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From: DrPi <314@drpi.fr>
Subject: Re: GtkAda : Trying to derive a widget
Date: Fri, 9 Apr 2021 13:42:02 +0200
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Le 09/04/2021 à 08:18, Dmitry A. Kazakov a écrit :
> 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
> 
Ah. I have to remember this.
That makes "Standard" some sort of reserved keyword.


>>> 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.
> 
Surprising behaviour for a strongly typed language but there surely is a 
good reason.

>>> 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].
> 
Ok.