Re: Problem With Self-Referential Access-to-Subprogram Type

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

On Wed, 05 Nov 2003 11:27:22 +0000, Marius Amado Alves
<amado.alves@netcabo.pt> wrote:

>On Wed, 2003-11-05 at 08:31, Dmitry A.Kazakov wrote:
>> >> Yes, there is a difference. Technically, for an out parameter the
>> >> compiler should still know the object constraints.
>
>> >Only at runtime. Consider Ada.Text_IO.Get (Item : out String). What
>> >constraints is the compiler knowing here?
>
>> It knows that there are bounds which cannot be changed. If Item were
>> of String (1..5) the compiler would probably choose another way of
>> parameter passing than for String (<>) with the consequence that the
>> stack frame would be different.
>
>Irrelevant. This applies to return types just as well.

They are "bad" anyway. A return value is potentially a newly allocated
object. An out parameter is not (copy out does not count). So a
difference does exist.

BTW, there is an AI for returning / initializing limited type objects.
Let's see (:-))

>The original poster has a very good question--which has found no real
>answer yet. My guess is it comes down to the hardware-orientedness of
>Ada.

C++ is also hardware-oriented, but solves this other way, which I do
not like much.

Returning to Ada let's compare:

   type Mysterious;

   function Create return Mysterious;
   Init : constant Mysterious; -- Illegal

   procedure Baz (X : Mysterious := Init); -- Illegal
   procedure Bar (X : Mysterious := 0);  -- Illegal, Ada is not C++!
   procedure Foo (X : Mysterious := Create); -- Illegal
   ...
   type Mysterious is new Integer;
   Init : constant Mysterious := 0;

with

   type Mysterious is private;

   function Create return Mysterious;
   Init : constant Mysterious;

   procedure Bar (X : Mysterious := Create);
   procedure Bar (X : Mysterious := 0);  -- Still illegal
   procedure Foo (X : Mysterious := Init);
   ...
private
   type Mysterious is new Integer;
   Init : constant Mysterious := 0;

Private types are much more prematurely usable! (:-))

BTW there is a more serious problem than premature use. That is an
inability to hide the parameter defaults of a subroutine if the type
has to be private. You have to expose Init and Create, even if their
direct use is undesirable.

> I was surprised the *first* form compiled.

Well, now the language lawyers have to say ... (:-))

---
Regards,
Dmitry Kazakov
www.dmitry-kazakov.de