Re: The "()" operator revisited.

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

On Thu, 15 Jan 2004 11:09:27 +0000 (UTC), Georg Bauhaus
<sb463ba@l1-hrz.uni-duisburg.de> wrote:

>Dmitry A. Kazakov <mailbox@dmitry-kazakov.de> wrote:
>: On Thu, 15 Jan 2004 01:21:16 +0000 (UTC), Georg Bauhaus
>:>Hm. How do you know then that you get an Apple from some Set, and
>:>not an Orange? Is there a run time penalty?
>: 
>: Which one? Templates do it at compile time anyway, so the type tags
>: are statically known.
>
>That is, if I have, in a unit
>procedure generic_array_shuffle(box: Fruit_Array);

No, a "generic" shuffle should be:

procedure generic_array_shuffle(box: Fruit_Array'Class);

>and then in another unit
>declare
>   A: Apple_Array;
>begin generic_array_shuffle(A); end;
>
>declare
>   O: Orange_Array;
>begin generic_array_shuffle(O); end;
>
>the compiler would not be informed about component sizes when
>compiling the unit containing generic_array_shuffle, right?
>Will it then have to, uhm, instantiate a procedure for the two
>calls above with component sizes adjusted, behind the scenes?

Maybe. In your example generic_array_shuffle might be inherited or
overridden. In which case the compiler could take an advantage of
knowing the constraint. Then, the procedure could be inlined, so again
the compiler could re-instantiate its body. Presently optimizations of
this kind are not required. I would define some of them in RM to make
them mandatory.

Futher, if neither applied, what would be a penalty as compared with a
variant based on a generic procedure, which instances are reusing the
body? Actually in the static case, either the constraints have to
propagate into the body, making its reuse impossible, or not. Whether
the body itself is a template or a class-wide is of little matter.

BTW, as Knuth said: "We should forget about small efficiencies, say
about 97% of the time: premature optimization is the root of all
evil." What is the real weight of possible penalties of this kind in a
software project of a real size and application field? I have no
figures, but I suppose it is vanishingly small.

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