Re: Elaboration in GNAT

[dewar@gnat.com (Robert Dewar)]

Mark Johnson <Mark_H_Johnson@Raytheon.com> wrote in message news:<3C39D954.58672C0A@Raytheon.com>...
> Robert Dewar wrote:
> > 
> > Some things to remember here:
> > 
> > 1. The static elaboration model of GNAT is primarily intended for the development of new code.

> Hmm. I guess I disagree with that statement. See reasons 
> below.

You can't really disagree with the designer of a feature
telling you its primary design intention :-) Of course it
is just fine if it can be used for legacy code, and it is
always good to hear of such successes.

> > 3. If you are working with legacy code, it often makes 
> > no sense to try to use the static model of GNAT

Often /= never! My point, which I guess was confusing was
that it often does not make sense to put in a huge amount
of work to restructure code to meet the static model
 
> Yes and no. Two examples follow.

i.e. you are agreeing, sometimes yes, and sometimes no, 
which is exactly what I was saying

> As Robert's message goes on to say - complicated
> elaboration code is likely filled with bugs anyway. =

That's misreading my message, the bugs I was talking about
are failure to have all the pragma Elaborate's in place, 
but that is very specific.

The paradigm that causes most trouble with the static model
is:

    with X;
    package Y is
       ...
       Q : integer := X.Func;
       ...

where X is a giant package with a large number of with's,
but you happen to know that Func is a little function
that does not use any of these with's. So it is in fact
safe to use pragma Elaborate, instead of pragma Elaborate_All and
everything will work.

The coding philosophy of the static model of GNAT rejects
this coding style on the grounds that it is improper for
a client of X to know anything about the implementation
of the body of X (and any of its subprogram). Yes, that's
a legitimate coding philosophy, but that does not mean that
any code that does not follow this philosophy is buggy :-)

That being said, any program which does a lot of computation at
elaboration time is worrisome. That's
especially so if you have library tasks, which can
start up before elaboration is complete :-( (see special
section on elaboration and tasking in the GNAT docs).

Robert Dewar