  If you keep the 
Gtk::Window in your glade file, you don't need to have a class that 
derives from Gtk::Window (unless you need more than one window for your 
program).

> Damon Register
> _______________________________________________
> gtkmm-list mailing list
> gtkmm-list gnome org
> http://mail.gnome.org/mailman/listinfo/gtkmm-list


--
José Alburquerque
jaalburquerque cox net