Re: problems with classes

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

On Thu, 30 Sep 2004 23:20:47 +0200, Rick Santa-Cruz wrote:

> First of all I wanna thank all people in this group for the help. I am just 
> a beginner in Ada and before I started Ada, I thought that cause I know C++ 
> very well, it would be easy for me to understand Ada (in fact I made this 
> experience with learning programmin languages like Java, Perl, C#, etc. 
> after working a lot with C++). But this assumption was totally wrong. The 
> syntax of Ada seems to me a bit strange, at least at the moment ;), but I am 
> working hard to become better in Ada...
> 
> Here are 2 examples of source-codes:
> 1. Source1:
> package Source_1 is
>     type Parent_Class is tagged
>         record
>             Number: Integer;
>         end record;
>     type Child_Class is new Parent_Class with private;
> 
>     procedure Parent_Proc(P: Parent_Class);
>     procedure Child_Proc(C: Child_Class);
> 
>     private
>         type Child_Class is new Parent_Class with
>             record
>                 Number2: Integer;
>             end record;
> end Source_1;
> ---------------------------------------------------------------------
> 1. Source2:
> package Source_2 is
>     type Parent_Class is tagged
>         record
>             Number: Integer;
>         end record;
>     type Child_Class is new Parent_Class with
>             record
>                 Number2: Integer;
>             end record;
> 
>     procedure Parent_Proc(P: Parent_Class);
>     procedure Child_Proc(C: Child_Class);
> end Source_2;
> 
> My question is now, why in the seconde code-example I get an error, that 
> Parent_Class has to be declared ready, before I define the Child_Class? Why 
> I don't get this error in the first case, cause there I specify the 
> Child_Class with: "type Child_Class is new Parent_Class with private;", 
> before I declare the procedure of the class Parent_Class?

In the first case the type Child_Class is not fully defined at the
declaration point of Parent_Proc. In the second case it is, which in effect
freezes Parent_Class, so that Parent_Proc cannot be declared as a primitive
operation. You can modify Source_2 as follows:

package Source_2 is
   type Parent_Class is tagged
      record
         Number: Integer;
      end record;
   procedure Parent_Proc(P: Parent_Class); -- Before freezing point, now

   type Child_Class is new Parent_Class with
      record
         Number2: Integer;
      end record;
   procedure Child_Proc(C: Child_Class);
end Source_2;

> If this would be C++, I think I would know the answer... so in C++ 
> "something like this" exists, too. It is called pre-definition of a type. 
> That means that, for example if I need the name of a type before I wanna 
> specify it, I can just write:
> class MyType; and specify it later. Is this the case in Ada too?

Close, but not quite. In your example you define the public view of a type
and later define its implementation. What you meant is a forward type
declaration, it looks exactly like in C++:

type Something; -- Forward declaration

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