Re: problems with classes

[Martin Krischik <krischik@users.sourceforge.net>]

Rick Santa-Cruz wrote:

> Hi,
> 
> 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?
> 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?

Because die declared Parent_Proc after the "full view" of Child_Class. To
declare the "full view" of Child_Class Parent_Proc need to be "frozen".

When a type is "frozen" you can't make any changes to it. And for a class
that means, you can't add another method.

Try:

package Source_2 is
    type Parent_Class is tagged
        record
            Number: Integer;
        end record;

    procedure Parent_Proc(P: Parent_Class);

    type Child_Class is new Parent_Class with
            record
                Number2: Integer;
            end record;

     procedure Child_Proc(C: Child_Class);
end Source_2;

And:

package Source_1 is
     type Parent_Class is tagged private;
     type Child_Class is new Parent_Class with private;
 
    procedure Parent_Proc(P: Parent_Class);
    procedure Child_Proc(C: Child_Class);
 
private
    type Parent_Class is tagged
        record
            Number: Integer;
        end record;

        type Child_Class is new Parent_Class with
            record
                Number2: Integer;
            end record;
end Source_1;

And see what happens.

Last not least: Usually one defines only one class per package. That's
unlike C++ where tons of classes share one namespace.

With Regards

Martin

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