Re: Idiom for a class and an object in Ada

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

On Wed, 20 Oct 2004 07:24:31 GMT, Matthew Heaney wrote:

> For reasons I can't fathom, many Ada95 developers still have a very
> Ada83 mindset.

Probably because Ada 83 was good! (:-)) 

> package P is
> 
>    type T (<>) is limited private;
> 
>    procedure Op (O : in T);
> 
>    function Object1 return T;
>    function Object2 return T;
> 
> private
> 
>    type T is limited record ...;
> 
> end P;

This approach is difficult to follow with read-write objects, because it
then forces to use indirection.

> This is exactly how Text_IO works.  The objects returned by functions
> Standard_Input, Standard_Output, etc, correspond to the functions
> Object1 and Object2 above.

Yes and the price is heavy:

   procedure Put(File : in File_Type; Item : in Character); 

writes into *in* File! Even if File_Type is considered to be an "iterator",
"access window" etc, rather than the file, then definitely Put and Get
should *change* it. So what is File_Type? It is a language construct to
provide a work-around for the pattern above, i.e. a hack. A bit watery for
an IDIOM.

> This is the canonical Ada95 idiom for controlling instance creation, and
> for declaring well-known objects.

Yes, but it lacks an ability to create non-constant objects on the stack,
and it requires an extra indirection level. There should be a way for
forward object declarations.

   type X (<>) is limited private;
   A : X renames private; -- Forward rename
or so
   A : X := private; -- Forward initialization
or
   A : private X;
private
   type X is record
      Field : Integer;
   end record;
   A : X := (Field => 123); 

Then of course there should be a way for in-place modification of the
result for by-reference types and copy-out/copy-in for all others.
Anonymous access results are good, but I'd prefer a more elaborated
solution without access types.

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