Re: private classes

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

Rick Santa-Cruz wrote:

> Hi,
> 
> sorry for so many question, but always when I thought I understand it, I
> read further and see a new problem... so given the following source-code:
> package Classes is
>  type Base_1 is tagged private;
> 
>  type Derived_1 is new Base_1 with private;
> 
>  procedure Proc(B: Base_1);
> 
>  private
>   type Base_1 is tagged record
>    Number: Integer;
>   end record;
> 
>   type Derived_1 is new Base_1 with null record;
> end Classes;
> 
> package body Classes is
>  procedure Proc(b: Base_1) is
>  begin
>   null;
>  end Proc;
> end Classes;
> 
> with Classes;
> 
> procedure main is
>  D: Classes.Derived_1;

     D_Class : Classes.Base_1'Base renames Classes.Base_1'Base (D);

> begin
>  Classes.Proc(D);

     Classes.Proc(D_Class);

> end Main;
> 
> I get an error in calling Classes.Proc(D). Although I thought that cause
> Derived_1 inherits from Base_1 the call should be possible. In fact
> exactly this I found in the book from John English in chapter 14.5. Why
> can't I compile such, although I thought the function Proc is visible for
> clients of the package Classes.

It's the strong typing in Ada. For a C++ programmer used to using pointers
and references which convert itself this might come as a suprise. Here you
should remember that "this" is indeed a pointer.

> My second question is then, if the above does not work

Well it does work with the fix I have made. You should note that Ada does
not automaticly slice objects. This has advantages and disadvantages. I
show you the advantage:

type Base_1_Access is access Base_1;

D_Pointer = new Base_1'(D_Class);

In C++ the operator new would slice D down to Base_1 - that is all the
Derived_1 information will be lost and only the copy constructor of Base_1
is called. In Ada it isn't. Ada will copy the complete object and use the
Adjust from Derived_1. And that allows for the equivalent of "vector
<Base_1>" to be as usefull as "vector <Base_1*>". 

{Ada.Containers.Vector will be added in Ada 2005 - Test versions are
available on the net.}

As a price you pay for this quite handy feature you need to use Base_1'Base
whenever you don't know what actual class an object could be.

With Regards

Martin
-- 
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