Re: discriminant questions

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

On Sun, 18 Sep 2011 05:41:24 -0700 (PDT), ytomino wrote:

>> But you can have P.Element which is much more comfortable. There is no
>> reason to expose access semantics if you can avoid it. And smart pointers
>> are to hide pointers rather than to expose them.

> If it hides semantics of access type,
> Ada.Containers.Indefinite_Holders gives good interface.

One problem of Indefinite_Holders is the definition of "=",  it is wrong
because Holder is declared tagged. When tagged, "=" must be doubly
dispatching. The correct definition should at least be:

   function "=" (Left : Holder; Right : Holder'Class) return Boolean;
        -- Emulates double dispatch by cascaded dispatch

But this is only an indicator of a bigger problem, which is that the design
does not support Element_Type classes. I understand that they didn't want
Holder'Class but must allow it to support prefix notation. This is how
language design problems get reflected in the container design!

Anyway, in my experience smart pointers become almost useless without
classes. I.e. you want a hierarchy of elements accompanied by a hierarchy
of smart pointers/handles/holders (name it as you wish). You also need
*same* set of operations implemented by both, such that operations on the
pointer would be delegated to the object. I.e. you don't want:

   X.Element + Y.Element

you want

   X + Y

instead. Even less you want pseudo-functional mess like:

   X.Update (Invert'Access);

you do

   Invert (X);

> (if reference-counted implementation is provided.
> but http://gcc.gnu.org/svn/gcc/trunk/gcc/ada/a-coinho.adb is not.)

My implementation uses reference counting, because its intended use was to
be a smart pointer. The intended use of Indefinite_Holders is not smart
pointers. It is only a definite wrapper for an indefinite content, only
this nothing more. Smart pointer is also definite, but much more than just
that. From smart pointers you expect them being comparable, delegating
operations, managing shared objects, supporting serialization, task safety
and so on.

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