Re: GtkAda : Trying to derive a widget

["Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>]

On 2021-04-09 07:28, DrPi wrote:
> Le 09/04/2021 à 00:27, Dmitry A. Kazakov a écrit :
>> On 2021-04-08 21:27, DrPi wrote:
>>
>>> I'm trying to create a GtkAda widget derived from a standard widget.
>>>
>>> -- debug_panel.ads
>>> with Gtk.Scrolled_Window; use Gtk.Scrolled_Window;
>>> with Gtk.Text_View;       use Gtk.Text_View;
>>>
>>> package Debug_Panel is
>>>
>>>    type Debug_Panel_Record is new Gtk_Scrolled_Window_Record with 
>>> private;
>>>    type Debug_Panel is access all Debug_Panel_Record'Class;
>>>
>>>
>>>    procedure Gtk_New (Panel : in out Debug_Panel);
>>>    procedure Initialize (Panel : not null access 
>>> Debug_Panel_Record'Class);
>>>
>>> private
>>>
>>>    type Debug_Panel_Record is new Gtk_Scrolled_Window_Record with record
>>>       Text       : Gtk_Text_View;
>>>    end record;
>>>
>>> end Debug_Panel;
>>>
>>>
>>> -- debug_panel.adb
>>> package body Debug_Panel is
>>>
>>>    procedure Gtk_New (Panel : in out Debug_Panel) is
>>>    begin
>>>       Panel := new Debug_Panel_Record;
>>>       Initialize (Panel);
>>
>>    Debug_Panel.Initialize (Panel);
> I did try this notation and got this error :
> debug_panel.adb:6:07: invalid prefix in selected component "Debug_Panel"
> debug_panel.adb:6:18: prefixed call is only allowed for objects of a 
> tagged type
> 
> I've just realized this is because the package and the access type have 
> the same name.

You can disambiguate it so:

    Standard.Debug_Panel.Initialize (Panel)

    Standard.Debug_Panel - The package name
    Standard.Debug_Panel.Debug_Panel - The type name

>> Another way would be to remove "use Gtk.Scrolled_Window" and declare as
>>
>>    type Debug_Panel_Record is
>>       new Gtk.Scrolled_Window.Gtk_Scrolled_Window_Record with private;
>>
>> Without "use" Initialize is unambiguous.
>>
> That makes sense.
> However, I don't understand why there is ambiguity when using "use".
> Debug_Panel (the type) is of type Debug_Panel_Record, so Initialize 
> should resolve to the one using this type. Well, I guess I'm wrong.

Debug_Panel belongs to both Debug_Panel_Record'Class and 
Gtk_Scrolled_Window_Record'Class (and many other classes), so the ambiguity.

>> P.S. When you create new widget it is better to use a more general 
>> ancestor hiding insufficient details, e.g.
>>
>>    type Debug_Panel_Record is
>>       new Gtk.Widget.Gtk_Widget_Record with private;
>>    ...
>> private
>>    type Debug_Panel_Record is
>>       new Gtk.Scrolled_Window.Gtk_Scrolled_Window_Record with
>>    record
>>       ...
>>    end record;
> 
> I'm surprized this is possible to write such a thing in Ada.
> What does the compiler do with this ?

The public view is Gtk_Widget_Record [with no record members], the full 
view is Gtk_Scrolled_Window_Record [with record members you specified].

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de