Re: Elaboration Order

[Robert Dewar <robert_dewar@my-deja.com>]

In article <FL8IuH.677@sd.aonix.com>,
  "Wolf-Dieter Heker" <heker@aonix.de> wrote:
> Hence I would suggest that these packages should contain an
> elaborate_body pragma (except, of course, if the have no
body). But alas,
> most programmers - including myself ;-) - do not write these
pragmas until
> the get into trouble.

Well then they are not following standard coding guidelines,
which is to use pragma Pure if possible, and if not possible,
use pragma Preelaborate, and if not possible use pragma
Elaborate_Body.

> The lack of these pragmas is a portability issue.

Of course the static elaboration model of GNAT eliminates this
portability issue, especially if you follow the ELaborate_All
guidelines suggested by -gnatwl.

> And isn't it true that if
> a package body cannnot be elaborated immediately after the
spec we have
> generally a doubtful design?

No, it is common to have package bodies that are mutually
recursive, and there is nothing whatsoever doubtful about
such a design.


> Now my major question: Why didn't the language designers
choose an approach
> that would make the standard case easy and provide a pragma
> defer_elaboration_of_body for those rare cases, where the user
doesn't want
> the body elaborated immediately after the spec?

Because they are not so rare

> Is there more reason that
> just compatibility with the user unfriendly definition of
> Ada83?

I see nothing especially unfriendly here, and it is not at
all obvious what the right solution is here. Programs that
use elaboration sparingly usually have little trouble. THe
supposedly user friendly suggestion you give of the defer
pragma would be a huge pain in the neck and result in piles
of pragmas for programs that do not need pragmas now.

Remember you only need elaborate pragmas if you actually call
routines during elaboration. Your pragma would need to be used
all over the place even in a program that HAD no elaboratoin
code at all.

To see an alternative approach, read the chapter on elaboration
in the GNAT RM.


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