what does preelaborated package mean?

what does preelaborated package mean?

[mitch]

hi Ada experts,

I am studying gnat code, and I see it says in few places that this
package is 'preelaborated' followed by "can thus be with'ed by 
preelaborated library units".

what exactly does this mean? I know what elaboration means, which is
done at run time. So, a package that is 'preelaborated', when does
this happen? and if this is a good thing, why not do everything like
that? 

thanks,
mitch.

Re: what does preelaborated package mean?

[Matthew Heaney]

In article <7v3m2n$2cpo@drn.newsguy.com> , mitch@nospam  wrote:

> what exactly does this mean? I know what elaboration means, which is
> done at run time. So, a package that is 'preelaborated', when does
> this happen? and if this is a good thing, why not do everything like
> that?

Basically, it means that you can't do any initialization of local
objects.




--
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hypotheses that seem true and then testing them -- trying, almost
perversely, to negate them, elevating only the handful that survive to
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interested only in seeking out data that support it.

George Johnson, NY Times, 15 Aug 1999

Re: what does preelaborated package mean?

[Tucker Taft]

Matthew Heaney wrote:
> 
> In article <7v3m2n$2cpo@drn.newsguy.com> , mitch@nospam  wrote:
> 
> > what exactly does this mean? I know what elaboration means, which is
> > done at run time. So, a package that is 'preelaborated', when does
> > this happen? and if this is a good thing, why not do everything like
> > that?
> 
> Basically, it means that you can't do any initialization of local
> objects.

You can't do any *dynamic* initialization.
You can do static initialization.  As a rough equivalent, you
can do the kind of static initialization legal in "C" code, where
there is no run-time elaboration, though there is link-time
initialization of constants and variables.  

The goal of "preelaboration" is to eliminate the need to execute
a run-time elaboration routine for the specified package, and rely
strictly on the kind of initialization that most linkers/loaders can
perform.

-- 
-Tucker Taft   stt@averstar.com   http://www.averstar.com/~stt/
Technical Director, Distributed IT Solutions  (www.averstar.com/tools)
AverStar (formerly Intermetrics, Inc.)   Burlington, MA  USA

Re: what does preelaborated package mean?

[Robert Dewar]

In article <38162057.79E96DDC@averstar.com>,
  Tucker Taft <stt@averstar.com> wrote:
> The goal of "preelaboration" is to eliminate the need to
execute
> a run-time elaboration routine for the specified package, and
rely
> strictly on the kind of initialization that most
linkers/loaders can
> perform.


Maybe, but it got very confused in the design into serving
multiple purposes, related to distribution and dynamic
elaboration checking.

We added a restriction identifier to GNAT

pragma Restrictions (No_Elaboration_Code);

which achieves what Tuck talks about in a much more direct
manner without getting tied up in these other entanglements.

This pragma is both more and less restrictive than pragma
Preelaborate.


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